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Quarterly Actinide Research

3rd/4th quarter 2003

Los Alamos National Laboratory

N u c l e a r M a t e r i a l s R e s e a r c h a n d T e c h n o l o g y

Highlights of thePlutonium Futures—The Science Conference 2003

Nuclear Materials Technology/Los Alamos National Laboratory

Actinide Research Quarterly

NMT Division DirectorSteve Yarbro

Scientific Advisors,Seaborg InstituteDavid L. ClarkGordon D. Jarvinen

EditorMeredith S. Coonley, IM-1

DesignerSusan L. Carlson, IM-1

Contributing WritersVin LoPresti, IM-1Octavio Ramos, Jr., IM-1Charmian Schaller, IM-1

Contributing PhotographerMick Greenbank, NMT-3

Printing CoordinationLupe Archuleta, IM-4

Address correspondence toActinide Research Quarterlyc/o Meredith CoonleyMail Stop E500Los AlamosNational LaboratoryLos Alamos, NM 87545

If you have questions,comments, suggestions, orcontributions, pleasecontact the ARQ staff [email protected].

Phone (505) 667-0392Fax (505) 665-7895

Actinide Research Quarterly highlights recent achievements and ongoingprograms of the Nuclear Materials Technology (NMT) Division. We welcome yoursuggestions and contributions. ARQ can be read on the World Wide Web at:http://www.lanl.gov/orgs/nmt/nmtdo/AQarchive/AQhome/AQhome.html

In This Issue

1 Plutonium Futures—The Science Conference 2003

4 Plutonium: Whether boon or risk,both viewpoints must be respected

6 A plutonium potpourri

9 Plutonium-238 metal as a multiphase system

12 Nanoscopic approachesto aquatic plutonium chemistry

17 Comparison of aqueous phase binding constantswith tetra- and hexavalent actinides

20 Researchers use high-resolution inelasticx-ray scattering and a microbeam approach

23 Investigations of protactiniumand americium metals provide important insights

29 Researching the phytoremediation of plutonium

35 Plenary speakers discussthe many avenues of research

38 Point/Counterpoint

39 Five views on opportunities andconstraints for actinide research

44 The conference in a nutshell

Actinide ResearchQuarterly is producedby Los AlamosNational Laboratory

LALP-03-067


Nuclear Materials Technology/Los Alamos National Laboratory 1

Plutonium Futures—The Science Conference 2003


1Nuclear Materials Technology/Los Alamos National Laboratory

Conference participantsreceived a speciallydesigned coin (shownat far right) encased witha sample of Trinitite, agreenish, glasslikesubstance found onlyat ground zero at TrinitySite in southern NewMexico. The man-mademineral and was formedon July 16, 1945, whenthe desert sand wasmelted by the intenseheat of the world’s firstnuclear explosion andthen solidified. The coinwas designed by LosAlamos' Jay Tracy.

More than 1,000 tons of plutonium exist throughout theworld in the form of used nuclear fuel, nuclear weaponscomponents, various nuclear inventories, legacy materials

and wastes. It is clear that large inventories of plutonium must be pru-dently managed for many decades. A complex blend of global political,socioeconomic and technological challenges must be overcome tomanage these inventories efficiently and safely.

From a technological perspective, plutonium is one of themost complex elements in the Periodic Table. The metal exhib-its six solid allotropes at ambient pressure and its phases arenotoriously unstable with temperature, pressure, chemicaladditions and time. Plutonium sits near the middle of theactinide series, which marks the emergence of 5f electronsin the valence shell. Elements to the left of plutonium havedelocalized (bonding) electrons, while elements to the rightof plutonium exhibit more localized (non-bonding) character.Plutonium is poised in the middle, and for the delta-phasemetal, the electrons seem to be in a unique state of being neitherfully bonding or localized, which leads to novel electronic interactionsand unusual physical and chemical behavior. The concept of localizedor delocalized 5f electrons also pervades the bonding descriptions ofmany of the plutonium molecules and compounds. An understandingof the electronic structure of the pure element and its compoundscontinues to challenge both theorists and experimentalists in all areasof plutonium science.

The “PlutoniumFutures—The ScienceConference”was es-tablished to increaseawareness of the im-portance of pluto-nium research andfacilitate communica-tion among its inter-national practitioners.


Nuclear Materials Technology/Los Alamos National Laboratory2

PlutoniumFutures—

The Science2003

2 Nuclear Materials Technology/Los Alamos National Laboratory


Moreover, we hope that this series of conferences will stimulate the nextgeneration of scientists and students to study the fundamental proper-ties of plutonium. The 2003 conference, held in July in Albuquerque,N.M., was the third in this series and attracted more than 328 partici-pants from 63 institutions and 12 countries. We were also encouragedby the participation of 42 students, an increase over past years. Morethan 180 contributed presentations covered the latest results in pluto-nium condensed matter physics, materials science, compounds andcomplexes, environmental behavior, detection and analysis, separationsand purification, nuclear fuel cycles, and waste isolation and disposal.

In this issue of Actinide Research Quarterly, we present some highlightsof the 2003 conference in the form of short articles from conference par-ticipants, interviews with poster presenters, and reports from ARQ staff.

By increasing the international dialogue through conferences andpublications, we hope to facilitate the developing renaissance in pluto-nium science that will enable efficient solutions to Cold War legacyproblems and provide maximum benefits for society from the enormousenergy potential of plutonium.

—Gordon D. JarvinenDavid L. Clark



To assess the environmental impact oftransuranium elements (neptunium,plutonium, and americium) on nuclearwaste repositories, scientists mustbetter understand the solid-statechemistry of transuranium compounds.Researchers are particularly interestedin how these elements might react withfission-product radionuclides, such asiodine-129 in its oxidized and reducedforms, such as the iodide (I–) or iodate(IO3

–) ions. Iodate is of particularinterest for nuclear waste.

“We used a process known ashydrothermal synthesis (mineralsynthesis in the presence of heatedwater) to produce single crystals, whichwe used to obtain crystal structuresand Raman spectra of five noveltransuranium iodates,” said AmandaBean of Los Alamos. “Our end goal wasto look at the fundamental chemistry ofthese iodates for the first time.”

The five transuranium iodates are as follows: NpO2(IO3)2(H2O),NpO2(IO3)2•H2O, PuO2(IO3)2•(H2O), and two forms of Am(IO3)3.This research presents a vital first step in applying a syntheticapproach to unraveling the structural properties of materialsthat contain the series of the light actinide elements. With suchstructural properties in hand, researchers will be better able toaddress issues related to long-term storage and disposition ofnuclear materials.

—Octavio Ramos, Jr.

Structural Trends and Bonding of the 5f-Elements (U–Am)with the Oxoligand IO3

–

A. C. Bean,1 B. L. Scott,1 T. Albrecht-Schmitt,2 and W. Runde1

1Los Alamos National Laboratory and 2Auburn University

Postersession

highlights

AmandaBean



KeynoteSpeaker

Plutonium evokes the entire gamut ofhuman emotions, from good to evil,from hope to despair, from the salva-

tion of humanity to its utter destruction.”With typical dramatic flair, Los Alamos Senior

Fellow Sig Hecker cut the ribbon on theopening morning of the conference.

Adding the obvious, that “there’s noother element in the periodic table thatshares such a burden,” Hecker then surgedinto a historical perspective. Its backdropwas the dual-use scenario of domesticpower and nuclear weapons—drawingenergy from an atomic nucleus that can betapped for “a factor of increase in millionsof energy production.”

Beginning with President Eisenhower’s1953 Atoms for Peace initiative, Hecker out-lined the period of intense international col-

laboration on the peaceful uses of atomic en-ergy that spanned the period from the 1955First International Congress in Genevathrough the end of the Cold War. In Hecker’sview, the late 1980s marked a period of dimin-ished collaboration, while the first PlutoniumFutures Conference in 1997 symbolized the“attempt to regenerate” the spirit of interna-tional collaboration in plutonium science.

Hecker’s post-1997 review adopted asociopolitical angle, citing the cooperative en-deavors whereby the United States and Russiacontinue to reduce their arsenals of nuclearweapons. He reviewed such initiatives asburning excess weapons plutonium for peace-ful energy production and blending-downRussia’s declared 500-ton excess of highlyenriched uranium to low-enriched uranium.Some of the latter has already been purchasedby the United States for its light-water electric-ity-generating reactors. In addition, Heckerdiscussed what he characterized as significant

enhancements in the security of nuclear mate-rials and a reduction in the level of nuclear-materials trafficking.

On the flip side, Hecker raised concerns de-riving from the “horizontal proliferation” ofnuclear weapons in Asia, including radiologi-cal-terrorism concerns following the events of9/11/01, which “indicate that we do not livein a very peaceful world.” He also commentedon the challenges of dealing with the legacywaste of the nuclear era, citing specific in-stances in both the United States and Russiaand including the decommissioning anddismantlement of the latter’s nuclearsubmarine fleet.

Never one to end on a sour note, Heckernoted the resurgence in nuclear-power re-search in the United States, mentioning bothfuel-cycle initiatives and the positive connota-tions of “discussions about the construction ofnuclear power plants . . . for the first time in acouple of decades.”

Referring to opinions that plutonium is ei-ther our greatest boon or our greatest risk,Hecker cautioned that “we must respect bothpoints of view.” He then rolled into the day’sevents by declaring that “no matter what youbelieve about the political challenges, scientifi-cally, plutonium is without question the mostcomplex and fascinating element . . . andthat’s, after all, why we’re here this week.”

Finally, in calling for a continued interna-tional collaboration to solve scientific andsociopolitical challenges, Hecker concludedby expressing the hope that “this conferencewill, in particular, stimulate interest in theyounger generation in carrying on the chal-lenges and all those things that we have yetto learn associated with plutonium and theother actinides.”

—Vin LoPresti

Keynote speaker Sig HeckerPlutonium: Whether boon or risk,both viewpoints must be respected

Sig Hecker

“



PosterSession

Highlights

Theory of Positron Annihilation in Helium-Filled Bubbles in Plutonium

P. A. Sterne and J. E. Pask

Lawrence Livermore National Laboratory

Scientists at Lawrence Livermore National Laboratory (LLNL) have developed a first-principles, finite-element-based method to calculate positron lifetimes for defects inmetals. This method can treat system cell sizes of several thousand atoms, thusenabling researchers to model defects in plutonium that range in size from mono-vacancy to helium-filled bubbles of more than 1 nanometer in diameter.

“Our objective is to perform sometheoretical calculations that can helpus interpret positron annihilationexperimental data,” said Phillip Sterneof LLNL. “Recent experiments atLivermore have identified two lifetimecomponents in aged plutonium samples,one at around 182 picoseconds andanother at about 350 to 400 picoseconds.But what do these values mean? What’sthe interpretation? To understand whatthey mean, we’ve been doing first-principles calculations using a modelingtechnique that can help us interpretwhich defects are responsible for thelifetime components we are observingduring our experiments.”

Sterne noted that positrons are greatprobes for defects. Positrons areattracted to the nanosized defects foundin the alloy. As the positrons decay, theyrelease energy in the form of gammarays. Gamma rays produce a distinctsignature that helps scientists identify thesize, quantity, and type of defect in an alloy.

When helium atoms are introduced into vacancy clusters, the positron lifetime isreduced. The modeling studies indicate that the experimentally observed positronlifetimes in aged plutonium metal are consistent with the presence of two to threehelium atoms per vacancy.


RetiredLos AlamosScientist

Robert Penneman

(left) and

PhillipSterne



PlutoniumTutorial

TutorialA plutonium potpourri

Graham Andrew

Dave Clark

Challenges in plutoniumphysics and chemistry

Seaborg Institute Director David Clarkkicked off an approximately four-hourplutonium tutorial designed to

“stimulate the next generation of scientists andstudents.” He framed his discussion in themost interesting way possible, that is, in termsof the metal’s chemical and metallurgical pe-culiarities. From the uniqueness of its oxida-tion-state behavior (whereby, under certainconditions, plutonium solutions can simulta-neously exhibit the presence of three or fourdifferent oxidation states) to the different crys-tal structures and physical properties of its al-lotropes (physical phases), the discussion accu-rately reflected the reason why so many scien-tists worldwide are engaged in the study of asingle chemical element and its compounds.

“I’m going to walk you through some of theinteresting scientific challenges that will be ad-dressed at the conference,” Clark offered, andhe quickly delivered on the promise, coveringthe areas of stockpile, materials, and environ-mental stewardship. His focus was the phe-nomenon of nuclear materials aging, and heused an example of an aged tanker that hadrecently broken apart off the coast of Spain togenerally frame the issue’s importance.

Clark did not ignore the fact that, as scienceoften does, a seemingly completely negativecharacteristic has also been harnessed for posi-tive applications. Most notable was the use ofplutonium-238 as a heat and power source inspace missions, particularly the Mars Roverprojects in which Los Alamos plutonium scien-tists have been intimately involved. Addition-ally, Clark predicted that because of self-irra-diation-based changes occurring in plutonium-based superconductors over time, “plutoniumwill teach us quite a bit about superconductiv-ity mechanisms.”

IAEA activities in plutoniumnonproliferation and security

The tutorial’s second presentation, given byGraham Andrew of the International AtomicEnergy Agency (IAEA), proved to be a perti-nent segue, particularly in light of recent inter-national events precipitating concern overnuclear terrorism. He reviewed how theagency functions and interacts with nationalstates and emphasized the ongoing challengeimplicit in “checking whether what you mea-sure can possibly be what you’re told.”

Of particular interest were new analyticaltools developed to assay plutonium samplesfor isotope ratio,as well as to de-termine the con-tent of minoractinides suchas neptunium,americium, andcurium. Also ofinterest was therevelation that,in addition to itscentral offices inVienna, theagency main-tains laborato-ries at certainlarge nuclear-fuel reprocess-ing plants such as the one at Rokkashamura,Japan. This need was highlighted by the statis-tic that mixed-oxide (MOX) fuel requirementsfor commercial light-water reactors compriseabout 190 metric tons per year, compared withonly 24 tons per year currently being producedby reprocessing.

Andrew also discussed the limitations of tra-ditional safeguards and the efforts to expandsafeguards by getting nations to “sign on tomore wide-ranging access and forensic envi-ronmental sampling . . . from mines to nuclearwaste” and via expanded inspector training to



Annie Kersting

Edward Arthur

help enhance each individual inspector’s rec-ognition capabilities. This concern extended tothe proliferation resistance of future energysystems, in the sense of “making that nuclearmaterial less attractive to a proliferator.” ThisIAEA initiative is, of course, particularly rel-evant since the events of 9/11/01, with the sub-sequent increased concerns about sabotage andradiological terrorism (“dirty bombs”).

The nuclear fuel cycleand new initiatives

In comparing the broad outlines of so-called“once-through,” “European/Japanese,” and“advanced proliferation-resistant” nuclear fuelcycles, Edward Arthur’s presentation was emi-nently pragmatic. Arthur, former Los Alamosacting office director for nuclear technologyand applications, cited the requirement for re-processing of spent fuel as the first step in anyof the cycles. He then made it clear that thecost of spent-fuel reprocessing did not cur-rently compare very favorably with thereprocessed fuel’s electricity value.

He stressed that “front-end” operations suchas decladding, storage, and separations, andback-end waste-management componentscollectively accounted “for 75 percent of thecost of a fuel reprocessing facility.” But he wasalso unequivocal in his view that economicissues had to be “looked at from an overallsystem perspective.”

Regardless of the obstacles, the U.S. Senate’sFY04 budget includes a funding target of $78million for the multi-laboratory Advanced FuelCycle Initiative and authorizes a study of ademonstration hydrogen-producing nuclearpower plant. Ultimately however, to achievethe goals of Power for the Twenty-First Cen-tury, reprocessing must handle tens of thou-sands of tons of spent nuclear fuel. It must alsodeal with such factors as radiation, criticality,and chemical hazards such as strong acids andsolvents—what Arthur characterized as “majortechnical issues still facing reprocessing.”

Colloid-facilitatedtransport of plutonium

The afternoon’s final presenter wasAnnie Kersting of Lawrence LivermoreNational Laboratory. “What we reallywant to understand is the life cycle of acolloid,” Kersting said, and she pro-ceeded to explain why this was a vitalgoal to chemists trying to sort out thecomplexities of plutonium’s interactionwith the natural environment.

Mobile particulates generally smallerthan 1 micron, colloids originate from adiversity of environmental sources; andboth organic (carbon-based) and inorganic(mineral-based) colloids are ubiquitous in soiland water. Understanding colloids and their sig-nificance in environmental transport of pluto-nium and other actinides thus entails a combina-tion of geology, hydrology, and chemistry, asKersting’s presentation illuminated at every turn.

As such, she discussed the need for fieldstudies to firmly foot laboratory experimentsand theory in the local geology and hydrologyof sites such as Rocky Flats and SavannahRiver. Such local geochemistry determinesif colloids will or will not facilitate thetransport of plutonium or other actinidesthrough the environment.

Kersting explained that these studieshave so far painted an incredibly complexlandscape for actinide transport, since bothadsorption and desorption had to be as-sessed and because “measuring the con-centration of colloids in water is very diffi-cult.” For example, “you may see pluto-nium sorb very strongly to say silica andiron oxide, and then you go down a fewmeters, and it may desorb off that silica butstill stay on the iron oxide.”

Kersting summarized her talk by fram-ing the overall issue as an imperative tounderstand the fundamental chemistryof actinide nanoparticles.

—Vin LoPresti



PosterSession

HighlightsUnderstanding and Predicting Plutonium Alloys Aging: A CoupledExperimental and Theoretical Approach

N. Baclet,1 P. Pochet,1 Ph. Faure,1 C. Valot,1 L. Gosmain,1 Ch. Valot,2 J. L. Flament,3

and C. Berthier3

1CEA–Centre de Valduc (France), 2LRRS, Université de Bourgogne (France), and3CEA–Ile de France (France)

Defects in a plutonium alloy begin at theatomic level. Once created, such defectscan diffuse, leading to changes in thephysical properties of the alloy. Suchchanges in turn may affect the reliabilityand safety of a nuclear weapon.

To better understand and thus betterpredict aging effects of plutonium alloys,scientists in France have embarked on amultiscale approach that involves boththeoretical and experimentalcomponents. The two-pronged challengeis to identify which aging parameters arerelevant for modeling and developexperimental techniques that canmeasure such physical parameters.

One parameter relevant for modeling isthe interatomic potential, which is thekey to modeling displacement cascadesusing molecular dynamics. The

interatomic potential is intimately related to the elastic constants of the plutonium-gallium alloys of interest. Scientists in France are developing x-ray diffractiontechniques to determine these values.

“We have tested this technique on pure copper and rhodium samples becausethe mechanical properties of these materials are similar to those of delta-plutonium,”said Nathalie Baclet of CEA–Centre de Valduc. “Preliminary results are promising.We are now improving the technique so that it takes into account thematerial’s texture.”


NathalieBaclet



ContributedPapers

Sergey I.Gorbunov

Previous investigations at the ResearchInstitute of Atomic Reactors ofplutonium metal containing more

than 80 percent plutonium-238 show a num-ber of physical properties in the metal that areconsiderably different from those found in or-dinary “low-radioactivity” plutonium-239.For example, the hydrostatic densities of plu-tonium-238 samples are always substantiallylower than those of similarly treated samplesof plutonium-239 (by 1.1–2.3 grams per cubiccentimeter [g/cm3] and more). More detailscan be found in an article in Radiochemistry(English), 2001, Vol. 43, pages 111-117.

When a special thermomechanical treat-ment is used on plutonium-238 samples, thedensities increase only for a short time. Thedensities decrease quickly after pressure isreleased and the shapes of the samplesbecome distorted.

Other differences between the two isotopescan be seen in the thermograms of plutonium-238 as the temperature is increased. The inten-sities of peaks of the alpha to beta, beta togamma, and gamma to delta (α→β, β→γ, γ→ δ)allotropic transformations are always substan-tially lower than those of the correspondingplutonium-239 material. Plutonium-238 alsohas a temperature coefficient of electrical re-sistivity that is lower in the region of the al-pha-phase field as well as some other differ-ences in the temperature dependence ofelectrical resistance.

A combination of the effects observed inplutonium-238 metal can be explained by itscontinuous and intensive self-irradiation,which results in co-existence of the knownplutonium crystal modifications in this metalat room temperature. To understand the na-ture of these differences, samples of variousages were studied using x-ray diffraction toreveal the crystal structure of the plutonium-238 metal and to discern how specific alpharadioactivity and self-irradiation (aging)affect this structure.

ResultsThe metal samples each had different plu-

tonium-238 content and were not subjected tocompression loads. They were composed ofmicron-thick layers of plutonium-238 con-densed at high temperature and high vacuumonto flat tantalum substrates. Moreover, thesesamples did not contain radiogenic helium inthe initial state.

The table below lists the properties of thesesamples. Samples 1, 2, and 3 were consideredhigh-radioactivity metals, sample 4 was la-beled as medium radioactivity, and sample 5was low radioactivity.

The samples were prepared, kept, andinvestigated under similar conditions.All of them had a comparable purity and,according to the spectral analysis results,did not contain certain impurities in thequantities sufficient to stabilize the “tempera-ture” modifications of plutonium. The x-ray

This article wascontributed by SergeyI. Gorbunov of theFederal State UnitaryEnterprise “StateScientific Centre ofthe RussianFederation–ResearchInstitute of AtomicReactors” (FSUE“SSC RF RIAR”),Russia.

Plutonium-238 metalas a multiphase system

12345

86.586.586.55.81.1

3.50.521108

6–80.8–1.037–4618–2214–18

240240240174

SampleAtomic fraction

of 238Pu, %Mass, mg Thickness, µm

Alpha-radioactivity*

*With reference to 239Pu isotope

Characteristics of plutonium samples



ContributedPapers

analysis was performed within about a one-year aging timeframe of the samples atroom temperature.

Only the alpha-phase diffraction peaks wereobserved in the “low-radioactivity” plutoniumx-ray patterns (sample 5) during the whole pe-riod of the investigation (see table below). Indi-vidual diffraction peaks of the “high-tempera-ture” modifications (beta, gamma, delta, and,probably, delta-prime) were recorded in the

x-ray patterns of the “medium” specific alpha-radioactivity plutonium (sample 4) along withthe strong alpha-plutonium peaks.

A unique phenomenon of the dynamic co-existence of four, five, and even six allotropicmodifications of plutonium was observed insamples 1, 2, and 3 of high-radioactivity metal.

During the long-term aging of high-radioac-tivity samples 1, 2, and 3 and “mid-radioactiv-ity” sample 4, a slow process of changes intheir phase compositions was recorded: a rela-tive decrease in the content of the “ high-tem-perature” low-density modifications and anincrease in the content of the high-densityalpha-phase of plutonium.

In samples 1, 2, and 3, a ratio of the total in-tensity of the (020) and (211) alpha-plutoniumdiffraction peaks to the total intensity of the(111) delta-plutonium, (111) gamma-pluto-nium, and (101) delta-prime-plutonium peakswas designated as criterion C of this process. Anumber of transformation stages can be foundon the curves resulting from plotting the valueof criterion C versus the age of the samplesince fabrication. The figure on the next page

presents the curve for sample 3 asan example. A similar curve wasalso obtained for “mid-radioactiv-ity” sample 4.

The peak-by-peak analysis of thex-ray patterns of the high-radioac-tivity samples shows that the deltaphase in them disappears moreslowly than the other “high-tem-perature” plutonium modifications(see table at left). Nevertheless, at acertain stage of aging these modifi-cations can recur, apparently, as in-termediate phases in the (delta toalpha) transformation. So, insamples 1 and 2 within 40- to 60-day aging, the maximum number ofthe allotropic modifications is re-

corded (five and even six). At the end of long-term aging (about one year), only the alphaand delta phases are observed by x-ray analy-sis of the high-radioactivity samples, and noobvious signs of the “high-temperature”phases were revealed in the “mid-radioactiv-ity” sample.

Some peculiarities were revealed on thecurves showing the atomic volume changes ofthe delta-plutonium lattice in sample 3 and ofthe alpha-plutonium lattice in sample 4. A vol-ume increase at the beginning of self-irradia-tion results in a “hole” (an abrupt decreaseand subsequent recovery of this parameter)after some period of aging. The atomic vol-umes of the indicated lattices reduce slowlyin the course of further aging of the samples.

Sample

1

2

5

120406098

3431

4352

1013391

343

τ*, day α β γ δ δ' εAllotropic plutonium modification**

±+++++±++++++

+±±+±±±+++–––

++++± −±+++–––

++++++ +++++––

+–++–– –+±±–––

–––––– –+±−–––

Changesin the phasecompositions of thehigh-radioactivity(1, 2) and “low-radioactivity” (5)plutonium samplesduring their aging.

* Aging period

** «–» no diffractionpeaks of this lattice;«±» individual peaksof this lattice;«+» sufficient peaksto calculate theparameters of thislattice



DiscussionIn these experiments with thin layers of plu-

tonium-238 metal deposited on a metal sub-strate, we have considerably increased the ra-diation-induced self-damage rate of the metal,but we cannot similarly increase the annealingrate of radiation defects. Therefore, these ex-periments do not cover the accelerated agingof bulk plutonium metal. A quantitativechange in the radiation-defects concentrationleads to observable qualitative changes in themetal properties, but this does not changethe nature of plutonium. An increase in thespecific alpha radioactivity allows a deeperinsight into the fundamental propertiesof plutonium.

The analysis of the experimental resultsshows that the co-existence of the differentallotropic modifications in high-radioactivityplutonium at room temperature is caused bothby intensive self-irradiation and the complexphase diagram of plutonium metal. Radiation-induced vacancies and clusters of vacanciesappear to play a large role in the formationof the “high-temperature” modifications.

The bonding 5f-orbitals in plutonium havea small overlap; therefore, a considerable relax-ation of the lattice can take place near the va-cancy and an area with an increased atomicvolume (fluctuation of atomic volume) ap-pears. The potential energy of the bonding5f electrons belonging to the atoms locatednear the vacancy decreases, the electrons local-ize (they transfer to the fluctuation states), and“fluctuons” (bound states of electrons andfluctuation) appear.

The bonds typical of one of the “high-tem-perature” modifications arise inevitably be-tween these atoms with localized 5f electrons.Thus, vacancies in plutonium apparently arenonlocal; they represent an ensemble of atomswith a different bonding pattern forming in thelattice as an entity.

The behavior of “fluctuons” was investi-gated theoretically by Russian theorist M.A.Krivoglaz. A comparison of properties of thefluctuons with those of plutonium-238 metalallows the so-called “unusual” behavior of thissystem to be explained.

An increase in the vacancy concentrationdue to intensive plutonium self-damage leadsto an appearance of the “high-temperature”modifications already at room temperature.The crystal lattice around the large vacancyclusters arising in the deceleration regions ofrecoils is under tension. It provides relative sta-bility of the “high-temperature” phases.

To explain the slow change in the phasecomposition of high- and “mid-radioactivity”plutonium during long-term aging as well asthe complex multistage nature of this process,researchers proposed another mechanism. Thiswas related to accumulation of radiogenic he-lium and evolution of its forms in the metal,especially in the intragrain cavities. Heliumbehavior also contributes to the changes in theatomic volumes of plutonium lattices underself-irradiation.

Criterion C versusaging period andcalculated content ofradiogenic helium insample 3.

Aging period, day

Cri

teri

on C

Atomic fraction of helium, %

0

0 0.1 0.2 0.3 0.4 0.5

10-3

10-2

10-1

100

101

100 200 300

I II III IV V



ContributedPapers

ThomasFanghänel

The Institut für Nukleare Entsorgung (INE) in Karlsrühe Research Center, Germany, focuses on the basic research

relevant to the assessment of prospectiverepositories for radioactive waste, in-cluding both technical and scientificchemical aspects. The predictive model-ing of long-term actinide mobility in thegeosphere is contingent on basic knowl-edge of the aquatic chemistry of ac-tinides. In this context, plutonium isof special interest.

Plutonium appears in spent fuel insmall amounts (about 1 percent), but af-ter the decay of short-lived fission prod-ucts, plutonium-239 represents the domi-nant radioactive inventory for thousandsof years.

On the one hand, the solubility con-straints of plutonium have led to the per-ception that this element will be immobi-

lized easily in the repository environment. Onthe other hand, this particular property of lowsolubility induces the formation of colloids(tiny, nano-sized particles—either real colloidsor pseudo colloids), which may strongly en-hance plutonium’s potential for mobility in theaquifer. These characteristic properties are ofcardinal importance for our work.

ChallengesDifferent oxidation states (typically III–VI)

of plutonium can coexist in aqueous solutionunder the appropriate conditions, with therelative abundance of each oxidation state de-pending on the chemical conditions such aspH, Eh, and ionic strength. Plutonium exhibitsa complicated redox behavior that permitstransformation of one oxidation state intoanother. For example, the collision of twoplutonium(IV) ions generates one plutonium(III)and one plutonium(V) ion, and theplutonium(V) ion subsequently can be oxi-dized to a plutonium(VI) ion by an additionalcollision with a plutonium(IV) ion.

Since different oxidation states exhibit differ-ent chemical properties, a reliable speciation(with regard to the oxidation states) is re-quired. Our work focuses on the tetravalentplutonium ion, which, in aquatic solution, isknown to be unstable even at low pH in diluteconcentrations, not only because of dispropor-tionation—the interconversion of its ionic spe-cies—but also because of colloid formation.

This instability of dilute plutonium(IV) hasmade the appraisal of its thermodynamic solu-bility in aquatic systems extremely difficult,resulting in a large number of controversial re-sults. Thus, assessing the chemical reactions ofplutonium(IV) in dilute concentrations in thelow pH region requires sensitive speciationand colloid characterization methods.

Our present work combines two differentlaser spectroscopic speciation approaches, pro-viding the possibility of assessing the chemicalreactions of plutonium(IV) at concentrations ofonly a few micromoles per liter or even lower.

This article wascontributed byClemens Walther,Claudia Bitea,Jong Il Yun,Jae Il Kim, ThomasFanghänel, ChristianM. Marquardt, VolkerNeck, and AliceSeibert of theInstitut für NukleareEntsorgung, KarlsrüheResearch Center.

Predicting long-term actinide mobilityNanoscopic approaches toaquatic plutonium chemistry

Thomas Fanghänelof the Institut fürNukleare Entsorgung,Karlsrühe ResearchCenter, Germany,makes a point duringhis discussion ofnanoscopicapproaches toaquatic plutoniumchemistry.

Researchers from the Karlsruhe Research CenterInstitut für Nukleare Entsorgung are using a“homebuilt” laser setup to study the colloidal transportof plutonium in aqueous solutions. In the front row,from left to right, are: Claudia Bitea and Alice Seibert.In the back row, from left to right, are: ChristianMarquardt, Clemens Walther, and Jong Il Yun.



Experimental approachesWe use two methods in our research:

laser-induced breakdown detection (LIBD)for colloid quantification and laser-inducedphotoacoustic spectroscopy (LPAS) forchemical speciation.

LIBD is based on nonlinear interaction ofcolloids with a tightly focused laser beam (seesidebar on page 14). This leads to the forma-tion of a hot, dense plasma, detected eitheroptically (via light emission) or acoustically(via its expansion-generation shockwave).Each plasma event corresponds to a singlecolloid and, when compared with the numberof laser shots, this provides a measure ofcolloid concentration.

Compared with dynamic light scattering(the most frequently applied colloid detectiontechnique), the sensitivity, particularly for col-loids smaller than 50 nanometers, is enhancedby more than six orders of magnitude. Com-pared with the so-called single-particlecounter, a static light-scattering device, theaccessible size range is considerably extendedtoward smaller colloids.

For particles of 20-nanometer diameter, forinstance, the sensitivity corresponds to detect-ing a pinhead one millimeter in diameter in agood-sized hotel swimming pool.

The speciation methods are based on linearlight absorption. Plutonium’s oxidation statesIII, IV, V, and VI exhibit characteristic absorp-tion bands. The absorption strength is directlyproportional to the amount of species present.From the (visible) absorption spectra, a quanti-tative speciation is obtained, typically by UV-visible (UV-Vis) spectroscopy, which measuresthe extinction (i.e., absorption plus scatteringlosses) of white light wavelength resolved af-ter passing through the sample.

However, this method is limited to measur-ing plutonium(IV) concentrations of approxi-mately 10 micromolar. LPAS lowers this limitby a factor of up to 100 by detecting the effectsof the light absorbed by the plutonium ions

(photothermal method) rather than the trans-mitted light. A 10-nanosecond light pulse of atunable dye-laser is guided through thesample, and the energy of the absorbed pho-tons gives rise to a rapid temperature increasewith subsequent expansion and generation ofan acoustic shock wave.

Analogous to the LIBD method, this acousticwave is detected by a piezo detector, but be-cause its energy content is rather low (less thanone nanojoule), the signal is electronically am-plified prior to data recording. The acoustic(and electronic) signal is linearly proportionalto the absorption strength at the specific wave-length of the laser beam. Absorption spectraare obtained by scanning the laser over thedesired spectral range. In addition to the in-creased sensitivity down to the micromole-per-liter range, LPAS (in contrast to UV-Vis) is notinfluenced by the presence of colloids, whichgive rise to light scattering in the blue end ofthe spectrum.

A time evolutionof laser-inducedplasma plumesincited on singlecolloids as observedby an ultrafast (200picoseconds)charge-coupleddevice camera.

This figure shows the sensitivity of laser-inducedbreakdown detection (LIBD) compared withclassical characterization methods. LIBD’ssensitivity is enhanced by more than six orders ofmagnitude compared with dynamic lightscattering, and the accessible size range isconsiderably extended toward smaller colloidswhen compared with the single-particle counter(SPC).

102

10 100

REM/TEM/AFM

light scattering

LIBD

SPC

slope-2

slope-6

particle diameter [nm]

parti

cle

num

ber c

once

ntra

tion

[ml-1

]

1000

103104

105106

107108109

10101011

10121013



The basics of LIBD

Laser-induced breakdown detection (LIBD) is based on plasma formationdue to dielectric breakdown in the high-field region of a focused pulsedlaser beam when a colloid is present. We commonly experience low-energy plasmas in everyday life, ranging from “flames” over neon lightsto the blinding flash of lightning.

The plasma used for LIBD begins with a single (so-called “lucky”)electron, which is created by the high-electric field of the laser. In thelanguage of quantum physics, this is equivalent to the nonresonantabsorption of four to six photons (multiphoton ionization or MPI) withinonly some picoseconds. This process is very unlikely, and it takes onthe order of 1015 photons to free one electron.

By interaction with the electric field, this electron is accelerated (byinverse bremsstrahlung) to energies sufficient to ionize neighboringatoms by means of collisions, resulting in a second electron (second“generation”), which undergoes the same process. Within typically only30 such generations, a hot, dense plasma is created, which is observedby either its light emission or by detection of the acoustic shock wavegenerated by its rapid expansion.

Because each plasma event corresponds to one single colloid, therelative number of events per number of laser shots provides a measureof colloid concentration. The photon flux required for breakdowndecreases with the increasing number of molecules inside the colloid.By making use of that dependence, the colloid size can be ascertainedby varying the laser pulse energy.

ContributedPapers

Plasma

M+M+

e-

e-

e-

e-

e-e-

e-e-

e-M+

M+M+M+

M+

hν

hν

hνhν

hν

Energy gain

Avalanche viainverse

bremsstrahlung

Hotplasma

Optical detection

Acoustic waveparticle counting

First electronby multiphotonionization (MPI)

Particle

CCD camera Plasma

Quartz cell

Telescope

Laser beam u

t

e-



A plutonium-242 solution of pure oxidationstate (IV) was prepared by electrochemicalreduction in 0.5-molar hydrochloric acid toplutonium(III), followed by careful re-oxida-tion back to plutonium(IV). The oxidationstate was monitored by UV spectroscopy andthe plutonium concentration was determinedby liquid scintillation spectroscopy. The pHwas increased with an associated decrease inplutonium concentration by very slow,stepwise dilution with 0.5-molar sodium chlo-ride solution. The pH adjustment ultimatelyled to the formation of plutonium(IV) colloidkernels, as observed by a well-defined, sharpincrease of the LIBD signal. Such colloid for-mation is the most sensitive indication that thesolubility limit has been exceeded.

Macroscopically, the solubility limit is de-fined as the concentration at which the totalamount of a substance is no longer present inionic form but instead forms a precipitate.Here, our precipitate takes the form of col-loids, which are so small that they remainsuspended because of Brownian motion.

Analogous experiments were conducted atdifferent concentrations of the plutoniumstock solution. With decreasing plutonium(IV)concentration, the pH of colloid kernel forma-tion increased.

EvaluationDepending on pH, the tetravalent pluto-

nium ion is more or less hydrolyzed, meaningthat it is surrounded by up to four hydroxide(OH–) molecules. Only in very acidic condi-tions (a pH less than zero), does a non-hydro-lyzed Pu4+ aquo-ion exist in solution. The hy-drolysis does not change the spectral proper-ties of Pu4+, and so it cannot be detected di-rectly by LPAS. But from the crossing points ofcolloid formation—log of plutonium(IV) con-centration versus pH, we obtain the solubilitycurve with slope = –2, indicating that theplutonium(IV) dihydroxo complex Pu(OH)2

2+

represents the dominant species.

This mononuclear plutonium species un-dergoes further colloid formation and is thusin equilibrium with colloids. Knowing the hy-drolysis constant of Pu(OH)2

2+ from the litera-ture, the solubility product (log Ksp = –59 atzero ionic strength) of plutonium colloids(presumably oxy-hydrate) can be de-rived from our data.

Since the amount ofplutonium(IV) col-loids is relativelysmall at a given pH ofcolloid kernel forma-tion, there remains aconsiderable quantityof (hydrolyzed)plutonium(IV) ionicspecies undergoingdisproportionationwith time. This reac-tion is directly observed by LPAS forplutonium(IV) and (III), at a plutonium con-centration within the region of colloid forma-tion above the solubility curve.

The disproportionation reaction can bewritten as 3Pu(IV) + 2H2O � 2Pu(III) + Pu(VI)+ 4H+, which requires that, for three consumedplutonium(IV) ions, two plutonium(III) ions andone plutonium(VI) ion must be formed. The ratioof plutonium(IV) decrease and plutonium(III)/plutonium(VI) increase should scale as 3:2:1.

Our LPAS measurement however, corre-sponds to a ratio of 1, indicating a decreaseof plutonium(IV) that is too low by at least50 percent.

To explain this discrepancy, we must keep inmind that plutonium(IV) colloids are in equilib-rium with the plutonium(IV) ionicspecies, and that the plutonium(IV) tied upin colloids is spectroscopically invisible in theLPAS experiment.

Laser-inducedphotoacousticspectroscopy (LPAS)detects the oxidationstates of plutoniumions in solution bymeans of lightabsorption. Thisfigure showsplutonium(IV)undergoingdisproportionationwith time. The signalon the left at 470nanometer (nm) isdue to the electronicabsorption ofplutonium(IV). Thissignal decreaseswith time, while theplutonium(III) signalat 605nm increasesby the same amount.



00 0.5 1.0 1.5

-log[H+]

log[

Pu]

BD

P(E

=2.

5mJ)

2.0

BDP=1%

Onset of colloidformation

Dilution with0.5 M NaCl

+ [Pu]tot

o [PuIV]

2.5

-6

-5

-4

-3

0.02

0.04

0.06

o++

++ + ++

++

Diluting a plutonium-242solution with 0.5-molarsodium chloride resultsin a decrease inplutonium concentrationand an increase in pH(top), until the solubilityis exceeded andplutonium(IV) colloidkernels are formed(crossing point). Theformation of the colloidkernels is shown by awell-defined, sharpincrease of the laser-induced breakdowndetection (LIBD) signal(bottom).

0-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

0.5

slope-2+ [Pu]tot

[PuIV]

1.0 1.5

-log[H+]

log[

Pu]

2.0 2.5

+

+

+

+

The solubility productof plutonium colloidscan be derived from theexperimental data. Fordecreasing plutoniumconcentration, thecrossing point of colloidformation is shifted tohigher pH. From theslope = –2, we infer theplutonium(IV) to bepresent as dihydroxo-complex, Pu(OH)2

2+, inequilibrium withcolloids.

ContributedPapers

Over the course of 70 hours, as plutonium(IV)ionic species are consumed by the disproportion-ation reaction, the fraction of (spectroscopicallyinvisible) plutonium(IV) present as colloids redis-solved, and this addition to the spectroscopicallyobservable plutonium(IV) fraction was sufficientto account for the discrepancy. Once this wastaken into account, the true decrease ofplutonium(IV) could be calculated, and satisfiedthe required 3:2 ratio of the reaction. This wasconfirmed by LIBD measurements, where the col-loid dissolution with time was observed directly.

At lower concentrations (below 10 micromolartotal plutonium concentration), LPAS is no longercapable of detecting small amounts ofplutonium(IV) colloids present in the solution.However, such minute concentration of colloids(less than one micromolar) can still bedetected by LIBD, and the dissolution withtime has been demonstrated for solutions ofone micromolar total plutonium.

A combination of LIBD and LPAS facilitatesa sound assessment of chemical reactions ofplutonium(IV) involved at a rim of its solubility-constrained pH for the total plutonium(IV) con-centration down to micromoles per liter and forits colloids, about 100 times less (down to a par-ticle concentration of 10 nanomoles per liter).

The present experiment shows how compli-cated the thermodynamic assessment ofplutonium(IV) solubility is. For this reason, thereis a wide scattering of the plutonium(IV) solubil-ity product published in the literature—either forits oxide or for its hydroxide, with the differencesbeing a few orders of magnitude. Our researchhas demonstrated how the use of novel spectro-scopic approaches can reduce the uncertaintiesthat have, up to now, hindered the assessment ofplutonium(IV) solubility.



ContributedPapers

SergeiSinkov

Sergei Sinkov ofPacific NorthwestNational Laboratory(PNNL) presentedresults of researchon the developmenta new diamideligand with sometruly amazingextractioncapabilities.Compared toconventionaldiamide ligands, thenew ligand moleculeis one million to 10million times moreefficient at removingf-block metals fromprocess solutions.

Diamides have been studied exten-sively as agents for selectiveextraction of trivalent f-block metal

ions from aqueous solutions. They form thebasis for separation processes designed to par-tition the minor actinides from spent nuclearfuel for subsequent transmutation (with con-comitant closure of the nuclear fuel cycle) orimmobilization (for disposal).

One possible drawback associated with thecurrent generation of diamide extractants con-cerns the relatively modest actinide distribu-tion coefficients obtained even at high (greaterthan 1 molar) extractant concentrations. In arecently published report in the Journal of theAmerican Chemical Society, we described theextraction characteristics for a new type ofdiamide molecule. This new diamide (struc-ture 1b, below) is one million to 10 milliontimes more efficient at extracting trivalent lan-thanides and actinides from aqueous solutionthan previously examined diamides such astetraalkylmalonamides.

It was assumed that this extraction en-hancement reflects superior ligand-metalbinding of the bicyclic diamide as comparedto acyclic analogs such as tetraalkylmal-onamides. A fuller understanding of suchstructure-function aspects of actinide extrac-tion by amides is needed for the intelligentdesign of extractants with superior character-istics to those currently available.

Bicyclic and acyclic diamidesComparison of aqueous phasebinding constants with tetra-and hexavalent actinides

N N

OORR

A new bicyclicdiamide ligandarchitectureoptimized forbidentatelanthanide/actinide binding.R = methyl (1a),R = octyl (1b).

One of the possible approaches to quantifythe magnitude of ligand-to-metal binding isthe application of ultraviolet-visible (UV-Vis)optical absorbance spectroscopy to monitorspectral changes in-duced by 1a in theabsorption spectra ofactinide ions in aque-ous solution. Thespectral informationobtained by varyingthe ligand-to-metalratio not only allowsestimation of thenumber of light-ab-sorbing species insolution, but can beused for measurementof the binding (orformation) constantsof a ligand withmetal ions.

The following sections compare theformation constants for the dimethylbicyclic diamide (1a) and N,N,N’,N’-tetramethylmalonamide (TMMA), a relatednon-cyclic compound, with plutonium(IV),plutonium(VI) and uranium(VI) as measuredin the aqueous phase at 1.0-molar ionicstrength (HNO3).

Results with plutonium(IV)In this experiment, we conducted a spec-

trophotometric titration by exposing a singleportion of a plutonium-239, -240(IV) stock

solution (better than 99.5 percent valencepurity) to successive introduction and dis-solution of either solid crystals (in the caseof 1a) or tiny aliquots of a concentratedstock solution (in the case of TMMA). Nei-ther precipitation nor perturbations in theoxidation state of plutonium(IV) were ob-served with either ligand throughout thetitration procedure at a metal concentra-tion of 8 millimolar (mM).



With 1a, the most significant spectralchanges observed with increasing ligand con-centration were found in the region of themain plutonium(IV) peak (a 476→497 nanom-eter [nm] shift) and in the red part of theplutonium(IV) spectrum (a 660→684 nm shiftand a 2.4-times intensification of the absorp-tion band).

Although the spectral changes for TMMAwere similar, we found that such changes wereless pronounced and not identical to thoseobserved with 1a. Moreover, these changes oc-curred at much higher ligand-to-metal ratios.

Using the Singular Value Decompositionprocedure, we found no less than fourcomplexed species for the plutonium(IV)-1asystem and three major complexed species(and perhaps even a fourth minor species) forthe TMMA-plutonium(IV) system.

Results with plutonium(VI)Not only was the most intense and sharp

absorption peak of plutonium(VI) at 833.6 nmmonitored for expected complexation effects,but a wider range (330–950 nm) was examinedto evaluate possible changes in the oxidationstate of the plutonium (initially better than99.5 percent hexavalent plutonium).

As with plutonium(IV), the 1a ligandproduced pronounced spectral changesin the plutonium(VI) spectrum, withtwo, new, well-resolved peaks at 840.6nm and 846.3 nm emerging with even a

moderate excess of ligand. After reach-ing a ligand-to-metal molar ratio of 80, all

three peaks of plutonium(VI) gradually dis-appeared. Moreover, several new spectral fea-tures emerged as shown in the figure on page26. For example, the visible range of the spec-trum showed new major peak maxima at 497nm and 684 nm. This new spectral signatureclearly indicated a reduction of plutonium(VI)to plutonium(IV) in the presence of 1a.

A careful redox speciation analysis of theplutonium(VI) and (IV) spectra showed thatthis plutonium(VI) reduction was accompa-nied by the formation of plutonium(V)—aweak spectral feature of variable intensity at568 nm. Once we accounted for the presenceof plutonium(IV) and (V), we used the nonlin-ear spectra processing routine known asSQUAD to process the portion of the spectralset in which plutonium(VI) remains predomi-nant in terms of concentration. The formationconstants refined from this analysis are shownin the table on page 19.

In contrast to 1a, the complexation ofplutonium(VI) with TMMA did not induceany redox reaction (even after an overnightcontact time). Complexation effects were sig-nificantly weaker, with the 1:2 complex seennot as a separate peak but rather as a weakshoulder in the 842–846 nm range.

Results with uranium(VI)Both ligands induced significant spectral

changes in the UO22+ spectrum, accompanied

by a bathochromic shift and intensification ofthe absorption bands. Subsequent analysis in-dicated that 1a again acted as a much stronger

A three-dimensionalstructure of the newbicyclic diamide.Carbon atoms areblack, nitrogenatoms are blue,hydrogen atoms arelight grey, andoxygen atoms arered. The vectors oneach oxygen atom,which indicate thedirection required foroptimal interactionwith a metal ion,converge in thecomputer-designedstructure,in contrast toconventionalmalonamide withdiverging vectors.

ContributedPapers



Complexant Species

1:1 4.42 (0.08)

Pu(IV) Pu(VI)

2.36 (0.04)

3.34 (0.07)

0.44 (0.01)

-0.03 (0.03)

-

-

-

-

-

-

-

-

U(VI)

2.85 (0.02)

4.61 (0.05)

1.00 (0.01)

0.97 (0.03)

7.95 (0.14)

10.36 (0.13)

11.48 (0.14)

2.56 (0.08)

4.08 (0.11)

5.01 (0.14)

4.50 (0.38)

1:2

1:3

1:4

1:1

1:2

1:3

1:4

MiLj

logβij

1a

TMMA

binding agent than did TMMA. In both casesno spectral evidence was found for formationof higher complexes (1:3 and 1:4 stoichiometry).

ConclusionUsing UV-visible spectroscopy, we have

quantified binding constants for both bicyclicand acyclic diamide ligands with plutonium(IV),plutonium(VI), and uranium(VI) in acidicaqueous solution.

This work has yielded three key results.First, we showed that alternating the diamidestructure from acyclic to bicyclic increasedthe overall formation constants withplutonium(IV) by as much as seven orders ofmagnitude. Second, we found that comparingactinide(VI)-ligand binding affinities revealsenhanced binding to uranium(VI) versusplutonium(VI). And third, the plutonium(VI),but not uranium(VI), in the bicyclic diamidetriggers reduction of plutonium(VI) toplutonium(V) and (IV).

We are already at work evaluating theformation constants of TMMA and 1a withactinides in the +3 and +5 oxidation states.

The spectrophotometric titration of plutonium(VI) by 1a in 1.0-molar HNO3.Spectra shown in dark pink correspond to seven successive additionsof the ligand up to L/M =80. Red spectra are kinetics of theplutonium(VI)L+plutonium(VI)L2 reduction to plutonium(IV)L4. Black traceshows the reference spectrum of plutonium(IV) with no ligand presentat the same acidity and metal concentration as the initial spectrum ofplutonium(VI). The kinetic series spectra 08, 09, 10, 11, 12, 13, 14, 15,and 16 were taken at zero minutes, 10 minutes, 20 minutes, 30 minutes,40 minutes, 52 minutes, 63 minutes, 150 minutes, and 24.5 hours,respectively, after the last addition of the ligand.

Wavelength, nanometer (nm)

480 520 560 600 640 680 720 760 800 840 880

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Spectrum #0 2 4 6 8 10 12 14 16

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

568 nm (Pu(V))

Pu(IV)L4

08

1608

16

Pu(VI)

Pu(VI)L

Pu(VI)L2

Pu(VI)

Pu(VI)L

Pu(VI)L2

#01-077 additions

of the ligand

#08-16kinetics at L/Pu = const

Optical absorb

ance

Op

tica

l a

bso

rba

nce

A summary of all the refined formationconstants resulting from this work.Conditional formation constants ofplutonium(IV), plutonium(VI), anduranium(VI) complexes with 1a andtetramethylmalonamide in 1.0-molarHNO3 at 23±0.5o C. Values in bracketsrepresent one sigma standarddeviations.

This article wascontributed by SergeiSinkov, Brian Rapko,and Gregg Lumettaof Pacific NorthwestNational Laboratory(PNNL); and JamesHutchison and BevinParks of theDepartment ofChemistry, Universityof Oregon, Eugene.



ContributedPapers

Joe Wong

This article wascontributed by JoeWong, Daniel Farber,Florent Occelli, AdamJ. Schwartz, MarkWall, and Carl Boro ofLawrence LivermoreNational Laboratory;Michael Krisch of theEuropean SynchrotronRadiation Facility,Grenoble, France; andTai-C. Chiang andRuqing Xu, of theDepartment of Physicsand Frederick SeitzMaterials ResearchLaboratory, Universityof Illinois at Urbana-Champaign.

Landmark experiment opens up new research opportunities in actinide scienceResearchers use high-resolution inelastic x-rayscattering and a microbeam approach to determinethe first full phonon dispersion curves ever measuredfor any plutonium-bearing materials

Ph

on

on

En

erg

y (m

eV)

Editor’s note: The six scientists from LawrenceLivermore National Laboratory (LLNL) who co-authored this article have garnered an award fortheir groundbreaking research. The team recentlyreceived a Science and Technology Award, the high-est honor granted by LLNL for science and technol-ogy accomplishments.

Understanding the physical basis forthe intriguing properties of pluto-nium materials such as force con-

stants, sound velocities, elasticity, phase stabil-ity, and thermodynamic properties criticallyhinges on the ability to produce high-qualityexperimental data. Of these, phonon disper-sion curves (PDCs) are key to the elucidationof many of these physical phenomena.

A phonon is a lattice vibration, the acousticequivalent of a photon, the basic quanta oflight. PDCs in plutonium and its alloys havedefied experimental determinations for thepast forty years because of the inability togrow the large single crystals (at least a fewcubic millimeters in volume) necessary for in-elastic neutron scattering (INS) measurements.Another obstacle was the high thermal-neu-tron absorption cross section of the mostcommon isotope, plutonium-239.

Furthermore, only recently havetheoretical computations of pluto-nium PDCs begun to overcome thedifficulties in treating the f electronsaccurately within the standard first-principles methods. Thus, the PDCsfor plutonium-bearing systems haveremained essentially unknown ex-perimentally and theoretically.

The experimental difficulties asso-ciated with INS are circumvented inour experimental approach by em-ploying inelastic x-ray scattering(IXS) with milli-electron volt (meV)

energy resolution. With the advent of highlybrilliant x-ray sources and high-performancefocusing optics, samples with volumes assmall as one ten-thousandth of a cubic milli-meter can now be studied. These capabilitieshave opened up new experimental opportuni-ties for materials that are only available insmall quantities, as is the case for many ac-tinide systems. The research is covered in de-tail in a paper published in August in Sciencemagazine (see Science,Volume 301, page 1078 [2003]).

Our samples were large-grain polycrystal-line specimens prepared from a plutonium-gallium alloy containing about 0.6 percentby weight of gallium produced by a strain-enhanced recrystallization technique. To ex-tract the phonon energies, the spectra were fit-ted by convolving the experimentally deter-mined resolution profile with a model functionconsisting of a Lorentzian for the elastic contri-bution and a pair of Lorentzians, constrainedby the thermal phonon population factor, forthe inelastic part.

The resulting PDCs along the three principalsymmetry directions in the face-centered cubic(fcc) lattice are displayed in the figure below,



Phonon dispersions along high-symmetrydirections in a face-centered cubic plutonium-gallium alloy containing about 0.6 percent byweight of gallium. The experimental data areshown as circles. Along the [0ξξ ] direction, thereare two transverse branches [011]<01-1>(T1)and [011]<100> (T2). Note the softening of theTA[ξξξ ] branch toward the L point. The latticeparameter of our samples is a = 0.4621nanometer. The solid curves are the fourth-nearest neighbor Born-von Kármán model fit.The derived phonon density of states,normalized to three states per atom, is plotted inthe right panel. The dashed curves arecalculated dispersions for pure delta-plutoniumbased on dynamical mean field theory (DMFT).

together with a fit using a standard Born-vonKármán (B-vK) force constant model. An ad-equate fit to the experimental data is obtainedif interactions up to the fourth-nearest neigh-bors are included and the calculated phonondensity of states derived from the B-vK modelis also shown. The dashed curves are recentdynamical mean field theory (DMFT) results.

The elastic moduli calculated from slopes ofthe experimental phonon dispersion curvesnear the G point are: C11 = 35.3 ±1.4 giga pascal(GPa), C12 = 25.5 ±1.5 GPa and C44 = 30.53 ±1.1GPa. These values are in excellent agreementwith those of the only other measurement on asimilar alloy (1 percent atomic weight gallium)using ultrasonic techniques as well as withthose recently calculated from a combinedDMFT and linear response theory for puredelta-plutonium.

The small difference between C11 and C44 isvery unusual for a metal with a face-centeredcubic structure. The shear moduli C44 and C’ =(C11 - C12)/2 differ by a factor of six. This is incontrast to that of a “normal” fcc metal such asaluminum: 1.2, and is significantly higher than

values in other “unusual” fcc metals such asgamma-cerium: 2.8, lanthanum: 4.1, andthorium: 3.6.

Published results support the conclusionthat delta-plutonium–gallium is the mostelastically anisotropic fcc metal known.Furthermore, we observe a large deviation,D, from the Cauchy criterion. Our measure-ment yields D = (C44 –C12)/C12 = 0.9, whichdiffers significantly from zero and impliesthat the interatomic forces have a strongnoncentral component.

The most striking feature of the experimen-tal PDCs is a soft-mode behavior for the TAbranch along [111]. A similar feature (but oc-curring at about twice the energy and at ahigher crystal momentum toward the L point)is also seen in a recent DMFT calculation fordelta-plutonium. The cubic fcc crystal struc-ture of delta-plutonium can be viewed as beingcomposed of hexagonally close-packed atomicplanes stacked along the [111] direction with aspecific stacking arrangement. The soft trans-verse mode at L suggests that each atomicplane could easily slide relative to its immedi-ate neighboring atomic planes to form newstacking arrangements.

Joe Wong and hiscolleagues used thehigh-resolutioninelastic x-rayscattering beam lineat the EuropeanSynchrotronRadiation Facility inGrenoble, France,(shown above) toobtain the first-everexperimentallydetermined phonondispersion curves of aplutonium alloy. ESRFis a third-generationsynchrotron, and theonly facility with abeam line capable ofmeasuringplutonium’s latticevibration energy.



Pure delta-plutonium, on lowering tempera-ture, transforms into the gamma phase, whichhas a face-centered orthorhombic structure.This structure can be described in terms of astack of slightly distorted hexagonally packedatomic layers, and indeed is the type of struc-ture that could be easily obtained by layer par-allel shear in the fcc phase.

Thus, the soft mode at L is likely a key fea-ture associated with the delta-to-gamma tran-sition, even though this transition is sup-pressed by gallium stabilization in the presentcase. This notion is indeed substantiated bythe fact that the first interplanar force constant(derived from the B-vK model) of the T[111]mode is more than an order of magnitudesmaller than those of the transverse modes inthe other two directions.

The small amount of gallium in the sampleis sufficient to allow the system to bypass thegamma and beta phases and make a transitiondirectly to a monoclinic alpha-prime phase atabout -110oC. The alpha-prime phase, despiteits apparent complexity, is also a slightly dis-torted hexagonal close-packed structure, andis easily accessible from the fcc phase throughthe same shear mechanism.

A similar soft-mode behavior has been ob-served in gamma-cerium and lanthanum. Inthose cases, the transition leads, instead, to adouble-hexagonal-close-packed structure, butthe same general concept of layer parallelshear applies.

As evident by the data presented in the fig-ure on the previous page, our IXS experimentvalidates the main qualitative predictions of arecent DMFT calculation for delta-plutoniumin terms of a low shear elastic modulus C’, aKohn-like anomaly in the T1[011] branch, and alarge softening of the T[111] modes. Such ex-perimental-theoretical agreements give cre-dence to the DMFT approach for the theoreti-cal treatment of 5f-electron systems. However,while there is good qualitative agreement be-tween theory and experiment, quantitative dif-ferences are evident. These differences providethe benchmark for refined theoretical treat-ments and further experiments with pluto-nium and other 5f elements.

AcknowledgementsThis work was performed under the aus-

pices of the Department of Energy by the Uni-versity of California, Lawrence Livermore Na-tional Laboratory under Contract No. W-7405-Eng-48 and the Department of Energy by theUniversity of Illinois Frederick Seitz MaterialsResearch Laboratory under Grant No.DEFG02-91ER45439. We are thankful toFrancesco Sette, European Synchrotron Radia-tion Facility (ESRF) for his support and en-couragement in this project, and to PaulBerkvens and Patrick Colomp, also with ESRF,for their advice and technical assistance.

ContributedPapers



ContributedPapers

RichardHaire

The science question

Pressure, like temperature, is an im-portant variable in the chemistry,physics and materials science of ma-

terials. It has been found that the f electrons ofthe actinide elements—the so-called 5f-electron series of ele-ments—are also greatly af-fected by pressure. At atmo-spheric pressure, protactinium,uranium, neptunium, and plu-tonium have itinerant 5f elec-trons (electrons involved tovarying degrees in the metallicbonding), and they exhibit un-usual low-symmetry structuresand properties. In contrast, thetransplutonium metals nor-mally have localized 5f elec-trons, display symmetricalstructures, and their bondingconsists of three non-f-electronconduction electrons (i.e.,“trivalent metals”). In this re-gard they mimic the lanthanidemetals.

One obvious difference be-tween these f elements acrossthe series is seen in the variation of theiratomic volumes at normal pressure, as shownin the figure at right. The similarity of thetrivalent, localized 4f-and 5f-electron met-als’ volumes contrastswith the behavior ex-hibited by the fouritinerant 5f-electronmetals (i.e., protac-tinium-plutonium).The latter have muchsmaller atomic vol-umes in addition totheir lower crystalsymmetries.

The three divalent f-element metals—europium, ytterbium, and einsteinium—are special cases, and result in part due totheir higher f-electron promotion energies.

With the some of the early lanthanide met-als, it is interesting to notethat after acquiring 4f-elec-tron character in their bond-ing by applying high pres-sures, they adopt some of thesame low-symmetry struc-tures exhibited by the protac-tinium-plutonium metals thathave itinerant 5f electrons.This immediately raises thequestion of whether applyingpressure on the transpluto-nium metals, which have lo-calized 5f electrons, can bringabout significant changes intheir bonding.

As these actinides havemore spatially extendedf electrons than their lan-thanide homologs, it shouldbe easier to force their 5f elec-trons to become bonding byapplying pressure—that is,

they may be more sensitive to the affects ofpressure than their lanthanide counterparts.

This paper wascontributed byRichard Haire,Chemical SciencesDivision, Oak RidgeNational Laboratory;Steve Heathmanand Monique Idiri,EuropeanCommission,Joint ResearchCentre, Institutefor TransuraniumElements; TristanLe Bihan, EuropeanSynchrotronRadiation Facility;and AndreasLindbaum, ViennaUniversity ofTechnology, Institutefor Solid StatePhysics.

International team of researchers employshigh pressure to probe bonding in actinide metalsInvestigations of protactinium andamericium metals provide importantinsights into actinide bonding concepts

Oak Ridge’s Dick Hairediscussed research by aninternational team on theuse of high pressure toprobe actinide bonding.

Atomic volumesof the f-electronelements reflectdifferences inbonding present forthe elements. Thesmaller volumes forthe protactinium-plutonium metalsreflect their itinerant5f-electrons.



Selecting the f-electrons for studyStudies of the actinides can be difficult,

given their varying levels of radioactivity and availability. Of interest to researchers is the po-tential for forcing increased bonding in the ac-tinides by applying pressure. For example, can pressure force divalent einsteinium metal (ele-ment 99) to acquire a third bonding electron? Basically, can pressure force one element to mimic the bonding, properties, and structures of another—a sort of modern alchemy?

Two actinides were selected for recent ex-perimental studies to probe these questions. One study involved americium and a more recent study involved protactinium. (Reports have been published in Physics Review Letters, Vol. 85, 2961; Phys. Rev.B, 63, 214101, (2001) and 67, 134101, 2003.) The behavior of these two metals under pressure is of special inter-est; the studies looked at the actinide elements that “initiate” and “complete” the “dip” seen in the actinide volume curve in the figure on the previous page.

Protactinium has the distinction of being the first actinide metal with a 5f electron (actinium and thorium have d-electrons), and it already has partially itinerant 5f electrons. In contrast, americium is the first element in the series having fully localized 5f electrons at atmo-spheric pressure, following the four preceding actinides that have itinerant 5f electrons.

These recent studies were conducted to de-termine if pressure can force additional 5f-elec-tron bonding in protactinium—making it more like uranium—and if pressure can force the onset of 5f-electron bonding in americium—making it behave like the protactinium– plutonium metals, or specifically, its near neighbor plutonium.

An international team of scientists from Oak Ridge National Laboratory, the European Insti-tute of Transuranium Elements, the European Synchrotron Radiation Facility and the Vienna University of Technology carried out the studies.

The researchers examined the structural be-haviors of the protactinium and americium up to pressures as high as 130 giga pascals (GPa) using different types of diamond anvil cells to generate static pressures on the metals through different transmitting media (silicon oil or liq-uid nitrogen). Pressure calibrations employed ruby fluorescence, and/or copper or platinum metals via their known equation of states. Modern diffraction techniques employed mul-tiple wavelengths of high-intensity radiation from the synchrotron and charge-coupled de-vice (CCD) detectors.

What was found experimentally?

The research found that the tetragonal struc-ture of protactinium at atmospheric pressure, where itinerant 5f-electron behavior in the bonding already exists, converts to an ortho-rhombic structure at about 77 GPa that is iso-structural with that of alpha-uranium.

At this pressure, the protactinium’s atomic volume is reduced to about 62 percent of that at atmospheric pressure. Accompanying this first-order transformation from the protac-tinium-I (Pa-I) to protactinium-II (Pa-II) crystal structure is a small relative volume collapse of 0.8 percent. It is postulated that structural transformation with pressure (the Pa-I to Pa-II transformation) reflects an acquisition of ad-ditional 5f-electron character in the metallic bonding and alters its basic properties.

Contributed Papers



With americium under pressure, it was de-termined that the delocalization of its 5f elec-trons occurs in two steps, with the progressiveformation of lower-symmetry crystal struc-tures (i.e., the third and fourth structures).Overall there are three phases changes up to100 GPa; the first change from double hexago-nal close-packed (dhcp) to face-centered cubic(fcc) represents only a second-order phasechange. In the first transition involving delo-calization (third phase, Am-III), americiumacquires the structure of gamma-plutonium—an orthorhombic structure (space groupFddd)—at about 10 GPa, this structureis accepted as reflecting itinerant5f-electron character.

This first-order transformation (Am-II toAm-III) is accompanied by a small but signifi-cant relative volume collapse of about 2 per-cent, which signifies that a change in bondingoccurs. It is followed by formation of a fourthphase (Am-IV), a primitive orthorhombicstructure (space group Pnma) at about 18 GPaand exists up to 100 GPa. The latter is closelyrelated to a Cmcm structure, the alpha-uranium structure and the Pa-II structure);this structure is also accepted as reflecting thepresence of itinerant 5f electrons in the metal-lic bonding. Accompanying the formation ofAm-IV is a more significant relative volumecollapse of about 7 percent.

Thus, pressure has forced the delocalizationof the 5f electrons in americium after itsatomic volume and interatomic distances arereduced considerably (at 100 GPa the volumeis about 46 percent of that at atmosphericpressure). Under pressure, the Am-II phasebecomes like plutonium (its near neighbor),and at higher pressures, americium thenadopts the alpha-uranium structure.

An important difference between the twometals is that americium starts out havingfully delocalized 5f electrons, while the 5f elec-trons of protactinium already exhibit some 5f-electron itinerancy at atmospheric pressure.Both acquire additional 5f character in theirbonding with pressure. This initial differenceis reflected in specific behavior and propertiesunder pressure. Comparisons between thecompression and structural behaviors of am-ericium, uranium, and a 50 atomic percentamericium-curium alloy with pressure areshown in the figure on page 26.

The more compressible nature and overallvolume reduction of americium metal is dueto it starting as a fully localized 5f-electron,trivalent metal, whereas uranium and protac-tinium initially have itinerant 5f electrons. Thefigure on page 27 also shows that the slope ofthe compressibility curves for alpha-uranium,Pa-II, Am -IV, and the 50 atomic percent(Am,Cm)-IV structures (i.e., in the high-pressure regions where they exist) are allvery similar, as they should be, given thatall have either an alpha-uranium or a closelyrelated structure.

The latest published calculations based ontheory for the behavior of americium underpressure were in agreement with this experi-mental work. In addition, Per Söderlind(LLNL) and Olof Eriksson of the Universityof Uppsala, Sweden, have also used a calcula-tional approach involving Gibb’s free energiesto predict a series of structural transforma-tions for protactinium metal under pressure(see Physics Review B, Vol. 56, 10719, 1997).



Our recent experimental work on protac-tinium confirmed their prediction that an al-pha-uranium structure would be formed un-der pressure from the body-centered tetrago-nal form, with the differences between experi-ment and theory being only at which pressureand at what relative atomic volume the Pa-I toPa-II transition occurs. However, the story forprotactinium remains incomplete, as theorypredicts that additional structural transforma-tions will occur at much higher pressures(presently difficult to reach experimentally).It will be a difficult but interesting challengeto try to reach these ultra high pressures toconfirm the theoretical predictions.

ContributedPapers

A comparison of thecompressionbehavior of uranium,protactinium,americium, and a 50atomic percentamericium-curiumalloy is shown. Bulkmodulli and pressurederivatives extractedfrom the data arealso given. (Adaptedfrom the Journal ofCondensed MatterPhysics, Vol. 15,S2297, 2003.)

Additional AspectsSeveral other pieces of important informa-

tion can be acquired from such experimentalpressure studies (i.e., the bulk modulus and itspressure derivative acquired by applying theBirch-Murnaghan equation of state). In asimple picture, the bulk modulus indicates the“stiffness” of a structure’s lattice—the morerigid the structure, the less compressible it is,and the more likely contains a greater degreeof bonding. Bulk modulli and pressure deriva-tives are given in the figure at left for ameri-cium, uranium, protactinium, and the ameri-cium-curium alloy. Note that the modulus foramericium is about 29 GPa (derived from theAm-I phase), while those for alpha-uraniumand the Pa-I structure are more than 100 GPa.

It can be informative to compare the com-pression behaviors of the different metals, thepressures at which structural transformationsoccur, and the overall picture of the relativevolume behaviors.

The close relationship between the body-centered tetragonal structure of protactinium(Pa-I) and the orthorhombic Cmcm structureof alpha-uranium can be seen in the figures atright. The main difference between the struc-tures occurs from a buckling of the “chains” inthe latter due to the “y” parameter of the 4csites of Cmcm. When the Pa-I structurechanges to the Pa-II structure, the “y”parameter of the Pa-II structure differs byless than 0.02 from that of alpha-uranium.



The Am-IV structure also shows a close simi-larity to that of alpha-uranium; it is only neces-sary to shift the position parameter “z” of thePnma structure 4c sites to zero from “z=0.10.”The three structures in the top of the figure atright show the close relationship that exists be-tween the four phases. The structural sequencefor americium metal under pressure is shownin the lower part of that figure. The first twotransformation processes basically involvechanges in the stacking sequence of planes;and the formation of the Am-IV structurerequires an additional “buckling” of thehexagonal planes.

Special facets of americiumand protactinium

Important changes in the role of the 5f elec-trons in the actinides can be extracted fromperturbations in their atomic volumes and thestructures displayed under pressure. The workon americium and protactinium metals underpressure have shown that pressures increasesthe itinerant (bonding) nature of their 5f elec-trons. The onset in (americium) or increase in(protactinium) delocalized 5f-electron characterreduces the atomic volumes of the metals, aswell as producing structures of lower symme-try. In contrast, heating plutonium reduces the5f-electron contribution and increases itsatomic volume (over that due to expansion).

Using a simple picture, plutonium becomesmore like the localized 5f-electron metals uponheating and americium becomes more like plu-tonium under pressure. Both changes reflectthe changing influence of the 5f electrons inthe elements’ bonding.

Structures of selected actinides under pressure. The similarity of theprotactinium-II and americium-IV structure to that of the alpha-uraniumstructure is evident. The lower segment shows the structural sequence ofamericium under pressure to 100 giga pascals.



Future experimentsA number of experiments are envisioned to

further explore the actinide metals under pres-sure to better understand the science that isoccurring. Certainly, studies under higherpressures are desired to explore the potentialfor an onset of new interactions that maydrive the formation of new structures andalter the bonding.

There is also the need to study actinide ele-ments with higher Z values using these im-proved experimental techniques, and both cu-rium and californium metals are two potentialcandidates. Their behavior under pressure willbe affected by the withdrawal of their 5f elec-trons from their Fermi edges, due to the in-creased nuclear charge. The latter should makeit more difficult to force the delocalization oftheir 5f electrons, and should require higherpressures to bring about.

Einsteinium is the highest actinide forwhich such pressure experiments can be envi-sioned, given the quantities of actinides ex-pected to be available. The behavior of thismetal under pressure would be very interest-ing to study, but it is also an exceptionally dif-ficult material to study because of the veryshort half-life and intense radiation of its mostavailable isotope (einsteinium-253).

An attempt to study this metal under pres-sure was unsuccessful, due to crystal destruc-tion by its self-irradiation. A special interest ineinsteinium’s behavior under pressure is that,in principle, this divalent metal could beforced by pressure to become a trivalent metallike the americium through californium ele-ments at atmospheric pressure. With addi-tional pressure, it may be possible to force

ContributedPapers

einsteinium’s 5f electrons to delocalize, per-haps displaying the behavior observed withamericium metal under pressure. One canalso easily envision numerous other studiesof actinide alloys and/or compoundsunder pressure.

ConclusionStudies under pressure can provide impor-

tant insights into the 5f-electron behaviors andtheir changing roles. It appears that orthor-hombic structure-types (i.e., alpha-uranium orthe closely related Pnma structure-type) areencountered under pressures in the region of100 GPa. Whether this type of structure is adominant high-pressure structure in this re-gion remains to be determined. Theory pre-dicts more symmetrical structures may againbe encountered at even higher pressures, dueto an increase in dominance of electrostaticinteractions and Born-Mayer repulsions.

These studies with americium and protac-tinium metals will not only help other re-searchers to better understand the bondingand electronic structure of two elements, butalso that of other actinides. They should enablethe development of important systematics con-cerning the actinides. The experimental datashould also serve as a platform from whichone may evaluate and fine-tune theoreticalpredictions, thereby enhancing the overall un-derstanding of these complex actinide metals.In addition, such research may contribute to animproved understanding of other elements inthe Periodic Table.

This work wassupported in partby the EuropeanCommission andby the Division ofChemical Sciences,Geoscience andBioscience, OBES,USDOE, undercontract DEACOR-00OR22725 withOak Ridge NationalLaboratory, managedby UT-Battelle, LLC.The authors wish toacknowledge beamtime provided at theESRF (ID30 beamline) in Grenoble,France, where thediffraction studieswere performed,and the efforts ofpersonnel at theESRF. One author,Andreas Lindbaumwishes to thank theAustrian Academyof Sciences (APART10739) and theAustrian ScienceFund (P14932) forfinancial support.Special thanks alsogo to Gerald Lander(European Institutefor TransuraniumElements) fordiscussions and hisstrong interest inpromoting theseexperimental efforts.



ContributedPapers

ChristyRuggerio

Phytoremediation—using plants toclean up the environment—relies onthe fact that plants are essentially

solar-driven pumps, transpiring water fromtheir roots through their leaves. In the process,they can degrade organic chemicals and stabi-lize contaminants in their root zone, or eventake up contaminants into their leaves, thusallowing them to be removed from the soil.

Phytoremediation has distinct advantagesover traditional cleanup methods: it is safe,relatively inexpensive, usually simple toimplement with existing agricultural practices,and has a positive public image.

Phytoremediation of metals has been particu-larly successful. A few techniques are beingexplored commercially, includingphytostabilization and phytoextraction.

Phytostabilization relies on the plants to de-crease contaminant migration by decreasingwind and water erosion of soils, while theplants pull soluble species into their rootzone where metals can be sorbed onto rootsand stabilized.

Phytoextraction takes the process a step fur-ther; it relies on metal-accumulating plants toactually take up metals into their abovegroundparts, thereby removing toxic metals from soilsand water. Metal-loaded plants are harvested,dried, ashed, composted, or stored. The vol-ume of resulting waste is generally a fraction ofwaste produced by many current, more inva-sive remediation technologies, and associatedcosts are much less.

Most research on metal phytoextraction hasfocused on finding naturally occurring plantsthat can take up a large amount of metal com-pared to their biomass (called hyper-

accumulators), and then developing theseplant species through genetic alteration forimproved field performance, and for a widerrange of metal uptake.

Work with hyperaccumulators has focusedon commercially produced “heavy” metal con-taminants—such as lead, mercury, chromium,cadmium, zinc, copper, arsenic, and nickel—rather than radionuclides such as plutonium,

This article wascontributed by ChristyRuggiero, EliseDeladurantaye,Brandy Duran, andStephen Stout ofStructural InorganicChemistry (C-SIC);and Scott Twary ofthe BioscienceDivision SzilardResource (B-3).

Process may have advantages over traditional cleanup methodsResearching thephytoremediation of plutonium

Elise Deladurantaye, a student in the Structural Inorganic ChemistryGroup (C-SIC), checks to see if barley plants have enough nutrientsolution to grow for one more day before being harvested todetermine their uranium uptake levels. The barley seeds are firstsprouted on a small water-soaked mat and then transferred to anutrient-rich growth solution, where they grew hydroponically in acontrolled-atmosphere growth chamber in a Chemistry Divisionlaboratory. The bottles containing the plant solutions are equippedwith an air bubbler inlet to make sure the growth solution doesn’tstagnate and a HEPA-filtered outlet. The plants grow in the nutrientsolution with either no iron, normal iron levels, or high iron levels(well-fertilized). After adjusting to the iron levels for one week, asolution of uranium is added to the nutrient solution and the plantsgrow in (and take up) the uranium. The plants are harvested byclipping off the stems and the roots and letting them completely dryin small vials. The dry plant parts are digested in acid and thenanalyzed for uranium content using ICP (Inductively-coupled Plasmaatomic emission) spectroscopy and compared to uraniumconcentration standards.



thorium, and uranium. These groups of metalsare chemically very different, so it is unlikelythat any known hyperaccumulators will beable to hyperaccumulate plutonium or otheractinides. Researchers have even speculated

that no hyperaccumulator will ever be foundfor the actinides.

Fortunately for actinide phytoremediation,true hyperaccumulators are not absolutelynecessary. A plant with high biomass but alower percent of metal uptake will removethe same amount of metal from the soil as aplant that has low biomass (such as mosthyperaccumulators) but a higher percent ofmetal uptake.

Maximum efficiency is still achieved by in-creasing the percent of metal uptake into theplant. This can be done by increasing themetal solubility, increasing the ability of themetal to cross the root membrane, and increas-ing the amount of metal translocated into thehigher plant parts.

Increasing metal solubility alone hasbeen shown to dramatically improvephytoextraction efficiency. Chelators such asEDTA or citrate are added to soils to improvemetal solubility and increase plant uptake, aprocess called chelate-enhanced, or induced,phytoextraction. For example, researchers at

Phytotech Inc., a metal phytoremediationcompany, published work in “EnvironmentalScience and Technology,” [1998, 32(13), 2004]showing that added citrate can increase plantuptake of uranium more than 1,000-fold.

Unfortunately, adding such chelators canbackfire. Even though they increase metalsolubility, the rate of metal translocation intothe plant roots may still be limited. This canlead to large amounts of solubilized metalsthat are not able to get into the plant, leadingto increased soil migration and leaching toground waters. Luckily, there may be a betterway to “trick” plants into taking up pluto-nium, or at least trapping it in the plantroot zone.

Plutonium behaves similarly to iron, a nec-essary nutrient in biological systems. Bothiron(III) and plutonium(IV) readily hydrolyze,have rich redox chemistry, have high coordi-nation numbers, have similar charge-to-radiusratio, and often have predictably similar bind-ing affinities for the same chelating ligands.Both have extremely low solubility in the en-vironment. Successful development of manyplutonium complexation and decontaminationagents has relied on this iron-plutonium simi-larity by exploiting iron chelating ligands.

We hope to be able to use plant iron-uptakesystems to trick plants to take up largeamounts of plutonium, or at least trap itin the plant root zone.

Graminaceous plants (grasses such asbarley, oat, and wheat) produce and releasephytosiderophores, a family of nonproteinamino acid that enhances solubility anduptake of iron and possibly other metalsfrom the environment (see figure at left).Phytosiderophores, all derivatives ofmugineic acid, form stable complexes withnumerous metals.

Phytosiderophores have been shown toenhance soil mobility of copper, iron, manga-nese, and zinc as much as microbial

ContributedPapers

Graminaceousplants produceand releasephytosiderophores,a family ofnonprotein aminoacid that enhancessolubility and uptakeof iron and possiblyother metals fromthe environment.Phytosiderophoresare derivatives ofmugineic acid.Shown here isthe mugineicacid family ofphytosiderophores,nicotianamine (aphytosiderophorebiosyntheticprecursor), EDTA,and citric acid.



siderophores and morethan some syntheticchelators. Someresearchers haveshown thatphytosiderophores areup to 100 times moreefficient than anthro-pogenic or bacterialiron chelators for ironuptake intograminaceous plants,presumably becausethey are the chelateform recognized bythe plant roots. Theyare notably similar toEDTA in structure andmetal-binding chemis-try. Both have multi-carboxylate chelating groups and formextremely strong complexes with iron andother transition metal ions.

We have examined the binding ofplutonium(IV) to mugineic acid over a largepH range using ultraviolet-visible (UV-Vis)spectroscopy (see figure above). Mugineic acidprevents hydrolysis and precipitation ofplutonium(IV) to a pH greater than 10; spectraof plutonium(IV)-EDTA and plutonium(IV)-mugineic acid are nearly identical at all but thehighest pH. In this pH range, plutonium(IV)-EDTA is believed to be a hydroxo rather thanaquo complex, as it is under acidic pH.

The plutonium-mugineic acid complex maynot undergo this change, or it may occur at aneven higher pH. Possible structures of theplutonium(IV)EDTA and plutonium(IV)-mugineic acid complex are shown in the figureat right. These data suggest that mugineic acidwill be as powerful a ligand as EDTA for keep-ing plutonium species soluble in the environ-ment. We are currently directly testing the rate

at which phytosiderophores solubilize pluto-nium hydroxides in comparison to EDTA andthe resistance of plutonium-phytosiderophorecomplexes to microbial degradation.

To investigate whether we can use this iron-uptake system for plutonium and other ac-tinides uptake into these plants, we need toknow if plants recognize and take up actinide- Pictured are

proposed structuresof the plutonium-EDTA complex asthe pH increases,and possibleanalogous structuresfor the plutonium-mugineic acidcomplex. Datasuggest thatmugineic acid maybe as powerful aligand as EDTA forkeeping plutoniumspecies soluble inthe environment.

Researchers haveused ultraviolet-visible (UV-Vis)spectroscopy toexamine the bindingof plutonium(IV) tomugineic acid (MA)or EDTA over a largepH range.



siderophore complexes or whether they aretrapped in the plant root zone. Alternatively,we need to know if phytosiderophorescreate soluble species that aren’t taken upor broken down, and therefore contribute toplutonium migration.

To test if phytosiderophores can aid in ac-tinide uptake, we are examining the effects ofplant iron status. Iron-starved plants producemore phytosiderophores than iron-saturatedones, and have dramatically higher uptakes ofzinc, nickel, manganese, copper, and even cad-mium (under some growth conditions), thando high-iron nutritional plants.

We are growing both a phytosiderophore-producing plant (barley) and a non-phyto-siderophore-producing “hyperaccumulator”species (Indian mustard) under variable ironconditions, and with actinides added as vari-ous chelated forms (phytosiderophore-che-lated, citric acid-chelated, and unchelated) tosee how this affects uptake into the plant.

ContributedPapers

Our initial tests with uranium show someinteresting results. Barley plants grown in hy-droponic solutions with 1,000 parts per mil-lion uranium with or without citrate have lowuranium uptake into plant shoots (typically0.2–3.5 milligrams per gram plant dryweight), with no consistent response to plantiron status, presence of citrate, or pH (pH 5 or6), although on average, uptake with citratewas slightly higher (see figure above). Thiswas surprising because we expected citrateand decreased pH to cause significantlyhigher shoot uptake because of their ability tokeep uranium soluble. We did not expect theiron status of barley plants to have a majoreffect on uranium uptake into shoots because,although the phytosiderophore could contrib-ute to uranium solubility, it is unlikely theiron(III)-phytosiderophore uptake systemin plants would recognize and take up auranium(VI) phytosiderophore complex.

Initial tests withuranium show someinteresting results.Barley plants weregrown in hydroponicsolutions with 1,000parts per millionuranium with orwithout citrate;without added iron,with 7.5 micromolesadded iron(“normal”), and with100 micromoles ofadded iron (“high”);and at a pH of 5 and6. Error bars showone standarddeviation among thethree to four plantsgrown at eachcondition.

500

400

300

200

100

mg

ura

niu

m/g

ro

ot

dry

wei

gh

t

mg

ura

niu

m/g

sh

oo

t d

ry w

eig

ht

0No Fe

pH6, CitratepH6, No CitratepH5, CitratepH5, No Citrate

Normal Fe High Fe No Fe Normal Fe High Fe

2.5

2.0

1.5

1.0

0.5

0



Barley plant roots have very high sorptionof uranium from solution, as has been ob-served with other plant species. We observedvalues from 4 milligrams uranium to morethan 600 milligrams uranium per gram dryroot weight—one to two orders of magnitudemore than in shoots (see figure on page 32).Interestingly, we did observe a significantdifference in root uranium binding basedon growth conditions.

Barley roots have significantly more ura-nium bound without citrate present than withcitrate, regardless of pH and iron status. Thiscould simply be due to increased precipitationof uranium onto the roots without citrate.However, without citrate present, roots boundsignificantly more uranium at pH 5 than at pH6. This may be caused by such rapid precipita-tion of uranium from solution at pH 6 that it isunavailable for interaction with the plantroots. And at pH 5, roots of plants with excessiron bound more uranium than roots of plantsgrown without iron. This could have manycauses. There could be increased microorgan-ism presence on the “high-iron” roots increas-ing sorption of metals, or there could be in-creased solubilization of uranium on the rootsby chelators, such as phytosiderophores, whenplants are iron starved.

We will soon test plutonium uptake intoplants under these same conditions, where weexpect iron status and phytosiderophores tomake significant differences. We now knowphytosiderophores are good chelators forplutonium(IV), and have a good chance of be-ing recognized by the iron-uptake system ofthe plant because of the similarities betweeniron and plutonium.

If plutonium-phytosiderophore complexesare recognized and translocated by thephytosiderophore uptake system in plants,plutonium solubilization may be balancedwith uptake, eliminating a problem of chelate-induced phytoextraction.

Phytosiderophores may also improve trans-location into higher plant parts. Their biosyn-thetic precursor, nicotinamine, is ubiquitous inplants and is believed to affect cation (copper,iron, zinc, and manganese) mobilization anddistribution in plants, presumablyhaving a direct influence on plant metaluptake and translocation.

Even if efficient uptake into the higher partsof the plant cannot be achieved, we will deter-mine how phytosiderophores will affectphytostabilization. Phytosiderophores willchelate actinides. These chelated actinidescould be fluxed to the root zone of the plant, asare other metal-phytosiderophore complexes,and could have increased precipitation ontothe plant roots. Concentrating contaminantsand fixing them onto the plant roots may helpprevent migration and improve the efficiencyof phytostabilization. Alternately,phytosiderophores could chelate actinides andprevent precipitation onto plant roots, therebyincreasing the risk of contaminant migration.

We think this research holds enormous po-tential. Grasses grow anywhere and every-where, from arid and semiarid sites, like theNevada Test Site and the Rocky Flats Environ-mental Technology Site, to wet and subtropicalsites like Oak Ridge National Laboratory andthe Savannah River Site.



The plutoniumphytoremediationprocess is illustrated.

Phytosiderophore-chelated plutonium

Phytoremediation: Pu uptake

Phytostabilization:Pu association

Pu concentratedand stabilized

Pu removed from soil

Erosion and migration prevented

Grasses

Pu

Pu Pu Pu Pu Pu

Pu Pu Pu Pu

Pu Pu Pu

Pu

Mobile/soluble/chelated plutonium

Migration

Wind and watererosion

Plants degrade chelators

Chelators (anthropogenic and natural)

Plants produce

phytosid

erophores,

chelators,

acids

Soilamendments,

additionalchelators

Sorbed and "nonmobile"plutonium (PuO2)

Plant uptake of actinides is well documented,particularly from past research on human healthimpacts of nuclear weapons use and testing.We know far more about plutonium uptake into

plants from these past studiesthan we know about other

actinides. Relative plantuptake availability foractinides is neptunium> uranium > curium >americium > plutonium,which is a reflection oftheir solubility underenvironmentalconditions.

Many variables havebeen shown to affectplutonium uptake intoplants: plutonium formand concentration, soilchemistry, soil type, number

of successive plantings, plantspecies, and plant surface area

(when fallout and resuspended soil are majoraccumulation contributors).

As with most metals, it seems that thepredominant factor in plutonium uptakefrom soil by plants depends on chelation ofplutonium to increase plutonium solubility.Application of synthetic chelators (EDTA andDPTA), to either soil or hydroponic solutions,has been shown to increase uptake of plutoniumup to 1,300 times—resulting in a 20-foldconcentration. Ranking of various ligandsand counterions for enhancing uptake ofplutonium has been shown to be nitrate< acetate < glycolate < oxalate < citrate< EDTA < DPTA, which is areflection of ligand affinityfor plutonium.

Plutonium is translocated fromthe plant roots into the higherparts of the plants. Plants reduceplutonium(VI) upon uptake.Various studies have shownplutonium is present only asplutonium(IV) (greater than 90percent) in plant xylem, even ifsupplied as plutonium(VI). Thisreduction process may be similarto the purported requirement forreduction of iron(III) to iron(II)prior to root membranetranslocation.

Fe(III)-PS

Fe(III)

Fe(III)

Fe

PS

Root fragment

Soilmicrobes

Rhizosphere

PSFe(III)

Plant uptake of plutonium

Grasses producephytosiderphores(PS) to acquire theiron (Fe) they need.Could thesephytosiderophoresalso be used to trickplants into taking upplutonium?

These studies also indicate plutonium istransported as an anionic organic complexthat is different than the form supplied to root-bathing solutions. In hydroponic studies insoybeans using plutonium(IV) nitrate, xylemexudates analysis of samples with plutoniumadded both in vivo and in vitro (xylem exudatescollected then plutonium added) using anionand cation exchange columns and thin-layerelectrophoresis showed that both iron andplutonium are present primarily as organic acidcomplexes (instead of amino acid or peptide).

Nickel(II) and cadmium(II) were present primarilywith components of the amino acid/peptidefraction, again showing the differing uptakeand transport systems for hard and soft metals.The forms of plutonium transported in the xylemappear to change with plant age, and theamounts of plutonium parallel essential ionconcentrations.

The evidence suggests that plutonium may trackthe normal plant ligands used for metal uptakeand growth. Indeed, it has been suggested thatafter absorption in the plant, trace contaminantsare translocated, metabolized, and storedgenerally analogously to nutrient elements.In support of this, nutrient elements, especiallyiron, zinc, and copper, that compete for reactionsites and organic ligands affect the chemicalform of plutonium in the plant.

Phytosiderophore-mediated uptake could explainsome of the observations made on plutoniumuptake into grasses.

34 Nuclear Materials Technology/Los Alamos National Laboratory




PlenarySpeakers

Condensed Matter Physics

A parallel institution to Los Alamosin the sense of its broad diversityof nuclear-materials research, the

European Union’s Institute of TransuraniumElements (ITU) was ably represented byGerry Lander at the conference’s firstplenary presentation.

Lander was enthusiastic in elaboratingsome novel ITU approaches to the actinideelectronic structure conundrum. He describeda body of work that employed high pressureto alter the atomic volume in actinides.Displaying compressibility curves for ura-nium, americium, and an americium-curiumalloy, Lander presented evidence for atransition from localized to bonding behaviorin americium’s 5f electrons when the elementwas compressed to 50 percent of itsatomic volume.

He subsequently described a clever way ofperforming the reverse experiment, applying

negative pressure to a thin deposited film ofplutonium to illustrate what he described as a“pseudo phase change” in delta-phase pluto-nium. While he admitted that the exact inter-pretation was still controversial, he also en-thused, “these experi-ments are very beautiful,and they give us a nicehandle on the electronicstructure of plutonium.”

Continuing on thistheme of understandingelectronic structure,Lander described the dis-covery of superconduc-tivity in plutonium com-pounds as “the most ex-citing thing in actinidescience for the last fiftyyears.” He was animatedabout the ITU–LosAlamos partnership, describing it as a collabo-ration in which the two organizations contrib-uted complementary skills and were continu-ing to work very closely together.

Actinide Compounds and ComplexesLos Alamos staff member Wolfgang Runde

of the Isotope and Nuclear ChemistryGroup (C-INC) offered a primer in the selectedchemistry of actinide compounds, particularlythose relevant to modern methods of pluto-nium purification related to pit manufacturing.

Despite what he characterized as “intensivestudy of transuranic minerals,” Runde empha-sized that single-crystal structures of pluto-nium compounds—particularly those ofplutonium(V) and (VI)—remain rare. Becauseof their importance in plutonium processingand purifications, and in long-term nuclearwaste management, his presentation focusedon the oxalates, hydroxides, and silicates ofplutonium, which despite their importance,are poorly characterized on a structural level.

The future of plutonium sciencePlenary speakers discussthe many avenues of research

Gerry Lander

Wolfgang Runde



His talk covered aspects ofboth the preparation of suchcompounds and the insightsacquired into their molecularstructure.

In addition to the wealth ofspecific information presentedin this talk, it also conveyed anoverall sense of the interestingand complicated chemistrythat keeps actinide researchersengaged in trying to compre-hend the myriad of bonding

behaviors thatactinides display in both laboratory andnatural environments.

The Nuclear Fuel CycleOn Dec. 8, 1953, President Dwight

Eisenhower delivered his “Atoms for Peace”speech before the United Nations.Eisenhower’s goal was to apply “the miracu-lous inventiveness of man” to change “thefearful atomic dilemma” into something thatcould benefit humanity.

Fifty years later, Vic Reis postulated thatperhaps it was time to initiate what he calledan “Atoms for Peace II.” Before he outlined thepossible vision for such an endeavor, he notedthat this new vision would also be for fiftyyears, which is equivalent to the lifetime of onenuclear power plant. Reis, of Science Applica-tions International Corp., is a former DOEassistant secretary for defense programs.

Reis’ vision emphasized nuclear power.With the construction of a system of next-generation nuclear power plants, electricitycould become safe, plentiful, affordable, andenvironmentally friendly. This extensivegrowth of nuclear power would require coun-tries to work together to settle regional con-flicts and curtail international terrorism androgue states. Wars would have to be mini-mized if not eliminated.

Key tasks for the United States include ex-tending licenses of existing nuclear powerplants and opening the Yucca Mountain re-pository, Reis said. The United States govern-ment also must help industry with costs asso-ciated with construction of new plants, con-tinue the development of the advanced fuelcycle initiative, and design a fourth-generationnuclear reactor. (See ARQ 1st/2nd quarter 2003.)

Materials Science and PlutoniumProperties

One of the more interesting characteristicsof plutonium is how it ages. In essence, pluto-nium ages from “inside out” and “outside in,”which means that it not only changes its ownproperties, but also has the potential to changethe properties of surrounding materials.

“The three most important aging effects inplutonium are the radiogenic decay of thevarious plutoniumisotopes, the possiblethermodynamic insta-bility of the plutoniumalloy itself, and thecorrosion ofplutonium’s surfaceduring both storageand function,” said JoeMartz of Los Alamos’Materials Science andTechnology (MST) Di-vision. “These agingeffects accumulateslowly over decades.

Vic Reis andTheresa Fryberger

Joe Martz

PlenarySpeakers



Furthermore, such effects may not take place ina linear fashion.”

To ensure the performance, safety, and reli-ability of the U.S. nuclear weapons stockpile,researchers must be able to predict when agingeffects negatively influence a weapon’s perfor-mance. To accomplish this goal, scientists mustunderstand the properties of plutoniumthroughout its “lifetime,” from the time it ismanufactured to the time that it is “retiredfrom service.”

“Virtually all conditions in plutonium areripe to age-related damage,” Martz concluded.“Yet, we have found no first-order effects afterseveral decades. Hence, we feel reasonablycomfortable that the lifetimes of componentswill last at least several tens of years.”

Actinides in the EnvironmentTeresa Fryberger, director of the DOE’s

Division of Environment Remediation Sci-ences, Office of Biological and EnvironmentalResearch, Office of Science, told her audience:“It is clear that we won’t be able to remove allcontamination at all sites. We must understandwhat contaminants are mobile and underwhat conditions.”

Such understanding, she said, will make itpossible to develop science-based risk evalua-tions to produce better decision-making;more-effective remediation and containmentstrategies; and a solid understanding of what,when, and where to monitor at sites in long-term stewardship.

Current actinide research in the DOE weap-ons complex includes study of uranium migra-tion under the tank farms at Hanford Sitein Washington; plutonium transport at RockyFlats Closure Project in Colorado; plutoniumsources and mobility at Savannah River Site inSouth Carolina; and microbial effects on ac-tinide speciation and mobility.

“We have learned a great deal about themolecular interaction of actinides in the sub-surface,” Fryberger said. “We have been ableto provide scientific data that has impacted

remediation decisions for specific problems atspecific DOE sites; (but) we have far to gobefore we can describe the complex interactionsand processes that are important in transport.”

In the future, she said, scientists must usethe available DOE computing capabilities andinsert mechanistic and chemical informationinto models; use interdisciplinaryteams to do comprehensive, long-term field studies that include model-ing in design and interpretation; anddevelop improved characterizationtools.

Detection and AnalysisChristopher Puxley of the Atomic

Weapons Establishment atAldermaston in the United Kingdombrought scientists up to date on theapplication of vibrational spectros-copy to actinide analysis.

Vibrational spectroscopy can beused for in situ nondestructive analy-sis of both bulk and trace actinidecompounds—notably for the detec-tion and analysis of typical speciesand unexpected contaminants thatoccur on the surface of actinides.

Puxley made three points aboutusing fiber-optic analysis on actinides. First,fiber-optic midinfrared analysis is applicable toorganic materials and their degradation prod-ucts. Its applicability to inorganic compoundsis limited to oxyanion identification, low-mo-lecular-weight atoms in materials (e.g., hy-drides), and low-molecular-weight, multiply-bonded anions.

Second, fiber-optic Raman analysis is appli-cable to both inorganic and organic materials.It is extremely useful for the identification ofinorganic compounds but is difficult to applyto compounds that are black.

And third, because of its high sensitivity andsmall spot size, fiber-optic Raman spectroscopyis a valuable technique to detect contaminationand surface species on bulk materials.

—Vin LoPresti,Octavio Ramos, Jr.,

and Charmian Schaller

Christopher Puxley



CounterpointHelen Caldicott’s views on nuclear weapons did not gounchallenged. Sig Hecker, Los Alamos Senior Fellow, rosefrom the audience to present “another point of view.”

Hecker, a plutonium metallurgist, said, “You brought us back toreality because the last few days, we were having great fun…. Iappreciate your point of view. I happen not to agree with it.”And, he said, the viewpoint of plutonium scientists is “not less

noble in terms of service tohumanity” than Caldicott’s.

He said that if one looks at thestatistics, about a million peopledied as a result of wars in the 1500sand 1600s. In the first half of the 20th

century, 87 million people died inwars. “Soon,” Hecker said, “wewould have found a way to wipeourselves off the face of the Earth.”However, in the last half of the 20th

century there was a dramatic drop inthe number of deaths. “In science,we call that a discontinuity . . . a

discontinuity in the death of mankind,” said Hecker. This wasan apparent reference to Caldicott’s lack of scientific data inher discussion.

“Nuclear weapons got mankindto think differently,” Hecker said.“I view it as a good thing thatoccurred during the buildup ofnuclear weapons.” The questionnow, he said, is, “How does onemanage significantly reducingthe number of nuclear weaponswithout the number of deathssneaking up once again?”

Caldicott said, “I know you say you developed nuclear powerto maintain the peace, and it was a good idea, but you don’tknow” whether those weapons will eventually be used. Shesaid she believes that unless mankind abolishes nuclearweapons, “there will be a nuclear war” somewhere, somedaybecause of the “fallibility of human minds.”

Jon Schwantes of Lawrence Berkeley National Laboratoryasked Caldicott whether she had developed any figures on thetransport of plutonium particles and their resulting danger ascompared to the transport and resulting danger of smoke particles.

She said the risk of plutonium to workers is “quite high,” and,“The amount of plutonium in the world is huge.” She asked,“How can you quantify something that lasts half a millionyears?” Schwantes said, “You can do risk assessment.”The exchange continued, and Schwantes concluded, “If there’sno chance of the plutonium coming to your lungs, then there’sno risk….”

—Charmian Schaller

Helen Caldicott and Sig HeckerPoint/Counterpoint

PointHelen Caldicott, founder of Physicians for SocialResponsibility, sketched out a very dark picture of plutoniumand nuclear weapons in a speech that opened the “Actinidesin the Environment and Life Sciences” portion ofthe conference.

She told her audience of plutoniumscientists—many of them from thenuclear weapons laboratories—that,“Plutonium is incredibly carcinogenic.”Sufficient exposure, she said,kills cells, and the cells that survive are“likely to have mutations.” The resultinggenomic progression of instability cancontinue for more than 30 generations.

Caldicott, an Australian pediatricianwith a specialty in cystic fibrosis, said,“We find out what radiation doesby doing decent epidemiologicalstudies,” but not enoughepidemiological studies are being

done at autopsy, partly because of their expense.

She mentioned new work being done by Alexander Millerat the Armed Forces Radiobiology Research Institute inBethesda, Maryland, John Little at the Harvard School ofPublic Health, and Eric Wright at the University of Dundeein Scotland on “the bystander effect”—the development ofgenetic mutations by cells on the periphery of tissue exposedto radiation.

She said that plutonium in the particulate form that is less thanfive microns in diameter can be inhaled. It can settle into thelower part of the lungs, where it can cause mutations.About five tons of plutonium was released into the atmosphereduring weapons testing, she said, and one result is that, “Weare increasing genetic disease by the contamination of theatmosphere with plutonium,” and contamination lives on inthe genes.

Plutonium, which has a 24,000-year half-life, was first madefor nuclear weapons. Caldicott said she knew some of theearly scientists who worked on the bomb, but, “I becameterribly alarmed about what they did.”

And she concluded, “I’m talking to you as a physician, as apediatrician … You must stop building nuclear weapons! … Idon’t know if you’ve got 10 more years.”

0

25

50

75

100

1.5M6.2M 6.4M

20M

87M

17M

1500s 1600s 1700s 1800s 1900to

1948

1949to

1990

Mill

ions

of P

eopl

e

Helen CaldicottSig Hecker

Death due to war



PanelDiscussion

Gerry Lander

Monday evening’s panel discussionbrought together five preeminentnames in actinide science:

Graham Andrew, of the International AtomicEnergy Agency (IAEA); Theresa Fryberger,head of the DOE’s Division of EnvironmentRemediation Science; Gerry Lander of theInstitute of Transuranium Elements (ITU) inKarlsrühe, Germany; Senior Los AlamosFellow and former director Sig Hecker; andVic Reis of Science Applications InternationalCorp. (SAIC). The discussion was moderatedby Los Alamos’ Ed Arthur.

Gerry Lander: There are tremendously ex-citing prospects for actinide science . . . butthere are also some fundamental problemsthat we, as the actinide people, should ad-dress. The first is: how do we as institutes thathave special capabilities share those capabili-ties with people on the outside?

The pressure’s on us from the institutionalaspect to reduce the number of places, to re-duce the flexibility at those places. And at thesame time, we have to reach out to otherpeople, because otherwise, this field is goingto get smaller and smaller in the funding eye.Somebody, today, made the comment that ageprofile at this conference is much better . . .much younger. Bringing in students andshowing them things can be done is anothercrucial aspect that we have to be fullyaware of.

The other thing that has personally frus-trated me is an institutional problem again.Transport of our materials has become an ab-solute nightmare. In the old days we used tocarry around bits of uranium. I never wentanywhere without bits of uranium in mysuitcase. Now that’s absolutely forbidden.

And, you know, there are people who think . . .that a milligram of uranium will kill a wholeprovince in France. You can’t somehow getacross to these people that there’s uranium allover the place.

But the transport now, especiallypost-September 2001, has become moredifficult. There’s been a total crackdown.And that just means collaboration be-tween the labs is cut again. So there’sthis huge tendency to reduce things, tocut us off from the outside.

Let me just say a bit about reactor fu-ture. And all the countries, in my view,are not collaborating enough. I thinkthere is good commercial reason forthat. There is, of course, a lot of moneyin nuclear fuel . . . and so there is a lackof collaboration, which is driven partlyby money. But in the end, if there is notcollaboration, we all go down together. Iwould like to see a more-open forum ondiscussing the pros and cons of those fuelsrather than this sort of internal battling thatseems to be going on all the time.

I am enormously distressed by the huge cutsthat we are seeing in our neighboring institu-tions, and the whole atmosphere that we see inmany European countries with a workingnuclear power system—a constant reduction.It’s not just because of the waste legacy, butalso because of the whole question of how thethird world fits in . . . for power and for raisingthe standards of living.

Panel discussionFive views on opportunities andconstraints for actinide research



Vic Reis: The future of civilization reallydepends upon how actinides, and specificallyplutonium, are managed over the next fiftyyears. It’s that important; I don’t think there’sanything more important. I come at this fromboth the perspective of national security and

global climate change. And if you thinkabout the major things that can occur overthe next fifty years, how one deals with ac-tinides, plutonium in particular, will be keyto much of that.

I worked twenty years ago in the Office ofScience and Technology Policy in the WhiteHouse. I was really impressed with the as-tronomy community, because what as-tronomy did then—and I suspect, still does—is that they argue a lot among themselves.But they then present a unified researchagenda, and that makes life a lot easier forthe people in the Office of Management andBudget and those people in the National Sci-ence Foundation who have to put the policytogether for Congress.What I’ve learned over the past four years

since I’ve left the warm confines of the nuclearweapons complex and moved out in to thebroader world is that it’s very hard to get aunified research agenda.

I suspect that we have a good many of theentire community right here at this meeting.The good news, right, is that there aren’t verymany of you. The bad news is that you can’tseem to figure out what to do. At least by thetime it reaches the Department of Energy andCongress, it’s so complicated that the interplaybetween the politics and the research commu-nity frequently gets destroyed. But the re-search community ought to be able to figure...out what that agenda is; and if they do that,

much of the politics will . . . I won’t say takecare of itself . . . but it will be a lot easier.

Theresa Fryberger: If you look at thecleanup efforts of the weapons complex in theUnited States, a lot of people would say thatthis has not been as successful as it might havebeen. And part of the reason is because theydon’t know where contaminants will go; theydon’t know how they behave; they cannot pre-dict risk.

If we’re hoping to make a place for cleannuclear energy or safe deposition of waste, weneed to understand this. We’ve studied thetransport of contaminants for several decades,particularly forYucca Moun-tain. Andpeople ask:“Well, whydon’t we knowmore?” Onereason is that,until about adecade ago, wedidn’t have theability to dothings like spe-ciation and ac-tual environ-mentalsamples thatwe’re now ableto focus on.So we reallycouldn’t understand at the molecular levelwhat the real chemistry was. We’ve madegreat progress over the last decade, but weneed to continue to make more.

Vic Reis

Theresa Fryberger

PanelDiscussion



So where do I think we need to go in envi-ronmental sciences in the near future? Scalingis an issue; we need to get beyond the molecu-lar level. We need to make more progress, bothtemporal and spatial. We need better character-ization techniques in the field . . . and we needto focus science a little bit differently. We needlong-term comprehensive field studies that areconducted by teams—dedicated teams of scien-tists from various disciplines. And we need touse modeling, both for designing experimentsas well as to model systems.

Sig Hecker: The near future? It’s the f elec-trons that are exciting and that’s one of the ex-citing challenges—just exactly what are thef electrons doing? And that’s going to be hearda number of times: that plutonium sits right atthat knife edge between the very different kindof behavior—bonding or localized . . . youknow, participating in the process or beingchemically inert. And we still have lots of yearsof exciting research to try to soak that up.

To me, actinides mean new materials, newbehavior, and new thinking. Personally, what’sbeen of greatest interest . . . what’s so interest-ing and fascinating is plutonium’s instability.We saw today the enormous gyrations that theyfind as a function of temperature with all thesix allotropes. So it’s unstable with temperaturebut it’s also very unstable with pressure. Thenwith the six allotropes—when you start squeez-ing on them—those high volume states start tosqueeze out, and then, with any actinide, youmay end up with interesting results in the crys-tal structure. But if you get the right chemicaladditions like gallium and aluminum, you canstabilize nice metallic crystal structures.

Unfortunately, the problems of proliferationand terrorism threaten to set back all our ef-forts for decades. There’s a great need for in-ternational collaboration. Meanwhile,there’s the Patriot Act . . . supposed tobe keeping terrorists out, and is keep-ing academicians out instead.

Russia is a huge environmentallaboratory, and we should be overthere taking advantage of thingsthey’ve done wrong. It illustrates thatit’s almost impossible, today, to getexperimental work done.

Graham Andrew: What I’ve been hearingfrom my colleagues . . . and I agree . . . is thatthe basic science is driven by both strategicand policy requirements. We must make adecision early on and go from basic science tothe next step. Yet we can’t pursue all options.

An example of crucial decision-making isthe area of waste. Unless we resolve repositoryand waste-management problems, we won’tbe proliferating weapons, but rather,Yucca Mountains.

Beyond plutonium, we need to improve ourcapabilities in minor actinide science. Minoractinide science needs to be taken forward inthe next ten years; we need new forensics. Dowe have the same understanding of, for ex-ample, solution chemistry for the minor ac-tinides as we do for plutonium?

We need much better international coopera-tion with respect to fuel-cycle options. For ex-ample, do we want to proliferate reprocessingplants or coalesce those efforts into, for ex-ample, having three or four such facilitiesaround the world?

Most important, we need to be concernedabout capturing the knowledge base that al-ready exists . . . dealing with capturing theknowledge of people leaving the field.Else, we risk squandering what is an irre-placeable resource.

Sig Heckerand Vic Reis



PosterSession

HighlightsSynthesis of New Plutonium Main-Group Metal Materials

Peter K. Dorhout,1 Ryan F. Hess,2 Kent D. Abney,2 and Steven D. Conradson2

1Colorado State University and 2Los Alamos National Laboratory

“This project focuses on understandingthe electron density around plutonium ina variety of different matrices,” explainedPeter Dorhout, a professor at ColoradoState University. “We prepared a newcompound this past year that hasplutonium in a selenium matrix. Theselenium chan share electron density ina ‘balancing’ fashion. That is, dependingupon what the plutonium metal ‘wants,’the selenium can either accept electrondensity or push electronic density ontothe metal.”

This new compound has the formulaKPu3Se8-x. To determine the electrondensity of this new compound,researchers used x-ray absorption near-edge structure (XANES), which providesoxidation-state information about theabsorbing, or target, atom. The key resultfrom this work was that the absorptionedges of such heavy chalcogenidesystems appear close to the value

found for plutonium metal (1–2 electron volts). This result implies that the electronicenvironment around plutonium in this compound is very similar to that found inthe metal.

“You really have to understand the electron density around plutonium with theparticular ligands before you can make any bold assertions about what kinds ofoxidation states you’re going to find,” said Dorhout. “Most studies have been done inoxides and aqueous solutions, but the real environment isn’t always just oxygen orwater. So, I think that by digging real deeply into oxidation chemistry, we found somemore applicable aspects to our work with respect to how we can better interpretXANES spectroscopy.”


PeterDorhout



PosterSession

Highlights

The Localized and Itinerant Nature of 5f Electrons in Pu and Pu Compounds

J. J. Joyce, J. M. Wills, T. Durakiewicz, M. T. Butterfield, E. Guziewicz,J. L. Sarrao, L. A. Morales, D. P. Moore, and A. J. Arko

Los Alamos National Laboratory

Scientists are using photoelectronspectroscopy and a computationalmixed-level model to determine theelectronic structure of plutoniummetal and several plutoniumcompounds, including materials thathave magnetic and superconductingtransitions. “We are trying tounderstand the electronic structure ofplutonium in a broader context,” saidJohn Joyce of Los Alamos. “To dothis, we are looking at plutonium frommany different crystal structures andwith many different ligands.”

By examining a large number of alloysand compounds, these workers haveobserved the development of acommon electronic structure basedon the dual nature of plutonium 5felectrons: localized or itinerant.

“From a programmatic standpoint,we’re interested in developing anatomic-scale model that shows how oxides and hydrides work with plutonium.Such work is relevant to life sciences and long-term storage issues.”

By applying both experimental (photoelectron spectroscopy) and computational(mixed-level model) techniques to describe the electronic structure of a broad rangeof plutonium materials, researchers at Los Alamos anticipate a much betterunderstanding of the electronic properties of plutonium.


JohnJoyce



The conference in a nutshell

The “Plutonium Futures—The Science Conference 2003”closed with a tour-de-force summary of the entire five dayprogram by Gerd Rosenblatt, one of the founders of theconference. Rosenblatt, of Lawrence Berkeley NationalLaboratory, said he listened to all forty-five talks and lookedat all 125 posters. “The scientific quality of the papers(presentations and posters) has just gone up and up and up,”Rosenblatt said. On the other hand, he commented, the averageage of those attending the conference has gone downnotably—perhaps as much as 10 years—a fact that givesRosenblatt “a lot of hope” for the future of plutonium science.”Such diverse participation is vital for the field, he said. “Thishas been an exciting meeting, and we’re making excitingprogress,” Rosenblatt concluded, but he reminded hisaudience of the call by Dr. Helen Caldicott, founder ofPhysicians for Social Responsibility, for an end to all nuclearweapons. “We have to think about what we do and why andour responsibilities to our fellow human beings,” he said.

Gerd Rosenblattprovided twopostscripts for hisaudience: The nextplutonium conferencewill be the “Actinides2005 InternationalConference,” July 3-8,2005, in Manchester,England. To keep upwith developments,those interested inthe conference shouldvisit the Web site(www.actinides2005.com).And the InternationalUnion of Pure andApplied Chemistry andthe International Unionof Pure and AppliedPhysics ruled inAugust that GSI inDarmstadt, Germany,(a heavy ion researchcenter funded by theFederal Governmentof Germany and thestate of Hesse) hasproved the existenceof another element—number 110. Theelement, whichwas discovered in1994, will be knownas “darmstadtium”and have the chemicalsymbol Ds. Rosenblattsaid that GSI has alsodiscovered element111. But, he said, noother suggestedelements yet seemsufficiently reliableto be approved.

Gerd Rosenblatt

PlutoniumFutures

conference


Nuclear Materials Technology/Los Alamos National Laboratory

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Plutonium Futures—The Science Conference

Conference TransActionsEditorsK.L. DeLucasG.D. Jarvinen

Topical Conferenceon Plutonium and Actinides

Albuquerque, New Mexico, USAAugust 6–10, 2003

Sponsored by Los Alamos National Laboratory

in cooperation with the American Nuclear Society

Melville, New York, AIP CONFERENCE PROCEEDINGS 673

Plutonium Futures

—The Science Conference

Conference

TransActionsEditors

K.L. DeLucas

G.D. Jarvinen

Topical Conference

on Plutonium and Actinides

Albuquerque, New Mexico, USA

August 6–10, 2003




Plutonium Futures

—The Science Conference

Conference

TransActions

Editors

K.L. DeLucas

G.D. Jarvinen

Topical Conference

on Plutonium and Actinides

Albuquerque, New Mexico, USA

August 6–10, 2003




Copies of conferencetransactions available

If you were unable to attend the recent“Plutonium Futures—The Science Confer-ence 2003” but would like a copy of theconference transactions, please contactMeredith Coonley. Send requests via e-mailto [email protected] or phone 505-667-0392.Please include your name and completepostal address in the message.

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