6-1

Nuclear Forensics Summer SchoolChemical behavior of isotopes and radioelements

• Radioisotopes and radioelements of concern§ Fission products§ Actinides§ Actinide decay products

• Speciation in fuel• Trends by periodic group

§ Cs§ Sr§ Lanthanides§ Halides§ Noble Gases§ Polonium§ Actinides

• Provide basis for understanding chemical behavior

6-2

Fission Products• Fission yield curve varies with fissile isotope• 2 peak areas for U and Pu thermal neutron induced fission• Variation in light fragment peak• Influence of neutron energy observed 235U fission yield

6-3

Burnup: LWR UO2

3

Element moles/kg % Element % TotalU 3.99E+00 91.85 91.85Xe 4.69E-02 1.08 92.93Zr 4.68E-02 1.08 94.01Mo 4.05E-02 0.93 94.95Nd 3.06E-02 0.71 95.65Ru 2.73E-02 0.63 96.28Cs 2.51E-02 0.58 96.86Ce 2.34E-02 0.54 97.40Sr 1.27E-02 0.29 97.69Pd 1.26E-02 0.29 97.98Ba 1.21E-02 0.28 98.26La 1.04E-02 0.24 98.50Pr 9.23E-03 0.21 98.71

Element moles/kg % Element % TotalTc 9.19E-03 0.21 98.93Y 6.52E-03 0.15 99.08Sm 5.39E-03 0.12 99.20Kr 5.13E-03 0.12 99.32Rh 4.86E-03 0.11 99.43Rb 4.80E-03 0.11 99.54Te 4.15E-03 0.10 99.64Pu 4.14E-03 0.10 99.73Np 2.66E-03 0.06 99.79I 2.13E-03 0.05 99.84Pm 1.31E-03 0.03 99.87Eu 1.25E-03 0.03 99.90

39 MWd/kg (Siemens/KWU)Ignore oxygen contribution

6-4

Burnup: Fast reactorelement distribution for different burnup

4

Element 20% Element 40% Element 60%U 53.84 U 30.16 U 15.50Pu 12.40 Pu 11.86 Xe 9.03Xe 4.05 Xe 6.89 Ru 8.75Ru 3.72 Ru 6.53 Pu 8.54Mo 3.58 Mo 6.13 Mo 7.98Zr 3.05 Zr 5.22 Zr 6.86Pd 2.78 Pd 4.85 Pd 6.43Nd 2.74 Nd 4.81 Nd 6.39Cs 2.60 Cs 3.85 Ba 5.89Ce 1.80 Ba 3.80 Cs 4.30Ba 1.73 Ce 3.09 Ce 4.10La 0.94 La 1.57 La 2.01Pr 0.83 Pr 1.34 Cd 1.98Tc 0.58 Cd 1.20 Pr 1.65Te 0.55 Te 0.96 Te 1.28Sm 0.54 Sr 0.85 Gd 1.08

Sm 0.85 Sr 1.05Sm 0.98

6-5

Elements in fuel at burnup

• From oxide (39 MWd/kg)§ Actinides: U, Np, Pu§ Noble gases: Xe, Kr§ Group 1: Cs, Rb§ Group 2: Sr, Ba§ Group 4: Zr§ Lanthanides: (Y), Nd, Ce,

La, Pr, Sm, Pm, Eu§ Metal phase: Mo, Ru, Pd,

Tc, Rh à Degree in metal phase

varies§ Non-metals: Te, I

5

• From fast reactor• Actinides: U, Pu• Noble Gases: Xe, Kr, He• Group 1: Cs, Rb• Group 2: Ba, Sr• Group 4: Zr• Lanthanides: Nd, Ce, La,

Pr, Sm, (Y) • Metal phase: Mo, Ru, Pd,

Rh, Tc• Non-metals: Te, I

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6-7

Speciation in Spent Fuel

• Chemical form of actinides and fission products vary with fuel • Oxide fuel

§ Fuel for thermal reactors§ Speciation dictated by reaction with oxygen

à Noble gases

à Oxides as solid solutions in UO2 (Ln, Group 1, Group 2, Zr, Nb, Mo, Te)

à Separate oxide phase (Group 1, Group 2, Zr, Nb, Mo,Te)

à Metallic phases (Mo, Tc, Ru, Rh, Pd)§ Specific behavior dependent upon concentration

à Related to burnup and fuel composition• Metallic fuel

§ Fuel for fast reactors, non-aqueous cooling of reactor§ Elemental species most common§ Solid solutions and intermetallic phases§ Reactions with halides (I-)

7

6-8

Oxide Volatility

• For treatment of oxide fuel

• UO2 oxidized to U3O8

§ Heating to 400-600 °C in O2 containing atmosphere

§ Around 30% volume increase

• U3O8 reduction by addition of H2

• Kr, Xe, I removed§ Some discrepancies

8

Element Ref. 1 Ref. 2 Ref. 3 Ref. 4 Ag 80 0 0 0Cd 0 75 75 80Cs 99 90 100 99In 0 75 0 75Ir 0 0 75 0Mo 80 0 0 80Pd 80 0 0 0Rh 80 0 0 0Ru 80 90 100 80Se 80 0 0 99Tc 80 --- 0 0Te 99 75 75 99

1. AECL Technologies, Inc. “Plutonium Consumption Program-CANDU Reactor Projects,” Final Report, July 1994.2. SCIENTECH, Inc., Gamma Engineering Corp., “Conceptual Design and Cost Evaluation for the DUPIC Fuel Fabrication Facility,” Final Report, SCIE-COM-219-96, May 1996.3. Recycling of Nuclear Spent Fuel with AIROX Processing, D. Majumdar Editor, DOE/ID-10423,

December 1992.4. Bollmann, C.A., Driscoll, M.J., and Kazimi, M.S.: Environmental and Economic Performance of

Direct Use of PWR Spent Fuel in CANDU Reactors. MIT-NFC-TR-014, 44-45, June 1998.

6-9

Element Volatility

9

Element °CHe -272Kr -157Xe -111Cs 29Rb 39I 114Te 450Pu 640Ba 725Sr 764Ce 795Eu 822

Element °CLa 920Pr 935Nd 1010Pm 1042Sm 1072U 1132Y 1523Pd 1552Zr 1852Rh 1966Tc 2200Ru 2250Mo 2617

• Melting points correlate with vapor pressure• Zone refining can

have applications• Data for elements

• Need to consider solid solutions and intermetallics in fuel

Melting Points

6-10

Radionuclide Inventories• Fission Products

§ generally short lived (except 135Cs, 129I)§ ß,emitters§ geochemical behavior varies

• Activation Products§ Formed by neutron capture (60Co)§ ß,emitters§ Lighter than fission products§ can include some environmentally important

elements (C,N)• Actinides

§ alpha emitters, long lived

6-11

Fission products

• Kr, Xe§ Inert gases§ Xe has high neutron capture cross section

• Lanthanides§ Similar to Am and Cm chemistry§ High neutron capture cross sections

• Tc§ Redox state (Tc4+, TcO4

-)• I

§ Anionic§ 129I long lived isotope

6-12

Cesium and Strontium

• High yield from fission• Both beta

§ Some half-lives similar• Similar chemistry

§ Limited oxidation states§ Complexation§ Reactions

• Can be separated or treated together

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6-14

Alkali Elements• 1st group of elements

§ Li, Na, K, Rb, Cs§ Single s electron outside noble gas core§ Chemistry dictated by +1 cation

àno other cations known or expected§ Most bonding is ionic in nature

àCharge, not sharing of electron§ For elemental series the following decrease

àmelting of metalsàsalt lattice energyàhydrated radii and hydration energyàease of carbonate decomposition

6-15

Solubility

• Group 1 metal ions soluble in some non-aqueous phases

• Liquid ammonia§ Aqueous electron

àvery high mobility• Amines• Tetrahydrofuran• Ethylene glycol dimethyl ether• Diethyl ether with cyclic polyethers

6-16

Complexes• Group 1 metal ions form oxides

§ M2O, MOH• Cs forms higher ordered chloride complexes• Cs perchlorate insoluble in water• Tetraphenylborate complexes of Cs are insoluble

§ Degradation of ligand occurs• Forms complexes with ß-diketones• Crown ethers complex Cs• Cobalthexamine can be used to extract Cs• Zeolites complex group 1 metals• In environment, clay minerals complex group 1 metal ions

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6-18

Group 2 Elements• 2nd group of elements

§ Be, Mg, Ca, Sr, Ba, Ra§ Two s electron outside noble gas core§ Chemistry dictated by +2 cation

àno other cations known or expected§ Most bonding is ionic in nature

àCharge, not sharing of electron§ For elemental series the following decrease

àmelting of metals* Mg is the lowest

à ease of carbonate decompositionàCharge/ionic radius ratio

6-19

Complexes• Group 2 metal ions form oxides

§ MO, M(OH)2

• Less polarizable than group 1 elements• Fluorides are hydroscopic

§ Ionic complexes with all halides• Carbonates somewhat insoluble in water• CaSO4 is also insoluble (Gypsum)• Nitrates can form from fuming nitric acid• Mg and Ca can form complexes in solution• Zeolites complex group 2 metals• In environment, clay minerals complex group 2 metal

ions

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6-21

Technetium

• Electronic configuration of neutral, gaseous Tc atoms in the ground

• [Kr]4d55s2 [l] with the term symbol 6S5/2

• Range of oxidation states

§ TcO4-, TcO2

• Tc chemical behavior is similar to Re§ Both elements differ from Mn

• Tc atomic radius of 1.358 Å § 0.015 Å smaller than Re

6-22

Technetium

• Tc and Re often form compounds of analogous composition and only slightly differing properties§ Compounds frequently isostructural§ Tc compounds appear to be more easily reduced than analogous

Re species § Tc compounds frequently more reactive than Re analogues

• 7 valence electrons are available for bonding§ formal oxidation states from +7 to -1 have been synthesized

• Potentials of the couples TcO4-/TcO2 and TcO4/Tc are intermediate

between those of Mn and Re

§ TcO4 – is a weak oxidizing agent

6-23

Polonium

• Chemistry of Po is similar to Te and Bi• dissolves readily in dilute acids• PoH2 volatile

§ melting point −36.1°C§ Boiling point 35.3°C)

• Halides have structure PoX2, PoX4 and PoX6

• 2oxides PoO2 and PoO3 are the products of oxidation of polonium.[14]

• Some microbes can methylate Po§ Similar to Hg, Se and Te

• electron configuration of Po ground state atoms § 5s25p65d106s26p4 (3P2) § analogous to the configurations of Se and Te§ stable oxidation states of -2, +2, +4, and +6 would be expected

6-24

Lanthanides• Electronic structure of the lanthanides tend to be [Xe]6s24fn

• ions have the configuration [Xe]4fm

• Lanthanide chemistry differs from main group and transition elements due to filling of 4f orbitals§ 4f electrons are localized

à Hard acid metals* Actinides are softer, basis of separations

§ Lanthanide chemistry dictated by ionic radiusà Contraction across lanthanides

* 102 pm (La3+) to 86 pm (Lu3+), à Ce3+ can oxidized Ce4+ à Eu3+ can reduce to Eu2+ with the f7 configuration which has the extra

stability of a half-filled shell

6-25

Lanthanides

• Difficult to separate lanthanides due to similarity in ionic radius§ Multistep processes§ Crystallization§ Solvent extraction (TBP)

à Counter current method• larger ions are 9-coordinate in aqueous solution• smaller ions are 8-coordinate• Complexation weak with monodentate ligands

§ Need to displace water§ Stronger complexes are formed with chelating

ligands

6-26

Actinides• Occurrence

§ Ac, Th, Pa, U naturalà Ac and Pa daughters of Th and U

§ Traces of 244Pu in Ce ores• Properties based on filling 5f orbitals

6-27

6-28

Actinide Electronic Structure

6-29

Electronic structure

• Electronic Configurations of Actinides are not always easy to confirm § atomic spectra of heavy elements are very difficult to interpret

in terms of configuration • Competition between 5fn7s2 and 5fn-16d7s2 configurations

§ for early actinides promotion 5f 6d occurs to provide more bonding electrons much easier than corresponding 4f 5d promotion in lanthanides

§ second half of actinide series resemble lanthanides more closely à Similarities for trivalent lanthanides and actinides

• 5f orbitals have greater extension with respect to 7s and 7p than do 4f relative to 6s and 6p orbitals § The 5 f electrons can become involved in bonding

à ESR evidence for bonding contribution in UF3, but not in NdF3

* Actinide f covalent bond contribution to ionic bond* Lanthanide 4f occupy inner orbits that are not

accessible•

Basis for chemical differences between lanthanides and actinides

6-30

Electronic Structure

• 5f / 6d / 7s / 7p orbitals are of comparable energies over a range of atomic numbers § especially U - Am

à Bonding can include any orbitals since energetically similar

à Explains tendency towards variable valency• greater tendency towards (covalent) complex formation than for

lanthanides§ Lanthanide complexes tend to be primarily ionic

• Actinide complexes complexation with p-bonding ligands• Hybrid bonds involving f electrons• Since 5f / 6d / 7s / 7p orbital energies are similar orbital shifts may

be on the order of chemical binding energies§ Electronic structure of element in given oxidation state may

vary with ligand§ Difficult to state which orbitals are involved in bonding

6-31

Ionic Radii• Trends based on ionic radii

6-32

Absorption Spectra and Magnetic Properties

• Electronic Spectra § 5fn transitions

à narrow bands (compared to transition metal spectra) à relatively uninfluenced by ligand field effects à intensities are ca. 10x those of lanthanide bands à complex to interpret

• Magnetic Properties § hard to interpret § spin-orbit coupling is large

à Russell-Saunders (L.S) Coupling scheme doesn't work, lower values than those calculated

* LS (http://hyperphysics.phy-astr.gsu.edu/hbase/atomic/lcoup.html)

* Weak spin orbit couplingØ Sum spin and orbital angular momentum Ø J=S+L

§ ligand field effects are expected where 5f orbitals are involved in bonding

6-33

Pu absorbance spectrum

400 500 600 700 8000

1

2

3

4

5

A

bso

rba

nce

Wavelength (nm)

Normal Heavy Light

Pu4+ (489 nm)

Pu6+(835 nm)

6-34

Oxidation states and stereochemistry

6-35

Hybrid orbitals• Various orbital combinations similar to sp or d orbital

mixing§ Linear: sf§ Tetrahedral: sf3

§ Square: sf2d§ Octahedral: d2sf3

à A number of orbital sets could be energetically accessible

• General geometries§ Trivalent: octahedral§ Tetravalent: 8 coordination

6-36

Stereochemistry

C.N. Geometry O.N. e.g.

4 distorted +4 U(NPh2)4

5 distorted tbp +4 U2(NEt2)8

6 octahedral +3 An(H2O)63+, An(acac)3

    +4 UCl62-

    +5 UF6-, a-UF5

    +6 AnF6

    +7 Li5[AnO6] (An = Np, Pu)

  distorted octahedral +6 Li4UO5 , UO3

    +5/+6 U5O8

    +6 UO2(S2CNEt2)2(ONMe3)

       

6-37

Stereochemistry8 cubic +4 (Et4N)4[U(NCS)8], ThO2, UO2

    +5 AnF83-

  square antiprismatic +4 ThI4, U(acac)4, Cs4[U(NCS)8],

    +5 b-UF5

  dodecahedral +4 Th(ox)44-, Th(S2CNEt2)4

  bicapped trigonal prismatic +3 PuBr3

  hexagonal bipyramidal +6 UO2(h2-NO3)2(H2O)2

  ? +6 UF82-

9 tricapped trigonal prismatic +3 UCl3

  capped square antiprismatic +4 Th(trop)4(H2O)

6-38

Stereochemistry

10 bicapped square antiprismatic +4 KTh(ox)4.4H2O

11? fully capped trigonal prismatic? +3 UF3

12 irregular icosahedral +4 Th(NO3)62-

  distorted cuboctahedral +4 An(h3-BH4)4, (Np, Pu)

       

14? complex +4 An(h3-BH4)4, (Th, Pa, U)

6-39

Actinide metals

• Preparation of actinide metals§ Reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca or Ba

at 1100 – 1400 °C§ Other redox methods are possible

à Thermal decomposition of iodine speciesà Am from Am2O3 with La

* Am volatility provides method of separation• Metals tend to be very dense

§ U 19.07 g/mL§ Np 20.45 g/mL§ Am lighter at 13.7 g/mL

• Some metals glow due to activity§ Ac, Cm, Cf

6-40

Pu metal

• Some controversy surrounding behavior of metal http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818030.pdf

Plutonium a b g d ¢d e

Symmetry monoclinic monoclinic orthorhombic fcc bc tetragonal bcc

Stability < 122°C 122-207°C 207-315°C 315-457°C 457-479°C 479-640°C

r / gcm-3 19.86 17.70 17.14 15.92 16.00 16.51

6-41

6-42

Oxidation states• +2

§ Unusual oxidation state § Common only for the heaviest elements § No2+ and Md2+ are more stable than Eu2+

à 5f6d promotion§ Divalent No stabilize by full 5f14

à Element Rn5f147s2

§ Divalent actinides similar properties to divalent lanthanides and Ba2+

• +3§ The most common oxidation state § The most stable oxidation state for all trans-Americium elements

except No§ Of marginal stability for early actinides Pa, U (But: Group

oxidation state for Ac) § General properties resemble Ln3+ and are size-dependent

à Binary Halides, MX3 easily prepared, & easily hydrolyzed to MOX

§ Binary Oxides, M2O3 known for Ac, Pu and trans-Am elements

6-43

Oxidation states• +4

§ Principal oxidation state for Th à similar to group 4

§ Very important, stable state for Pa, U, Pu § Am, Cm, Bk & Cf are increasingly easily reduced - only stable in certain complexes

e.g. Bk4+ is more oxidizing than Ce4+

§ MO2 known from Th to Cf (fluorite structure) § MF4 are isostructural with lanthanide tetrafluorides § MCl4 only known for Th, Pa, U & Np § Hydrolysis / Complexation / Disproportionation are all important in aqueous phase

• +5§ Principal state for Pa (similar to group 5)§ For U, Np, Pu and Am the AnO2

+ ion is known § Comparatively few other AnV species are known

à fluorides fluoro-anions, oxochlorides, uranates, • +6

§ AnO22+ ions are important for U, Np, Pu, Am UO22+ is the most stable

§ Few other compounds e.g. AnF6 (An = U, Np, Pu), UCl6, UOF4 etc..., U(OR)6 • +7

§ Only the marginally stable oxo-anions of Np and Pu, e.g. AnO53-

6-44

Redox chemistry (Frost diagrams)

6-45

Redox chemistry

6-46

Redox chemistry

• actinides are electropositive • Pa - Pu show significant redox chemistry

§ all 4 oxidation states of Pu can co-exist in appropriate conditions• stability of high oxidation states peaks at U (Np) • redox potentials show strong dependence on pH (data for Ac - Cm)

§ high oxidation states are more stable in basic conditions § even at low pH hydrolysis occurs § tendency to disproportionation is particularly dependent on pH § at high pH 3Pu4+ + 2H2O PuO2

2+ + 2Pu3+ + 4H+ • early actinides have a tendency to form complexes

§ complex formation influences reduction potentials à Am4+(aq) exists when complexed by fluoride (15 M NH4F(aq))

• radiation-induced solvent decomposition produces H• and OH• radicals § lead to reduction of higher oxidation states e.g. PuV/VI, AmIV/VI

6-47

Actinide complexes

6-48

6-49

6-50

6-51

6-52

Organometallic

• Organometallic chemistry of actinides is relatively recent § Interest is expanding but still focused on U

• Similar to lanthanides in range of cyclopentadienides / cyclooctatetraenides / alkyls

• Cyclopentadienides are p-bonded to actinides

6-53

Uranocene• Paramagnetic • Pyrophoric • Stable to hydrolysis • Planar 'sandwich' • Eclipsed D8h conformation • UV-PES studies show that bonding in uranocene has 5f & 6d

contributions • e2u symmetry interaction shown can only occur via f-orbitals

6-54

Questions

1. What elements can be expected from nuclear fission?

2. What actinides can naturally be found in the environment?

3. What is a radioelement? Provide 10 examples.

4. What influences the speciation of actinides and fission products in spent nuclear fuel?

5. What are the similarities and differences between lanthanides and actinides?