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Los Alamos Critical Assemblies Facility

UNO.LOS ALAMOS SCIENTIFIC LABORATORYPost Office Box 1663 Los Alamos New Mexico 87545

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LOS ALAMOS CRITICAL ASSEMBLIES FACILITY

by

R. E. Malenfant

ABSTRACT

The Critical Assemblies Facility of the Los Alamos National Laboratory has been inexistence for thirty five years. In that period, many thousands of measurements havebeen made on assemblies of :"Li, : " U . a id ;'9Pu in various configurations, includingthe nitrate, sulfate. fluoride, carbide, and oxide chemical compositions and the solid,liquid, and gaseous states. The present complex of eleven operating machines isdescribed, and typical applications are presented.

I. INTRODUCTION

The Los Alamos Critical Assemblies lacilit\ consists

of three remote control laboratories, known as Kivas.

which arc located at sonic distance from the main

laborator> building that houses individual control rooms

for each Ki\a.

Each Kiva is surrounded by a security fence to keep

personnel at a safe distance during remote operations.

The Kivas are entered through a manned security station

with additional safeguards provided by approved access

lists, a strictly imposed "buddy" system, and a controlled

key procedure.

Several general purpose assembly machines occupy

each Kiva. However, material and even entire machines

can be moved from one Kiva to another. This is

probably the only facility in which all fissile species

( ; " U . : '?U. and :'"Pu) are routinely handled in a

multitude of configurations involving all physical forms(solid, liquid, and gaseous]. Machines have been run ator near critical, bare (as the 17-kg Jezebel) to massivelyreflected (as the .9 metric ton Big Ten), with loadingsfrom a few hundred grams to hundreds of kilograms, andat power levels from fractions of a milliwatt to near100 000 MW (for a few microseconds).

II. KIVA I

A. Honeycomb

'I he Hor.ewomb critical assembK is a universalsplit table machine containing a 1.83 by 1>3 b\I.S3 m id ::> matr ix of 76 by 76-mm (3 in.) a luminum

tubes (Fig. i). It is designed as a flexible system to

a c c o m m o d a t e initial mockup studies for basic critical

parameter investigations. Aluminum tubes are used to

support components because aluminum is relativelyneutral in either fast or thermal neutron systems. Variousmaterials of interest in reactor experiments are availablein blocks, bars, plates, and foils, with dimensions com-mensurate with the Honeycomb matrix. These includelarge quantities of graphite and beryllium and lesseramounts of iron, aluminum, niobium, molybdenum.tungsten, zirconium, zirconium hydride, and polyethylene. Fuel inventory consists of various fissile speciessuch as 330 kg of 0.05-mm-thick U(93) foils, with widthsand lengths appropriate to the aluminum matrix tubes, orextruded sintered VO: rods. Controls and safety rodsuse sections of the core or reflector materials, and themovable section of the table provides major disassembly.

Many reactor concepts have been mocked up in

I VOL.

Fig. I.Honevcomb is a general purpose critical assembly machine incorporating a horizontal split table. A cubical mocking upcurt1, n reflector. nnil a control system can casik he loaded into the matrix of 7f> mm sq aluminum tubes. The machine iscurrently loaded with 11 h5 kg I'O core, herv Ilium reflected mockup of a space power reactor.

Honeycomb. We have been able to include details muchsmaller ihan the 76 by 76 mm matrix tube dimensionsby providing fine structure within the tubes. Often wehave replaced a section of aluminum tubes wtih a realreactor section. These options demonstrate the extremeversatility ol Honeycomb. Systems that have beenstudied include the Kiwi. Phoebus, and Dumbo reactorsfor the Ro\er Program, the TREAT reactor for ArgonneNational Laboratory, and several assemblies of morebasic interest in the reactor field. The assembly that isjust being built up for the space program uses a L'O. Mocore with a beryllium reflector.

B. Mars

The Plasma Core Assembly (PCA) makes use of ourMars critical assembly machine (Fig. 2). which isbasically a heavv-duty stand with a hydraulic cylinder

recessed in the floor to provide primary assemblymotion. Mars is one of four similar general purposeassembly machines (Comet. Super Comet. Mars, andVenusl. PCA consists of a 1 m long by 1 m diam cavitysurrounded by a 0.5 m-thick beryllium reflector. Experiments under way are determining critical conditions for aLF6 gas core.

The beryllium reflector for PCA was assembled fromthe Rover Program reactor reflector segments. Many ofthese segments retain residual activation from theirhigh power operation several years ago. The outer0.20 m of beryllium is a complete Phoebus 2A reflector.Some adapter pieces of graphite are used to fill voidsbetween beryllium reflector sections. A 1 m-diam by0.5 m long beryllium end plug, mounted on a movablecart over the hydraulic piston, supports the core materi-als. The fueled core and beryllium end plug are insertedby remote control for critical operation.

The PC A cm our Mars critical a s s emhk machine, a

heav \ dul> stand with :i hydraulic c; Under recessed in the

floor for primary jissiitihK motion.

Experiments to date ha\c been with small quantities o(

Lh\. and criticality is achieved by adding a ring of solid

uranium graphite PARKA fuel elements outside the

Uf-',, containing canister. Recently. PC A was operated at

critical with circulating UFh. In another phase of our

plasma core reactor research, we investigated the neu

tronic properties of a vortex confined LT\ gas region in

the core ot our beryllium reflected critical assembly.

Argon was the confining buffer gas. and helium trans

ported the L'F,, fuel around the system. PCA in its

present "clean" configuration is critical with as little as 5

kg of L W ) .

Gas core reactors are of interest for space applications, as potential sources of fission-driven laser energy,and as reactor power plants presenting minimum pro-liferation risk. We recently completed a joint programwith United Technology Research Corporation to eval-uate the feasibility of gas core reactors. The criticalstudies with supporting neutronic calculations were con-ducted at Los Alamos National Laboratory. Initialexperiments used a driver section of PARKA fuelelements to effect criticality with smaJl amounts of UF6

in a central cavity. In continuing experiments, theamount of UF6 was gradually increased until the behavior of the system with flowing L'F6 fuel could be studiedrealistically with reference to possible future undrivenconcepts. A follow-up program will be conducted withthe University of Florida (UFLA-UFJ on a static systemwith up to 3 kg of UF,, gas.

C. Venus

The Venus machine is a general purpose hydraulicram and support structure to accommodate systemsunder study. Although unloaded at present, it was mostrecently used in a bench mark measurement of awater reflected L'(97) sphere.

III. KIVA 2

A. Big Ten

Big Ten (Fig. 3) is a cylindrical assembly comprising a:"U(10%) core surrounded by a depleted-uranium reflector. Overall, the cylinder is 0.84 m in diameter ard0.96 m long. The reflector has 0.15 m-thick walls and0.21 m thick ends. Major disassembly is provided by amovable reflector section 0.39 m long; the stationarysection is 0.57 m long.

The fissile core is composed of a central section ofhomogeneous :'*U(lO°/o) surrounded by a region where3 mm -thick :''U(93%) plates alternate with 27-mm-thicknatural uranium plates (making 18 pairs) simulating 10%material. The total : "U inventory in the core is 226 kg.

Twelve movable 88.9 mm-diam rods of depleteduranium are located in the peripheral reflector. Six rodsin the movable section and five rods in the stationarysection operate in the "in/out" mode with no in-termediate positions available and act as safety rods. The

Hg. 3.Big Ten. a cylindrical asu'mbK with a movable and a stationary section to provide disassembly.

sixth rod in the stationary section and a 38 mm diam!5?U(10%) rod along the axis act as control rods. Theserods are continuously adjustable, and their positions areread out accurately in the control room.

The composition of Big Ten was selected to give aneutron spectrum at the center comparable to that in theLiquid Metal Cooled Fast Breeder Reactor (LMFBR) sothat verification of cross-section sets in Big Ten wouldvalidate those sets for LMFBR calculations as well. Themedian energy for fission in Big Ten is computed to be-0.4 MeV.

Measurements of the neutronic properties of Big Tenare being investigated in the same fashion as those ofFlattop and Jezebel assemblies. Comparison of calculations with measurements constitutes an unequivocalintegral test of input cross sections in the calculationsand complements the results for Jezebel. Godiva. andFlattop. Big Ten is presently being used for a series ofreplacement measurements of transuranic isotopes andfor diagnostic irradiations.

B. Comet

The Comet is a general purpose assembly machinethat has been used for many critical determinations am)nuclear safety studies (Fig. 4). It is basically a supportstand with means for bringing two parts of a configu: ation together. Secondary vernier motion on top of thetable is accomplished by a screw-driven platform pewered by a stepping motor, with accurate positionread-out in the control room. Various stationary tablesare available for the top of the support stand. One ofthese is a 0.05 cm thick steel membrane to provide a"massless" supporting plane for a section of the assembly. Often the top support is custom designed for theparticular application. Various top support structures areavailable, including hardware having a vertical actuatorand a control rod. Experiments to establish criticalconditions are generally subcri'ica], but some expertments have been operated at critical. Comet provides aflexible facility for conducting many different expertments on short notice.

Fig. 4.Comet, a general purpose assemhh machine used fur manycritical determinations and nuclear safetv studies.

Comet is an assembly machine rather than a criticalassembly: generally the configurations on Comet arevaried and short-term. Use is made of the Flattop cores,the Thor core, and uranium foil for a variety of criticality

safety and critical mass determinations. The Thor core isparticularly useful in Comet operations and has come tobe associated with the machine.

At present, Comet is set up for a demonstrationmultiplication measurement and critical determinationfor a criticality safety class. This basic experiment, with arectangular array of Lucite plates and uranium foil, isconducted periodically as a graphic hands-on trainingaid. During 1979, Comet was employed in a series ofmeasurements to evaluate 242Pu cross sections and invarious stackings of interest in criticality safety. A recentComet application was an extensive series of measurements of critical dimensions for spherical systems con-taining a central 9.8-kg 23'Pu-alloy core surrounded by10-mm-thick shells of various materials and reflectedwith polyethylene. These data were used to checkneutronic cross sections for calculations.

Our Thor assembly, which is of particular currentinterest, consists of a 9.8 kg 239Pu ball in a thick thoriumreflector. Thorium is in the limelight now as breedingmaterial for 233U in place of uranium-plutonium. In thatcontext, another series of Comet experiments is ofinterest; in these experiments, critical conditions wereestablished for cylinders of plutonium diluted with steel,aluminum, thorium, and depleted uranium, refected bydifferent thicknesses of uranium and thorium.

Many other experiments have been run on Cornel,including tests on the safety of weapons in storage orimmersion and additional criticality safety measure-ments. The lowest critical mass ever measured wasdetermined with the Comet machine. In apolyethylene-uranium core with a thick beryllium re-flector, a <300-g quantity of :1!U was critical.

C. Flattop

Flattop (Fig. 5) is at the opposite extreme from ourunreflected critical assemblies, as it is a thick naturaluranium reflector (0.48-m o.d.) around spherical cores ofuranium (93.2% 23JU and 98.1% 233U) and plutonium(94.9% ;<*Pu). Different adapter shells of naturaluranium allow adjustment of the central reflector cavityto fit cores of various sizes. The two quarter sections ofthe reflector that are movable serve as scrams. One ofthese is moved in or out by hydraulic pressure, and theother is motor driven, with position indicated in thecontrol room. Three control rods of natural uranium areinserted from below into the stationary half of thereflector. The fissile core is mounted on a 127-mm-diam

Fig. 5.Flaltop. showing the two movable quarter sections of thereflector, several spherical cores, and the hemispherical cap forthe ?"U sphere.

natural uranium pedestal that may be moved into itscentral position by hand cranking a screw. The 233U and231|Pu spheres have I2.7-mm-diam central channels; the235U sphere has a 25-mm-diam channel. Access to theseexperimental volumes is through the stationary reflectorsection. Cylindrical mass adjustment buttons of eitherreflector or fissile material fit into cavities in the reflectorcomponents immediately surrounding the active balls. Athin (2.8-mm) hemispherical shell positioned on the tophalf of the core between the natural uranium reflectorand the fissile material provides additional reactivityadjustment for the :l*U sphere. This cap may be of eithernatural uranium or enriched-uranium pieces. A completeset of data has been obtained for the neutronic character-istics of each of the three core materials. Critical massesare 6.0 kg for the :'"Pu (94.9%) ball. 5.7 kg for the 233U(98.1%) ball, and 17.8 kg for the :1'U (93.2%) ball aftercorrection to a completely spherical geometry. The 233Usphere for Flattop is in storage and is no longer usablebecause of its high gamma activity, resulting fromdaughter products of the ; |?L' impurity in the originalmaterial.

Flattop assemblies are characterized by a fast-neutronspectrum at the center of the core and a degradedspectrum in the reflector. An index of the neutron energyis provided by the 21?U'./2<IIU fission detector countingratio, which is as low as ^6 in the fissile core andincreases to -100 near the outside of the uraniumreflector. This varying neutron spectrum characteristic isparticularly useful for the activation studies and calibra-tions conducted by Group CNC-I1 for radiochemicaldiagnostics.

Flattop is one of our bench-mark critical assemblieswhose characteristics have been meticulously establishedover a period of years. Flattop assemblies are usedprincipally in a continuing program of neutron activationand reactivity coefficient measurements, the appropriateneutron energy spectrum being selected by the radialposition of the sample. For example, the central reactivi-ty coefficient of 237Np was recently checked in the 235UFlattop cue to supplement the same measurement madein Jezebei, which has a different neutron spectrum.Flattop is used also for replacement measurements andservice irradiations.

We have determined the neutronic properties of thevarious Flattop assemblies, thereby establishing preciseintegral checks for the cross-section verification used byweapons and reactor programs. These include detaileddata for 233U, 235U. 238U, and 239Pu, as well as centralreactivity coefficients for other fissionable isotopes andmany nonfissionable isotopes. The National Bureau ofStandards and other agencies have amplified the statusof Flattop as a bench mark through fast-neutrondosimetry intercomparison and calibration studies. Theservices of Flattop will continue to be in demand fordosimetry intercomparisons and for spectrum-sensitivedetector calibrations. Because of its generally softerspectrum, Flattop complements Jezebel and Godiva inthis regard.

D. Jezebel

The Jezebel assembly (Fig. 6) was established todetermine the neutronic behavior of an unreflectedcritical sphere of delta-phase plutonium. Support andcontrol structures were designed to keep neutron re-flection to an absolute minimum. The effects of reflectionare small enough to allow confident extrapolations to theunreflected sphere. In that respect, the nickel protectivecoating on the plutonium metal must be taken intoaccount as a component of extraneous reflection.

Major features of Jezebel, a plutonium sphere cladwith 0.13-mm-thick nickel, include a stationary t:ntralsection, upper and lower movable blocks, and a singleplutonium control rod. The central section is actuallymade up of two separate plutonium pieces clampedtogether. The reactivity of Jezebel may be adjusted inincrements by 12 plutonium filler buttons (12.7-mmdiam by 6.3mm thick) that fit into cavities machinedinto the top surfaces of the lower block and the central

I ig. 6.Jezebel, showing the plutonium sphere, stationary centralsection, upper and lower movable blocks, and plutoniumcontrol rod.

section that presents another perturbation. Jezebel ismounted on a portable aluminum stand, and extensioncables allow operation even outside the building. In fact,measurements have been made with Jezebel supported~10 m from the ground to avoid neutron room-returneffects.

Jezebel measurements supply the same basic informa-tion for plutonium that Lady Godiva provided foruranium. Both the weapons and reactor divisions of theDepartment of Energy (DOE) consider Jezebel a stan-dard for continuous confrontation of neutronics calcu-lations and experiments, and its neutron field is useful fordetector development and calibrations. Jezebel also constitutes an invaluable training tool for Los Alamossponsored Nuclear Criticality Safety courses.

Early measurements concentrated on establishing aprecision value for the critical mass of an unreflectedplutonium sphere and on determining various neutroniccharacteristics such as neutron spectrum, neutron lifetime, and relative fission cross sections for isotopes in theJezebel neutron spectrum. These still serve to verifycomputations. Subsequently, central reactivity coefficients were measured for some 40 isotopes, and theirabsorption cross sections for the Jezebel neutron spectrum were deduced from the resulting data. Results ofJezebel measurements have appeared frequently in theliteral ure.

In the past, the Jezebel machine has also been used toassemble critical unreflected metal spheres of plutonium(20.1% ;40Pu) and uranium (98.13% 2"U). Detailedneutronics measurements were performed with eachcritical sphere, the measurements of highest precisionbeing interassembly comparisons. Thus, although thehigh ;4"Pu and the 2"U spheres have long since beendestroyed, our understanding of the neutronics of theseassemblies continues to improve as we improve theaccurancy of measurements with the present 4.5% ""Pusphere. The Jezebel components are now being recastand remachined. Jezebel continues a; the ?;knowledgedstandard for dosimetry calibrations ai •' for spectrum -sensitive detector comparisons.

section, and by plutomum plugs that fit into the 12.7-mmcylindrical channel passing through the central section.Polar discs of plutonium may be added to the supportends of the upper and lower blocks. For [his purpose, thepolar surface of these blocks has been machined flat toaccommodate the polar mass adjustment discs. Thus,there is significant deviation from sphericity at the poles.In addition, there is a flange machined around the central

IV. KIVA 3

A. Godiva IV

Godiva IV is the latest in the line of unreflectedenriched uranium critical assemblies. The originalspherical Godiva (Lady Godiva) established an accurate

value for the critical mass of a "!LJ unreflected sphere,thereby providing a calculational check for cross-sectionsets. Such early critical measurements contributed signif-icantly to computations on nuclear weapons. In fact,computational sets must still satisfy Godiva criticalconditions. The reactivity self-quenching features ofmetal critical assemblies were also established withGodiva I. We started the fast-burst reactor evolution byexperiments in which we took Godiva past promptcritical and demonstrated the inherent shutdown fromexpansion as energy is liberated. Godiva I was then usedroutinely for burst production, primarily for simulating anuclear weapons radiation environment. Maximumbursts of 2 x 10" fissions with a half-width of 35 uswere produced, resulting in a peak power greater than10 000 MW. Subsequent work with Godiva IV gavepeak powers of about 100 000 MW.

Because Godiva II was designed specifically for theproduction of prompt bursts, there was no incentive topreserve the spherical shape. Therefore a cylindricalsystem was fabricated with emphasis on structuralstability and reduced thermal shock. A second model ofthe Godiva II core was made at Los Alamos foroperation in the Sandia Pulse Reactor Facility (desig-nated Godiva III by Los Alamos and SPR-I by SandiaNational Laboratories).

Godiva IV (Fig. 7), an improved version of Godiva II,was designed to give maximum pulse yield. The designuses a U-Mo (5%) alloy for increased tensile strengthand resistance to deformation and cracking under theoperating thermal shock conditions. Godiva IV, which isoperating today, had new U-Mo rings installed recently.

Godiva IV bursts are in demand when intense radi-ation pulses are required, both for weapon- and non-weapon-related experiments. Peak levels approaching10n fissions per burst are possible. In a recent program,bursts were used to induce nuclear pumping of lasers.Nuclear energy was coupled to the laser gas by fissionfragments from the fission reaction or by the n-3Hereaction. Typically, a laser tube containing a U(93) foillining was surrounded by a polyethylene moderator andexposed to the neutron flux from a Godiva IV burst. Themoderator slows down the incident fast neutrons, result-ing in a high fission rate in the laser foil, several times therate in Godiva IV itself. A mixture containing UF6 gascould serve the same purpose as the foil. Enough Iasingaction has been observed with nuclear pumping tostimulate considerable interest in this direct conversionof nuclear energy into light energy. The obvious applica-tion is advanced-concept, high-energy lasers.

Fig. 7.Godiva IV, the latest unreflected enriched uranium criticalassembly.

For diagnostic instrumentation development studies,Godiva IV bursts can simulate fast-reactor accidenttransients by exposing fuel pins in a polyethylene mod-erator or by inducing a transient in the PARKA criticalassembly with a Godiva IV burst. For the latter experi-ment, a test bundle of fuel pins is located in the centralregion of PARKA. From these and other tests we hopeto specify instrumentation to monitor fuel failurephenomena caused by destructive transients infast-reactor safety tests.

Other applications of Godiva IV bursts include fluorevaluation for electro-optical imaging systems, codedaperture imaging with neutron and gamma radiation,radiation response of instrumentation, radiation muta-tions for plant life genetic modifications, and isotopeproduction and analysis using an on-line mass separator.

Godiva IV is a major component in the weapons-supported critical assembly program in Group Q-14.Service is supplied routinely to the Laboratory for

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various studies. The nuclear-pumped laser programsupported by the National Aeronautics and Space Ad-ministration is completely dependent on Godiva IV as asource of radiation. Godiva IV is also used extensively indiagnostic instrumentation evaluation for the Safety TestFacility (STF) program. During 1980, Godiva IV pro-vided pulse neutrons as a probe to study fission densitydistributions on moving geometries.

B. Skua

Skua, a burst-mode critical assembly under construc-tion (Fig. 8), will be an approximate unit-height-to-diameter cylinder of annular U (93) fuel rings, in-ternally moderated by ZrH, 6 and externally reflected bycopper. The ZrH16 moderator was chosen overpolyethylene because of superior high-temperature quali-ties.

Skua has an axial moderated-neutron experimentalsample volume 51 mm in diameter with clear accessfrom top to bottom. The sample volume diameter isvariable, the tradeoff being that the central neutron flux

Fig. 8.Skua, a burst-mode critical assembly under construction.

spectrum becomes progressively harder as the moderatoris removed to increase the sample volume diameter.Rapid vaporization of uranium to the flux trap is one ofthe design goals of the system. From the axial samplevolume outward, there is a graphite thermal barrier; aZrH, 6 moderator; a cadmium barrier to protect the nextregion, which is a boral low-energy-neutron barrier, fromoverheating because of excessive neutron captures; aUCJO0 sleeve; annular U(93) fuel rings; and a76-mm-thick copper reflector. The copper reflector is 51mm thick at the top and bottom. The region from thegraphite thermal barrier to the UC]00 sleeve is designedto be packaged as a unit to insure system safety. The fuelrings have a 235-mm i.d. and a 314-mm o.d. Reactorcontrol is by reflector movement. Gross control andshutdown are effected by radial movement of reflectorregions, where the reflector worth of — 1 $/12kg gives ashutdown of >5 $ reactivity. The fine control and"burst" mechanism will be rotary copper cylinders asshown in Fig. 8.

The 175 kg of uranium will permit bursts of 2 X 10p

fissions with a mean system temperature rise of ~300°C.With the centra! "g'ory hole" at its smallest diameter (51mm), the "amplification factor" (fission ratio, central tomean core) is 170. This, of course, decreases as thesample diameter increases to its maximum of 234 mm,the fuel inside diameter. For the dimensions given,neutron lifetime is ~0.1 ms and the pulse width is < 1 ms.

Skua is designed to provide high thermal fluxes in thecentral cavity. It is expected that uranium foil can easilybe vaporized or fast-reactor fuel pins can be melted.Weapons program applications of Skua are vulnerabilitystudies and spectroscopic measurements of uraniumvapor. Fast-reactor safety applications include destruc-tive testing on fuel pins and providing a test bed for fuelmotion diagnostic instrumentation. Nuclear pumping oflasers has been demonstrated with Godiva IV for fissionfragment excitation of the gas and for excitation byproton and triton particles from the 3He(n,p) reaction.Because nuclear pumping seems to be a thresholdphenomena, the order of magnitude increase in excitationthat is possible in Skua should open up new areas forinvestigation.

C. PARKA

PARKA is a Kiwi-type reactor with a special coremade up of hexagonal, one-hole, bead-loaded fuel ele-ments (Fig. 9). The core is reflected bv 50 mm of

Fig. 9.PARKA, showing Ihc core of hexagonal, one holeuranium graphite fuel elements.

graphite and I 14 mm of beryllium; the ends arc essentially unreflected. Control is pro\ ided by 12 rotatingberyllium drums, each with a '"B Al vane covering 130°of the surface, which aiso serve as "scram" rods, usingsprings as the energizing force. The control drums arerotated by stepping motors whose angular position isread out precisely in the control root.. All componentsof PARKA are leitovc-s from the Rover Program.

PARKA presents a classic example of the payoff ofnot scrapping a good critical assembly simply becausethere is no apparent program for it. Until recently.PARKA served as a convenient radiation source, providing gamma and neutron characterisiics different fromthose of our other eruical assemblies and thereby beingattractive for certain experiment. Measurements toevaluate fuel motion diagnostics were made in PARKAduring J y79. More recently. PARKA has been adaptedto drive an assembly of fast reactor-like fuel pins in-stalled in a cavity along its axis. This application gives anaccurate representation al low power of conditions in anSTF :hat :s being planned by DOH for destructive•'i'.v.ir:. :th\- .: l.MIBR core sections. We recently had5 \'J:!£2.- Rie.-.i:'.1.". Commission program to explore'.-.i- -.:'•- '.-.•- ,rr-:r. ration for these safety tests.

PARKA is the only facility capable of simulating STFbecause it can serve as an epithermal driver for largearrays of fast-reactor fuel pins, which contain enricheduranium oxide rods to simulate fast-reactor fuel. Current-ly we are doing experiments with a 37-pin bundle, andcalculations show that much larger arrays can behandled. We recently tested a method of operatingPARKA in a transient mode, whereby it is driven by apulse from Godiva. This experiment demonstrated theversatility provided by a multiplicity of critical assemblies. The cost of setting up a system equivalent toPARKA specifically for this reactor safety investigationwould be prohibitive.

PARKA experiments necessitated a shielded instru-ment room to permit observation of conditions in the testfuel pin array without radiation exposure from thePARKA driver. This shield room has been equally usefulfor experiments with our pulsed Godiva IV assembly onnuclear-pumped lasers. This sharing of facilities (andcosts) is a powerful justification for our flexible, multi-purpose critical assembly instaJlation. In fact, we antici-pate that PARKA will have some application to nucle-ar-pumped laser studies as well as to the reactor safetyprogram.

PARKA hexagonal fuel elements are made of graphiteloaded with pyrolytic-carbon-coated UC beads. They are19 rnm across flats and have a 12-mm central hole.These fuel elements were made for mockup studies of thePhoebus 2 reactor. A fraction of the PARKA fuelelements described here are being used with the PCA.

Some unused Rover ZrC composite elements havebeen retained for use in PARKA. A small number ofthese are in the present core, and the rest are being heldfor potential us;.

D. Super Comet

Super Comet is a general-purpose heavy duty standsomewhat larger than Comet. It is presently unloadedand being held in storage.

V. SHEBA

Our new solution assembly, Sheba, which is housed inan extension of Kiva I, achieved first critical on Septem-ber 6, 1980, just 4 1/2 months after we received fundingapproval for the program. Our purpose was to provide aradiation source similar to those in proposed centrifuge

processing plants to set a standard for calculations andto evaluate criticality accident alarm detectors andpersonnel dosimetry. A series of experiments, whichincluded steady-state and ramp operation, was com-pleted for Goodyear Atomic Corporation and OakRidge Gaseous Diffusion Plant personnel who were hereat Pajarito Site to test their equipment. In addition, sevendetector vendors participated in evaluations of theirproducts, and Lawrence Livermore National Laboratorywas involved with spectrum and dosimeter measure-ments.

Sheba (Fig. !0) is a bare assembly fueled with asolution of ~ 5 % enriched uranyl fluoride initially storedin two horizontal 25.4-cm-diam by 1.5-m-long "safe"stainless steel tanks. The solution is transferred to thereactor cavity (a 56-cm-diam by 1.3-m-high stainlesssteel tank with 0.64-cm-thick walls) by evacuating thecavity while applying helium pressure to the storagetanks. A completely clean geometry is provided in thissimple cylindrical system, because reactivity control iseffected by varying the solution level. A safety rod maybe inserted in a central thimble to provide fast shutdown.The critical solution height is 36.5 cm at 25 °C. Thiscorresponds to 85.5 ( of solution in the cavity becausethe specific volume for the system is 2.34 (Icm.

For the just-completed Goodyear Atomic and vendorirradiations, we operated Sheba at powers ranging up to5 kW. Detectors were positioned at various locationsfrom 2 to 275 m from the assembly. Attenuating shields,used either singly or in combination, included 20-cmconcrete. 10-cm Lucite. and steel up to 28 cm thick.Evaluations of the experimental results will be madeavailable to us when they are completed.

F ig . 10.

Sheba. a hart' ;*sH'mb]v ;n whjcb Cinirm) is providedvarying the solution level.

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