A coincidence neutron counting method has been developed for assaying batches of plutonium oxalate. Using counting data from two concentric rings of /sup 3/He detectors, corrections are made for the effects that water has on the coincidence neutron count rate. Batches of plutonium oxalate varying from 750 to 1000 g of plutonium and from 34 to 54% water are assayed with an average accuracy of +-3%

Erkkila, B. H.

Marshall, R. S.

UNT Digital Library

ILA-uR .nq -2?s~TITLE: THEMEASUREMENTOF PLUTONIUM OXALATE IN THERMALNEUTRONCOINCIDENCE CO1.lNTERSAUTHOR(S): R. S. Marshalland B. 1+.ErkkilaSUBMI’ITED TO: Oak RidgeNationalLaboratoryConferenceonAnalyticalChemistryin EnergyTechnologyOctober9-11Gatlinburg,TNBy acceptance of this article, tbe publisher recognizes that the U.S. Government retainsa non-exclusive, royalty-free limw to publish orrepmduce the published form of this contribution, or toallow others to do so, for U.S. GovernmentprqaTtm2AMAlamos$ktentlflc Iabomtory requests thatthepubUsherIdentll’ythlsarticle m work performedunder the msplees of the Department of Energy.? alamosscl-ntiflc laboratoryof tho Unlvordty of California10S ALAMOS, NEW MEXICO 87045An Aflirmativo ActIon/Equal Opportunity Employ-rForm No. 836 R2SI, No. 2629Ij ~ DEPARTMENTOF ENERGYCONTRACTW-7405.ENG. 36THE MEASUR-T OF PLUTONIUMOXALATETHERMALNEUTRONCOINCIDENCECOUNTERSR. S. Marshalland B. H. ErkkilaUniversityof California,Los AlamosLos Alamos,NM87545INScientificLaboratory,ABSTRACTA coincidenceneutroncountingmethodhas been developedforasz?yingbatchesof plutoniumoxalate. Using countingdata fromtwo concentricringsof 3He detectors,correctionsare madefor the effectsthat water has on the coincidenceneutroncountrate. Batchesof plutoniumoxalatevaryingfrom750 to 1000gof plutcmiumand from 34 to 54% waterare assayedwith an aver-age accuracyof~3%.INTRODUCTIONPurificationof plutoniumby the selectiveprecipitationofplutoniumas plutoniumoxalateis used extensivelyat the LosAlamosScientificLaboratory(LASL)and SavannahRiver plutoniumrecoveryfacilities. Plutoniumoxalateprecipitationnot onlyservesas a purificationstep,but also producesa productthatis easilycalcinedto form low-firedPu02.The plutoniumoxalateprecipit.a~l-,.~Krocessused at LASLprecipitatesplutoniumas plutonium-111oxalatefroma nitricacid solution. The slurryis filteredin a vacuumfilterboatand air dried for severalhours. Water contentin the filtercake variesover e rangeof 34 to 54%, while the plutoniumcon-tent variesfrom 30 to 42$. A typicalbatch size is about800 gof plutoniumand about 2200 g of totalmaterial.Both the precipitationprocessand subsequentcalciningstepsare lengthyprocesses. To maintainreal-the account-abilityof the plutoniummoving throughtheseprocesses,it isimportantthat the plutoniumcxalatebe assayedby anin-linenondestructiveassay (NDA)❑ethod between the precipi-tationprocessand calciningsteps. The highlyvariablewatercontentof the PlutoniumoxalatemakesaccurateNDA measurementsdifficultto attain. Methodsbased on weighingor gamma count-ing provideonlyapproximateinformation.Coincidenceneutroncountingprovidesaccuratem=surenientsonly if correctionsare❑ade for water effects.Coincidenceneutroncountingmethodsfor plutoniumanalysisare based on countingcoincidencen utronscomingfrom sponta-# 240~u,~d242pu.neousfissioningof the isotopes23 Pu,The isotope2jgPu ‘!n contributeto the coincidentneutronsignalif spontaneousfissioningneutronsor (ajn)neutronsinteractwith the 23gPunuclelthat causethe fissions. Spon-taneousfissionneutronsand (@ln)neutronst icallyare bornwith energiesin the 1- 3!to 2-MeV range;the 2 Pu fissioncross section for neutronsin this energyrange is about 1barn. With this smallneutroncrosssectjon,one would expectthe coincidenceneutronsignalfrom23gPu fissionsto be atrivialpart of the overallcoincidentneutronsignalinweapons-gr=deor reactor-gral?plutonium. However,the 23gPufissioncrosssectionrises rapidlyfor neutronsof lowerenergy.The water componentin the plutoniumoxalatestudiedeffec-tivelylowersthe spontaneous-fissionand (a,n)neutronenergiesby proton-neutronelasticscattering. Consequently,23gPufissioningbecomesa ❑ajor contributorto the overallcoinci-denceneutronsignaland, ❑ore significantly,becomesa functionof the amountof water in the plutoniumoxalate.This paper describes❑easurementsmade on a seriesof p..~to-nium oxalatebztches,and the ❑easurement❑ethodand calcula-tionswe derivedto successfullycorrectfor the watereffectonthe plutoniumnxalate.INSTRUMENTATIG:dThe neutroncounterused for this studyis a double-ringthermalneutroncoincidencecounter(TNC .JThe term ““double-ring”refersto two concentricrlcgsof He detectorsthatsurrounda centralcountingWeil. The 3He detectorsareembeddedin polyethylene,which servesas a ❑oderatorto de-creasethe energyof neutronsari’ivlneat the detectors,therebyincre~singthe countingefficic’ncyof the TNC. Figure 1 showsthe lo~ationand numberof 3He detectorsin the TNC; the me-chanicalcomponentsare describedin detailelsewhere.1The purposeof the double-ringdesignis to give a signalthat is sensitiveto the averageenergyof neutronscountedbythe 3He detector~.z The thiclcne~s of the polyethylenebe-tween the Inner ring of the 3He detectorsand the countingwell Is insufficientto reduce 1- to 2-MeV neutronsto theoptimumenergyrange for deteution. Conversely,the poly-ethylenethicknessbetweenthe outerring of deteotorsand the~,countlngwell is too thick for optimummoderation. Con9e-“ quently,if the averageneutronenergyfrom materialbeing meas-ured Is in the 1- to 2-MeV ,ange, the ratio of count ratesbetween the inner and outer rings of detectorswill be rela-tivelylow. If, on the other hand, the average neutronenergyfrom materialbeing oountedis low becauseof moderaticmofneutronswithin the material,then the ratio of i.rmer-toouter-ringoount rateswill be relativelyhigh.The 16-cm-diamcountingwell shown’lnFig. 1 is bolted tothe bottomof a gloveboxand opens into the glovebox. Contain-ers of plutoniumoxalateto be ❑easuredare loaded into a samplechnber in the gloveboxand loweredInto the countingwell by amotor-drivencable. The outer shielding,consistingof 10-cm-thickpolyethylenelinedwith a O.hnm-thick cadmiumsheet,shieldsthe detectorarray from backgroundneutrons.~he TNC electronicsconsistsof a high-voltagepower supply,preamplifiers,amplifiers,and shiftregistercountingctr-cuits.s Using the shift registers,we are able to counteither singleneutronevents or coincidenceneutro~events.Furthermore,by using two shift registers,we can count singlePTERHIELDING$> MODERATORr’% DETECTORS,.,,;(<:+:“-f’~irkl(,~.4,:,,,. ..,.:. . . . , —COUNTING WELL-,.,},,.. ,fY /::4S Xk&$’TORFig. 1Cross sectionof the dual-ringTNC, showingthe 31iedetectorsand the countingwell.and coincidenceneutroneventsseparatelyfor the innerandouterconcentricringsof 31iedetectors, The TNC has a singleneutroncountingefficiencyof aLoutc20%.We are able ti countover 100 000 neutronsper secondwithoutlargedeadtimelossessincethe T?JCelectronicsdeadtlmeconstantis onl about3.5 Vs. {A ❑icroprocessor,calledthe THENCSunit, is builtinto,the instrumentto storecalibrationinformation,to makedeadtimeand backgroundcorrections,and to calculategrams?lu-toniumfor measuredsamples.CALIBRATIONFive Pu02 standards,rangingfrom5 to 660 g of plutoniumwereused to calibratethe TNC. The THENCSmicroprocessorcor-rectedthe coincidencecounting data for backgroundand deadtimelosses,and fittedthe correctedcount rates to a best-fitcurveusingregressionanalysis. The coincidencecountingdatausedfor calibrationpurposeswas taken from the innerring of 3Hedetectorg.SAMPLINGA!!DPREPARATIONFOR COUNTINGEach batchof plutoniumoxalatewas sampledbeforecount-ing. Becausethe plutoniLuoxalatewas not likelyto beuniformlydry acrossthe filterboats,a numberof core sampleswere taken from differentlocationsacrf~ssthe filterboatsandcombined. Thesesampleswere wrighed,dried,and 10w-temperaturecalcinedto Pu02. The driedsampleswere weighedand sent to the LASL AnalyticalChemistryLaboratoryfor pluto-nium assay Dy coulometrictitrationand for Isotopicdetermina-tion by mass spectroscopy.After sampling,the PIU’lium oxalatewas removedfrom thefilterboatswith a spatula.,,ddumpedinto a tared 12-cm-diam,30-cm-highstainlesssteel can. The dark blue-greenplutoniumoxalate,In the form of largelumps,filledthe cans to within10 cm from the top. A close-fittinglid was put on the can, andthe can was weighedand loweredinto the TNC countingwell for❑easurement.CALCULATIONSFROM ANALYTICALRESULTSUsingthe analyticallab data and weightdata,we calculatedthe percentwaterand percentplutoniumfor each plutoniumoxa-latebatch. ‘i’heplutoniumweightfractionfor each batchwascalculatedas follows:(1)g Pu g Pu R driedsampleg Pu oxala= = g driedsample x g Pu oxalate “Gramsof plutoniumfor each batchwere then calculated:(2)g PugPu=g Pu oxalatex batchweight .The assayand weighingerrorsare quite small;❑ore signifi-ca~t erro?sare associatedwith cross-contaminationand othersamplingproblem. We estimatethat the mass plutoniumvalueslisted~n-TableI have associateduncertaintie~of about~l%.TABLE IMEASUREMENTDATA FOR PLUTONIUMOXALATEBATCHESBatchNumber12345678910111213141516ActualMaSsPu (g)887797756781851855917880799925861864912803770787MeasuredMassPu (I?)12911025108812601318133211641564123814501631156010199911037960Ratio ofMeasured!4assto ActualMass1.461.291.441.611.551.561.271.781.541.571.901.811.121.241.351.22Water(Wt%)44.342.743.048.446.547.137.452.747,,547.753.953.734.436.241.94G.5RingRatio1.0701.0531.0681.0951.0751.0911.0541.0971.0731.o831.1101.1011.o361.0291.0451.036The weightfractionof waterfor each batch was calculatedas follows:RHO(3) ~pu :Xalate = 1 -[’Pu(1 + 0.552 Roxalateg Pu oxalate g Pu )] 9where the constant0.552 is based on the assumptionthatplutmium oxalateis pu2(C@4)3 “ X H@. Thisassumptionis not entirelyvalid,becausesurfaceoxidationtoPu(C204)2“ x H20 was observed.Combiningthe plutoniumoxldaticnstateuncertaintyandsamplinguncertainties,the percentwater valuesshowninTableI are probablyaccurateto only~l% absolute.MEASUREMENTRESULTSAND CALCULATIONSEach can of plutoniumoxalatewas countedin the TNC for500 s. The countrateswere very high (singles,approximately40 000 comtsis; coincidence,approximately3 300 count9/s).Hence the precisionof the countingdata Is good. The THENCSmicroprocessorused the innerring coincidencecountrate foreach plutoniumoxalatebatch to calculateplutonium❑ass versusthfiPu02 calibrationcurve.TableI lists the actualmass plutonium,~he measuredmssplutonium,the measured-massto actual-massratio,and theweightpercentwater for each batch. Notice that the measuredmass was alwaysbiasedhigh and that therej? a correlationbe-tween the amountof bias and the percentwatercharacteristicofeachbatch. A correctionto the m-su-zd mass based on percentwaterwould reducethe ❑easuredmass bias,but percentwater isnot measureddirectlyby the Tt.JCAs discussedearlier, the ratioof neutroncountsbetweenthe innerring of detectorsand the outerring shouldbe sensi-tive to the averageneutronenergyand, therefore,to the watercontentof the plutoniumoxalate. The ring ratlasand ❑easured-mass to actual-massratiosfromTableI are plotte~in Fig. 2; agood correlationis apparent. To evaluatethe correlation,wefittedthe data to an exponentialcurveof the followingform:(4) Y= &ebx ,wherey = measured•as:~dividedby actualmass,a= 0.0015372,5= 6.38986,?.ndx= ring ratio.The coefficientof determinationvaluefor this fit is0.56.25,indicatin~that the exponentialform fits the dataquite\Jell.‘%E.L10“108- ●106-@I02 I I I I1.2 1.4 1.6 1.8 . 20TNC-T&ANALHICAL RATIOFiR: 2.Correlationof the ring ratiosand themeasured-massto actual-massratios.Usinu Eq. 4 and the ring ratios,we then calculatedthevaluesfor each plutoniumoxalatebatch. We next correctedmeasuredplutoniummass valuesspendingy valuesas follows:(5) correctedmass =Table II shows the resultsYthein Table I with their corre-+ (measuredmass) .of applyingthis correctiontothe 16 batchesof plutoniumoxalate. Notice that the averagedifferencebetweenthe correctedmass and actualmass is 2.7$,and the threeworst caseshad errorsof 5.4%, 4.9%, and 4.5%.DISCUSSIONThe ring ratio correctionworkedwell for the 16 batchesofplutoniumoxalateinvestigated.It is importantto note thatthe batchesrangedove~ a limited❑ass range of 756 to 925 g ofplutonium. We have done some preliminarymeasurementsof smal-ler massesof plutoniumoxalate;the ring ratio correctiongenerateda significantbias for plutoniummasses below 500 g.The 16 batchesInvestigatedrepresenteda wide spectrumof watercontent. We suspectthat the correctionequationconstants~reinoorrectfor the very dry plutoniumoxalatethatmight be gen-eratedby vacuumdrying.The ring ratioswere calculatedfrom Inner-and outer-ringsingleneutroncounts. Usingring ratiosof coincidencecountsyieldssimilar,but lesspreciseresults. The presenceof highcross-section(ajn)nucleisuch as beryllium,litnium,orfluorinecouldshiftthe averageneutronenergyspectrumandbias the ring ratiocorrection.Generally,the oxalateprecipitationprocessprovidesgood purificationof theseTABLE IIRING RATIO CORRECTEDMASSPLUTONIUMBatch IDFox 1177FOX 1180RIX 1178FOX 1181FOX 1185FOX 1183FOX 1182FOX 115C3FOX 115c6FOX 115C7FOX 116C3FOX 116c4FOX 116C5FOX 116C6Fox llCCFOX JRC-1ActualMassPu (g)887797756781851855917880799925861864912903770787Measured MassaPu (g)886787756739877803886907835918871883872816771820Errorb(%)-0.2-1.30.0-5.43.1-6.1-3.43.14.5-0.81.22.14.51.70.14.9aRing ratiocorrected.bAverageerror= 2.7%.elements,but shouldtheseel’mmts be presentIn 0.1% orgreaterconcentrations,the ‘orrectionmay be biased. Nme oftheselimitationspose probl@a for the particularoxalatepreclpitatim process investigatedat LASL becausethe processis operated within the limitingparameters.CONCLUSIONUsinga double-ringTNC, we have developeda ring ratiocorrectionmethodthat successfullycompensatesfor the effectthatwater has on the coincidencecountrate of plutoniumoxalate. Batchesof plutonium oxalate can now be assayed withan average measure~nt error of about 3%. The methodprovidesthe rapidin-linemeasure~entcapabilityrequiredto maintainreal-timeSUM accountabilf.tyarounda piutoniumoxalatepuri-ficationprocess. At LAS],,plutoniumoxalatemeasurementsareperformedby productiontechnicians,and requirea 500-scountingtime ald approxfilately10 min for calculatingandrecordingthe data.ACKNOWLEDGEMENTSThe authors wish to thankEldon Christensenand MiltonHaas~or providingthe batchesof plutoniumoxalateused in thisstudy.REFERENCES1. N. Baron, ‘A Correctionfor VariableModerationandMultiplicationEffectsAssociatedwith ThermalNeutronCoincidenceCounting,W IAEA Symposiumon NuclearMaterialSafeguards,Vienna (1978).2. R. S. Marshall, Los Alamos Scientific Laboratory ReportLA-7616-PR,70-73 (1978).3. M. S. Krick and H. O. Menlove.Los AlamosScientificLaboratoryReportLA-7779-M(197!)).4. B. H. Erkkilaand R. S. Marshall,‘A ThermalNeutronCoincidenceCounttngSystem.flTo be presentedat theAmericanNuclearSociety1979WinterMeeting,San Francisco,Calif.

Measurement of plutonium oxalate in thermal neutron coincidence counters

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Measurement of plutonium oxalate in thermal neutron coincidence counters

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