USNRDL-TR-9243 November 1985

QUANTITATIVE RADIOCHEMICAL DETERMINATION OFNICKEL-683 IN SEAWATER

,4byM. G. LaiH. A. Goya

CLEAR!rfC H 0FOR FEDERAL T-1' " 7,. ,'-

FHardcory .

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U.S. NAVAL RADIOLOGICAL

DEFENSE LABORATORY

SAN FRANCISCO • CALIFORNIA • 94135

APPLIED RESEARCH BRANCHD. Love, Head

CHEMICAL TECHNOLOGY DIVISIONR. Cole, Head

ADMINISTRATIVE INFORMATTION

The work reported was part of a projectsponsored by the Atomic Energy Commission, underContract No. AT(49-5)-2084.

DDC AVAILABILITY NOTICE

Distribution of this document is unlimited.

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Eugene Pfc per D.DC. Campbell, CAPT UcSNScientific Director Commanding Officer and Director

ABSTRACT

A liquid scintillation method for the quantitative determination! of Ni63 in seawater is described. The method consists in cumplexing

the nickel in seawater with dimethylglyoxime,, extracting it intochloroform,, and then back-e~xtracting the nickel into an aqueous phaseusing dilute sulfuric acid. An aliquot of this sample is added to ascintillattion solvent and counted in a liquid scintillation counter. ;

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APJ

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! A rapid method for determining NiS in seawater has been developed.

S~This method will be used for studying the corrosion of bMAP fuel encap-S~sulant materials., using neutron activation analysis. In this procedure

t a sample of the nickel alloy encapsulant i•terial is irradiated to pro-S~duce the beta-emitting product, nuclide Ni 3. The activated alloy is

i ~then placed in the corroding seawater medium. Periodical•y the. seawateris drained from the sample chamber and the concentration of Ni63 is

i determined. The rate of corrosion of the alloy sample is then calcu-Slated from the rate at which its principal constituent, Ni, is released. I-

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INThO)DUCTION :

A radiochemicel procedure for measuring Ni63 in seawater wasneeded for determining accurately, in a short-term study, the rate ofcorrobion in seawater of a nickel alloy used to encase radioactivefuels in SNAP (Space Nuclear Auxiliary Power) reactors.

BACKGROE)

Corrosion-resistant materials are used for encasing radionuclidefuels to prevent, in an accident situation, the leakage of radioactivematerials to the enviromwnt.

Several types of fuelt being consided hbave long half-lives, inthe order of a Inm.-rad yenra. A major en-vironment in ihich these ftelsmight be immersed in case of an accident is the ocean. In this ccne,the container vouId have to maitatin its integrity for mery hundreds ofyears. However, piedicting the long-term risistance of a suitablematerial reqaircs highly accurate short-term measurement of its corro-sion rate. Oresent reacurement methods, for exanple differential weigh-ing tecbniquez, bave proved generally -nsatinfactory. The problem in-volved here is t'.ofeoli: 1. There is difficulty in detecting theextremely small wvight I=3 due to ccrrosion of the specimen, 2. theaccumulation of organic or oither matter requires abrasive cleaning of thesample before final reweighing; this may cause sow dsmzage to the sur-face and conseciently interfere witb the differential veighing technique.

OB.JECTIV

A program has been initiated at the U. S. Raval Radiological DefenseLaboratory to attempt to refine the measurement technique so as to be able

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to obtain accurate, short-term data on corrosion rates of a number ofhighly corrosion-resistant encapsulant materials. One alloy presentlybeing studied is -aste1loy C, a metal composed of 57 % •i, i6 % Cr,17% % o, 4 % W and 5 %'Fe. The metal sample is to be .irradiated withthermal neutrons and the rate oC corrosion is to be followed by measuringthe activities (primarily of Ni03 ) of the radioactive products leachedinto the seawater solution.

APFROACH

Ni6 is a beta-emitting isctope with a half-life of 92 years. Sincethe highest energy radiation associated with the nuclide is a 67-Keybeta-ra•y, difficulty is encountered in the ordinary counting techniquesreqyiring solid sources. Self-absorption reduces the counting efficiencyof solid samples and makes the interpretation of the data difficult.Gaseous samples, such as nickel carbonyl, on the other hand, presentdifficult problems of handling and system decontamination. Fbr theseand other reasons, a liquid-scintillation counting technique seemed themost prumising approach. A previously reported studyl had demonstratedthe feasibility of the liquid-scintillation counting of Ni63.

In the technique devised here, Ni63 activity in seawater is chelatedwith dimethylglyoxime. The activity is concentrated by extraction witha small volume of chloroform. The sample is then prepared for countingby back-extraction into an aqaeous medium.

3ince a high concentration of Ni in the final scintillator solutionacts as a quenching agent, the operations are performed without theeadition cf nickel carrier. Consequently, quantitative removal of thenickel activity is required.

EXPERIM~ENTAL

MA2ERIAJ.S

All' chemicals used were ACS Reagent Grade.

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Scintillator Solution: The scintillator solution contained 10 g of2,5-diphenyloxazole (PPO) and 0.25 g of 1,i4-bis-2(4-methyl-5-phenyloxa-

zoly)-benzene (dimethyl POPOP) dissolved in toluene to make one liter of .solution. The solution was stored in an amber bottle in the dark.

Standard Solutions: A secondary standard of Ni63 in dilute Iydro-chloric acid was obtained from the Oak Ridge National Laboratory. Theradiochemical purity vas certified to be 99 %. Its specific activitywas 1.57 x 105 dpm/pg Ni. This solution was check-calibrated against anintercalibrated2 primary standard obtained from the Atomic Weapons ReseaachEstablisbment, Aldermaston, Berkeshire, England. Alicuots of the second-ary standard were used throughout in the experimental procedure.

Tracers: The radioactive tracers were used in the form furnishedby Oak Ridge National Laboratory.

APPARATUS

The counting system used was a Tri-Carb liquid scintillation spectro-meter, Model 314-A (Packard Instrument Co., Ia Grange, Ill.). The freezersection was maintained at 0°C.

Samples were counted in 22-mi screw-capped vials.

The counting instrument was caibrated to optimm settings of windowwidth and high voltage using the Ni.63 secondary standard. First, optimumhigh-voltage settings were determined at a fixed window width. Then op-timam window settings were obtained at a fixed optimum high-voltage set-ting. The final conditions chosen were a high-voltage tap setting of1.5 and a window-width setting of 6.00. Background at these settingsaveraged 20 + 2 cpam.

Counter efficiency was established by diluting the Ni 6 PrOastandard so that 1 ml of the final solution contained 6.80 x ilo dpm.One-ml aligiots were placed in counting vials and the scintillator solu-tion was added. The average measured counting rate was 1.66 + 0.01 X i04cpm, or a detection efficiency of 21.5 + 0.15 %.

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4 f Place 100 ml of seawater sample conainin the Ni63 activity into asepaatory funnel. Add 0.5 ml of an ethanolic solution of dimetbylg3y-cotime (1 $). Mix the solution thoroughly and allow to stand for at least10 min. Extract the nickel dlmet-bylg3yoxime by shaking with 10 ml ofchloroform for 2,-3 min. Drain the organic phase into another separatoryfunnel. Then wash the aqueous phase with 10 ml of chlorofom and cnm-bine the organic extracts. Discar'd the aqueous phase. Back-extract thenickel into an aqueous phase by adding 10 ml of 0.1 N sulfuric acid andshaking for 1-2 miin. Drain the organic phase into a clean separatoryfunnel and the aqueous phase into a 50-ml teflon beaker. Wash the org-anic phase with 10 ml of 0.01 N sulfuric acid. Discard the organic phase.Combine the aqueous phase with the previous one in the teflon beaker.Evaporate the solution to approximately 3 ml. Transfer quantitativelyinto a 5-ml volumetric flask and dilute to the mazk with water rinses ofthe teflon beaker. Pipet 1 ml of the solution into a vial containing 10ml of scintillator solution and-10 ml of ethanol. Mix the system thor-oughly. Place the sample in the refrigeration chamber of the counterfor 30 miin before measuring its activity.

PROCEDURE DEVEOPHEINT

In the development of this procedure for the determination of Ni63in seawater, two groups of parameters were investigated.

The first comprised those factors that affected the counting rateof the liquid scintillation sample: concentrations of scintillators,aqueous volume, linearity of counting, reproducibility of counting, tem-perature, acidity, and stability of the counting sample.

The second comprised those factors concerned with the extraction ofthe nickel activity from the seawater: concentration of dimethylglyoximereagent, pH of the seawater solution, recovery factors, back-extraction,and extraction of possible interfering radionuclides.

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PARAMETERS AFFECTIG COUNTIM RATE OF LIQUID SCInTILLAT=ON SAMPLES

Concentrations of Scintillators

To determine the optimum concentrations of the primary and second-ary scintillators, identical radioactive samples containing varyingamounts of scintillators were prepared and counted. The results indi-cated that a PPO concentration of 10 g per liter of toluene and a TOPOPconcentration of 0.25 g per liter of toluene would be satisfactory(Fig.

Effect of Agaeous Volume

Since liquid scintillation systems generally do not tolerate largevolumes of aqueous solution, it was necessary to study the volume limitsof samples being used in the toluene-ethanol system. In this sWdy 0.5to 1.8 ml of solution containing known, identical amounts of Nio3 weremixed with 2D ml of the scintillator-cthanol solution and counted. Theresults are presented in Table I. It appeared that if aqueous solutionali.puots of larger than 1.2 ml were added to the scintillator solutionthere would bs. a decrease in the total mmber of observed counts. Apart of the decrease in counting rate was probab2y due to some effectcaused by the dilution of the scintillator as the aqueous volume incre-ased. The aqueous aliqiot volume was therefore set at 1 ml.

Linearity of Counting with Increased Concentration of Ni 6 3

To determine the linearity of counting ratio with vary4n activitylevels, standard solutions containing varying amounts of Nio3 were pre-pared. One-ml alipuots of each solution were added to 20 ml of thescintillator-ethanol solution and counted. The results (FI. 2) indi-cate a linear relationship between the true and observed counting rateover a wide range of activities. Counting errors were less than 1 %.

Reroducibility

To study the reproducibility of sample preparation, five identicalNi6 3 samples were individually prepared and counted. The results aregiven in Table I3. The reproducibility of this method appears to beexcellent.

Effect of Temperature

The effect of temperature on the counting rate was determined bymeasuring counting rate as a function of elapsed time after the sample

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0 0. 50w~u) A 0.25g0 ~>O.lOg

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PPO CONCENTRATION (g/LITER)

Fig. 1 Effect of Concentration of Scintillators on Coumting Rate

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TABLE I

Effect of Aqueous Volume

Aqueous Volume Loss in Counting Rate(2,,) M•a

0.5 0.01.0 0.01.2 0.21.3 0.81.4 1.81.5 3.01.6 3.21.7 3.41.8 3.5

a. Compared to aqlueous volume of 1 ml.

TABIE 31

Reproducibility of the Method

Sample Number Observed Counting Rate

1 178,1002 178,5983 177,8514 177,6P&35 179,246

Average 178,287 + 576

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TABIZ III

Effect of Temperature on Counting Rate

Time in Counter Counting Rate(mri) (c/r,)

1 172.,6925 1711,200

10 176,84915 177,10520 178,o90430 178,21060 178,150

was placed in the freezer comnlrtment of the counter at O°C. A standardNi63 solution was freshly prepared and immediately placed in the freezercompartment. Counts were taken at various intervals. Table III showsthat a steady-state counting rate was reached and maintained after 20minutes.

Effect of Acidity

To determine the effect of acid concentration on the counting rate,,one-ml aliqaots of Wi63 solution containing varying quantities of sul-furic acid* were mixed with 20 ml. of the scintillator-ethanol solutionand co•uted. The results (Table IV) show that the counting decreasesas the acidity increases. The system would tolerate 1.5 N sulfuricacid with approximately I % loss in counting rate. Solutions with highacidity tended to decompose the scintillator, thus decreasing the count-ing efficiency. However. as can be seen in the following section.,"Stability of Sample," a minimal acid concentration is necessary formaintaining the stability of the counting sample.Stability of Sample

In order to assess the effect of pH on the stability of the samples,samples of various pH were prepared and stored in the dark at roam tem-perature for varying periods of timn up to 6 days. The results (Table V)demonstrated that solutions of 0.1 N JEl were stable fox more than 6days, vhile the counting rate of the other solutions (pH 5-6.5) graduallydecreased. In solutions of low acidity, nickel was probably adsorbed

*Sulfuric acid solution, described in the later section "Back-Ectraction.,is reqoxred to back-extract nickel into the aqueous phase.

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S• • cone Ioss in Couwting Rate

S0.5 0S1.0 o.6

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i2.0 3.63.0 6.35.0 9 -9

TABLE V

EfeStability of SAples

412804cn 'si Count ing Rate

Storage Ti•e ()(da~s) P11 1 PH 6

0 178,120 178,2600.08 178s,150 177,.4510.17 178.522 178.5080.25 178,03.0 1784621.0 178, 6.3 175o4312.0 178 9.0 9 172,6143(s0 178p,105 170p 0O6.0 178.,005 168,540

0210

TABIE VI1

Effect of pH on Extraction of Nickel From Seawater

pH Percent Extracted

2.0 0.0.0 o0.0

6.0 17.37.0 96.87.5 99.48.0 99.59.0 99.3

onto the wa3ls of the liquid-scintillation counting vials, thereby re-sulting in 2x geoetry and the consequent reduced counting efficiency.

PARA�SDNCERM V WI E M UANTITA RECOVY O NICKEL

WMW SENATER

Concentration of Dimtbylg3_oxime for Extraction

To determine the optilum concentration of the organic reagent,varying amounts of 1 % solution of dimetbylglyoxie were used for theextraction. The recovery was essential•y 100 % within the range inves-tigated (0.1 to 5 ml BW per 100 ml seawater).

E.fect of pH on Extraction

Extraction of nickel from seawater solutions of varying pH wadinvestigated. The results shown in Table VI indicate that aanttetiverecovery was not obtained from solutions of pH less than 7.5. Since -:hepH of natural seawater generally lies between 7.5 and 8.5, no furtheradjustment of pH is necessary for the extraction procedure.

Recovery of Ni bWy Dimethygglyoxi Extraction

The recovery of Ni "frc Dimetbylglyoxime was studied. Table VIIgives recovery values of Nio3 found in the two organic phases and theone residual aqueous phase of a typical extraction. The initial mixing All;time was 3 min, but a time study showed that an interval of 2 min issufficient for jaantitative recoveries.

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TABI VII

Extraction of Nild with Dimetrlvglgyoxlne in ChloroformFrom Seawater (Seawater, 100 ml; IM., 0.5 ml; Chloroform,

102ml)

SExtraction Ni63 RecoveredTimeM(min)

CHCl 3 extract 1 3 97.7CMI[1§ extract 2 2 2.1Resi2ual Aqueous 0.3

PhaseTotal 100.1

_Back-Extraction

Since chloroform acts as a qaenching agent in liquid scintillationcounting., it was necessary to back-extract nickel into an aqueous phase.This was easily done with acid eolutions. However, since the scintilla-tor solution could not tolerate a high acid concentration, it was neces-sary to determine the acid conditions for stripping that would not causeinterference in the final counting senple. In this experiment a seriesof chloroform extracts containing Ni t '3 were baak-extracted with l0-mlportions of solutions of varying concentrations of sulfuric acid. Theresults (Table VIII) indicated that nickel could be back-extracted fromthe organic phase with 20 ml of 0.1 N sulfuric acid. This acid concan-tration was compatible with the final counting conditions.

TABLE VIII

Back-Extraction of Nickel From Organic Phase IntoAqueous Phase

H^ Concentration Percent Stripped

(normality)

0.035 8.10.070 4.0O.lO8 100.0o.,1o 100.o0o.18o 100.0

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Extraction of Other Radionuclides

When the SNAP metal samples are initially irradiated with neutrons,0other radionuclides besides NiO3 are produced. These ma include:

Fe-58 n,7) Fe-59Mo-98 nj.7) Mo-99w-184 nn,.) w-185Cr-50 n,7) Cr-51Ni-58 n,p) Co-58Ni-6D np) Co-60Fb-4 In.p) Mn-5

In studying the corrosion behavior of the metal in seawater, then,activities other than Ni63 will be found. Consequently it was necessaryto determine whether these other activities would interfere with the Nianalysis. This was done by studying the extractability of these radio-nuclides into dimethVrglyoxime-chloroform and the stripping with 0.1 Nsulfuric acid. The amount of each radionuclide recovered finelly inthe stripped aqueous phase is given in Table IX. Among the nuclidestested, none showed any significant interference with the nickel extrac-tion. The data indicated that a minium dedontwmination factor of ap-proximately 103 can be achieved by the foregoing procedure.

TABLE IX

Extraction of Elements with 0.1 % DMV in

CR1 3 From Seawater

Nuclide Percent Extracted

Fe-59 I11) o.13co-6O In) 0.17Mn-54 I3) 0.09Cr-51 III) 0.17w-184 VI) < 0.01Mo-99 VI) < 0.01

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FGMUI P" A SEMI3 OF aAM1 ANAIYSES

To demonstrate the accuracy and precision of the above procedure aseries of seven 100-ml seavater samplJs, to which were added from5.0 x 102 C/, to 1.75"x 106 c/M of Nt63vwere ana3,yzed. The resultsare shown in Table X. The average recovery vas 99 %, with a standarddeviation of O.9 1 . %. The small loss probably was due to incomplete trans-fer of the separated phases during the extraction procedure. ibo samplesof very low nickel activity, increased sensitivity could be obtained byefther using larger seawater samples or by evaporating the final aqueousvolume to 1 ml in the counting vial.

TABLEX

Recovery of Nickel 6 From Seawater

Ni63 Added Ni63 Recovered(c/r) (M)

5.00 x.o0 97.8

1.00 x 98.51.70 x 103 101.56.79 x 103 99.1

6.79 x 10o 98.

1.75 x 105 98.11.75 20 6 99.4

Average 99.0 + .94 %

111

1. C. E. Gleit, J. Dwnot, "Liquid Scintillation Counting of Nickel-63,"Int. J. Appl. Red. Isotopes 12:66, 1961.

2. R. L. G. Keith, D. W. Watt, F. Brown, "The Intercomparison of 41xPropcrtional Counting Tecbniques in Canada, UK and USA. Part 2:S-35 and Ni-63j" United Kingdom Atomic Weapons Research 'Establishment

AWREo-26/Of, may 19&e.

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I. ORIGINATING ACTIVITY (Corporate author) 20. REPORT 9ECURITY C LASSuI$CATION

U. S. Naval Radiological Defense Laboratory UNCLASSIFIEDSan Francisco, California 94135 2b GRouP

3. REPORT TITLE

QUATITATIVM RADIOCHEMICAL DETERMINATION OF NICKEL-63 IN SEAWATER

4. DESCRIPTIVE NOTES (Typ, of report and inclueive date*)

S. AUTHOR(S) (Last name. first name. initial)

Lai, King G.Goya, Harry A.

6. REPORT DATE 7a. TOTAL NO. OF PAGES 7 b. NO. air REtaS

31 January 1966 20I 2Ila. CONTRACT OR GRANT NO. $0. ORIGINATOR'S REPORT NUMKEE(S)

AT(49-5) -2084& USNmDL-TR-924b. PROJECT NO.

C. ,b. THER PORT N.(S) (Any other numb.ersdht may be a..i.,ed

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U. S. Atomic Energy CommissionWashington, D.C. 2054&5

13. ABSTRACT

A liquid scintillation method for the quantitative determination of Ni6 3in seawater is described. The method consists in canplexing the nickel inseawater with dimethylgjyoxime, extracting it into chloroform, and then back-extracting the nickel into an aqueous phase using dilute sulfuric acid. Analiquot of this sample is added to a scintillation solvent and counted in aliquid scintillation counter.

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Eadiochmical analysisNickel-63Sea water ~•1Metal corrosion

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