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Experimental evaluation of plutonium-239 spike for determining plutonium concentration by isotope dilution-thermal ionization mass spectrometry
International Journal of Mass Spectrometry and Ion Processes, 69 (1986) 137-151 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
137
EXPERIMENTAL EVALUATION OF PLUTONIUM-239 SPIKE FOR DETERMINING PLUTONIUM CONCENTRATION BY ISOTOPE DILUTION-THERMAL IONIZATION MASS SPECTROMETRY
S.K. AGGARWAL, H.C. JAIN Fuel Chemistry (First received
G. CHOURASIYA,
Division, Bhahha Atomic
R.K. DUGGAL.
RADHIKA
Research Centre, Tromhay,
12 June 1985; in final form 16 September
RAO and
Bombay 400 085 (India)
1985)
ABSTRACT The use of 239Pu as a spike in isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) for determining the plutonium concentration in irradiated fuels is demonstrated. The method is based on the high precision better than 0.1% determination of the 240Pu/23yPu atom ratio in the sample, the spike, and the spiked mixture. The precision and accuracy of the results obtained on different plutonium samples using a 239Pu spike are compared with those obtained using a 14*Pu spike. A mean precision of O.l-0.2% is achieved using a 2.7uPu or 242Pu spike and a factor close to unity is obtained for the ratio ID-TIMS ( 23”Pu)/ID-TIMS ( 14*Pu). This study provides a viable alternative spike material to those laboratories which do not have access to enriched 242Pu. which is generally used in determining the plutonium concentration by ID-TIMS, and at the same time, offers certain advantages over the use of a 242Pu or 244Pu spike.
INTRODUCTION
Determination of the plutonium concentration is important at different stages in the nuclear fuel cycle and, in particular, at the input point of a reprocessing plant. At this point, plutonium is associated with large quantities of uranium and fission products and the solution is highly radioactive. Further plutonium is present in very low concentrations and may be in different chemical forms (e.g. different oxidation states, or polymers). The composition of the dissolver solution of irradiated fuel therefore demands the selection of methods which give high precision and accuracy, require a small amount of sample and do not depend on quantitative separation and purification of plutonium. The isotope dilution methods using mass spectrometry [I] and alpha spectrometry [2,3] meet all these requirements and are therefore most commonly used for this purpose. Isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) using a 242Pu spike has been used for more than two decades and is capable of providing an accuracy of 016%1176/86/$03.50
0 1986 Elsevier Science Publishers
B.V
138
O.l-0.5%. A 244Pu spike has also been proposed [4,5] for determining the plutonium concentration by ID-TIMS and it has the potential of simultaneously providing data on the isotopic composition of plutonium. However, the production of enriched isotopes of 242Pu and 244Pu requires high flux isotope reactor (HFIR) and calutron facilities which exist only in a few countries. Thus, enriched 242Pu and, particularly, 244Pu are not easily accessible. With the availability of present generation thermal ionisation mass spectrometers capable of giving high internal precision (better than 0.1%) in the isotope ratio measurements ranging from 0.01 to 100, it was considered interesting to explore the utility of routinely using 239Pu, which is easily available from reactors, as an alternative to enriched 242Pu or 244Pu spikes. This involves the measurement of small changes in the 240Pu/239Pu atom ratio with high precision (better than 0.1%) and provides an alternative spike to those laboratories which do not have access to enriched 242Pu or 244Pu. Irrespective of the spike used, the two problems generally encountered in ID-TIMS of plutonium are: (i) incomplete chemical exchange between the sample and the spike isotopes, and (ii) isotope fractionation in the ion source during the mass spectrometric analysis. Incomplete chemical exchange between the sample and the spike isotopes leads to an unequal recovery of the two isotopes during the purification step and thus erratic results are obtained on replicate aliquots taken from the same plutonium solution (spike solution or unknown dissolver solution). An evaluation of chemical exchange treatments has been published by Marsh et al. [6], but this still remains the limiting factor, which at times leads to either totally inconsistent results or a large spread ( > 0.5%) in the concentration values. In our laboratory, complete chemical exchange is ensured by first treating the spiked mixture with concentrated nitric acid at least twice for the depolymerisation of plutonium and subsequently converting all the plutonium to Pu(IV) using H,O, in 3M HNO,. We have tried this procedure on more than 100 different plutonium samples and have not experienced any difficulty except for one or two cases where erratic results were obtained. This could be attributed to the incomplete depolymerisation, since on treating fresh spiked samples 4-5 times instead of twice with concentrated nitric acid, consistent results were obtained even in those samples. Isotope fractionation in the ion source of a thermal ionisation mass spectrometer is another problem and it is a much less understood function for plutonium due to the non-availability of absolute isotopic standards of 242Pu/ 239Pu ratios. Fractionation causes the measured isotope ratios to change with time. The extent of isotope fractionation is proportional to the mass difference between the isotopes and is dependent on sample composition and size, sample purity, sample acidity, sample loading procedure,
139
heating temperatures of the sample and ionisation filaments and the time of measurement. Isotope fractionation is difficult to control and is a cause of variable systematic error. With the advent of present generation high sensitivity thermal ionisation mass spectrometers capable of giving high internal precision (0.01%) in the isotope ratio measurements, it is now possible to study some of the factors influencing isotope fractionation. For example, in our laboratory, differences of 0.6% and 0.2% were observed in measuring *42Pu/ 239Pu and 240Pu/ 239Pu isotope ratios, respectively, when the amount of plutonium on the filament was changed from 4 pg to 25 ng [7]. From this viewpoint, the use of 239Pu as a spike has the advantage of less isotope fractionation in addition to giving a lower mass discrimination factor while using a secondary electron multiplier (SEM) as a detector. On the other hand, these effects are accentuated while using 242Pu or 244Pu spikes due to three and five mass unit differences, respectively, with respect to 239Pu. Moreover, the isotopic reference materials with the same mass difference as in the spiked sample are not available for determining the calibration factor. The two problems in the ID-TIMS of plutonium mentioned above are also evident in the round robin experiment IDA-80 conducted by the Safeguards Project at Karlsruhe [8,9]. A detailed description of this interlaboratory experiment has been published. Only 12 of the 31 participating laboratories could report the plutonium concentration values within 0.4% of the certified value and there were a number of laboratories with experience of more than five years whose values had larger deviations when compared with the reference value. In addition to smaller isotope fractionation, the use of 239Pu as a spike has certain other advantages. For example, in this case, the unknown dissolver solution need not be diluted, which eliminates the dilution errors. The dilution is mandatory while using 242Pu or 244Pu spikes since these enriched isotopes are expensive. Furthermore, the use of 239Pu as a spike will eliminate the errors involved in 242Pu or 244Pu spike calibration since a solution from a high purity stoichiometric compound of plutonium or plutonium metal, i.e. a chemical assay standard of plutonium, can be used directly as a spike. 239Pu (about 99 atom W) can be prepared by irradiating natural uranium in a reactor: 239Pu( > 90 atom %) and 239Pu( G 80 atom %a) can be obtained from irradiated fuels. These advantages in using a 239Pu spike were recognised in the 1970s and an attempt on its use is reported in literature [lO,ll]. In these studies, plutonium containing about 97% 239Pu was used as a spike for determining the plutonium concentration in undiluted input solutions having plutonium with 70% 239Pu. A spread of about 2% was observed in the results at that time. Feasibility studies using plutonium with 239Pu of - 95 atom % and 239Pu of - 99 atom % as spikes in ID-TIMS have also been reported from our laboratory [12].
140
This paper describes the experience gained in using 2’9Pu as a spike in ID-TIMS for determining the plutonium concentration in different plutonium samples over a period of about one year, presents the results obtained on different plutonium samples, compares them with those obtained by using an enriched 242Pu spike and discusses the advantages and limitations of using 239Pu as a spike. Choice of spike The ideal spike for determining the plutonium concentration by ID-TIMS should satisfy the following criteria. (i) It should be easily accessible to all international laboratories without any restrictions and should not be very expensive. (ii) Its mass number should be adjacent to the major isotope ( 239Pu) in the sample. This minimises the variable systematic error due to isotope fractionation in the ion-source as isotope fractionation is a mass-dependent effect. (iii) There should not be any isobaric interference at the mass number of the spike isotope. (iv) The spike isotope should have a sufficiently long half-life. This minimises the error in accounting for the radioactive decay of the isotope and thus avoids frequent fresh calibration of the spike solution. (v) The spike isotope should preferably be absent or least abundant in the sample so that random errors are minimised. The isotope itself should be highly enriched. (vi) It should b e possible to either completely eliminate the spike calibration by using a chemical assay standard of plutonium or sufficient amount of the enriched isotope should be easily available so that spike calibration can be checked independently by the methods based on other physical principles, e.g. by electro-chemical methods. This will eliminate the possibility of any systematic error due to spike calibration. These criteria along with different isotopes of plutonium which are available in weighable quantities are listed in Table 1. It can be seen that none of the isotopes satisfies all the criteria. Furthermore, 236Pu and 24’Pu are obviously not suitable candidates to be used as spikes. 23RP~ cannot be used routinely as a spike in ID-TIMS due to the ubiquitous isobaric 238U at 238Pu. Thus, 239Pu, 240Pu, 242Pu and 244Pu are the interference of isotopes which may be used as spikes. In fact, 242Pu has been used as a spike for more than two decades. 244Pu has also been proposed: this appears promising for obtaining simultaneously the data on the isotopic composition of plutonium in the sample. But, to the best of our knowledge, no detailed studies have been carried out in the use of this isotope on a routine basis.
141 TABLE
1
Criteria
for selection
S. no. 1
2
3 4 5 6
of the spike in ID-TIMS
Criterion
Spike isotope 23hpu
Accessibility to all the laboratories Adjacent to the major isotope ( 239Pu) in the sample Free from isobaric interference Half-life > 5 X 103y (decay < 0.01% per y) Absent/least-abundant in the sample Possibility of checking spike calibration by other methods
a + indicates
that the criterion
of plutonium a
23Rpu
239~~
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
240Pu
24’Pu
+
+
+
242~~
244
Pu
+
is satisfied.
240Pu and 239Pu are promising from the point of view of smaller isotope fractionation and the possibility of eliminating the spike calibration step. But the production of enriched isotopes of 240Pu, 242Pu and 244Pu requires high flux isotope reactor (HFIR) and calutron facilities which exist only in a few countries. Hence, under these constraints, one is left with no option but to use the easily available 239Pu isotope as a spike. Principle of the method The principle of ID-TIMS using 239Pu as a spike is similar to that using a 242Pu spike except for the measurement of the 240Pu/239Pu atom ratio instead of the 242Pu/239Pu atom ratio in the spiked mixture. It can be shown that if C denotes the plutonium concentration, W denotes the aliquot sizes, R denotes the 240Pu/239Pu atom ratios, (At.Wt.) denotes the average atomic weight of plutonium, and (A.F.) denotes the atomic fraction, then the plutonium concentration in the sample is given by
I- Rm/Rs, RnI - RS
(At.wt.), (At.Wt.),,
where the subscripts s, sp and m stand and the spiked mixture, respectively.
x (A.F.240P~),p
(1)
(A.F.~~~PU), for the unspiked
sample,
the spike
142 EXPERIMENTAL
Instrument The mass spectrometer used in the present work is a MAT-261 thermal ionisation mass spectrometer controlled by a HP-9835A calculator. In this instrument, the ion beam leaves the ion source via a 0.2 mm wide fixed slit, accelerated to an energy of 8 keV (used in the present work), and is projected at an angle of 26.5” into the 90” magnetic sector field, from which it emerges at the same angle. This arrangement doubles the dispersion of a conventional system with an ion path radius of 46 cm. A collector slit width of 0.6 mm is used which results in a resolution of about 500 (10% valley definition). Up to 13 samples can be analysed in one run due to the turret-type sample magazine. The instrument is equipped with a Faraday cup as well as a secondary electron multipier. A pressure of < 10-h torr in the ion source and a pressure of < lo-’ torr in the analyser region is obtained by using turbo molecular pump and vat-ion pumps. An abundance sensitivity of better than 10” is obtained. The sample (1 ~1) is loaded onto the sample filament by using a push-button type of micro-pipette and is dried by using a progammable heating device. Determination of plutonium / “‘Pu atom ratio < 0.15)
concentration
in low burn-up samples
(i.e. ‘40Pu
Two different plutonium samples containing about 80% 239Pu were used of the spikes were determined as the spike materials. Isotopic compositions and the data are given in Table 2. The 23xPu in these mass spectrometrically The 240Pu/239Pu atom ratios spikes was determined by alpha spectrometry.
TABLE 2 Isotopic
composition
Isotope
of plutonium
Atom percent 242Pu spike
2% Pu 239~~
240Pu 24’Pu 242Pu a Determined
in the 242Pu and 239Pu spikes
0.0226 ” 1.5186 2.9347 1.5402 93.9838 by alpha spectrometry.
239Pu spike Rl
R2
N
0.322 ’ 83.5910 12.9320 2.2725 0.8825
0.252 B 77.0460 18.7870 2.8145 1.0999
0.0136 a 94.4030 5.3644 0.2025 0.0165
143
in the two 239Pu spikes were 0.1547 and 0.2433 and these spikes are referred in this paper. The spikes were to as 239Pu-R1 and 239Pu-R2, respectively, calibrated by ID-TIMS using a chemical assay standard solution of plutonium with about 95 atom % 239Pu. The 242Pu spike was also calibrated using the same chemical assay standard as that used for the 239Pu spike materials. Different plutonium samples with low burn-up (about 1000 MWD/TU) containing uranium and fission products, were taken for determining the plutonium concentration. Five or six samples were taken from each of the dissolver solutions. Of these, two or three were spiked with 239Pu and the remaining were spiked with 242Pu. The amount of 239Pu spike added to the samples was such as to obtain a 240Pu/239Pu atom ratio in the spiked mixture corresponding to the geometric mean of the 240Pu/239Pu atom ratio in the unspiked sample and the spike. The amount of 242Pu spike added was such as to obtain a 242Pu/ 239Pu atom ratio in the spiked mixture close to unity. This was done with the objective of minimising the random errors in the factor (1 - R,/R,,)/( R, - R,) in Eq. (1). The spike samples were treated twice with cont. HNO, in order to break any polymer present, and were then subjected to a redox treatment using H,O, in 3M HNO,. The purification of plutonium was achieved by using 0.1 M TTA in benzene [3]. The plutonium was back-extracted from the organic phase using 7 M HNO,. The solution containing plutonium was evaporated to near dryness and was taken up in 0.5 M HNO, for mass spectrometric determination of isotope ratios. The 240Pu/ 239Pu or 242Pu/ 239Pu atom ratios in the purified spiked aliquots were determined mass spectrometrically using a double rhenium filament assembly. Plutonium ions were obtained by heating the ionising filament to a temperature corresponding to a current of 5-6 A, whereas the sample filament current ranged from 1.6-2 A. The exact temperature of the ionising filament was fixed in each mass spectrometric determination by monitoring the i*‘Re+ signal (3 X lo-‘* A). A single Faraday cup was used as a detector in all studies reported in this paper. The various parameters, such as sample acidity, sample amount, sample loading procedure, heating temperatures of the filaments and the time of data acquisition, were kept fixed as far as possible, with the objective of reproducing identical conditions in all the mass spectrometric determinations. In each determination, two runs of isotope ratios, each run consisting of a set of 12 scans, were taken. No correction was applied to the observed isotope ratios due to the K-factor or mass discrimination factor (K equals the certified isotope ratio/observed isotope ratio) [13] as the spikes were also calibrated by ID-TIMS and hence the K-factor is cancelled.
144
Determination -‘-;“Pu> 0. IS)
of plutonium
concentration
in high burn-up samples
(‘40Pu/
A plutonium solution with about 95 atom% 2’9Pu was used as a spike. This is referred to as 2.79Pu-N in this paper and data of the isotopic composition are given in Table 2. This was a working chemical assay standard which was calibrated earlier by using ID-TIMS and by electrochemical methods where a primary chemical assay standard of plutonium (SRM-949 d) was also analysed simultaneously. In fact, this working chemical assay standard was used for the calibration of ““Pu-RI, ‘39Pu-R2 and ‘42Pu spikes. samples with high burn-up (2 7000 Several different plutonium MWD/TU) were taken for the determination of plutonium concentration. Replicate aliquots were taken from each of the dissolver samples, spiked separately with ‘j9Pu-N and 242Pu, subjected to a redox treatment and taken up for mass spectrometric determination of the desired isotope ratio as discussed above. RESULTS
AND DISCUSSION
The results obtained on spike calibration using the same chemical assay standard of plutonium are shown in Table 3. It is seen that a relative precision of 0.1% is obtained in all cases. Thus the error due to uncertainty in the spike calibration in the present case is nearly the same while using
TABLE 3 Summary
of data on spike calibration
Aliquot no.
Concentration ‘“9PU_Rl
'39pu_~2
342pu
1 2 3 4 5 6
22.9488 22.9254 22.9451 22.9680 23.0070 22.9327
7.1204 a 7.0841 7.0951 7.0769 7.0800
6.8671 6.8946 6.8772 6.8821
Mean
22.9545 kO.027 h (0.12%)
7.0840 + 0.007 h (0.10%)
6.8802 _+O.OlO h (0.14%)
,’ Excluded
while calculating
h Standard
deviation
replicate
aliquots
of plutonium
of plutonium)
the mean concen!ration.
s calculated
taken.
(pg/g
from s2 = 1 (x, - .Y)2/( n -l), ,=,
where n is the number
of
145
either a 239~~ or a 242 Pu spike. This error can, however, be eliminated in the case of the 239Pu spike by using a chemical assay reference material. It may be noted that one of the values in 2”9Pu-R2 spike calibration is excluded based on the two statistical tests, viz Student’s t criterion and Grubb’s R criterion before calculating the mean. This is also in accordance with the experimental observation as the data on isotope ratios used for calculating the concentration for this aliquot were acquired after an appreciable delay of time (i.e. fifth and sixth runs compared to the common practice of taking the first two runs), as the first four runs did not yield the desired precision of better than 0.1%. Furthermore, the positive bias in the rejected value when compared to the mean is attributable to isotope fractionation which leads to an increase in the 240Pu/239P u a t om ratio. Such a critical examination of data is extremely important for high accuracy measurements since any error in the spike calibration constitutes a systematic error in the determination of
TABLE 4 Concentration as determined 742Pu spikes
of plutonium in various low burn-up samples ( 240Pu/23yPu atom ratio < 0.15) by isotope dilution-thermal ionisation mass spectrometry using 23’Pu-Rl and
S. no.
Sample code
ID-TIMS
DS-16 DS-17 DS-18 DS-19 DS-20 DS-21 DS-22 DS-23 DS-24 Mean relative precision a Mean
( ‘3yPu-Rl)
1.923+(0.34%)’ 3.140+ (0.08%) 3.395*(0.18%) 3.708 + (0.39%) 3.692 f (0.23%) 2.875+(0.07%) 2.719 f (0.24%) 2.850+(0.11%) 2.775 + (0.49%)
0.24%
ID-TIMS
(3) {3) (2) { 2) { 3) (2) (3) (2) {3)
ID-TIMS
(“‘Pu-RI)
ID-TIMS
ID-TIMS
’
(242Pu)
( 24’ Pu)
1.937+(0.12%) 3.152 f (0.06%) 3.39O_t(O.19%) 3.719 + (0.22%) 3.701 f (0.26%) 2.882+(0.11%) 2.741 k(O.ll%) 2.863+(0.01%) 2.774* (0.19%)
b (3) (3) (3) (2) (2) (2) (2) (3) { 3)
0.9928 0.9962 I.0015 0.9970 0.9976 0.9976 0.9920 0.9955 1.0004
0.14%
(239P~-R1)
ID-TIMS b Standard
Concentration of plutonium (pg/g of solution)
deviation
= 0.9967 + _ 0.0023 ‘. ,1 s calculated from s2 = c (x, - x)‘/(
(242Pu)
replicate
aliquots
taken and is given by the number
’ Gives
95% C.L. calculated
as ts/fi
t = 2.31 for 8 degrees of freedom.
where
in Fkets
s2 = ,g,(x,
n - 1) where n is the number
of
{}. - Z)2/(n
-1)
with
n = 9 and
146
the plutonium concentration of an unknown sample. Hence, it is mandatory that the time of data acquisition must, as far as possible, be kept the same. One of the ways of overcoming the problem of isotope fractionation is by integrating the ion current corresponding to each of the isotopes, over the total lifetime of the beam. For this purpose, it is preferable to use a multiple collector system existing in recently available commercial thermal ionisation mass spectrometers. However, this facility does not exist in the MAT-261 mass spectrometer existing in our laboratory. It may be noted that the integration method will increase the time of mass spectrometric analysis by an order of magnitude and hence cannot be adopted routinely. Another practical problem may be the limited lifetime of the ionisation filament compared to the time required for complete evaporation of the sample from the vaporisation filament, as the ionisation filament is heated to a higher
TABLE 5 Concentration as determined 242Pu spikes S. no.
of plutonium in various low burn-up samples ( 240Pu/239Pu atom ratio < 0.15) by isotope dilution-thermal ionisation mass spectrometry using 239P~-R2 and
Sample code
Concentration of ulutonium (pg/g of solutionj ID-TIMS
1 2 3 4 5 6 7 8 9 10
DS-6 DS-7 DS-8 DS-9 DS-10 DS-11 DS-12 DS-13 DS-14 DS-15
Mean relative precision a Mean
5.500*(0.01%) 5.800~(0.10%) 4.850 + (0.05%) 5.230*(0.03%) 5.572t_(O.17%) 5.526 f (0.03%) 5.921+ (0.08%) 6.265 f (0.07%) 5.711 &(O.OS%) 5.183 f (0.35%)
b (3) (2) { 3) (2) (3) (3) { 3) { 3) {2) (3)
0.09%
ID-TIMS
(239P~-R2)
ID-TIMS ’ Standard
( 239Pu-R2)
ID-TIMS ID-TI&
ID-TIMS
(*s9Pu-R21 a (242Pu)’
( 242Pu)
5.500*(0.09%) 5.805 + (0.04%) 4.860 i (0.04%) 5.252 + (0.10%) 5.569+(0.18%) 5.529 f (0.17%) 5.939+(0.11%) 6.287 + (0.16%) 5.714+(0.06%) 5.196 + (0.27%)
b {3}_ { 2) (2) (2) (3) (3) (2) {3) (3) { 3)
1 .oooo 0.9991 0.9979 0.9958 1.0005 0.9995 0.9970 0.9965 0.9995 0.9975
0.12% = 0.9983 + 0.0011 =.
(242Pu)
n from s2 = c (x, - X)2/( n - 1) where n is the number I=1 taken and is given by the number in irackets { ).
deviation
s calculated
replicate
aliquots
’ Gives
95% C.L. calculated
as ts/fi
t = 2.26 for 9 degrees of freedom.
where
s* = c (x, - x)*/(n !=I
- 1) with
n =lO
of
and
147
temperature ( - 2500 K) compared to that of the vaporisation filament (- 1200 K). The results of plutonium concentration on different irradiated fuel samples with low burn-up ( 240Pu/239Pu atom ratio < 0.15) using 239Pu-R1 and 239Pu-R2 as spikes in ID-TIMS are summarised in Tables 4 and 5, respectively. For comparison, the results obtained by ID-TIMS using 242Pu spike are also included in these tables. It is seen that 239Pu-R2 leads to a mean precision of 0.09% (Table 5) compared to a mean precision of 0.24% (Table 4) using 239Pu-R1 This is obviously due to the fact that the values for the 240Pu/239Pu atom ratio in the sample and the spike (239Pu-Rl) are quite close to each other which demands still better precision in the isotope ratio measurements. Furthermore, the factors of 0.9967 f 0.0023 and 0.9983 + 0.0011 (95% confidence limit) for the ratios ID-TIMS( 239Pu-R1)/IDTIMS( 242Pu) and ID-TIMS( 239Pu-R2)/1D-TIMS( 242Pu), respectively, show that the method is free from any bias within the experimental uncertainties. The results of plutonium concentration on different irradiated fuel samples with high burn-up ( 240Pu/239Pu 2 0.15) using 239Pu-N and 242Pu spikes
TABLE 6 Concentration as determined 242Pu spikes
of plutonium in various high burn-up samples ( 240Pu/239Pu atom ratio < 0.15) by isotope dilution-thermal ionisation mass spectrometry using 239Pu-N and
S. no.
240pu K
Concentration of plutonium (pg/g of solution)
atom ratio
ID-TIMS
0.1929 0.2433 0.2568 0.2869 0.2897
7.863 *(0.22%) 7.124*(0.10%) 35.916 f (0.36%) 14.788*(0.16%) 22.346 f (0.28%)
1 2 3 4 5
Sample code
DS-29 DS-30 DS-31 DS-32 DS-33
Mean relative precision
( 2’9Pu-N) ’ (4) (4) { 3) (3) (4)
ID-TIMS
(239Pu-N) a
ID-TIMS
ID-TIMS
(242Pu)
( 242Pu) h
7.880 f (0.21%) 7.139 f (0.05%) 35.860* (0.10%) 14.801+(0.12%) 22.342 + (0.15%)
(4) (3) (3) (3) { 3)
0.9978 0.9979 1.0016 0.9991 1.0002
0.13%
0.23%
B Mean
IDWTIMS (239pU-N) = 0.9993 f 0.0017 ‘. ID-TIMS (242Pu) ,I ’ Standard deviation s calculated from s2 = c (x, - X)2/(n ,=I replicate aliquots taken and is given by the number in pkets
- 1) where n is the number
’ Gives
- Z)2/(n
95% C.L. calculated
as ts/&
t = 2.78 for 4 degrees of freedom.
where
s2 = ,F,(x,
of
{}. - 1) with
n = 5 and
148 TABLE 7 Statistical
treatment
of the data in Tables 4-6
Data used from
Source of variation
Table 4
Between methods Between samples Error Total
Table 5
Table 6
Degrees of freedom
Sum of squares
Mean sum of squares
Calculated F values
8 8 17
0.00037 4.94405 0.00026 4.94468
0.00037 0.62 0.000033
11.43 a 18910
Between methods Between samples Error Total
1 9 9 19
0.00043 2.94488 0.00037 2.94568
0.00043 0.33 0.000042
10.40(NS)* 7873
Between methods Between samples Error Total
1 4 4 9
0.000023 1138.121 0.00189 1138.1226
0.000023 284.5302 0.00474
O.O47(NS) b 600591
1
a Significantly different at 1% and 5% significance levels. b (NS)* means not significant at the 1% significance level but significant level. (NS) means not significant at the 1% and 5% significance levels.
b
at th 5% significance
in ID-TIMS are summarised in Table 6. It is seen that mean precision values of 0.23% and 0.13% are obtained using 239Pu-N and 242Pu spikes, respectively. Furthermore, a factor of 0.9993 ) 0.0017 (95% confidence limit) for the ratio ID-TIMS ( 239Pu-N)/ID-TIMS( 242Pu) shows that the method does not have any bias. A statistical treatment of the data presented in Tables 4-6 was carried out by employing the method of analysis of variance(ANOVA), the details of which are shown in Table 7. It revealed that there is a significant difference at the 1% and 5% significance levels between the results obtained by spikes 239Pu-R1 and 242Pu. In the case of results obtained by the spikes 239Pu-R2 and 242Pu there is no significant difference at the 1% significance level but there is a significant difference at the 5% significance level. Also, there is no significant difference at the 1% and 5 % significance levels between the results more experiments by obtained by 239Pu-N and 242Pu spikes. However, independent laboratories on the use of the 239Pu spike are essential to arrive at any meaningful conclusion from this type of statistical analysis. The estimates of error in ID-TIMS using 239Pu and 242Pu spikes are given in Table 8. It is seen that the overall error is the same in the two cases. This is because the two main factors, viz (i) smaller isotope fractionation, and (ii) small change in the 240Pu/ 239Pu isotope ratio, while using 239Pu as a spike, work in the opposite direction. In other words, the disadvantage of a smaller
149 TABLE
8
Estimates
s.
of errors (lo)
Parameter
no.
Remarks
Error (W) IDMS using 239~"
IDMS using Pu
242
C’P
0.10
0.10
w,,( - 250 mg) W,( - 250 mg) R,-R,
0.02 0.02 0.10 (R y9=
0.02 0.02 0.15 (R2,/9~10R;/~)
2R;j9)
1 -(R,/R,,)
0.10
(At.Wt.),
0.10
0.10
(At.Wt.),,
0.10
0.10
(A.F. 240Pu) “p
0.05
0.05
T..F. 242Pu) “p (A.F.‘s9Pu),
0.05
0.05
0.24%
0.24%
Combined error
Spike calibrated with respect to chemical assay standard Error of 0.00005 g Error of 0.00005 g Error in R”’ is taken as three times the error in R O/9 due to isotope fractionation R0,(9=1/2Rf; R’,/’ << R;;,/’ Based on isotopic composition Based on isotopic composition Experimentally determined Experimentally determined Computed by error propagation
difference of R m from R, and R,, in the case of the 239Pu spike is more-or-less compensated for by the favourable feature of a smaller isotope fractionation. The present work, therefore, demonstrates the use of 239Pu as a spike in ID-TIMS for determining the plutonium concentration in a dissolver solution of irradiated fuels on a routine basis in the event of non-availability of enriched 242Pu or 244Pu. It may be mentioned that ID-TIMS using 239Pu as a spike is quite sensitive to any error in the 240Pu/ 239Pu atom ratio in the unspiked sample which may exist either due to poor precision or due to small cross-contamination. Thus, the consistency in the concentration data obtained by using a 239Pu spike provides confidence about the high reliability of the isotopic composition data particularly on the major isotopes 239Pu and 240Pu.
150
These isotopic composition data are extremely useful in isotope correlations, e.g. [ 242Pu X 239Pu/( 240Pu)2] vs. Pu/U, which provide valuable information regarding verification of the total plutonium at the input point of a reprocessing plant. However, optimum spiking (i.e. sample/spike ratio) is essential for achieving high precision and accuracy in determining plutonium concentration. This difficulty can be circumvented by using enriched 240Pu as a spike; this retains all the attractive features of 239Pu. CONCLUSIONS
The present work shows that under the optimum spiking conditions (i.e. sample/spike ratio) precision and accuracy in determining plutonium concentration using 239Pu as a spike in ID-TIMS are comparable to that obtained by using 242Pu. The use of 242Pu or 244Pu as spikes have the advantage that optimum spiking is not mandatory. Also, the use of a 244Pu spike can simultaneously provide data on the isotopic composition of plutonium. However, the availability of 242Pu and, in particular, of 244Pu is limited to a few international laboratories. The use of 239Pu as a spike offers the possibility of eliminating error due to spike calibration which has been identified as a major source of error in the international intercomparison experiment IDA-80 [8,9]. Furthermore, it has the potential of reducing the measurement uncertainties due to mass-dependent error sources such as isotope fractionation in the ion source. Moreover, the available isotope reference materials (SRM-947 and 948) with one unit of mass difference can be used for calibrating the instrument for isotope fractionation while measuring the 240Pu/239Pu ratio. There exists scope for improvement by employing recently available multi-Faraday-cups. ACKNOWLEDGEMENTS
The authors are thankful to Dr. M.V. Group and Dr. D.D. Sood, Head of the constant encouragement and keen interest to Sh. C.P. Singh and Sh. A.S. Rawat for of the work.
Ramaniah, Director, Radiological Fuel Chemistry Division for their in the work. Thanks are also due their help during the initial stages
REFERENCES 1 S.A. Chitambar, V.D. Kavimandan, B. Saha, S.K. Aggarwal, G. Periaswamy, K. Bagyalakshmi, P.M. Shah, P.A. Ramasubramanian, A.I. Almaula, H.C. Jain, C.K. Mathews and M.V. Ramaniah, BARC-865, Bhabha Atomic Research Centre, Bombay, 1976. 2 M.V. Ramaniah, H.C. Jain, S.K. Aggarwal, S.A. Chitambar, V.D. Kavimandan, A.I. Almoula, P.M. Shah, A.R. Parab and V.L. Sant, Nucl. Technol., 49 (1980) 121.
151 3 G. Chourasiya, SK. Aggarwal, R.K. Duggal, C.P. Singh, A.S. Rawat and H.C. Jain, Radiochim. Acta, 36 (1985) 187. 4 P. DeBievre, 4th SALE Program Participants Meeting, CONF-810738, July 8-9. 1981, p. 188. 5 J.D. Fassett and W.R. Kelly, Anal. Chem., 56 (1984) 550. 6 S.F. Marsh, R.M. Abernathey and J.E. Rein, Anal. Lett., 13 (1980) 1487. 7 Unpublished results from this laboratory. 8 W. Beyrich, W. Golly, G. Spannagel. P. DeBievre, W. Walters. M. Gallet, L.A. Machlan, J.W. Gramlich and J.D. Fassett, Proc. Int. Symp. Recent Adv. Nucl. Mater. Safeguards, IAEA/SM-260/33, Vienna, Austria, Nov. 1982. 9 W. Beyrich, W. Golly, G. Spannagel, P. DeBievre and W. Walters, The IDA-80 Measurement Evaluation Programme on Mass Spectrometric Isotope Dilution Analysis of Uranium and Plutonium, Vols. I, II and III, 1984, EUR 7990e-7992e. 10 P. DeBievre, J. Broothaerts, M. Gallet, A. Loopmans, E. Sattler and J. Van Audenhove, Proc. 17th Annu. Meet. INMM June 1976; Nucl. Mater. Manage.. 5 (1976) 478. 11 W. Lycke, M. Gallet, F. Peetermans. R. Damen, E. Bouwmeester, P. DeBievre and J. Van Audenhove, Natl. Bur. Stand. (U.S.) Spec. Publ., 582 (1980) 93. 12 S.K. Aggarwal. G. Chourasiya, C.P. Singh, A.S. Rawat, S.A. Chitambar and H.C. Jain, J. Radioanal. Nucl. Chem. Lett., 93 (1985) 141. 13 S.K. Aggarwal, A.I. Almaula, P.S. Khodade, A.R. Parab, R.K. Duggal, C.P. Singh, A.S. Rawat, G. Chourasiya, S.A. Chitambar and H.C. Jain, J. Radioanal. Nucl. Chem. Lett.. 87 (1984) 169.
137
EXPERIMENTAL EVALUATION OF PLUTONIUM-239 SPIKE FOR DETERMINING PLUTONIUM CONCENTRATION BY ISOTOPE DILUTION-THERMAL IONIZATION MASS SPECTROMETRY
S.K. AGGARWAL, H.C. JAIN Fuel Chemistry (First received
G. CHOURASIYA,
Division, Bhahha Atomic
R.K. DUGGAL.
RADHIKA
Research Centre, Tromhay,
12 June 1985; in final form 16 September
RAO and
Bombay 400 085 (India)
1985)
ABSTRACT The use of 239Pu as a spike in isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) for determining the plutonium concentration in irradiated fuels is demonstrated. The method is based on the high precision better than 0.1% determination of the 240Pu/23yPu atom ratio in the sample, the spike, and the spiked mixture. The precision and accuracy of the results obtained on different plutonium samples using a 239Pu spike are compared with those obtained using a 14*Pu spike. A mean precision of O.l-0.2% is achieved using a 2.7uPu or 242Pu spike and a factor close to unity is obtained for the ratio ID-TIMS ( 23”Pu)/ID-TIMS ( 14*Pu). This study provides a viable alternative spike material to those laboratories which do not have access to enriched 242Pu. which is generally used in determining the plutonium concentration by ID-TIMS, and at the same time, offers certain advantages over the use of a 242Pu or 244Pu spike.
INTRODUCTION
Determination of the plutonium concentration is important at different stages in the nuclear fuel cycle and, in particular, at the input point of a reprocessing plant. At this point, plutonium is associated with large quantities of uranium and fission products and the solution is highly radioactive. Further plutonium is present in very low concentrations and may be in different chemical forms (e.g. different oxidation states, or polymers). The composition of the dissolver solution of irradiated fuel therefore demands the selection of methods which give high precision and accuracy, require a small amount of sample and do not depend on quantitative separation and purification of plutonium. The isotope dilution methods using mass spectrometry [I] and alpha spectrometry [2,3] meet all these requirements and are therefore most commonly used for this purpose. Isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) using a 242Pu spike has been used for more than two decades and is capable of providing an accuracy of 016%1176/86/$03.50
0 1986 Elsevier Science Publishers
B.V
138
O.l-0.5%. A 244Pu spike has also been proposed [4,5] for determining the plutonium concentration by ID-TIMS and it has the potential of simultaneously providing data on the isotopic composition of plutonium. However, the production of enriched isotopes of 242Pu and 244Pu requires high flux isotope reactor (HFIR) and calutron facilities which exist only in a few countries. Thus, enriched 242Pu and, particularly, 244Pu are not easily accessible. With the availability of present generation thermal ionisation mass spectrometers capable of giving high internal precision (better than 0.1%) in the isotope ratio measurements ranging from 0.01 to 100, it was considered interesting to explore the utility of routinely using 239Pu, which is easily available from reactors, as an alternative to enriched 242Pu or 244Pu spikes. This involves the measurement of small changes in the 240Pu/239Pu atom ratio with high precision (better than 0.1%) and provides an alternative spike to those laboratories which do not have access to enriched 242Pu or 244Pu. Irrespective of the spike used, the two problems generally encountered in ID-TIMS of plutonium are: (i) incomplete chemical exchange between the sample and the spike isotopes, and (ii) isotope fractionation in the ion source during the mass spectrometric analysis. Incomplete chemical exchange between the sample and the spike isotopes leads to an unequal recovery of the two isotopes during the purification step and thus erratic results are obtained on replicate aliquots taken from the same plutonium solution (spike solution or unknown dissolver solution). An evaluation of chemical exchange treatments has been published by Marsh et al. [6], but this still remains the limiting factor, which at times leads to either totally inconsistent results or a large spread ( > 0.5%) in the concentration values. In our laboratory, complete chemical exchange is ensured by first treating the spiked mixture with concentrated nitric acid at least twice for the depolymerisation of plutonium and subsequently converting all the plutonium to Pu(IV) using H,O, in 3M HNO,. We have tried this procedure on more than 100 different plutonium samples and have not experienced any difficulty except for one or two cases where erratic results were obtained. This could be attributed to the incomplete depolymerisation, since on treating fresh spiked samples 4-5 times instead of twice with concentrated nitric acid, consistent results were obtained even in those samples. Isotope fractionation in the ion source of a thermal ionisation mass spectrometer is another problem and it is a much less understood function for plutonium due to the non-availability of absolute isotopic standards of 242Pu/ 239Pu ratios. Fractionation causes the measured isotope ratios to change with time. The extent of isotope fractionation is proportional to the mass difference between the isotopes and is dependent on sample composition and size, sample purity, sample acidity, sample loading procedure,
139
heating temperatures of the sample and ionisation filaments and the time of measurement. Isotope fractionation is difficult to control and is a cause of variable systematic error. With the advent of present generation high sensitivity thermal ionisation mass spectrometers capable of giving high internal precision (0.01%) in the isotope ratio measurements, it is now possible to study some of the factors influencing isotope fractionation. For example, in our laboratory, differences of 0.6% and 0.2% were observed in measuring *42Pu/ 239Pu and 240Pu/ 239Pu isotope ratios, respectively, when the amount of plutonium on the filament was changed from 4 pg to 25 ng [7]. From this viewpoint, the use of 239Pu as a spike has the advantage of less isotope fractionation in addition to giving a lower mass discrimination factor while using a secondary electron multiplier (SEM) as a detector. On the other hand, these effects are accentuated while using 242Pu or 244Pu spikes due to three and five mass unit differences, respectively, with respect to 239Pu. Moreover, the isotopic reference materials with the same mass difference as in the spiked sample are not available for determining the calibration factor. The two problems in the ID-TIMS of plutonium mentioned above are also evident in the round robin experiment IDA-80 conducted by the Safeguards Project at Karlsruhe [8,9]. A detailed description of this interlaboratory experiment has been published. Only 12 of the 31 participating laboratories could report the plutonium concentration values within 0.4% of the certified value and there were a number of laboratories with experience of more than five years whose values had larger deviations when compared with the reference value. In addition to smaller isotope fractionation, the use of 239Pu as a spike has certain other advantages. For example, in this case, the unknown dissolver solution need not be diluted, which eliminates the dilution errors. The dilution is mandatory while using 242Pu or 244Pu spikes since these enriched isotopes are expensive. Furthermore, the use of 239Pu as a spike will eliminate the errors involved in 242Pu or 244Pu spike calibration since a solution from a high purity stoichiometric compound of plutonium or plutonium metal, i.e. a chemical assay standard of plutonium, can be used directly as a spike. 239Pu (about 99 atom W) can be prepared by irradiating natural uranium in a reactor: 239Pu( > 90 atom %) and 239Pu( G 80 atom %a) can be obtained from irradiated fuels. These advantages in using a 239Pu spike were recognised in the 1970s and an attempt on its use is reported in literature [lO,ll]. In these studies, plutonium containing about 97% 239Pu was used as a spike for determining the plutonium concentration in undiluted input solutions having plutonium with 70% 239Pu. A spread of about 2% was observed in the results at that time. Feasibility studies using plutonium with 239Pu of - 95 atom % and 239Pu of - 99 atom % as spikes in ID-TIMS have also been reported from our laboratory [12].
140
This paper describes the experience gained in using 2’9Pu as a spike in ID-TIMS for determining the plutonium concentration in different plutonium samples over a period of about one year, presents the results obtained on different plutonium samples, compares them with those obtained by using an enriched 242Pu spike and discusses the advantages and limitations of using 239Pu as a spike. Choice of spike The ideal spike for determining the plutonium concentration by ID-TIMS should satisfy the following criteria. (i) It should be easily accessible to all international laboratories without any restrictions and should not be very expensive. (ii) Its mass number should be adjacent to the major isotope ( 239Pu) in the sample. This minimises the variable systematic error due to isotope fractionation in the ion-source as isotope fractionation is a mass-dependent effect. (iii) There should not be any isobaric interference at the mass number of the spike isotope. (iv) The spike isotope should have a sufficiently long half-life. This minimises the error in accounting for the radioactive decay of the isotope and thus avoids frequent fresh calibration of the spike solution. (v) The spike isotope should preferably be absent or least abundant in the sample so that random errors are minimised. The isotope itself should be highly enriched. (vi) It should b e possible to either completely eliminate the spike calibration by using a chemical assay standard of plutonium or sufficient amount of the enriched isotope should be easily available so that spike calibration can be checked independently by the methods based on other physical principles, e.g. by electro-chemical methods. This will eliminate the possibility of any systematic error due to spike calibration. These criteria along with different isotopes of plutonium which are available in weighable quantities are listed in Table 1. It can be seen that none of the isotopes satisfies all the criteria. Furthermore, 236Pu and 24’Pu are obviously not suitable candidates to be used as spikes. 23RP~ cannot be used routinely as a spike in ID-TIMS due to the ubiquitous isobaric 238U at 238Pu. Thus, 239Pu, 240Pu, 242Pu and 244Pu are the interference of isotopes which may be used as spikes. In fact, 242Pu has been used as a spike for more than two decades. 244Pu has also been proposed: this appears promising for obtaining simultaneously the data on the isotopic composition of plutonium in the sample. But, to the best of our knowledge, no detailed studies have been carried out in the use of this isotope on a routine basis.
141 TABLE
1
Criteria
for selection
S. no. 1
2
3 4 5 6
of the spike in ID-TIMS
Criterion
Spike isotope 23hpu
Accessibility to all the laboratories Adjacent to the major isotope ( 239Pu) in the sample Free from isobaric interference Half-life > 5 X 103y (decay < 0.01% per y) Absent/least-abundant in the sample Possibility of checking spike calibration by other methods
a + indicates
that the criterion
of plutonium a
23Rpu
239~~
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
240Pu
24’Pu
+
+
+
242~~
244
Pu
+
is satisfied.
240Pu and 239Pu are promising from the point of view of smaller isotope fractionation and the possibility of eliminating the spike calibration step. But the production of enriched isotopes of 240Pu, 242Pu and 244Pu requires high flux isotope reactor (HFIR) and calutron facilities which exist only in a few countries. Hence, under these constraints, one is left with no option but to use the easily available 239Pu isotope as a spike. Principle of the method The principle of ID-TIMS using 239Pu as a spike is similar to that using a 242Pu spike except for the measurement of the 240Pu/239Pu atom ratio instead of the 242Pu/239Pu atom ratio in the spiked mixture. It can be shown that if C denotes the plutonium concentration, W denotes the aliquot sizes, R denotes the 240Pu/239Pu atom ratios, (At.Wt.) denotes the average atomic weight of plutonium, and (A.F.) denotes the atomic fraction, then the plutonium concentration in the sample is given by
I- Rm/Rs, RnI - RS
(At.wt.), (At.Wt.),,
where the subscripts s, sp and m stand and the spiked mixture, respectively.
x (A.F.240P~),p
(1)
(A.F.~~~PU), for the unspiked
sample,
the spike
142 EXPERIMENTAL
Instrument The mass spectrometer used in the present work is a MAT-261 thermal ionisation mass spectrometer controlled by a HP-9835A calculator. In this instrument, the ion beam leaves the ion source via a 0.2 mm wide fixed slit, accelerated to an energy of 8 keV (used in the present work), and is projected at an angle of 26.5” into the 90” magnetic sector field, from which it emerges at the same angle. This arrangement doubles the dispersion of a conventional system with an ion path radius of 46 cm. A collector slit width of 0.6 mm is used which results in a resolution of about 500 (10% valley definition). Up to 13 samples can be analysed in one run due to the turret-type sample magazine. The instrument is equipped with a Faraday cup as well as a secondary electron multipier. A pressure of < 10-h torr in the ion source and a pressure of < lo-’ torr in the analyser region is obtained by using turbo molecular pump and vat-ion pumps. An abundance sensitivity of better than 10” is obtained. The sample (1 ~1) is loaded onto the sample filament by using a push-button type of micro-pipette and is dried by using a progammable heating device. Determination of plutonium / “‘Pu atom ratio < 0.15)
concentration
in low burn-up samples
(i.e. ‘40Pu
Two different plutonium samples containing about 80% 239Pu were used of the spikes were determined as the spike materials. Isotopic compositions and the data are given in Table 2. The 23xPu in these mass spectrometrically The 240Pu/239Pu atom ratios spikes was determined by alpha spectrometry.
TABLE 2 Isotopic
composition
Isotope
of plutonium
Atom percent 242Pu spike
2% Pu 239~~
240Pu 24’Pu 242Pu a Determined
in the 242Pu and 239Pu spikes
0.0226 ” 1.5186 2.9347 1.5402 93.9838 by alpha spectrometry.
239Pu spike Rl
R2
N
0.322 ’ 83.5910 12.9320 2.2725 0.8825
0.252 B 77.0460 18.7870 2.8145 1.0999
0.0136 a 94.4030 5.3644 0.2025 0.0165
143
in the two 239Pu spikes were 0.1547 and 0.2433 and these spikes are referred in this paper. The spikes were to as 239Pu-R1 and 239Pu-R2, respectively, calibrated by ID-TIMS using a chemical assay standard solution of plutonium with about 95 atom % 239Pu. The 242Pu spike was also calibrated using the same chemical assay standard as that used for the 239Pu spike materials. Different plutonium samples with low burn-up (about 1000 MWD/TU) containing uranium and fission products, were taken for determining the plutonium concentration. Five or six samples were taken from each of the dissolver solutions. Of these, two or three were spiked with 239Pu and the remaining were spiked with 242Pu. The amount of 239Pu spike added to the samples was such as to obtain a 240Pu/239Pu atom ratio in the spiked mixture corresponding to the geometric mean of the 240Pu/239Pu atom ratio in the unspiked sample and the spike. The amount of 242Pu spike added was such as to obtain a 242Pu/ 239Pu atom ratio in the spiked mixture close to unity. This was done with the objective of minimising the random errors in the factor (1 - R,/R,,)/( R, - R,) in Eq. (1). The spike samples were treated twice with cont. HNO, in order to break any polymer present, and were then subjected to a redox treatment using H,O, in 3M HNO,. The purification of plutonium was achieved by using 0.1 M TTA in benzene [3]. The plutonium was back-extracted from the organic phase using 7 M HNO,. The solution containing plutonium was evaporated to near dryness and was taken up in 0.5 M HNO, for mass spectrometric determination of isotope ratios. The 240Pu/ 239Pu or 242Pu/ 239Pu atom ratios in the purified spiked aliquots were determined mass spectrometrically using a double rhenium filament assembly. Plutonium ions were obtained by heating the ionising filament to a temperature corresponding to a current of 5-6 A, whereas the sample filament current ranged from 1.6-2 A. The exact temperature of the ionising filament was fixed in each mass spectrometric determination by monitoring the i*‘Re+ signal (3 X lo-‘* A). A single Faraday cup was used as a detector in all studies reported in this paper. The various parameters, such as sample acidity, sample amount, sample loading procedure, heating temperatures of the filaments and the time of data acquisition, were kept fixed as far as possible, with the objective of reproducing identical conditions in all the mass spectrometric determinations. In each determination, two runs of isotope ratios, each run consisting of a set of 12 scans, were taken. No correction was applied to the observed isotope ratios due to the K-factor or mass discrimination factor (K equals the certified isotope ratio/observed isotope ratio) [13] as the spikes were also calibrated by ID-TIMS and hence the K-factor is cancelled.
144
Determination -‘-;“Pu> 0. IS)
of plutonium
concentration
in high burn-up samples
(‘40Pu/
A plutonium solution with about 95 atom% 2’9Pu was used as a spike. This is referred to as 2.79Pu-N in this paper and data of the isotopic composition are given in Table 2. This was a working chemical assay standard which was calibrated earlier by using ID-TIMS and by electrochemical methods where a primary chemical assay standard of plutonium (SRM-949 d) was also analysed simultaneously. In fact, this working chemical assay standard was used for the calibration of ““Pu-RI, ‘39Pu-R2 and ‘42Pu spikes. samples with high burn-up (2 7000 Several different plutonium MWD/TU) were taken for the determination of plutonium concentration. Replicate aliquots were taken from each of the dissolver samples, spiked separately with ‘j9Pu-N and 242Pu, subjected to a redox treatment and taken up for mass spectrometric determination of the desired isotope ratio as discussed above. RESULTS
AND DISCUSSION
The results obtained on spike calibration using the same chemical assay standard of plutonium are shown in Table 3. It is seen that a relative precision of 0.1% is obtained in all cases. Thus the error due to uncertainty in the spike calibration in the present case is nearly the same while using
TABLE 3 Summary
of data on spike calibration
Aliquot no.
Concentration ‘“9PU_Rl
'39pu_~2
342pu
1 2 3 4 5 6
22.9488 22.9254 22.9451 22.9680 23.0070 22.9327
7.1204 a 7.0841 7.0951 7.0769 7.0800
6.8671 6.8946 6.8772 6.8821
Mean
22.9545 kO.027 h (0.12%)
7.0840 + 0.007 h (0.10%)
6.8802 _+O.OlO h (0.14%)
,’ Excluded
while calculating
h Standard
deviation
replicate
aliquots
of plutonium
of plutonium)
the mean concen!ration.
s calculated
taken.
(pg/g
from s2 = 1 (x, - .Y)2/( n -l), ,=,
where n is the number
of
145
either a 239~~ or a 242 Pu spike. This error can, however, be eliminated in the case of the 239Pu spike by using a chemical assay reference material. It may be noted that one of the values in 2”9Pu-R2 spike calibration is excluded based on the two statistical tests, viz Student’s t criterion and Grubb’s R criterion before calculating the mean. This is also in accordance with the experimental observation as the data on isotope ratios used for calculating the concentration for this aliquot were acquired after an appreciable delay of time (i.e. fifth and sixth runs compared to the common practice of taking the first two runs), as the first four runs did not yield the desired precision of better than 0.1%. Furthermore, the positive bias in the rejected value when compared to the mean is attributable to isotope fractionation which leads to an increase in the 240Pu/239P u a t om ratio. Such a critical examination of data is extremely important for high accuracy measurements since any error in the spike calibration constitutes a systematic error in the determination of
TABLE 4 Concentration as determined 742Pu spikes
of plutonium in various low burn-up samples ( 240Pu/23yPu atom ratio < 0.15) by isotope dilution-thermal ionisation mass spectrometry using 23’Pu-Rl and
S. no.
Sample code
ID-TIMS
DS-16 DS-17 DS-18 DS-19 DS-20 DS-21 DS-22 DS-23 DS-24 Mean relative precision a Mean
( ‘3yPu-Rl)
1.923+(0.34%)’ 3.140+ (0.08%) 3.395*(0.18%) 3.708 + (0.39%) 3.692 f (0.23%) 2.875+(0.07%) 2.719 f (0.24%) 2.850+(0.11%) 2.775 + (0.49%)
0.24%
ID-TIMS
(3) {3) (2) { 2) { 3) (2) (3) (2) {3)
ID-TIMS
(“‘Pu-RI)
ID-TIMS
ID-TIMS
’
(242Pu)
( 24’ Pu)
1.937+(0.12%) 3.152 f (0.06%) 3.39O_t(O.19%) 3.719 + (0.22%) 3.701 f (0.26%) 2.882+(0.11%) 2.741 k(O.ll%) 2.863+(0.01%) 2.774* (0.19%)
b (3) (3) (3) (2) (2) (2) (2) (3) { 3)
0.9928 0.9962 I.0015 0.9970 0.9976 0.9976 0.9920 0.9955 1.0004
0.14%
(239P~-R1)
ID-TIMS b Standard
Concentration of plutonium (pg/g of solution)
deviation
= 0.9967 + _ 0.0023 ‘. ,1 s calculated from s2 = c (x, - x)‘/(
(242Pu)
replicate
aliquots
taken and is given by the number
’ Gives
95% C.L. calculated
as ts/fi
t = 2.31 for 8 degrees of freedom.
where
in Fkets
s2 = ,g,(x,
n - 1) where n is the number
of
{}. - Z)2/(n
-1)
with
n = 9 and
146
the plutonium concentration of an unknown sample. Hence, it is mandatory that the time of data acquisition must, as far as possible, be kept the same. One of the ways of overcoming the problem of isotope fractionation is by integrating the ion current corresponding to each of the isotopes, over the total lifetime of the beam. For this purpose, it is preferable to use a multiple collector system existing in recently available commercial thermal ionisation mass spectrometers. However, this facility does not exist in the MAT-261 mass spectrometer existing in our laboratory. It may be noted that the integration method will increase the time of mass spectrometric analysis by an order of magnitude and hence cannot be adopted routinely. Another practical problem may be the limited lifetime of the ionisation filament compared to the time required for complete evaporation of the sample from the vaporisation filament, as the ionisation filament is heated to a higher
TABLE 5 Concentration as determined 242Pu spikes S. no.
of plutonium in various low burn-up samples ( 240Pu/239Pu atom ratio < 0.15) by isotope dilution-thermal ionisation mass spectrometry using 239P~-R2 and
Sample code
Concentration of ulutonium (pg/g of solutionj ID-TIMS
1 2 3 4 5 6 7 8 9 10
DS-6 DS-7 DS-8 DS-9 DS-10 DS-11 DS-12 DS-13 DS-14 DS-15
Mean relative precision a Mean
5.500*(0.01%) 5.800~(0.10%) 4.850 + (0.05%) 5.230*(0.03%) 5.572t_(O.17%) 5.526 f (0.03%) 5.921+ (0.08%) 6.265 f (0.07%) 5.711 &(O.OS%) 5.183 f (0.35%)
b (3) (2) { 3) (2) (3) (3) { 3) { 3) {2) (3)
0.09%
ID-TIMS
(239P~-R2)
ID-TIMS ’ Standard
( 239Pu-R2)
ID-TIMS ID-TI&
ID-TIMS
(*s9Pu-R21 a (242Pu)’
( 242Pu)
5.500*(0.09%) 5.805 + (0.04%) 4.860 i (0.04%) 5.252 + (0.10%) 5.569+(0.18%) 5.529 f (0.17%) 5.939+(0.11%) 6.287 + (0.16%) 5.714+(0.06%) 5.196 + (0.27%)
b {3}_ { 2) (2) (2) (3) (3) (2) {3) (3) { 3)
1 .oooo 0.9991 0.9979 0.9958 1.0005 0.9995 0.9970 0.9965 0.9995 0.9975
0.12% = 0.9983 + 0.0011 =.
(242Pu)
n from s2 = c (x, - X)2/( n - 1) where n is the number I=1 taken and is given by the number in irackets { ).
deviation
s calculated
replicate
aliquots
’ Gives
95% C.L. calculated
as ts/fi
t = 2.26 for 9 degrees of freedom.
where
s* = c (x, - x)*/(n !=I
- 1) with
n =lO
of
and
147
temperature ( - 2500 K) compared to that of the vaporisation filament (- 1200 K). The results of plutonium concentration on different irradiated fuel samples with low burn-up ( 240Pu/239Pu atom ratio < 0.15) using 239Pu-R1 and 239Pu-R2 as spikes in ID-TIMS are summarised in Tables 4 and 5, respectively. For comparison, the results obtained by ID-TIMS using 242Pu spike are also included in these tables. It is seen that 239Pu-R2 leads to a mean precision of 0.09% (Table 5) compared to a mean precision of 0.24% (Table 4) using 239Pu-R1 This is obviously due to the fact that the values for the 240Pu/239Pu atom ratio in the sample and the spike (239Pu-Rl) are quite close to each other which demands still better precision in the isotope ratio measurements. Furthermore, the factors of 0.9967 f 0.0023 and 0.9983 + 0.0011 (95% confidence limit) for the ratios ID-TIMS( 239Pu-R1)/IDTIMS( 242Pu) and ID-TIMS( 239Pu-R2)/1D-TIMS( 242Pu), respectively, show that the method is free from any bias within the experimental uncertainties. The results of plutonium concentration on different irradiated fuel samples with high burn-up ( 240Pu/239Pu 2 0.15) using 239Pu-N and 242Pu spikes
TABLE 6 Concentration as determined 242Pu spikes
of plutonium in various high burn-up samples ( 240Pu/239Pu atom ratio < 0.15) by isotope dilution-thermal ionisation mass spectrometry using 239Pu-N and
S. no.
240pu K
Concentration of plutonium (pg/g of solution)
atom ratio
ID-TIMS
0.1929 0.2433 0.2568 0.2869 0.2897
7.863 *(0.22%) 7.124*(0.10%) 35.916 f (0.36%) 14.788*(0.16%) 22.346 f (0.28%)
1 2 3 4 5
Sample code
DS-29 DS-30 DS-31 DS-32 DS-33
Mean relative precision
( 2’9Pu-N) ’ (4) (4) { 3) (3) (4)
ID-TIMS
(239Pu-N) a
ID-TIMS
ID-TIMS
(242Pu)
( 242Pu) h
7.880 f (0.21%) 7.139 f (0.05%) 35.860* (0.10%) 14.801+(0.12%) 22.342 + (0.15%)
(4) (3) (3) (3) { 3)
0.9978 0.9979 1.0016 0.9991 1.0002
0.13%
0.23%
B Mean
IDWTIMS (239pU-N) = 0.9993 f 0.0017 ‘. ID-TIMS (242Pu) ,I ’ Standard deviation s calculated from s2 = c (x, - X)2/(n ,=I replicate aliquots taken and is given by the number in pkets
- 1) where n is the number
’ Gives
- Z)2/(n
95% C.L. calculated
as ts/&
t = 2.78 for 4 degrees of freedom.
where
s2 = ,F,(x,
of
{}. - 1) with
n = 5 and
148 TABLE 7 Statistical
treatment
of the data in Tables 4-6
Data used from
Source of variation
Table 4
Between methods Between samples Error Total
Table 5
Table 6
Degrees of freedom
Sum of squares
Mean sum of squares
Calculated F values
8 8 17
0.00037 4.94405 0.00026 4.94468
0.00037 0.62 0.000033
11.43 a 18910
Between methods Between samples Error Total
1 9 9 19
0.00043 2.94488 0.00037 2.94568
0.00043 0.33 0.000042
10.40(NS)* 7873
Between methods Between samples Error Total
1 4 4 9
0.000023 1138.121 0.00189 1138.1226
0.000023 284.5302 0.00474
O.O47(NS) b 600591
1
a Significantly different at 1% and 5% significance levels. b (NS)* means not significant at the 1% significance level but significant level. (NS) means not significant at the 1% and 5% significance levels.
b
at th 5% significance
in ID-TIMS are summarised in Table 6. It is seen that mean precision values of 0.23% and 0.13% are obtained using 239Pu-N and 242Pu spikes, respectively. Furthermore, a factor of 0.9993 ) 0.0017 (95% confidence limit) for the ratio ID-TIMS ( 239Pu-N)/ID-TIMS( 242Pu) shows that the method does not have any bias. A statistical treatment of the data presented in Tables 4-6 was carried out by employing the method of analysis of variance(ANOVA), the details of which are shown in Table 7. It revealed that there is a significant difference at the 1% and 5% significance levels between the results obtained by spikes 239Pu-R1 and 242Pu. In the case of results obtained by the spikes 239Pu-R2 and 242Pu there is no significant difference at the 1% significance level but there is a significant difference at the 5% significance level. Also, there is no significant difference at the 1% and 5 % significance levels between the results more experiments by obtained by 239Pu-N and 242Pu spikes. However, independent laboratories on the use of the 239Pu spike are essential to arrive at any meaningful conclusion from this type of statistical analysis. The estimates of error in ID-TIMS using 239Pu and 242Pu spikes are given in Table 8. It is seen that the overall error is the same in the two cases. This is because the two main factors, viz (i) smaller isotope fractionation, and (ii) small change in the 240Pu/ 239Pu isotope ratio, while using 239Pu as a spike, work in the opposite direction. In other words, the disadvantage of a smaller
149 TABLE
8
Estimates
s.
of errors (lo)
Parameter
no.
Remarks
Error (W) IDMS using 239~"
IDMS using Pu
242
C’P
0.10
0.10
w,,( - 250 mg) W,( - 250 mg) R,-R,
0.02 0.02 0.10 (R y9=
0.02 0.02 0.15 (R2,/9~10R;/~)
2R;j9)
1 -(R,/R,,)
0.10
(At.Wt.),
0.10
0.10
(At.Wt.),,
0.10
0.10
(A.F. 240Pu) “p
0.05
0.05
T..F. 242Pu) “p (A.F.‘s9Pu),
0.05
0.05
0.24%
0.24%
Combined error
Spike calibrated with respect to chemical assay standard Error of 0.00005 g Error of 0.00005 g Error in R”’ is taken as three times the error in R O/9 due to isotope fractionation R0,(9=1/2Rf; R’,/’ << R;;,/’ Based on isotopic composition Based on isotopic composition Experimentally determined Experimentally determined Computed by error propagation
difference of R m from R, and R,, in the case of the 239Pu spike is more-or-less compensated for by the favourable feature of a smaller isotope fractionation. The present work, therefore, demonstrates the use of 239Pu as a spike in ID-TIMS for determining the plutonium concentration in a dissolver solution of irradiated fuels on a routine basis in the event of non-availability of enriched 242Pu or 244Pu. It may be mentioned that ID-TIMS using 239Pu as a spike is quite sensitive to any error in the 240Pu/ 239Pu atom ratio in the unspiked sample which may exist either due to poor precision or due to small cross-contamination. Thus, the consistency in the concentration data obtained by using a 239Pu spike provides confidence about the high reliability of the isotopic composition data particularly on the major isotopes 239Pu and 240Pu.
150
These isotopic composition data are extremely useful in isotope correlations, e.g. [ 242Pu X 239Pu/( 240Pu)2] vs. Pu/U, which provide valuable information regarding verification of the total plutonium at the input point of a reprocessing plant. However, optimum spiking (i.e. sample/spike ratio) is essential for achieving high precision and accuracy in determining plutonium concentration. This difficulty can be circumvented by using enriched 240Pu as a spike; this retains all the attractive features of 239Pu. CONCLUSIONS
The present work shows that under the optimum spiking conditions (i.e. sample/spike ratio) precision and accuracy in determining plutonium concentration using 239Pu as a spike in ID-TIMS are comparable to that obtained by using 242Pu. The use of 242Pu or 244Pu as spikes have the advantage that optimum spiking is not mandatory. Also, the use of a 244Pu spike can simultaneously provide data on the isotopic composition of plutonium. However, the availability of 242Pu and, in particular, of 244Pu is limited to a few international laboratories. The use of 239Pu as a spike offers the possibility of eliminating error due to spike calibration which has been identified as a major source of error in the international intercomparison experiment IDA-80 [8,9]. Furthermore, it has the potential of reducing the measurement uncertainties due to mass-dependent error sources such as isotope fractionation in the ion source. Moreover, the available isotope reference materials (SRM-947 and 948) with one unit of mass difference can be used for calibrating the instrument for isotope fractionation while measuring the 240Pu/239Pu ratio. There exists scope for improvement by employing recently available multi-Faraday-cups. ACKNOWLEDGEMENTS
The authors are thankful to Dr. M.V. Group and Dr. D.D. Sood, Head of the constant encouragement and keen interest to Sh. C.P. Singh and Sh. A.S. Rawat for of the work.
Ramaniah, Director, Radiological Fuel Chemistry Division for their in the work. Thanks are also due their help during the initial stages
REFERENCES 1 S.A. Chitambar, V.D. Kavimandan, B. Saha, S.K. Aggarwal, G. Periaswamy, K. Bagyalakshmi, P.M. Shah, P.A. Ramasubramanian, A.I. Almaula, H.C. Jain, C.K. Mathews and M.V. Ramaniah, BARC-865, Bhabha Atomic Research Centre, Bombay, 1976. 2 M.V. Ramaniah, H.C. Jain, S.K. Aggarwal, S.A. Chitambar, V.D. Kavimandan, A.I. Almoula, P.M. Shah, A.R. Parab and V.L. Sant, Nucl. Technol., 49 (1980) 121.
151 3 G. Chourasiya, SK. Aggarwal, R.K. Duggal, C.P. Singh, A.S. Rawat and H.C. Jain, Radiochim. Acta, 36 (1985) 187. 4 P. DeBievre, 4th SALE Program Participants Meeting, CONF-810738, July 8-9. 1981, p. 188. 5 J.D. Fassett and W.R. Kelly, Anal. Chem., 56 (1984) 550. 6 S.F. Marsh, R.M. Abernathey and J.E. Rein, Anal. Lett., 13 (1980) 1487. 7 Unpublished results from this laboratory. 8 W. Beyrich, W. Golly, G. Spannagel. P. DeBievre, W. Walters. M. Gallet, L.A. Machlan, J.W. Gramlich and J.D. Fassett, Proc. Int. Symp. Recent Adv. Nucl. Mater. Safeguards, IAEA/SM-260/33, Vienna, Austria, Nov. 1982. 9 W. Beyrich, W. Golly, G. Spannagel, P. DeBievre and W. Walters, The IDA-80 Measurement Evaluation Programme on Mass Spectrometric Isotope Dilution Analysis of Uranium and Plutonium, Vols. I, II and III, 1984, EUR 7990e-7992e. 10 P. DeBievre, J. Broothaerts, M. Gallet, A. Loopmans, E. Sattler and J. Van Audenhove, Proc. 17th Annu. Meet. INMM June 1976; Nucl. Mater. Manage.. 5 (1976) 478. 11 W. Lycke, M. Gallet, F. Peetermans. R. Damen, E. Bouwmeester, P. DeBievre and J. Van Audenhove, Natl. Bur. Stand. (U.S.) Spec. Publ., 582 (1980) 93. 12 S.K. Aggarwal. G. Chourasiya, C.P. Singh, A.S. Rawat, S.A. Chitambar and H.C. Jain, J. Radioanal. Nucl. Chem. Lett., 93 (1985) 141. 13 S.K. Aggarwal, A.I. Almaula, P.S. Khodade, A.R. Parab, R.K. Duggal, C.P. Singh, A.S. Rawat, G. Chourasiya, S.A. Chitambar and H.C. Jain, J. Radioanal. Nucl. Chem. Lett.. 87 (1984) 169.