Thermal fission yield of 137Cs in 233U

J. inorg, nucl. Chem. 1966, Vol. 28, pp, 1763 to 1767. Pergamon Press Ltd. Printed in Northern Ireland

T H E R M A L FISSION YIELD OF la~Cs IN 2~U* R. S. ONDREJCIN Savannah River Laboratory, E.I. du Pont de Nemours & Co., Aiken, South Carolina (Received 3 December 1965)

Abstract--The thermal neutron fission yield of 1~7Cs in 233U was found to be 6.13 ± 0.13 per cent. 235U and 2~U samples were irradiated in the ORNL Graphite Reactor. The samples were analysed radio-chemically to determine the amount of 137Cs formed per unit weight of uranium. In the case of the 235U samples, the number of fissions was also determined. The apparent number of fissions for both isotopes was calculated from flux monitor data. Curves for flux depression in the uranium samples were developed from reactor parameters and comparison of fission product gamma activity with thick and thin 23sU and 233U foils irradiated in an SRL reactor. From these curves, flux depression corrections were determined for the 233U samples so that the number of fissions could be calculated ; the validity of the flux correction was demonstrated with the 2~5U samples. MEASUREMENT o f the ~3rCs c o n t e n t o f spent r e a c t o r fuel is a widely a c c e p t e d m e t h o d o f d e t e r m i n i n g fuel e x p o s u r e o r b u r n - u p , F o r r e a c t o r fuel types in w h i c h the p r i m a r y s o u r c e o f fission is 2~5U, the m e t h o d is well f o u n d e d since the fission yield o f 137Cs has b e e n r e l i a b l y d e t e r m i n e d . ~a) H o w e v e r , f o r p o t e n t i a l fuel types t h a t utilize 233U as the fissionable nuclide, t h e d e t e r m i n a t i o n o f b u r n - u p t h r o u g h m e a s u r e m e n t o f 137Cs will be h a n d i c a p p e d by the u n c e r t a i n t y in the r e p o r t e d v a l u e s for the fission yield w h i c h r a n g e f r o m 5.39 to 7.16 p e r c e n t ~2-7) ( T a b l e 1). TABLE I . - - T H E R M A L FISSION YIELDS OF 137Cs IN 233U

Ivlethod

Fission yield (~)

Source

Mass spectrometer integral* Revised value for Ref. 2 Mass spectrometer isotope dilution Mass spectrometer integral* Radiochemical separation and 47r t3 count Mass spectrometer isotope dilutions Mass spectrometer selected corrected value

6"87 ± 0'36 7.16 ~: 0.21 5.80 -k 0.30 6.16 ~: 0.14 :5.39 i 0.11 6.58 ~: 0.20 6.64 =k 0.26

Ref. 2 Ref. 3 Ref. 4 Ref. 4 Ref. 5 Ref. 6 Ref. 7

* Relative yields measured, then yield of 187Cs calculated on the basis of one or more isotopes determined on absolute basis. * The information contained in this article was developed during the course of work under contract AT(07-2)-I with the U.S. Atomic Energy Commission. ~1~W. J. MAECK, R. M. AaERNATHYand J. E. REIN, Trans. Am. NucL Soc. (1965). 12~ W. FLEMIrq6, R. H. TOMLINSONand H. G. THODE, Can. J. Phys. 32, 522 (1954). ~3~Nucleonics Data Sheet No. 24, Nucleonics 16, (4), 79 (1958).. c4~ M. P. ANIKINA et. al. Proc. 2rid bit. Conf. Peaceful Uses Atomic Energy 15, 446 (1960) P/2040 USSR. ~s~D. C. SANTRYand L. YAFFEE,Can. J. Chem. 38, 421 (1960). t~) D. R. BIN1NOSTI,O. E. IRISHand R. H. TOMLINSON,Can. J. Chem. 39, 628 (1961), also Nucleonics 18, (11), 201 (1960). ~71R. L. FERGUSONand G. D. O'KELLEV, ORNL-3305 (July 1962). 1763

1764

R . S . ONDRE.ICIN

A t the S a v a n n a h River L a b o r a t o r y (SRL), d a t a were d e v e l o p e d t h a t p e r m i t t e d precise c a l c u l a t i o n o f the yield o f 13~Cs in the fission o f 233U. The d a t a were subs t a n t i a t e d b y the a37Cs yield in the fission o f ~35U. EXPERIMENTAL Discs of isotopically pure ~85U (99.71 per cent) and 28aU(99.76 per cent) were pickled in nitric acid, dried, weighed, and immediately encapsulated in high purity (99.9995 +per cent) aluminium. All uranium metal was in the form of 0-020-in. thick sheet. Each uranium sample was placed in an aluminium container, as shown in Fig. 1.* An aluminium~0.08 per cent cobalt alloy wire monitor

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FIG. 1.--Irradiation capsule. (0"030-in. dia. × 2-5 in.) containing about 40 mg of cobalt was sealed in a stainless steel tube and

placed in the milled groove on the outside of the container. The entire assembly was placed in the Oak Ridge National Laboratory (ORNL) Graphite Reactor at a flux of ~1012 n/(sec)(cm~) for three weeks. After irradiation with the ~sU and ~z~U samples, the cobalt wire monitors were counted for 6°Co and were then analysed for total cobalt content by a special analytical method. (8~ The analysis for total cobalt was required because of the nonuniform distribution in the wire. Because of the small volume and thermal flux depression of the samples, the monitors could not be used to measure the exposure in the conventional manner; they were used only to enable calculation of the apparent number of fissions in the equivalent samples of 2asU and 285U. Each sample of both 2~sU and 23~Uwas dissolved in nitric acid and analysed three to five times for 1~7Cs by tetraphenylboron separation and gamma pulse height analysis. A standard of 187Cs (standardized to 4-2 per cent at the 95 per cent confidence level by the National Bureau of Standards, Nuclear Chicago, and Vallecitos Laboratory) was also analysed in replicate with the samples. RESULTS AND DISCUSSION The n u m b e r o f fissions p e r g r a m o f 2asU was calculated f r o m the s a m p l e weight, the m e a s u r e d z37Cs content, a n d the k n o w n fission yield, tl~ A c o m p a r i s o n o f these values with the a p p a r e n t n u m b e r o f fissions (calculated f r o m 6°Co c o n t e n t o f the c o b a l t wire flux m o n i t o r s , T a b l e 2) shows the m a g n i t u d e o f the flux d e p r e s s i o n in the ~35U samples. * A second heavier sample was also placed in each container for other studies. Effects from these samples were compensated for in the flux-depression calculations. is) R. S. ONDREJCIN, Analyt. Chem. 36, 937 0964).

T h e r m a l fission yield of ~37Cs in 233U

1765

TABLE 2.--FISSIONS PER GRAM OF 885U Sample number

187Cs D e t e r m i n a t i o n * (fissions/g z~sU)

1 2 3 4 5 6 7 8

1.16 1"07 1-16 1"12 1"16 1"17 1"17 1"19

× 10 TM × 10 TM × 1018 × 1018 × 10 TM × 10 TM ~< 10 TM × 10 TM

8°Co D e t e r m i n a t i o n (fissions/g 285U) 2-87 2"75 2'90 2"97 2"86 2"87 2"98 3"14

× x × × × × × ×

10 TM 1018 10 TM 10 TM 10 TM 10 TM 10 TM 1018

* O n the basis of the ~37Cs s t a n d a r d the precision of the ~37Cs determination was 0.9 per cent at P = 0.95.

Corrections were made for flux depression caused by the relatively high density of 233U and zzsU in the irradiation capsules. Irradiations of the uranium isotopes and the cobalt wire monitors were made at a graphite moderator temperature of 125-150°C. The cadmium ratio for a 5-mil indium foil was 20. The index of the ratio of epithermal to thermal neutrons, WESTCOTT'S" r " factor, was calculated to be 0-03. WESTCOTT'S values ~9)for the thermal cross section (a) at 200°C, g values (accounting for the non-1/v character of the cross section at 200°C), and cobalt resonance integrals were used in the calculations. The dilute epicadmium fssion resonance integrals for ~3U and 235U were taken to be 720 and 270 barns, respectively. Maxwellian-averaged activation depressions in the flux were calculated for the thermal, the eipthermal-subcadmium, and resonance energy regions. These depression factors were computed by standard methods ~1°)and were verified by activation comparison of thick and thin samples in an SRL test reactor, where r is known. Comparisons were made for both ~asU and 23zU, and for both epicadmium and subcadmium components. No flux depressions were caused by the cobalt wire monitors. Figure 2 shows the calculated curves for both 235U and 2~U. In Table 3 the flux depression calculated in the 235U samples is compared with that measured by the gamma activity of n°Co and 13rCs. The yield of a37Cs in 235U fission was taken as 6-23 per cent. ~) The agreement within experimental accuracy between the ~37Cs:6°Co ratio of fissions/g ~35U and the calculated depression factor verifies the choice of the WzsxcoxT " r " factor and the method of calculating the flux depression. The number of fissions per gram of 2zzu was calculated from the data of Fig. 2 and from the measured cobalt wire monitor results. The data are shown in Table 4. The e~U samples were dissolved in nitric acid volumetrically diluted, and analysed for ~37Cs by the tetraphenylboron method. The data in the fourth column of Table 5 were calculated from the expression lzrCs disintegrations/(min)(ml) × 100 2187cs (fissions/ml) where 213,cs = 4-52 × 10-s rnin -a. To fission yield =

~9~ C. H. WEsv¢oa'r, W. H. WALKER a n d T. K. ALEXANDER, Proc. U.N. int. Conf. peaceful uses Atomic Energy 16, 70 (1958). cao~ N. P. BAUMANr~ a n d M. B. STROUD, Nucleonics 23 (8), 98 (1965).

1766

R.S.

ONDREJCIN

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235 U "r" Factor = 0.03

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r I 0.08 0.09 0.10 Weight of Uronium, g

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I 0.12

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TABLE 3.--MEASURED VS. CALCULATED FLUX DEPRE~IONS IN 2a~USAMPLES

Sample number

W e i g h t ~ssU (g)

1 2 3 4 5 6 7 8

0-1070 0"1006 0-1226 0"1267 0'0869 0'0817 0"1173 0"0982

D e p r e s s i o n factor 137Cs: 6°Co Activities* Calculated¢ 0-404 0"389 0"400 0"377 0"405 0"407 0"392 0'379

0"402 0"406 0"394 0-391 0-414 0"418 0"397 0"407

* A v e r a g e value = 0.394 4- 0.009 (P = 0"95, n = 8). I" A v e r a g e value = 0.404 4- 0.008 ( P = 0.95, n = 8) f r o m Fig. 2.

TABLE 4.--FISSIONS PER GRAM OF 2aaU

Sample number

Samples weight (g 2aaU)

9 10 11 12 13 14

0"1097 0"1163 0'0794 0'0811 0'1149 0.1247

C o wire (fissions/g ~3U) Measured Calculated 2"62 2"61 2'60 2"56 2"74 2-61

x x x x x x

1018 10 is 10 is 1018 1018 10 is

1"21 1"19 1"22 1"20 1-24 1-18

x x x x x x

1018 1018 10 is 10 is 10 is 10 is

1767

Thermal fission yield of 137Cs in 2s3U TABLE 5.--THERMAL FISSION YIELD OF l:lTfs IN 2aaU

Sample number 9 10 I1 12 13 14

233U Sample Fissions/ml 1~7Cs d/(min)(ml)* 1-33 1-38 0.968 0.973 1.42 1.47

× × × × × x

1018 1015 1015 1015 1015 1015

3.73 3-78 2.76 2.69 3"94 3.97

× × × x × x

106 106 106 106 l0 s l0 s

Fission yield of 137Cs ( ~ ) 6.20 6.06 6.30 6.12 6-13 5.98

* On the basis of the NBS 137Cs standard the precision of this determination was 0'9 per cent at P = 0.95. T h e a v e r a g e e x p e r i m e n t a l v a l u e f o r t h e t h e r m a l f i s s i o n y i e l d o f 1arCs in 23zU w a s 6.13 ~ 0"13 a t P = 0.95 f o r n

6.

Acknowledgement--The author thanks Dr. N. P. BAUMANNfor calculating the WESTCOTT "'r" factor plots and for helpful discussions.