Evidence of fissiogenic Cs estimated from Ba isotopic deviations in an Oklo natural reactor zone

Earth and Planetary Science Letters, 114 (1993) 391-396

391

Elsevier Science Publishers B.V., Amsterdam [CH]

E x p r e s s Letter

Evidence of fissiogenic Cs estimated from Ba isotopic deviations in an Oklo natural reactor zone Hiroshi Hidaka a Philippe Holliger b and Akimasa Masuda c a Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan o Centre d'Etudes Nucleaires de Cadarache, BP1, 13108 Saint-Paul-les-Durance, France c Department of Chemistry, University of Electro-communications, Chofu, Tokyo 182, Japan

Received September 25, 1992; revision accepted November 27, 1992

ABSTRACT Isotopic studies of many elements from the uranium ore natural nuclear reactors at Oklo provide useful information on the migration of radioactive nuclides. The fissiogenic isotopic composition of Ba is particularly interesting, as it is an important indication in the search for fissiogenic Cs. In this report we detail the detection of remarkable isotopic deviations of Ba in the Oklo samples and estimate the geochemical behaviour of fissiogenic Cs from excess Ba isotopes. Six samples systematically collected from borehole SF84 (zone 10) at the Oklo uranium mine have been analyzed. Isotopic deviations of Ba indicate the existence of fissiogenic Cs and Ba. A good correlation between the elemental abundance of Cs and isotopic abundances of excess X35Baand 137Ba suggests that fissiogenic 135Ba and 137Ba behaved as Cs rather than Ba.

I. Introduction T h e b e h a v i o u r of r a d i o a c t i v e n u c l i d e s in geological m e d i a is c u r r e n t l y of g r e a t i n t e r e s t b e c a u s e o f its r e l e v a n c e to p o s s i b l e l e a k a g e of rad i o n u c l i d e s into t h e b i o s p h e r e . In o r d e r to study the l o n g - t e r m b e h a v i o u r of r a d i o n u c l i d e s which c a n n o t be followed by l a b o r a t o r y e x p e r i m e n t s , we m u s t study t h e effects in existing " n a t u r a l a n a logues". U r a n i u m o r e f r o m O k l o in the R e p u b l i c o f G a b o n is o n e of t h e b e s t n a t u r a l a n a l o g u e s for r a d i o a c t i v e waste disposal, b e c a u s e l a r g e - s c a l e fission r e a c t i o n s have s p o n t a n e o u s l y o c c u r r e d within t h e o r e b o d y d u r i n g geologic time [1,2]. M a n y e l e m e n t s in s a m p l e s f r o m Oklo, e.g., Zr, Mo, Ru, Pd, Ag, Cd, Sn, T e a n d r a r e e a r t h e l e m e n t s ( R E E ) , show isotopic a n o m a l i e s d e r i v e d

Correspondence to: H. Hidaka, Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan.

from fission a n d / o r n e u t r o n c a p t u r e . A c c u r a t e isotopic m e a s u r e m e n t s m a k e it possible to d e t e r m i n e t h e c h a r a c t e r i s t i c s of t h e r e a c t o r s [3-6], a n d to d e d u c e t h e g e o c h e m i c a l b e h a v i o u r of fissiogenic nuclides in t h e o r e q u a n t i t a t i v e l y [7-9]. H o w e v e r , a few e l e m e n t s , such as Rb, Sr a n d Ba, have few isotopic a n o m a l i e s , in spite o f t h e i r high fission p r o d u c t yields. In t h e past, several isotopic studies w e r e c a r r i e d out to s e a r c h for fissiogenic alkaline a n d alkaline e a r t h e l e m e n t s , such as Rb, Sr, Cs a n d Ba, in ores f r o m the O k l o n a t u r a l reactors, b u t with little success [9]. B r o o k i n s et al. [10] c a r r i e d o u t Ba isotopic m e a s u r e m e n t s on O k l o s a m p l e s a n d d e t e c t e d a slight d e v i a t i o n in t h e 135Ba/t37Ba ratio. H o w e v e r , they could n o t d e t e c t fissiogenic 138Ba a n d did not clarify the r e t e n t i o n o f fissiogenic Cs a n d Ba. T h e e x t r e m e l y low retentivity of such fissiogenic elements, c o u p l e d with t h e high c o n t a m i n a t i o n of n a t u r a l e l e m e n t s f r o m the s u r r o u n d i n g s , m i g h t a c c o u n t for t h e low values o f possible a n o m a l i e s in t h e isotopic c o m p o s i t i o n s of t h e s e e l e m e n t s .

0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

392

The retentivity of fissiogenic nuclides in the ore depends upon several factors: diffusion coefficient, adsorption by organic matter [11], the complexation ability of the ion in question [12], pH and redox conditions in the ore [13], etc. Some nuclides with long-lived precursors in the fission chains behave differently from the end products, due to the difference in the chemical properties between the precursor and its endproduct; for example, precursor 99Tc (TI/2 = 2.1 × 10 5 yr) to 99Ru, and precursor 1265n (T1/2 = 1 × 10 5 yr) to 126Te [8,14]. Fissiogenic 135Ba and 137Ba have long-lived precursors (135Cs (T1/2 = 2.3 × 10 6 yr) and 137Cs (Tt/z = 30 yr), respectively) in their fission chains, and therefore there is a possibility that fissiogenic 135Ba and 137Ba have behaved differently from other Ba nuclides during the time that these two isotopes existed as long-lived Cs. The main purpose of this study is to clarify the retentivity of fissiogenic Cs and Ba and to define the chemical fractionation between fissiogenic Cs and Ba.

2. Samples The series of six samples used for this study were taken from the borehole SF84 at Oklo reactor zone 10. This reactor zone is located underground, at the southeast end of the Oklo ore body, and is in the process of being mined out [15]. The number of the sample (1400, 1469, 1480, 1485, 1492 and 1640) indicates the distance from the beginning of borehole in centimetres. Four samples (1469, 1480, 1485 and 1492) are from the reactor core, while samples 1400 and 1640 are host rocks located respectively below and above the reactor core, and from the so-called FA and FB layers. A detailed description of these samples has already been given [16].

3. Experimental procedures Each sample was leached by H N O 3 and the residue was decomposed by H F and HC10 4. The sample solution was dried and redissolved with 1 ml of 2 M HC1. This solution was loaded onto a cation exchange resin column (Dowex AG50WX8, 200-400 mesh, 2.5 ml). The separation of Ba from the sample solution was cariied out following the method of Eugster et al. [17]. For the

H. H1DAKAET AL. TABLE 1 Abundances of Cs, Ba, Nd and U in the Oklo samples Element Sample 1400

1469

1480

1485

1492

1640

Cs (ppm) Ba (ppm)

0.321 12.5 6.02 14.8 30.4 11.1 247 453 290 322 574 822 (1.35) (20.7) (-) (8.60) (30.6) ( - ) Nd (ppm) 224 357 144 382 575 1.95 ( < 6) (343) (135) (351) (550) (-) U(%) 2.99 17.2 7.06 14.9 24.3 0.240 The values in parentheses indicate concentrations of fissiogenic elements calculated from isotopic data.

purpose of determination of relative retentivity in the samples, Nd was also measured. It is generally known that fissiogenic R E E have been well retained in the Oklo reactor [8,9], therefore the retentivity of fissiogenic R E E is a good starting point with which to estimate the behaviour of other fissiogenic nuclides. The measurements for Ba, Nd and U isotopic ratios were made on a thermal ionization mass spectrometer equipped with an electron multiplier (JEOL JMS-05RB) and a VG Sector 54 mass spectrometer with a Daly detector. Elemental abundances of Ba, Nd and U were determined by isotope dilution and Cs concentration was determined by I C P - M S (VG Plasma Quad). The concentrations of elements are presented in Table 1.

4. Results and discussion Fissiogenic Cs cannot be directly distinguished from the total Cs fraction because Cs is a monoisotopic element. However, information on the behavior of fissiogenic Cs could be obtained from excess 135Ba and 137Ba, since the half lives of t35Cs and 137Cs are relatively long. As shown in Fig. 1, fission production and neutron capture reactions are the main contributions to isotopic anomalies. 135Ba, 137Ba and |38Ba are produced by fission, while the four other Ba isotopes are non-fissiogenic. 134Ba is a non-fissiogenic nuclide, but might have been produced in the Oklo ore body by neutron capture of 133Cs. However, the abundance of 136Ba has remained nearly the same, regardless of nuclear reactions. Taking into account this situation, the deviation of 134Ba/136Ba,

393

E V I D E N C E O F F 1 S S I O G E N I C Cs E S T I M A T E D F R O M B a I S O T O P I C D E V I A T I O N S

t35Ba/136Ba, t 3 7 B a / 1 3 6 B a a n d 138Ba/136Ba r a t i o s primordial

will indicate the amounts of fissiogenic Ba. The results of Nd and Ba isotopic measurements are given in Table 2. Mass fractionation corrections for the isotopic ratios were not carried out in this study, because it is difficult to define a normalization factor for correction of the Oklo samples. However, isotopic deviations of Oklo samples from terrestrial standard values, the 6 values in Table 2, largely exceed the deviation derived from mass fractionation a n d / o r mass discrimination which is of the order of 0.5-1%. As was expected, clear positive deviation by addition of fission products can be easily recognized: The increases of 135Ba/136Ba, 137Ba/136Ba and 138Ba/136Ba indicate that the addition of fission products has occurred and the e x c e s s 134Ba indicates the production of this isotope due to neu-

fissiogenic neutron

capture

._¢2 o. 0

130

132

134

135

136

137

138

m a s s number

Fig. 1. Ba isotopic composition and inventory of excess isotopes.

TABLE 2 Isotopic data on Ba, Nd and U in Oklo samples Isotope

Sample 1400

Ba Ba Ba Ba

134/136 135/136 137/136 138/136

0.31036-+51 0.83998_+108 1.4299_+20 9.1941_+160

Deviation (%o) 2 6134Ba +3.8-+1.7 613~Ba 0_+1.3 ~137Ba +1.1_+1.4 3138Ba +7.0_+1.8

STD 1 1469 0.31483-+57 0.90775_+159 1.5228_+32 9.5773_+178 +18.3_+1.8 +80.7_+1.9 +66.2_+2.2 +49.0_+2.0

Relative retentivity (%) 3 135Ba 137Ba 138Ba

2.2 3.2 14.2

Nd (atom %) 142 26.67 _+6 143 12.47_+3 144 24.00_+5 145 8.53_+2 146 17.08_+4 148 5.782 _+1 150 5.473 _+2

1.033 _+4 25.86_+4 28.55_+5 18.25_+3 14.90_+2 8.04 _+1 3.364 _+5

U 235/238

0.007254_+15

0.006048_+9

1480

1485

0.31028_+70 0.84238_+144 1.4303_+34 9.1290_+158 +3.6-+2.2 +2.9_+1.7 +1.4_+2.4 -0.1_+1.7 <0.1 <0.1 0 1.666 _+6 24.35_+4 29.68_+8 17.90_+3 15.04_+4 8.01 _+2 3.364 _+7 0.005069_+7

1492 0.31326_+62 0.98199_+152 1.5596_+36 9.1986 _+179

1640 0.31365_+80 1.04218_+192 1.6182_+35 9.4483 -+ 123

+13.2_+2.0 +169.1_+1.8 +91.9_+2.5 +7.5+2.0

+14.5_+2.5 +240.8_+2.2 +133.0_+2.4 34.9_+1.3

3.3 3.3 1.6

5.3 5.3 8.1

2.166 _+9 24.86-+4 28.82_+6 17.81 _+4 14.97_+3 7.93 _+1 3.440 _+7 0.005649_+12

1.436 _+7 25.54-+5 28.76_+11 18.11 _+7 15.08_+3 7.86 _+4 3.222 _+2 0.005797-+17

0.30902 _+61 0.83970_+ 141 1.4301 _+18 9.1254 _+116

0.30918_+34 0.83994 _+81 1.4283 _+15 9.1298 + 88

-0.5_+2.0 -0.3_+1.7 +1.3_+1.2 -0.5_+1.3 0 0 0 28.45 _+10 11.95-+4 23.33_+8 8.19_+3 17.03_+6 5.66 _+2 5.39 _+2

27.19 _+6 12.18-+3 23.80_+4 8.29_+2 17.17_+3 5.74 _+1 5.62 _+1

0.007250_+10 0.007252_+13

Standard errors are 20- m. a STD values were obtained from the measurements of standard solution produced by Wako Pure Chemical Industries Ltd. 2 ~iBa =[( tB a / 136 B a ) s a m p l e / (t B a / 136 Ba)STD- 1]x 1000. 3 The relative retention factor to the retention of Nd in each sample (see text).

394

tron capture by 133Cs. Samples 1469, 1485 and 1492 show large isotopic anomalies for Ba, while excess Ba isotopes in sample 1480 could not be clearly observed in spite of its being a sample from the reactor core. We believe that these remarkable isotopic deviations of Ba in Oklo samples are the first to be reported. It would appear that one of the reasons why we have been able to obtain such clear results is that the abundance of "normal" elements have been maintained at a low level in this particular reactor. The observed compositions resulted from the mixture of fissiogenic and natural isotopes. The amounts of fissiogenic and natural fractions in each element are distinguishable from the total, if it is assumed that the abundance of an isotope, which is hardly influenced by fission a n d / o r neutron capture, has remained constant. Simple subtraction of natural isotopic compositions for Ba from the total leads to the approximate fissiogenic composition retained in the reactor (Fig. 1). In this respect, ]36Ba can be used for the estimation of the amount of fissiogenic Ba. Figure 2 shows the patterns of fissiogenic Ba compositions obtained after simple subtraction. The patterns of three samples, 1469, 1485 and 1492 (Figs. 2B-D), differ from the theoretical fission pattern shown in Fig. 2A and, moreover, are different from each other. Three fissile nuclides, 235U, 238U and 239pu, contribute fission in the Oklo reactor and these fission proportions are somewhat different for each sample. In practice, these proportions of 235U, 238U and 239pu a r e determined from the fissiogenic isotopic ratios l°2Ru/l°lRu, l°4Ru/ l°lRu and (155Gd +156Gd)/(L~VGd + 158Gd) in each sample (Table 3). It is reasonable to conclude that the differences among these four patterns in Fig. 2 are caused by the difference in geochemical behaviour between Cs and Ba. Fissiogenic ~35Ba and ~37Ba might have behaved as Cs because of the existence of long-lived precursors in the fission chains, while fissiogenic 138Ba, which has no long-lived precursor in its fission chain, behaved as Ba. In addition, it can be seen that the abundance of excess t35Ba is nearly the same as that of ~37Ba in each sample, despite the large difference in half-lives between precursor 135Cs and 137Cs. There is little chemical fractionation between fissiogenic 135Ba and 137Ba, while the retention of

H. HIDAKA

(A)fission for 235U 6.5 0

6.3

,..........

:.:+:+:

.. .. .. .. .. .. .. .. .. .. :.:.:.:+

.:+:+:

. ...... .......,.

:.:+:+:

:::::::::::

135

137

(C)1485 .142

.131

3

('~sCs) ('~'Cs)

==

(B)1469 .448

6.7

.......... ..........

....... ....... ......

138

.068

.095

135

137

138

(D)1492

>*

.319 .069

:::::::::::

ET AL.

.202

.190

135

137

:5::::::::

i i i!i i l i 135

137

138

138

m a s s number

Fig. 2. Fission patterns of Ba evaluated from isotopic data. (A) Neutron-induced fission of 235U by England and Rider [18]. (B) Sample 1469. (C) Sample 1485. (D) Sample 1492. For convenience, theoretical fission data is represented by the fission data from 235U (A), but two other fissile nuclides, 23Su and 239pu also contribute fission in the Oklo samples (see text). Shaded columns = isotopes 135Ba and 137Ba in (B-D). These isotopes possibly behaved as Cs and differently the isotope 138Ba (unshaded columns). The numbers in (A) indicate fission yields, and those in (B-D) indicate the differences in isotopic ratios between the sample and the natural standard.

t3SBa is considerably different from that of both t35Ba and ~37Ba. It is of interest to speculate on the possibility that most of the chemical fractionation between Cs and Ba occurred during the first stage of reactor operation, rather than after the reactor stopped working. If the chemical fractionation between Cs and Ba had occurred after the cessation of the reactor, there should be some measurable difference in the isotopic abundance between excess 135Ba and t37Ba. In this case, the

TABLE 3 Relative proportions of fission events Event

Sample 1469

1480

1485

1492

e35U(thermal) 0.889_+5 0.906_+10 0.924_+8 0.924_+8 e3Su (fast) 0.074 0.051 0.038 0.049 239pu (thermal) 0.037 0.043 0.038 0.027 Errors refer to uncertainties in the last significant figures. Errors of less than one half unit of the last order are omitted.

EVIDENCE

OF FISSIOGENIC

Cs E S T I M A T E D

retentivity of t37Ba would be similar to that of 138Ba, because 137Cs becomes stable 137Ba after the reactor ceases working. The retentivity of 138Ba varies from 1.6 to 14.2 in this reactor. These data of 138Ba reveal that fissiogenic Ba has been greatly redistributed in the reactor. As described previously, Brookins et al. [10] could not successfully find fissiogenic t38Ba in spite of the detection of fissiogenic t35Ba and t37Ba in the same samples for their study. In this paper, a retentivity for fissiogenic nuclide in the Oklo ore is defined as a "relative retentivity" to that of Nd. The calculation of relative retentivity is as follows: R i = (CilCNd) X (YNd/Yi)

395

F R O M Ba I S O T O P I C D E V I A T I O N S

X (MNa/M/)

where R i = the relative retentivity of nuclide i; lind, Y, = the fission product yield of Nd and i, respectively [18]; C N d , C i = the abundance of fissiogenic Nd and i, respectively; MNd , m i = the atomic mass of Nd and i, respectively. The results are listed in Table 2. The retentivities of fissiogenic Ba are variable even within the reactor core. Fissiogenic Ba appears to be preserved to some degree in the reactor core, but not out of the reactor (FA and FB layer). It is clear that most of the nuclides have diffused within the reactor core and that they show a slight concentration towards the outside with respect to the center of the reactor. It therefore appears that the retentivities of fissiogenic Ba are higher at the boundary between the reactor core and the adjacent layers (FA a n d / o r FB). The major reason for this p h e n o m e n a would appear to be the neoformation of apatites within the reactor, which preferentially retained fissiogenic alkaline, alkaline earth and rare earth elements [publication in preparation]. The reason for the difference between sample 1480 and the other three reactor core samples in terms of detectability of fissiogenic Ba may be explained by the gradient of diffusion in this reactor and the existence of neoformed apatite. Moreover, the containment of these elements, especially Cs, was enhanced in the organic-rich samples 1485 and 1492. In order to understand the contribution of precursor Cs to fissiogenic Ba, it is necessary to study the correlation between the degree of excess Ba and the elemental abundance of Cs

(A) ~35Ba(.), '3'Ba(o) 8 1492

8 1492

1485 8

1485 :=

o

$

o

14~9

1

148o

1480

o CS

(ppm)

(xlO'ppm)

Ba

(B) '38Ba(-) 15

14•9

14e9 1492

10

1492

~s 1485

1485

1480

0

• 1() Cs

2'0

3'0

(ppm)

i

2 3 d g Ba

(xlO=ppm)

Fig. 3. Relationship between retentivities of fissiogenic Ba and the elemental abundances of Ba and Cs. Retentivities are given by the proportions of the amounts of (A) fissiogenic 13SBa and 137Ba, and (B) 13SBa, to the calculated values of fissiogenic Nd (see text).

a n d / o r Ba. It may be possible to clarify the relationship between fissiogenic Cs and natural Cs by use of retentivity values evaluated from isotopic abundances. Figure 3 shows a relationship between the abundance of Cs and those of excess 135Ba, 137Ba and t38Ba. Since the amount of fissiogenic Ba isotopes in each sample depends upon the uranium content and the degree of fission, comparison of the absolute amount of fissiogenic Ba between samples is meaningless for understanding the behaviour of fissiogenic Ba. On the one hand, a good correlation between Cs concentration and the retentivities of fissiogenic 135Ba and 137Ba (Fig. 3A) suggests fissiogenic t35Ba and 137Ba behaved as Cs rather than as Ba. On the other hand, the retentivity of fissiogenic 138Ba is better correlated with Ba than Cs (Fig. 3B). Further, it can be presumed that the observed Cs (133Cs) is largely derived from fission and only a small amount of natural Cs is present in the reactor. Judging from the data from sample 1480, which contains little fissiogenic Ba, the amount of natural Cs in this reactor is at most 6 ppm. For the other samples, the increase in Cs

396

abundance with e x c e s s 135Ba and 137Ba (Fig. 3A) suggests the existence of fissiogenic 133Cs.

H. H I D A K A E T A L .

5

5. Conclusion The geochemistry of radionuclides, especially their migration processes in the geosphere, are important for the evaluation of the long-term safety of a radioactive waste storage. The information relevant to radionuclide mobility must be largely obtained from natural analogues. Based upon the deviations of Ba isotopic abundances in a series of samples from fission reactor SF84 (zone 10), we have been able to find a chemical fractionation between fissiogenic Cs and Ba in this reactor. In addition, our data on the lack of fractionation between fissiogenic 135Ba and 137Ba in reactor core samples, suggests that the major part of the fractionation between Cs and Ba possibly occurred during the early stage of reactor operation.

Acknowledgements We wish to thank Mr. K. Suzuki for his assistance with I C P - M S measurements and Mr. R. Oliver for the reading of the manuscript. This work was supported by the "Oklo as a Natural Analogue" Working Group, organized by the CEC (Commission of the European Communities) and CEA (Commissariat ~ l'Energie Atom-

ique ). References 1 International Atomic Energy Authority, The Oklo Phenomenon, STI/PUB/405, 647 pp., IAEA, Vienna, 1975. 2 International Atomic Energy Authority, Natural Fission Reactors, STI/PUB/475, 753 pp., IAEA, Vienna, 1978. 3 J.C. Ruffenach, J. Menes, C. Devillers, M. Lucas and R. Hagemann, Etudes chimiques et isotopiques de l'uranium, du plomb et de plusieurs produits de fission dans un &hantillon de mineral du r6acteur naturel d'Oklo, Earth Planet. Sci. Len. 30, 94-108, 1976. 4 M. Loubet and C.J. All~gre, Behavior of the rare earth

6

7

8

9

10

11

elements in the Oklo natural reactor, Geochim. Cosmochim. Acta 41, 1539-1548, 1977. P. Holliger and C. Devillers, Contribution a l'&ude de la temperature darts les r6acteurs fossiles d'Oklo par la mesure du rapport isotopique du lutetium, Earth Planet. Sci. Lett. 52, 76-84, 1981. H. Hidaka and A. Masuda, Nuclide analyses of rare earth elements of the Oklo uranium ore samples: a new method to estimate the neutron fluence, Earth Planet. Sci. Lett. 88, 330-336, 1988. J.R. De Laeter, K.J.R. Rosman and C.L. Smith, The Oklo natural reactor: Cumulative fission yields and retentivity of the symmetric mass region fission products, Earth Planet. Sci. Lett. 50, 238-246, 1980. D.B. Curtis, T.M. Benjamin, A.J. Gancarz, R.D. Loss, K.J.R. Rosman, J.R. De Laeter, J.E. Delmore and W.J. Maeck, Fission product retention in the Oklo natural fission reactors, Appl. Geochem. 4, 49-62, 1989. H. Hidaka, T. Konishi and A. Masuda, Reconstruction of cumulative fission yield curve and geochemical behaviors of fissiogenic nuclides in the Oklo natural reactors, Geochem. J., in press, 1992. D.G. Brookins, M.J. Lee, B. Mukhopadhyay and S.L. Bolivar, Search for fission-produced Rb, Sr, Cs and Ba at Oklo, in: The Oklo Phenomenon, IAEA, Vienna, 401-413, 1975. B. Nagy, F. Gauthier-Lafaye, P. Holliger, D.W. Davis, D.J. Mossman, J.S. Leventhal, M.J. Rigali and J. Parnell, Organic matter and containment of uranium and fissiogenic isotopes at the Oklo natural reactors, Nature 354, 472-475, 1991.

12 G.R. Choppin, Humics and radionuclide migration, Radiochim. Acta 44/45, 23-28, 1988. 13 D.G. Brookins, Geochemical Aspects of Radioactive Waste Disposal, Springer, Berlin, 1984. 14 R.D. Loss, J.R. De Laeter, K.J.R. Rosman, T.M. Benjamin, D.B. Curtis, A.J. Gancarz, J.E. Delmore and W.J. Maeck, The Oklo natural reactors: cumulative fission yields and nuclear characteristics of Reactor Zone 9, Earth Planet Sci. Lett. 89, 193-206, 1988. 15 V. Ledee, Suivi des travaux miniers et ~chantillonnage: mission ~ Oklo de septembre 1989 ~ novembre 1990, Rep. CEA/IPSN, 130 pp., 1991. 16 H. Hidaka, P. Holliger, H. Shimizu and A. Masuda, Lanthanide tetrad effect observed in the Oklo and ordinary uraninites and its implication for their forming processes. Geochem. J., in press, 1992. 17 O. Eugster, F. Tera and G.J. Wasserburg, Isotopic analyses of barium in meteorites and in terrestrial samples. J. Geophys. Res. 74, 3897-3908, 1969. 18 T.R. England and B.F. Rider, Chainy for MITO-TAL, Los Alamos Rep. LA-UR-88-1696, 1988.