The Oklo natural reactors: cumulative fission yields and nuclear characteristics of Reactor Zone 9

Earth and Planetary Science Letters, 89 (1988) 193-206 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

193

[1]

The Oklo natural reactors: cumulative fission yields and nuclear characteristics of Reactor Zone 9 R.D. Loss 1,*, J.R. De Laeter 1,* * K.J.R. Rosman 1, T.M. Benjamin 2, D.B. Curtis z, A.J. Gancarz z, J.E. Delmore 3 and W.J. Maeck 3 i Department of Applied Physics, Curtin University of Technology, Perth, W.A, 6001 (Australia) 2 lsotope Geochemistry Group, Los Alamos National Laboratory, N M 87545 (U.S.A.) 3 ldaho National Engineering Laboratory, Idaho Falls, ID 83401 (U.S.A.)

Received December 4, 1987; revised version accepted April 21, 1988 The isotopic composition of molybdenum, ruthenium, palladium, silver, cadmium, tin, tellurium, neodymium and uranium have been measured by solid source mass spectrometry in eight uraninite samples from Reactor Zone 9 at the Oklo natural reactors. Cumulative fission yields have been derived for most of these elements, after correcting where necessary for the primordial component of the dement concerned. Neutron capture reactions on a number of nuclides with significant thermal cross sections, and fission chains in which one of the precursor nuclides has a lengthy half-life, have also been examined to provide information on the relative mobilities of the elements involved. The Pd fission yields have been utilised to calculate the proportions of 235U, 238U and 239pu fission to be approximately 88%, 8% and 4% respectively in Reactor Zone 9, It has also been shown that nearly 50% of the fissioning 235U nuclides were produced from the a-decay of 239pu. The integrated neutron flux in this zone was calculated to be approximately 3.6 × 10 20 n cm -2. Estimates of the spectral index show that approximately two-thirds of the neutrons in Zone 9 were thermalised. Although it is unlikely that the Oklo reactors operated continuously, this study has shown that the duration of criticality in Zone 9 was approximately 2.2 × l0 s years. During this period the average fission density was 0.92 × 10 20 fissions cm 3, which represent a total energy output of 4 × 10 8 J g a of sample with an average power output of 8.1 × 10 -5 W g 1 of sample. The accurate analysis of the isotopic abundances of nine elements, has enabled the nuclear characteristics of Reactor Zone 9 to be established, thus providing a base for estimating the nuclear inventory of the various fission products. This will allow a comparison to be made with the present-day elemental abundances and hence an evaluation of the mobility of these elements in the geological environment at Oklo.

1. Introduction E f f o r t s to e v a l u a t e t h e e f f e c t i v e n e s s o f g e o l o g i cal m e d i a in c o n t a i n i n g r a d i o a c t i v e w a s t e s are s e v e r e l y c o n s t r a i n e d in t h a t t h e t i m e p e r i o d s inv o l v e d are g r e a t e r t h a n t h a t r e c o r d e d b y h u m a n experience. Although many fission products decay to l o w levels in t e n s o f years, a n u m b e r o f l o n g e r l i v e d r a d i o n u c l i d e s , like 137Cs a n d 9°Sr, t a k e h u n d r e d s o f y e a r s b e f o r e t h e y a r e e f f e c t i v e l y extinct. H o w e v e r , t h e a c t i n i d e s , w i t h t h e i r l o n g e r half-lives, t a k e h u n d r e d s o f t h o u s a n d s to m i l l i o n s

* Present address: Geological Research Division, Scripps Institute of Oceanography, University of California, San Diego, CA 92093, U.S.A. ** Author to whom correspondence should be addressed. 0012-821x/88/$03.50

© 1988 Elsevier Science Publishers B.V.

o f y e a r s to d e c a y to l o w levels, so t h a t n o a r t i f i c i a l r e p o s i t o r y c a n p r o v i d e d e f i n i t i v e a n s w e r s to the long-term problems of radioactive waste containment. I n 1972 it w a s d i s c o v e r e d t h a t fission r e a c t i o n s spontaneously occurred some two billion years a g o in t h e u r a n i u m m i n e at O k l o , w h i c h is s i t u a t e d in t h e s o u t h e a s t p a r t o f G a b o n in S o u t h W e s t A f r i c a [1]. T h e p o s s i b i l i t y o f n a t u r a l n u c l e a r react o r s o c c u r r i n g in t h e p a s t h i s t o r y o f t h e e a r t h h a d b e e n e x a m i n e d b y K u r o d a [2] f o l l o w i n g a suggest i o n b y W e t h e r i l l a n d I n g h r a m [3]. S i n c e t h e disint e g r a t i o n r a t e o f 235U is g r e a t e r t h a n t h a t o f 238U, t h e a b u n d a n c e o f 235U d e c r e a s e s w i t h t i m e r e l a t i v e t o 238U. I f o n e e x t r a p o l a t e s b a c k t w o b i l l i o n y e a r s ago, the 235U a b u n d a n c e w a s a p p r o x i m a t e l y 3.65% as c o m p a r e d to its p r e s e n t - d a y a b u n d a n c e o f

194

0.72%. The high uranium concentration at Oklo in quantities exceeding the critical mass, the absence of pile poisons, and the availability of water as a moderator, were all factors contributing to the operation of the reactors. However, the most important aspect of the Oklo reactors is that they have been well-preserved throughout the past two billion years of geological history. This allows the measurement of the isotopic composition of samples from the reactor zones, and an interpretation of their isotopic abundances in terms of neutron physics. This in turn enables the nuclear parameters of the reactors to be determined. Of key importance to this process are the various fission products which were produced in the reactors, as they serve as isotopic tracers, not only in characterising the reactors, but in deciphering the geochemical mobility of the various elements in the geological environment. The mobility of the fission products produced at Oklo is therefore a key factor in the assessment of the effectiveness of natural repositories for radioactive waste containment. Uranium mineralisation at Oklo is located in a sandstone conglomerate lens which has a mean slant of 40 o and is concordant with the crystalline basement relative to the sedimentary cover. Criticality was achieved in a number of localities in the deposit and several reactor zones have been identified. Each reactor zone consists of a compact accumulation of rich uranium ore, flattened in the direction of the stratum, with diameters ranging from 10 to 20 m and thicknesses of a few tens of N "~'----, .

.

"

"

20N REACTOR ZONE7

Z:305m DEPTH Z: 3 0 0 m

Z:310m 10N REACTOR ZOIqE 9 metres 35&4

0

%::

15&16 ~ - - - - 3 6 30~'

28 10S REACTOR ZONE 8

I

I

lOW

Fig. 2. Location of Reactor Zone 9 samples relative to Reactor Zones 7 and 8.

centimetres. These uranium segregations are embedded in a clay gangue which is contained in the sandstone-conglomerate lens. The uranium is present as pitchblende, except in the reactor zones themselves, where it occurs as coarse crystalline uraninite. The uraninite is remarkably deficient in elements possessing isotopes of high neutron capture cross-section which would have prevented the reactors operating for any length of time. Most of our knowledge of the nuclear and geochemical characteristics of the Oklo reactors

REACTOR ZONE 9

• ,

." . .)".i:. .:.:.. :::: ..-...........-..i '.":-".:": :... i. -.... : i! .i " : ...... i

-"-"."x=0.-.. y-0 " ~ . ~ . ~ ' "~ :i . / y: "_"4: . :~-."' " ":

1 meire I ~ ]

Congl . . . . .

IOE

metres

-

te

'

~

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Coarse sandstone

~

Green shale

Fine sandstone

~

High grade o r e

Fracture

~

Shear

' "

..

'"

"

" '

"" ~ '

~

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.

:"

Q

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zone

Fig. l. Schematic view of the portion of the Oklo mine site from which samples were taken. Location of the Reactor Zone 9 samples listed in Table 1 are referenced to the coordinates (0,0) in this diagram.

195 TABLE 1 Locations and uranium concentrations of samples from Reactot Zone 9 Sample

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 a Natural U

Relativecoordinates Uranium 235U (metres from 0,0) (g/g) (atom %) E-W

N-S

- 0.05 -0.15 -0.08 0 - 0.05 0 0 0

- 1.4 -1.3 -0.75 0 0 +1.6 - 1 - 1

-

-

0.671 0.663 0.400 0.552 0.584 0.581 0.479 0.588

0.559 0.595 0.581 0.678 0.683 0.688 0.690 0.698

-

0.719 + 0.003 b

Sample 9-16 is 0.1 m downdip from sample 9-15. b Averageof 235Uin 9 duplicate analyses of NBS 960.

neodymium. The uranium isotopic abundances listed in Table 1 were measured at the Los Alamos National Laboratory. Samples were taken into solution using successive treatments of aqua regia, H F and HC1 in teflon beakers. The various elements were extracted in a form suitable for mass spectrometric analysis by ion exchange chemistry. Details of the chemical extraction techniques, the tracers used for the isotope dilution measurements, and the mass spectrometric techniques are as follows: Pd, Ag and Te [5], Cd [6], Sn [7], Ru [8], Mo and Nd [9] and U [10].

3. Results

a

have been obtained from a detailed study of Zone 2 [4]. However, in 1979 Reactor Zone 9 was exposed during mining operations. This reactor was disc-shaped and conformable with the surrounding strata. The mine floor cut across the stratigraphic section and samples were taken from the exposed reactor (see Figs. 1 and 2). The aim of the present project is to analyse the isotopic composition of a number of elements using solid source mass spectrometric techniques, in order to characterise some of the nuclear parameters of Reactor Zone 9.

2. Experimental procedure Eight samples of Reactor Zone 9 were taken from the floor of the pit. Relative locations of each sample together with their uranium concentrations and isotopic abundances, are presented in Table 1. All samples consist mainly of uraninite, the average concentration of uranium in the Zone being approximately 64%. Positive values represent samples from north and east of the coordinates (0,0) shown in Fig. 1. Samples were selected, ground and sieved at the Los Alamos National Laboratory. Aliquots were sent to Curtin University of Technology for the determination of the isotopic abundances of palladium, silver, cadmium, tin and tellurium, and to the Idaho National Engineering Laboratory for isotopic analysis of molybdenum, ruthenium and

Table 2 shows the measured isotopic compositions of the elements in the eight Reactor Zone 9 Oklo uraninite samples described above. The ratios have not been corrected for instrumental mass fractionation effects. The errors quoted with each ratio are 95% confidence intervals and refer to the last significant figure in the respective isotopic ratios. The isotopic composition of the respective laboratory standards, as measured under similar instrumental conditions, are also listed. It should be noted that the Sn data are only representative of the non-cassiteritic Sn in the samples, as it is unlikely that the digestion procedure employed dissolved tin oxide. Each fission product element in the reactor zone has an isotopically distinct, but chemically identical primordial counterpart. The term "primordial" refers to that portion of the element that was in the samples prior to the time of nuclear criticality or subsequently introduced from regions that did not sustain nuclear fission. We use the term "primordial", instead of the more commonly used terms "natural" or "terrestrial", because the fission products are also natural components of elements in these rocks. Some analyses of the elements in the reactor zone samples showed little evidence of a primordial component (Ru, Pd and Te), whereas other elements reveal significant primordial contributions (Mo, Ag, Cd, Sn and Nd). The fraction of the primordial contribution can be gauged from those isotopes which are not produced by the fission process. The only element which does not have a non-fissiogenic isotope is Ag, and thus it is

196 TABLE 2 Isotopic composition of Mo, Ru, Pd, Cd, Ag, Sn, Te and Nd for Reactor Zone 9 samples Sample

Molybdenum 94/92

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Standard Sample

0.57 0.60 0.60 0.55 0.60 0.50 0.61 0.61 0.6284

±1 ±1 ±1 ±1 ±1 ±1 ±1 ±1 ±1

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Standard

0.2941 ± 1

Sample

Palladium

Sample

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Standard Sample

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

96/92

97/92

3.00 ±1 4.40 ±1 4.40 ±1 2.22 ±1 2.00 ±1 5.75 ±1 1.31 ±1 1.31 ±1 1.074 ± 1

1.14 ±1 1.20 +1 1.20 ±1 1.11 +1 1.10 ±1 1.00 ±1 1.15 ±1 1.15 ±1 1.128 ± 1

2.43 3.80 3.80 1.78 1.50 5.00 0.85 0.85 0.6486

98/104

99/104

100/104

101/104

102/104

-

1.67 1.73 1.60 2.15 1.92 2.67 2.00 2.15 0.6791

-

2.13 2.13 2.20 2.46 2.62 2.50 2.54 2.46 0.9091

1.87 ±1 1.80 ±1 1.87 ± 1 2.08 ±1 2.15 _+1 2.17 ±1 2.15 ±1 2.08 ±1 1.690 ± 1

±1 ±1 ±1 _+1 ±1 ±1 ±1 ±1 ±1

98/92

100/92

3.43 ±1 4.80 ±1 4.80 ±1 2.67 ±1 2.40 +1 6.00 ±1 1.85 ±1 1.85 ±1 1.628 ± 1

2.71 4.00 4.20 1.78 1.60 5.50 0.92 0.92 0.6486

±1 ±1 ±1 ±1 ±1 ±1 ±1 _+1 ±1

Ruthenium 96/104

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Standard

95/92

-

0.1016 ± 1

±1 ±1 ±1 ±1 ±1 ±1 +1 ±1 ±1

0.6738 ± 1

Silver

102/105

104/105

106/105

108/105

< < < < < < < <

0.1890 0.1582 0.1639 0.0624 0.0592 0.0825 0.0332 0.0284 0.4998

0.597 ± 1 0.5741 ± 5 0.5653 + 7 0.5080 ± 8 0.5074 ± 8 0.490 ± 1 0.4787 ± 9 0.476 ± 1 1.2163 ± 2

0.1749 0.1566 0.1499 0.1111 0.1098 0.0922 0.0898 0.0862 1.169

0.00004 0.0003 0.0003 0.0001 0.00005 0.0001 0.0001 0.0003 0.0463 ± 1

±1 ±1 ±1 ±1 ±1 ±1 ±1 ±1 ± 1

±2 ±1 ±1 ±1 ±1 ±1 ±2 +2 ±9

± ± ± ± ± ± ± ± ±

110/105 5 5 8 5 1 1 3 3 2

0.0543 0.0491 0.0470 0.0379 0.0373 0.0321 0.0326 0.0315 0.5117

± ± ± ± ± ± ± ± +

109/107 1 2 5 1 2 2 1 1 9

0.5808 0.5923 0.5955 0.5742 0.5843 0.5925 0.600 0.5976 0.918

+5 ±6 ±5 ±1 ±1 ±7 +1 ±5 ±2

Cadmium 108/106

110/106

111/106

112/106

113/106

114/106

116/106

0.74 0.72 0.76 0.71 0.69 0.81 0.73 0.70 0.705

10.3 ±1 10.1 ±1 10.6 ± 3 9.9 ± 2 10.4 ± 3 12.2 ± 3 9.8 ± 2 9.7 ± 2 9.78 ± 1

17.7 + 9 12.5 _+1 14.8 ± 4 15.4 ± 4 14.6 ± 4 13.3 ± 3 10.1 ± 2 10.2 ± 2 9.97 ± 2

21.9 20.1 22.0 22.2 22.2 23.7 18.6 18.7 18.67

8.8 9.5 9.9 9.9 10.3 11.8 9.3 9.4 9.41

26 22.0 26.2 26.6 26.1 28.0 21.8 22.0 22.02

8.0 ± 6 6.5 ±1 7.4 ± 2 8.4 ± 2 8.0 ± 2 7.4 ± 2 6.1 ±1 5.7 ±1 5.69 ± 1

±16 ± 2 ± 3 ± 3 ± 5 ___ 3 ± 2 ± 3 ± 1

+9 ±2 ±6 ±5 +_5 ±6 ±4 ±4 ±3

±6 ±1 ±3 ±2 ±3 ±3 ±2 ±2 ±1

±2 ±2 ±7 ±6 ±6 ±7 ±4 ±4 ±3

Tin 112/116

114/116

115/116

117/116

118/116

119/116

0.0659 + 5 0.0658±7 0.067 ± 1 0.070 ± 3 0.070 ±1 0.070 ± 2 0.0682±5 0.071 ± 2

0.0440 ± 3 0.0444±6 0.0457 ± 7 0.047 ± 2 0.048 ± 2 0.048 ±1 0.0456±7 0.043 ±1

0.0492 ± 3 0.05 ±1 0.0501 ± 9 0.045 ± 4 0.037 ± 4 0.055 +1 0.0372±7 0.043 __+1

0.949 ± 3 0.966±9 0.960 ± 6 0.77 ±1 0.732±2 1.028±9 0.75 ± 2 0.79 ± 1

1.987 ± 8 2.00 ±1 1.993 ± 2 1.87 ± 6 1.831±2 2.05 ± 4 1.84 ±1 1.82 ±1

0.982 ± 1 0.9952±8 1.001 ± 5 0.8292+6 0.7702+8 1.045 ± 6 0.802 ± 2 0.826 ± 3

197 T A B L E 2 (continued) Sample

Tin (continued)

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Standard Sample

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Standard Sample

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Standard

120/116

122/116

124/116

2.531-+5 2.55 + 6 2.570_+ 1 2.47 _+6 2.395_+5 2.67 _+2 2.4 + 1 2.43 + 1 2.186 -+ 5

0.867 _+4 0.892 -+ 3 0.882 _+3 0.63 _+2 0.56 _+3 0.947 _+5 0.606 -+4 0.645 + 5 0.3096 -+ 9

1.333_+5 1.377_+ 4 1.36 _+1 0.924_+2 0.813_+8 1.44 _+2 0.893+2 0.957+4 0.379 _+1

Tellurium 120/130

122/130

123/130

124/130

125/130

126/130

128/130

<0.000002 <0.000001 <0.000020 < 0.000001 <0.000001 < 0.000002 <0.000002 <0.000010 0.00303_+8

0.00065-+ 1 0.00046-+ 3 0.00059-+ 1 0.00038-+ 1 0.00041-+ 1 0.00055 -+ 6 0.00051-+ 4 0.00073-+ 1 0.08201_+21

0.00006-+ 1 0.00002-+ 1 0.00005-+ 1 0.00009 -+ 1 0.00009-+ 1 0.00014 _+ 4 0.00014-+ 2 0.00012_+ 1 0.02851_+11

0.00046-+1 0.00032-+1 0.00043-+1 0.00051 -+ 1 0.00057-+1 0.00062 _+6 0.00062-+5 0.00077-+1 0.1513 _+4

0.0204 -+2 0.01994-+2 0.02032-+6 0.01915 -+ 5 0.01970-+6 0.02043 _+6 0.0172 -+1 0.01828_+3 0.21122-+6

0.0181 -+1 0.01615-+3 0.01904-+3 0.02768 -+ 7 0.06349-+5 0.02961 -+ 7 0.0769 -+1 0.0960 _+1 0.5607 -+1

0.2024-+3 0.1988-+1 0.1975-+1 0.1940 -+ 1 0.1934-+1 0.1896 -+ 3 0.1894-+3 0.1910_+2 0.9376-+1

Neodymium 143/142

144/142

145/142

146/142

148/142

150/142

1.223 + 1 1.162 _+l 1.294 + 1 0.619 + 1 0.643 -+1 0.685 -+ 1 0.528 -+ 1 0.586 -+1 0.4492 -+ 1

1.790 + 1 1.708 +1 1.853 + 1 1.061 _ 1 1.112 -+1 1.162 -+ 1 0.968 -+1 1.029 -+1 0.8788 -+ 1

0.872 + 1 0.825 + 1 0.916 + 1 0.425 + 1 0.451 -+1 0.481 n-+ 1 0.364 + 1 0.402 -+1 0.3066 -+ 1

1.095 1.058 1.126 0.732 0.750 0.773 0.684 0.703 0.6361

0.465 + 1 0.442 + 1 0.483 + 1 0.265 + 1 0.275 -+1 0.289 -+ 1 0.236 -+ 1 0.249 -+1 0.2135 -+ 1

0.313 -+1 0.303 + 1 0.318 + 1 0.229 -+1 0.233 -+1 0.239 -+ 1 0.216 -+ 1 0.216 -+1 0.2095 +_1

not possible to estimate the undoubtedly significant fraction of the primordial component for this element. Of all the elements analysed Cd has the least fissiogenic component, and two samples (9-15 and 9-16), showed no detectable fissiogenic Cd. The relative amounts of the fissiogenic component to the primordial component for Mo, Cd, Sn and N d are listed in Table 3 for each of the eight samples analysed. In order to obtain the cumulative fission yields (which are listed in Table 3), it is necessary to correct for the primordial component using the measured laboratory standard ratios. The cumulative fission yields have been corrected for instrumentally induced mass fractionation effects, so

+1 +1 +1 +1 -+1 -+ 1 -+ 1 -+1 -+ 1

that they may be directly compared to the selected cumulative fission yields for 235U, 238U and 239pu, which are also listed in Table 3. These selected yields have been derived from published work as discussed by De Laeter et al. [11]. A full list of the cumulative fission yields for 101 ~
198 TABLE 3 Relative cumulative fission yields for Mo, Ru, Pd, Cd, Sn, Te and N d for Reactor Zone 9 samples a Sample

Molybdenum 95

97

98

100

Mot/MOp

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

1 1 1 1 1 1 1 1

0.94 0.95 0.95 0.98 0.91 0.93 0.86 0.86

0.93 0.95 0.95 0.91 0.84 0.94 0.90 0.90

1.07 1.02 1.07 0.98 1.02 1.03 1.21 1.21

2.3 1.92 2.7 0.98 0.59 3.6 0.15 0.14

235U 238U 239pu

1 1 1

0.914 1.20 1.09

0.883 1.27 1.19

0.959 1.44 1.40

Sample

Ruthenium 99

101

102

104

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

1.67 1.73 1.60 2.15 1.92 2.67 2.00 2.15

2.13 2.13 2.20 2.46 2.62 2.50 2.54 2.46

1.87 1.80 1.87 2.08 2.15 2.17 2.15 2.08

1 1 1 1 1 1 1 1

235U 238U

3.20 1.29 1.02

2.70 1.31 0.99

2.26 1.30 1.01

1 1 1

239pu Sample

Palladium 105

106

108

110

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

1 1 1 1 1 1 1 1

0.600 ± 1 0.577 _+1 0.569 _+ 1 0.511 ± 1 0.510 _+1 0.493 _+1 0.481 ± 1 0.479 + 1

0.1781 _+6 0.1594 _+6 0.1526 _+9 0.1132 _+5 0.1118 _+4 0.0939 _+2 0.0914 -+ 3 0.0876 -+ 3

0.0559 _+1 0.0507 _+2 0.0484 _+5 0.0390 _+1 0.0384_+ 2 0.0331 -+ 2 0.0336 -+ 1 0.0325 _+2

235U 238U 239pu

1 1 1

0.406 0.70 0.75

0.056 0.12 0.37

0.026 0.032 0.11

Sample

Cadmium 111

112

113

114

116

Cdf/Cdp

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

3.2 + 6 3.0 + 3 2.7_+4 1.9 ___2 1.9+2 1.8_+3 . .

1.4 + 7 1.7 + 3 1.9+4 1.3 + 2 1.5_+3 2.9_+5 . .

0.0 ± 3 0.06 + 9 0.3 + 2 0.19 + 9 0.4 _+1 1.4 _+2

1.6 _+8 2.0 + 3 2.4+5 1.7 + 3 1.8_+3 3.4_+6

1 1 1 1 1 1 1 1

0.21 0.62 0.18 0.21 0.19 0.23 Nil Nil

235U 238U

1.40 1.93 5.89

1.00 1.24 1.60

1.00 1.07 1.19

1 1 1

239pu

. .

1.22 1.51 2.79

. .

199 TABLE 3 (continued) Sample

Tin 117

118

119

120

122

124

Snf/Snp

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

0.429 5:4 0.43 5:1 0.429 5:8 0.43 5:3 0.46 + 2 0.45 + 1 0.431 5:6 0.43 5:3

0.344 5:9 0.36 5:3 0.342 5:7 0.39 5:9 0.41 + 7 0.37 + 3 0.37 5:1 0.30 5:2

0.408 + 3 0.41 5:1 0.416 5:8 0.45 5:5 0.43 + 5 0.423 5:9 0.420 5:6 0.4125:6

0.353 5:8 0.33 5:2 0.38 + 2 0.51 5:9 0.47 + 4 0.45 5:2 0.42 5:2 0.40 _+2

0.580 5:5 0.58 5:1 0.580 5:7 0.59 5:4 0.59 _+1 0.59 5:1 0.574 + 8 0.5775:9

1 1 1 1 1 1 1

0.46 0.47 0.46 0.29 0.22 0.54 0.21 0.28

235U 238U

0.55 0.89 0.35

0.40 0.87 0.35

0.46 0.87 0.38

0.49 0.87 0.33

0.55 0.91 0.55

1 1 1

239pu Sample

Tellurium 125

126

128

130

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

0.0199 + 2 0.0193 5:1 0.0196 5:1 0.0186 + 1 0.0191 5:1 0.0198 5:1 0.0167 5:1 0.0177 + 1

0.0177 + 1 0.0157 5:1 0.0185 5:1 0.0270 5:1 0.0619 5:1 0.0288 + 1 0.0750 + 1 0.0950 5:1

0.1998 5:3 0.1962 + 1 0.1950 + 1 0.1915 + 1 0.1909 5:2 0.1871 + 3 0.1869 5:3 0.1885 + 2

1 1 1 1 1 1 1 1

235U 238U 239pu

0.018 0.026 0.043

0.033 0.034 0.086

0.180 0.24 0.34

1 1 1

Sample

Neodymium 143

144

145

146

148

150

Ndf/Ndp

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

1 1 1 1 1 1 1 1

1.178 5:4 1.163 5:4 1.153 5:4 1.07 5:2 1.20 + 2 1.20 5:1 1.13 5:4 1.10 5:2

0.731 5:3 0.727 5:4 0.721 + 3 0.70 5:1 0.74 5:1 0.74 5:1 0.72 5:3 0.70 + 2

0.593 5:3 0.592 _+3 0.580 _+3 0.56 5:2 0.59 5:1 0.58 + 1 0.61 + 3 0.49 5:2

0.325 5:3 0.321 5:3 0.319 5:3 0.30 5:1 0.32 5:1 0.32 5:1 0.29 5:3 0.26 ± 2

0.134 5:3 0.131 5:3 0.128 5:2 0.11 +1 0.12 5:1 0.13 5:1 0.08 5:3 0.05 + 2

0.83 0.76 0.89 0.17 0.21 0.25 0.08 0.13

235U

1 1 1

0.92 1.00 0.84

0.66 0.82 0.67

0.50 0.75 0.56

0.28 0.45 0.37

0.11 0.28 0.22

238U

239pu

a The stated uncertainties are 95% confidence intervals and refer to the last significant figure in the respective cumulative yields.

y i e l d s . T h e r a t i o s v a r y f r o m 0 . 1 4 f o r s a m p l e 9-16 t o 3.6 f o r s a m p l e 9-5. D e s p i t e t h e l a r g e r a n g e i n primordial Mo contamination, the cumulative yields derived from the isotopic data in Table 2 are remarkably consistent for each of the samples. Ruthenium comprises seven stable isotopes of w h i c h 96Ru, 98Ru a n d l ° ° R u a r e n o t p r o d u c e d i n the fission process. These isotopes were not de-

tected in the mass spectrometric analyses and therefore no correction for a primordial Ru component was necessary. Using the values for the p r o p o r t i o n o f 235U, 238U a n d 239pu f i s s i o n as l i s t e d in Table 4 for each of the eight samples, one can calculate the expected cumulative fission yields a n d c o m p a r e t h e m w i t h t h e m e a s u r e d yields. Fig. 3 a is a p l o t o f t h e r e l a t i v e c u m u l a t i v e y i e l d s 99Ru

200

99Ru3.0 {Q)

235U

• 4,16

1'0 g-2agPu

1:0 3.0 lOlR~

1'5

2:0

2"5

~°2Ru

2!5

l°2Ru

(b)

416

1.3o~ 2:isU 5

30 3 ~

2-~

1.0

~239pu 1:0

1:5

2:0

Fig. 3. (a) Cumulative fission yields 99Ru versus l°2Ru (relative to a°4Ru), showing the deficiencies in 99Ru with respect to the 235U-238U-239pu triangle of expected yields. (b) Cumulative fission yields lmRu versus ]°2Ru (relative to m4Ru), which show a good correlation with the 235U-238U-239pu triangle of expected yields.

versus l ° 2 R u (with respect to a°4Ru) for the eight

reactor Zone 9 samples listed in Table 3. The cumulative yields of 235U, 238U and 239pu are also shown, and it would be expected that the experimentally determined data would fall within the t r i a n g l e 235U-238U-239pu. It is apparent that this is not the case. In contrast, Fig. 3b depicts the corresponding situation with respect to the relative cumulative yields l°lRu versus l°2Ru. The experimentally determined data for each of the eight samples fall close to the 2 3 5 U - 2 3 8 U - 2 3 9 p u triangle. It needs to be recognised that the relative cumulative yields for the fissile nuclides 235U, 23Su and 239pu are subject to uncertainties because of the paucity of experimental data. In particular the relative cumulative yields of 238U have never been measured mass spectrometrically. The uncertainties in the cumulative fission yields of the fission nuclides in a limiting factor in the evaluation of many of the parameters discussed in this paper. It is nevertheless apparent that the

measured yields for l°]Ru and m2Ru are consistent with the predicted yields, taking into account the experimental uncertainties, whereas the 99Ru yields are lower than predicted, the extent of the loss varying from sample to sample. A precursor to 99Ru is 99Tc, which has a half-life of 2.1 × 10 5 y. Within approximately 10 6 y after the end of criticality all 99Tc produced in the reactor will have decayed to its stable daughter 99Ru. Thus for a geologically significant period of time the fission produced isobars at mass 99 existed as two different chemical species--To and Ru. The results indicate that a significant proportion of Tc must have been mobilised from Reactor Zone 9, in contrast to Ru which must have been retained to a much greater extent in the uraninite. The amount of primordial Pd in the Zone 9 samples is negligible, as indicated by the measured l°2pd/l°Spd ratios. Although m4pd is not a fission product, it is produced by neutron capture on l°3Rh. Unfortunately Rh is monoisotopic and one therefore cannot distinguish between primordial and fissiogenic components. However, since Rh is rare in nature, it is likely that a significant proportion of ]°4pd has been produced from the neutron capture reaction on fissiogenic m3Rh. Samples 928, 9-36 and 9-30 have the largest l°4pd/l°sPd ratios which indicates that these samples have been exposed to the largest integrated neutron flux (or had better Rh retention during criticality). This neutron fluence hypothesis is supported by the fact that these three samples have suffered the greatest 235U " b u r n - u p " as indicated by their present 235U content as listed in Table 1, and by estimates of the neutron fluence (as discussed later). Although the measured isotopic ratio (1°gAg/ 1°rAg) is listed for each of the eight Zone 9 samples in Table 2, the data is of limited value since it is impossible to identify the relative amounts of terrestrial Ag contamination from fission product Ag in the samples. Thus we have not listed any cumulative fission yields for Ag in Table 3. The mass spectrometry of Cd was difficult because the low concentration of Cd (0.014-0.054 ppb [13]), together with the size of the samples used in the analysis (0.1-0.5 g), only provided nanogram amounts of Cd for analysis. Furthermore most of the Cd present in the samples is primordial (see Table 3), which limits the accuracy

201 of the fission yields. Fortunately l°6Cd can be used to correct for the primordial contribution, whereas it is not possible to use 1°8Cd or 1]°Cd because these nuclides have been enhanced due to neutron capture effects on mTAg and l°9mg respectively. The enhancement of m8Cd and n ° C d can be observed in the isotopic data in Table 2. The thermal neutron capture cross sections of l°7Ag and l°9Ag are 37.6 and 91 barns (b), respectively [14]. In addition, l°9Ag has a large resonance neutron cross section of 1400 b. The net effect is that neutron capture on m9Ag converts a proportion of this isotope to 11°Cd. The effect on 1°8Cd is comparatively smaller, not only because of the smaller cross sections involved, but also because much of the fissiogenic 1°TAgis held up at l°Tpd (half-life of 7 × 106 y). Thus only a small fraction of fissiogenic 1°TAg is converted to 1°8Cd. The magnitude of the enhancement in 1°8Cd and n ° C d is also dependent on the elemental A g / C d ratio in the samples. The concentrations of Ag in the eight samples range from 2.0 to 6.9 ppm [13], so that the A g / C d ratio is greater than 100 for every sample analysed. An examination of the cumulative yields for Cd in Table 3 reveals that n3Cd is severely depleted, despite the fact that the fission yield distribution across the bottom of the valley of symmetry is relatively constant. This is due to the large thermal neutron-capture cross section of 113Cd of 20,600 b [14]. Thus 113Cd is progressively converted to 114Cd, as is shown in the enhanced cumulative yields for ll4Cd. However the amount of 113Cd present in the reactor zone was very small, so that it did not constitute a significant "pile poison". The amount of depletion in the 113Cd fission yield is closely correlated with the 235U depletion and neutron fluence. Although the n3Cd cumulative fission yield for sample 9-28 is shown as zero, in fact the calculated yield was negative, indicating the presence of primordial 113Cd in this sample during the reaction period. In contrast, since no significant amounts of Cdf were found in samples 9-15 and 9-16, the observed Cd in these samples must have been introduced after the period of the nuclear reactions. Although Sn does not possess any isotopes which have a large capture cross section, ll6Sn is produced by neutron capture on nSIn. This reac-

tion has a thermal neutron capture cross section of 202 b and a resonance cross section of 3300 b. The fact that most of the n 6 s n / n Z s n ratios in Table 2 do not show any significant enrichment indicates that the 115In produced by fission did not reside in the uraninite for a sufficient period of time to produce excess ll6Sn. Because excess H6Sn would also be produced by primordial nSIn, the results also indicate that only small amounts of primordial indium were present in Reactor Zone 9 during the reaction period. The isotopic composition of Te indicates a very low level of the primordial component, so that only minor corrections were necessary to obtain the cumulative fission yields. It should be noted that some of the relative cumulative fission yields for 126Te as listed in Table 3 are less than and others greater than any of the yields of the fissile nuclides producing the fission products. This is in marked contrast to almost every other fission yield listed in Table 3 where the measured yield is some intermediate value between the dominant 235U thermal fission yield and the minor, but nevertheless significant fission yields of 238U (fast fission) or 239pu (thermal fission). This can be seen in Fig. 4 where the mass 126 fission yields show considerable scatter in comparison to the mass 125 and mass 128 yields. The

/

SAMPLE 26 30 NUMBERS 346

..,.

..

35 a ' 235U 16 ...'15 0"I

I§I .. 15m .-" 35 • /.......""

RELATIVE YIELD

.....'"451

.."235 U

'

235

U

36 281 30•

0'01

I t25

I

I

126

128

MASS

I 130

NUMBER

Fig. 4. Relative cumulative fission yields for Te in samples from Reactor Zone 9.

202 precursor to 126Te in the mass 126 isobaric fission chain is 126Sn which has a half-life of approximately 10 3 y. If Sn exhibited some mobility in the geological environment at Oklo, some of the 1268n may have escaped from the sample into the surrounding material, thereby reducing the amount of 126Te in some samples (ORZ-28, -30 and -36) and enhancing the amount of 126Te in others (e.g. samples ORZ-15, -16 and -35). It therefore seems likely that Sn was partially mobile in the uraninite at the time of reactor criticality and a proportion redistributed itself within the reactor zone while some escaped from the reactor zone, before Sn was able to decay completely to 126Te. An examination of the cumulative fission yields for 125Te indicates that for some of the samples the yields are less than the 235U fission yields and significantly less than the expected fission yield for the samples, taking into account the actual amounts produced by the fission process of 235U, 238U and 239pu. The precursor to 125Te is 125Sb, which has a half-life of 2.73 y. It is therefore possible that some of the Sb may have been mobilised before all of it had decayed to a25Te thus explaining the small, but nevertheless significant variations, in the 125Te cumulative fission yields. This result is surprising when one considers the brevity of the half-life of a25Sb. The 122Te/a3°Te, 123Te/13°Te and 124Te/13°Te ratios are listed in Table 2 but a2°Te could not be measured in any of the samples. Since lZZTe,123Te and 124Te are not fissiogenic nuclides, one would expect to find 123Te depleted and 124Teenhanced due to the neutron capture on 123Te, which has a thermal cross section of 418 b and a resonance cross section of 5630 b [14]. Although the uncertainties are large, ~23Te appears depleted with respect to 122Te, the magnitude of the depletion corresponding to some extent with the 235U depletion and neutron fluence. However 124Te also appears depleted with respect to 122Te. The ratio of 124Te/aZ3Te shows the expected increase with 235U depletion and neutron fluence although the uncertainty is larger. The reason for the apparent depletion in 124Teis not understood at this stage. The simplest explanation is an error in the measured isotopic abundance of 122Te. The remaining element analysed was the rare earth element Nd which has seven isotopes with all but a42Nd being produced by fission. This

isotope was used to correct for the primordial contribution to provide the relative cumulative fission yields listed in Table 3. The amount of correction required varied enormously, sample 9-36 having a N d f / N d p ratio of 0.89 whilst sample 9-15 has a ratio of 0.08. In fact the small amount of fissiogenic Nd in samples 9-15 and 9-16 and to a lesser extent 9-4 makes their calculated cumulative yields suspect. Neodymium is situated on the high mass side of the high-mass fission hump distribution and thus the fission product yields are diminishing with increasing mass. The cumulative yields listed in Table 3 have little significance in that some of the isotopic ratios are affected by neutron capture. In particular 143Nd and 145Nd both possess significant cross sections and thus are depleted, whilst their daughter products 144Nd and 146Nd are correspondingly enhanced. The major reason for measuring the Nd isotopic composition is to enable the neutron fluence to be calculated (see later discussion). 4. Reactor parameters The measured isotopic abundances and cumulative fission yields for the eight uraninite samples enable a number of significant reactor parameters to be determined for Zone 9 using the principles of neutron physics and radioactive decay.

4.1. Proportion of fissioning nuclides A comparison of the measured fission yields of Pd with the selected fission yields of 235U, Z38U and 239pu show that in each of the eight samples the measured yields are larger than the 235U yields, which indicates that the actual fission yields are a mixture of 235U, 238U and 239pu fission. As the mass of the fissioning nuclide increases, the lowmass hump shifts progressively towards a higher mass, whilst the high-mass hump stays essentially in the same position. Thus Pd is an excellent element to determine the relative amounts of 23~U, 238U and 239pu fission, since its fission products are located on the high wing of the low-mass hump and therefore the yields vary from one fissioning nuclide to another, as seen by the selected fission yields listed in Table 3. If a a n d / 3 are the proportion of fission events due to 238U (fast fission) and a39pu (thermal fis-

203 TABLE 4 Proportion of fissile nuclides in Reactor Zone 9 samples

238U

239pu

1-a-fl

a

fl

0.821+0.012 0.805 + 0.003 0.811+0.021 0.902 + 0.001 0.900 + 0.004 0.911 + 0.009 0.933 + 0.001 0.935 + 0.001

0.083+0.008 0.114 + 0.003 0.117+0.010 0.066 + 0.001 0.069 + 0.004 0.073 + 0.010 0.050 + 0.002 0.051 + 0.001

0.096 + 0.0809 + 0.072 + 0.0328 + 0.0312 + 0.0160 + 0.0168 + 0.0139 +

0.078

0.045

Sample

23SU

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16 Mean

0.877

0.001 0.0003 0.001 0.0001 0.0006 0.0009 0.0001 0.0001

sion) respectively, then one can analyse the measured Pd fission yields Y to calculate the values of a and fl by the following equation: y = P235 (1 -- @ -- /3) "1- P2380t "[- P239/3 p~35(1 - ~ - / 3 ) + p~38~ + p~./3 where P235, P238, 10239, 10235, 10~38 and 10~39 represent cumulative yields for 935U, 238U and 2 3 9 p u for two isobaric mass chains of an dement. (The denominator is used for normalization, in this case ~°sPd.) Using all combinations by twos of the three measured relative cumulative yields from each sample, three pairs of a a n d / 3 values can be determined. The mean a and 13 values for each reactor zone sample have been listed in Table 4. The proportion of E38U (fast fission) varies from 5.0% to 11.7%, whilst the proportion of 239pu (thermal fission) varies from 1.4% to 9.6%. The average values of a a n d / 3 for Reactor Zone 9 are 7.8% and 4.5% respectively. The uncertainties listed with the a and /3 values in Table 4 give an indication of the range of the three computed values. It should be pointed out that the reliability of the calculated values for a and /3 depend to a large extent on the selected cumulative fission yield values for Pd listed in Table 3. Although Shima et al. [15], have measured the fission yields for 235U and 239pu, no measurements of 238U fast fission have been carried out. Thus the selected values for 238U are open to question. Nevertheless these are the most precise values of a and /3 which have been determined for any of the Oklo reactor zones. The same calculations can be carried out for some of the other isotopic pairs listed

in Table 3, but the situation is not as favourable as in the case of Pd since the other elements are either in the valley of fission, where the mass distribution is essentially constant, or on the low wing of the high-mass hump, where the fission yields of the fissile nuclides do not vary to any great extent.

4.2. Neutron fluence, partitioning coefficient and spectral index The integrated neutron flux (or neutron fluence F), the fraction of 235U produced from 2 3 9 p u by alpha decay (C), and the fraction of non-thermal neutrons in the neutron spectrum ( R ) can be calculated using techniques such as computer modelling [16] or the direct solution of equations in F, R and C [17,18]. The latter technique is based on an d e m e n t such as N d which has at least one isotope with a considerable thermal and resonance neutron capture cross section, and another isotope not produced in the fission process which can be used to correct for the presence of the primordial contribution. The isotopes used were 143Nd/144Nd and 145Nd/146Nd. Cross sections were the same as those used by Ruffenach et al. [171. Table 5 lists the values of F, R and C for the eight Reactor Zone 9 samples, using the technique of Ruffenach et al. [17] for all but sample 9-5. It was found that no physically meaningful values could be derived for this sample, presumably because of the more complex irradiation history for this sample or because of the redistribution of Nd.

TABLE 5 N e u t r o n fluence (F), spectral index ( R ) and fraction of 235U from 239pu ( C ) for Reactor Zone 9 samples Sample

Fluence, P (10 20 n cm - 2 )

Spectral index, R

Fraction of

235U from 239Pu, C

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

7.58 6.04 6.71 1.92 1.76 2.1 1.76 1.05

0.119 0.174 0.134 0.213 0.355 0.38 0.932 0.404

0.471 0.504 0.494 0.515 0.552 0.077 0.688 0.564

Mean

3.62

0.339

0.483

204

The values of F, R and C for sample 9-5 listed in Table 5 were derived using computer modelling techniques similar to Cowan et al. [16]. Sample 9-5 is situated away from the other seven samples as indicated by the coordinates in Table 1. Some of the isotopic data for sample 9-5 is difficult to interpret. For example the largest enhancement of 11°Pd is found in this sample although it has one of the lowest 235U deficits and the lowest neutron fluences. The cadmium data for this sample is also difficult to interpret and the only explanation seems to be that it has either suffered a complex radiation history or some of the elements have been significantly redistributed. Because of the large number of variables involved in these calculations, it is difficult to determine the uncertainties for these parameters. An estimate of the errors based on the reproducibility of results, calculated using different pairs of Nd isotopic abundances, indicate that the uncertainties are of the order of 15%. Some confidence in the results is provided by the excellent correlation of the calculated fluence values with the abundances of 235U listed in Table 1. The calculations for I', C and R assume that the effects of any Nd and U redistribution in the Reactor Zone 9 samples has been minimal. As we have already seen, this does not appear to be true for sample 9-5, and may not be entirely correct for some of the other samples.

TABLE 6 Period of criticality (T), fission density (FD), total energy and average power output of the Reactor Zone 9 samples Sample

Period of criticality, T a

Fission density, FD b

Total energy output c

Average power output d

9-28 9-30 9-36 9-4 9-35 9-5 9-15 9-16

1.42 1.81 2.16 1.68 1.90 0.64 5.56 2.78

2.50 1.93 0.93 0.43 0.45 0.50 0.35 0.26

10.1 7.93 5.19 2.01 2.00 2.26 1.74 1.17

22.7 13.9 7.63 3.80 3.32 11.2 0.99 1.31

Mean

2.24

0.92

4.05

8.11

a b c d

x l 0 5 years. X1020 fission cm -3 of sample. x l 0 8 j g t of sample. X 1 0 - 5 W g - 1 of sample.

Table 6 gives the calculated period of criticality ( T ) for the Zone 9 samples. In earlier determinations of this parameter [18], the principal source of uncertainty in T was the error in the value o f / 3 / . The results from the present work have significantly reduced the errors associated with a and/3 to a point where the uncertainties in the fluence, fraction of 235U from 2 3 9 p u and cross sections are the more dominant error sources when determining the period of criticality.

4.3. Duration of the nuclear reactions

Ruffenach et al. [17] have shown that the duration of the reactions T can be estimated by using the following expression: T = 90"f(50f + 5 0 a ) ( 1 -

Or - - / 3 ) C I "

50f/3239)k

where 9of = fission cross section for 239pu; 5of = fission cross section for 235U; 50"a= absorption cross section for 235U; 239~k=239pu alpha decay constant, and F, C, a and /3 are as previously defined. The 235U and 2 3 9 p u c r o s s sections are dependent on temperature and the degree of thermalization of the neutron flux (R). These cross sections, defined as polynomials of R, for a temperature of 350 ° C are given by Ruffenach [18].

4.4. Parameters relating to the fission density

The fission density parameter (number of fissions cm-3 of present day reactor zone material) is a reasonable indicator of the degree of thermal and radiation damage to which UO 2 was subjected during the period of the reactions. Fission densities in pure uraninite in excess of 0.5 × 10 20 cm -3 produce significant amounts of swelling which leads to significant fracturing of this mineral [19]. The total number of fission events per gram of reactor sample (FD) is given by Ruffenach et al. [17] as: F D = S Nf + 8 Nf + 9Nf

where 5Nf, 8Nf and 9Nf are the numbers of 235U,

205 238U and 239pu fissions per gram of reactor zone material given by:

'N0

(1-

[e

.5N (1- ~-jS)

9Uf

/ -SUf

(1 -.

where sN0 = number of =35U atoms per gram of reactor material at the commencement of the nuclear reactions; 5ot= fission plus absorption cross section for =35U; and ~,/3, C, F and R have been defined previously. Using the known energy output per fission event of 32 pJ per fission [19], and the fission densities calculated above, the total energy output of each sample has been determined. Finally, by assuming that the reactor samples ran continuously over the periods described in the previous section, their average power output was determined. Table 6 lists these parameters for all eight Reactor Zone 9 samples. 5. Conclusions

The reactor zone samples contained essentially pure fissiogenic Ru, Pd and Te, whereas Mo, Ag, Cd, Sn and Nd contained a mixture of fissiogenic and primordial components. Very little fissiogenic Cd was found in the Reactor Zone 9 samples, and in two of the samples there was virtually no fissiogenic component at all. It was possible to correct for the primordial component by using nuclides which were not produced in the fission process, for all the elements except Ag which contains only two nuclides, both of which are produced in the fission process. Cumulative fission yields have been calculated for all the other elements and compared to selected cumulative yields from the fission of 235U (thermal fission), 238U (fast fission) and 239pu (thermal fission). This indicated that the cumulative fission yields for the Oklo samples resulted from a mixture of 235U, 238U and 239pu fission. Palladium is particularly well suited to decipher the relative proportions of the three fissile nuclides, as its fission-produced nuclides occur on the low

mass side of the fission valley, where significant variations in fission yields occur as the mass of the fissioning nuclide increases. Calculations showed that approximately 88% of the fissiogenic Pd was produced by the thermal neutron fission of 235U with the fast fission of 238U and the thermal fission of 239pu contributing approximately 7.8% and 4.5% to the fission product inventory respectively. This is in good agreement with the values calculated for Zone 2 by Loss et al. [20]. However, there were quite large variations from sample to sample, particularly for the case of 239pu where there was a seven-fold variation across the eight samples. These results were then used to calculate the integrated neutron flux to which Reactor Zone 9 had been exposed during the periods of criticality. Again there were quite large variations across the eight samples analysed (approximately seven-fold), but the average neutron fluence was 3.6 × 10 2° n cm -2. This was somewhat smaller than the neutron fluence of 1 × 10 21 n cm -2 reported for Reactor Zone 2 at Oklo [4]. The fraction of neutrons at non-thermal energies was 0.34 for Reactor Zone 9, which indicates that approximately two-thirds of the neutron flux was thermalised. The fraction of fissioning 235U produced by alpha decay from 239pu was 0.48, indicating almost 50% of the 235U fissions resulted from this process. The reactors at Oklo did not operate continuously, but nevertheless the duration of the Zone 9 reactions has been estimated to be 2.2 × 105 years, which is a somewhat shorter period than for Zone 2 which had a duration of 8 x 105 years [4]. During this time the average fission density was 0.92 × 1020 fissions cm -3 of the uraninite samples in Zone 9, which represents a total energy output of 4 × 10 8 j g-1 of sample. This gives an estimate for the average power output of 8.1 × 10 -5 W g-1 of sample. These reactor parameters can be used to estimate the fission product inventory for the various elements analysed in this study. A comparison of these calculated fissiogenic abundances with the measured abundances can be used to estimate the retentivity of these fission products in the uraninite samples [13]. An examination of the isotopic abundances of the Zone 9 samples also reveals that a number of

206

isotopes have been altered by neutron capture processes. Variations observed in the relative abundances of X°4Pd, 1°8Cd, 11°Cd, n3Cd, 114Cd, t16Sn and probably 123Te and 124Te can be explained by neutron capture processes, and give some insight into the behaviour of elements such as Rh and In which have not been isotopically analysed in this study.

Acknowledgements This work was supported by the Australian Research Grants Scheme and the Office of Nuclear Waste Isolation, Battelle Laboratory, Columbus, Ohio. We would like to thank Dr. F. Begemann for a number of critical comments which has improved the final presentation of this paper, and to acknowledge the technical assistance provided by Mrs. P.R. Harris.

References 1 R. Bodu, H. Bouzigues, N. Morin and J.P. Pfiffelmann, Sur l'existence d'anomalies isotopiques recontr~es darts l'uranium du Gabon, C.R. Acad. Sci. Paris, Ser. D 275, 1731-1732, 1972. 2 P.K. Kuroda, On the nuclear stability of uranium minerals, J. Chem. Phys. 25, 781-782, 1956. 3 G.W. Wetherill and M.G. Inghram, Spontaneous fission in uranium and thorium, in: Nuclear Processes in Geological Settings, pp. 30-32, University of Chicago Press, Chicago, Ill., 1953. 4 J.C. Ruffenach, R. Hagemann and E. Roth, Isotopic abundance measurements--a key to understanding the Oklo Phenomenon, Z. Naturforsch. 35a, 171-179, 1980. 5 R.D. Loss, K.J.R. Rosman and J.R. De Laeter, The measurement of Ag, Te and Pd in geochemical reference materials by mass spectrometric isotope dilution analysis, Talanta 30, 831-835, 1983. 6 K.J.R. Rosman and J.R. De Laeter, The isotopic composition of cadmium in terrestrial minerals, Int. J. Mass Spectrom. Ion Phys. 16, 385-394, 1975. 7 J.R. De Laeter, M.T. McCulloch and K.J.R. Rosman, Mass spectrometric isotope dilution analyses of tin in stony meteorites and standard rocks, Earth Planet. Sci. Lett. 22, 226-232, 1974.

8 W.J. Maeck, J.R. Delmore, R.L. Eggleston and F.W. Spraktes, The measurement of ruthenium in uranium ores and 238U spontaneous fission yields, in: Natural Fission Reactors, Proc. Tech. Comm., Paris, 1977, STI/PUB/475, pp. 521-541, 1AEA, Vienna, 1978. 9 R.L. Tromp, J.E. Delmore, R.A. Nielson and T.C. Chapman, Mass spectrometric systems and techniques for fission product analysis, Exxon Nuclear Idaho Co., ENICO-1094, 1981. 10 D.J. Rokop, R.E. Perrin, G.W. Knobeloch, V.M. Armijo and W.R. Shields, Thermal ionisation mass spectrometry of uranium with electrodeposition as a loading technique, Anal. Chem. 54, 957-960, 1982. 11 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. 12 R.D. Loss and J.R. De Laeter, Selected absolute cumulative fission yields for masses 101 to 130 from 235U and 239pu thermal fission and 238U fast fission, W.A. Inst. Technol., Rep. SPG421/1986/AP123, 1986. 13 D.B. Curtis, T.M. Benjamin, A.J. Gancarz, R.D. Loss, Kj.R. Rosman, J.R. De Laeter, J.E. Delmore and W.J. Maeck, Fission product retention in the Oklo natural reactors, Appl. Geochem., in press, 1988. 14 S.F. Mughabghab, M. Divadeenan and N.E. Holden, Neutron Cross Sections, Vol. 1, Part A. Academic Press, New York, N.Y., 1981. 15 M. Shima, H.G. Thode and R.H. Tomlinson, Cumulative yields of stable and long-rived isotopes of ruthenium and palladium in neutron-induced fission, Can. J. Phys. 56, 1340-1352, 1978. 16 G.A. Cowan, E.A. Bryant, W.R. Daniels and W.J. Maeck, Some United States studies of the Oklo phenomenon, Int. Symp. of the Oklo Phenomenon, Gabon, IAEA SM204/5, 341-356, 1975. 17 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 enchantillon de mineral du rracteur natural d'Oklo, Earth Planet. Sci. Lett. 30, 94-108, 1976. 18 J.C. Ruffenach, Les Rracteurs Nucl~alres Naturels D'Oklo, Docteur et Sciences Physiques Thesis, l'Universite de Paris VII, 1979 (unpublished). 19 S. Glasstone and A. Sesonske, Nuclear Reactor Engineering, Van Nostrand Reinhold Co., New York, N.Y., 1981. 20 R.D. Loss, K.J.R. Rosman and J.R. De Laeter, Transport of symmetric mass region fission products at the Oklo natural reactors, Earth Planet. Sci. Lett. 68, 240-248, 1984.