Geochemical Characteristics of an Ancient Nuclear Reactor “Oklo”

1. Introduction
The Oklo uranium deposit at the Republic of Gabon, central Africa, had partly functioned as natural fission reactors. Large-scale fission chain reactions spontaneously occurred at 16 separate areas in the Oklo deposit, so-called "reactor zones (hereafter RZs)”, two billion years ago, and sustained for 24,000 to 200,000 years.  Two more RZs have been identified at the Okelobondo and Bangombé uranium deposits close to Oklo.  Figure 1(a) shows a map of the Oklo and Okélobondo uranium deposits. Many elements of the Oklo RZs and the related samples show the variations in the isotopic composition caused by a combination of nuclear fission, neutron capture, and radioac- tive decay.  Isotopic studies provide useful information to esti- mate reactor conditions and to understand behavior of radionuclides in geological media. 1,2  The data are also of value in particle physics to examine time-variations of fundamental physical constants.³ Most of previous isotopic analyses for the Oklo studies were based on bulk analysis of the rock samples with chemical sepa- ration for individual elements.  As the first stage of the Oklo study, retentivities of the most of the fissiogenic isotopes such as rare earth elements (REE), platinoid elements, and alkaline elements in the Oklo reactors were quantitatively determined from the isotopic data of the bulk analyses using thermal ion- ization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS).  Although the bulk anal- yses have provided precise isotopic data, the results indicate average information in the samples. It is known that some kinds of fission products are heteroge- neously distributed in uranium matrices, even if they have Figure 1.  (a) A map of the Oklo-Okélobondo uranium deposits, Republic of Gabon.  (b) A cross section of RZ 10.  The sample used in this study was collected from one of drifts D81N that could be accessed to RZ 10. (above the reactor) 1050 (130 cm above): coffinite, uraninite 1170 (10 cm above): florencite (vicinity of the reactor) 1180 (reactor): uraninite 1215 (35 cm below): uraninite 1225 (45 cm below): coffinite, françoisite 1240 (60 cm below): coffinite retained well in the reactor.  In addition, even fission products having chemically good affinity with uranium might have partly remobilized out of the reactor in association with disso- lution of reactor uraninite due to late weathering.  From the mineralogical observation using scanning electron microscope (SEM) and transmitted electron microscope (TEM), partial dissolution of uraninite and recrystallization of uranium miner- als in and around the RZs were confirmed.  Therefore, for bet- ter understanding the migration mechanism of fission products out of the RZs, it is essential to perform microscopic isotopic observation on the Oklo samples.  In-situ isotopic observation with secondary ion mass spectrometry (SIMS) has been widely used even in the field of geology and geochemistry. Secondarily formed minerals bearing fissiogenic products were found in peripheral rocks around the RZs. 4-6  Instrumental development of micro-region analyses by SIMS offers great advantages in determining the migration and incorporation processes of fission products into specific minerals in and around natural fission reactors at Oklo and Bangombé.  In my recent work, in-situ isotopic analyses of individual tiny miner- als in and around reactor zones have been performed using a Sensitive High Resolution Ion Micro-Probe (SHRIMP). The aim of this paper is to summarize the recent studies of natural reactors.  This paper consists of two parts.  Sections 2 and 3 review the current topics of geochemical behavior of fis- siogenic isotopes migrated from the natural reactors on the basis of my isotopic studies.  Section 4 reviews the applications of isotopic data to other research fields in connection with radioactive waste containment and particle physics.
2.  Analytical Method
2.1. Geological background of samples.  One of the sam- ples, D81-13, was collected from the Oklo RZ 10.  RZ 10 is in the mine around 310 m depth, and the most preserved from geological weathering and alteration.  D81N is one of five drifts to be accessed to RZ 10.  Figure 1(b) shows a cross section of RZ 10.  D81-13 was collected from a part of the boundary layer between RZ 10 and sandstone layer just beneath the RZ, and contains high-grade uraninite and massive apatite concretions. 7 The sample was used for U isotopic analyses for evidence of 239 Pu adsorption. A series of samples BAX3.1050 to BAX.1290 were collected from one of drilling-cores, BAX3 at the Bangombé site. 8  More than twenty boreholes were drilled to investigate the geological setting of the Bangombé RZ, but only three boreholes (BA145, BAX3, and BAX8) could intersect the RZ.  The samples BAX3.1215, 1225, 1240, and 1290 are sandstones beneath the reactor.  The samples BAX3.1190 and 1050 are clay and black shale layers, respectively, taken from above the reactor. Petrologic type of stratigraphic section of the BAX3 drilling- core is shown in Figure 2.  The Bangombé RZ is located at 11.80 m depth within the groundwater discharge area, and is affected by weathering and chemical reactions due to the groundwater flow.  This suggests that a part of the fission prod- ucts was released from the reactor and distributed in the wall rocks.  Migration behavior of fissiogenic REE can be discussed from the distribution profile of isotopic variations with depth. Several kinds of secondarily formed uranium minerals have been found in peripheral rocks of the RZ. The sample SD37.10 was taken from the sandstone layer near RZ 13.  RZ 13 is located close to a dolerite dyke which intruded the strata 860 million years ago.  The sandstone includes a number of calcite veins possibly caused by hydrothermal activ- ity due to the dolerite intrusion.  The sample was used for Pb isotopic study for evidence of 226Ra adsorption.  Several tiny illite grains sized about 100 μ m were found in fine calcite veins with 0.1 to 2 mm width in quartz matrix texture For comparison of the isotopic data between fissiogenic and non-fissiogenic components, standard materials without fissio- genic component are required as standard materials.  In this study, the Faraday Mine uraninite, PRAP apatite, and NIST610
*PRAP apatite 7 and Faraday mine uraninite 7 were used as standard minerals for ²³⁵U/²³³U isotopic analyses.
standard glass were used.7.8
2.2. Preparation of polished sections.  Each sample was cut and mounted in an epoxy resin disk of 1-inch diameter. The surface of the sample was polished with 1/4 μm diamond paste.  Before in-situ isotopic analyses, mineral observations of all polished section samples were done by an optical micro- scope and an electron probe micro-analyzer (JEOL XA-8200). In particular, several kinds of phosphate minerals such as flor- encite ((REE)Al 3 (PO 4 ) 2 (OH) 6 ), françoisite ((REE)(UO 2 ) 3 O(O H)(PO 4 ) 6 H 2 O) and P-enriched coffinite (USiO 4 ) were identi- fied from the BAX3 drill-core samples (see Figure 2). 2.3. In-situ isotopic analysis by SHRIMP.  Isotopic analy- ses of lighter REE (LREE) in micro-region using SIMS have been effectively used to discuss the migration processes of fis- siogenic isotopes into U- and REE-bearing minerals, because LREE are highly produced by U fission in the natural reactors. Here I focus on Pb, U, and REE isotopic analyses of the indi- vidual minerals by SHRIMP. SHRIMP was originally designed for the analysis of geologi- cal materials and constructed at Australian National University. Physically large mass analyzer consists of a 72.5° sector mag- net with 100 cm turning radius and a cylindrical 85° electro- static analyzer with 127.2 cm radius, which allows for high mass resolution.  The instrument used in this study, SHRIMP II was installed at Hiroshima University in 1997.  2 to 2.5 nA of O 2־primary ion beam focused on a 20 µ m diameter spot was used for the analysis in this study. For Pb and U isotopic analyses, the masses of ²04 Pb, ²06 Pb, 208pb, 235U, and 238U were scanned with a mass resolution about 5800 (M/ΔM at 1% of the peak height).  On the other hand, the mass resolution was set to higher than 9000 to avoid isobaric interferences of oxide and some unknown species onto atomic + ¹44Nd, ¹45Nd, ¹46Nd, ¹47Sm, ¹48Nd, ¹49Sm, ¹50Nd + ¹50Sm, ¹51Eu, 152 Sm, and 153 Eu were scanned for LREE isotopic analyses. 142 Ce, 144 Sm, and 150 Sm isotopic abundances can be obtained from subtraction of ¹ 42 Nd, ¹ 44 Nd, and ¹ 50 Nd isotopic abundances, respectively, because isotopic abundances of ¹ 42 Nd, ¹ 44 Nd, and 150Nd can be expected from those of ¹45Nd, ¹46Nd, and ¹48Nd.8
3. Discussion
3.1. U isotopic ratios of apatite.  In the Oklo samples, 235U depletion ( ²³³U/²³³U < 0.00725) has been generally found, because 235 U was consumed by fission reactions in the reactors. Depleted U migrated from the reactor to peripheral rocks in association with dissolution of reactor U provides U isotopic anomalies even in wall rocks of the reactor.  However, in an extremely rare case, 235U excess ( 235U/238 U > 0.00725) was reported from the Oklo samples. 9.10  The 235U excess is inter- preted as a result of selective uptake of 239 Pu in association with chemical differentiation between U and Pu.  239Pu with a half- life of 2.4 × 10 4 years was produced from the neutron capture of ²³³U in the RZs, and decays to ²³³U. 235U/238U isotopic ratios of apatite and uraninite in D81-13 are shown in Table 1.  Although the data of apatite include large analytical errors because of low U content, significant 235U excesses ( 235U/238U = 0.00944 to 0.01707) are observed from four of five analytical spots in the apatite.  On the other hand, the isotopic data of uraninite coexisting with apatite in the sample show depletion of ²³⁵U (²³⁵U/²³⁸U = 0.00659 to 0.0066), which indicates that the uraninite is a reactor mate- rial that experienced fission reaction.  The ²³³U/²³³³U ratios of apatite in this study are much higher than those previously reported in clay minerals from the sandstone layer around RZ 10 and apatite from RZ 16.9.10 This suggests heterogeneous dis- tribution of ²³⁹Pu out of the reactors. REE data also support the Pu migration in apatite.  High amount of fissiogenic Nd was also observed in the apatite grain. 7 Since one of REE, Nd has been often used as an ana- logue of Pu in geochemical fields, 11,12 it is reasonable that fis- siogenic Nd and nucleogenic 239 Pu were incorporated into the apatite grain.  The petrologic texture of the D81-13 sample shows that a part of uraninite grains were recrystallized, sug- gesting that the sample was subjected to the interaction with thermal fluid that flowed after operation of the reactor. Chemical fractionation between U and Pu might have occurred in association with the partial dissolution of uraninite. 3.2. Distribution of fissiogenic LREE isotopes with depth. Fission and neutron capture reactions are main reactions that cause the significant isotopic anomalies of the elements in a reactor.  140 Ce, 142 Ce, 143 Nd, 146 Nd, 147 Sm, 149 Sm, 151 Eu, and 153 Eu of the samples used in this study include fissiogenic component as well as non-fissiogenic component, because they are not shielded from β - decay.  Considering the fission product yields of individual isotopes, 140 Ce/ 142 Ce isotopic ratios of the sam- ples are expected to be lower than those of non-fissiogenic standard sample.  On the other hand, 143 Nd/146 Nd and 151 Eu/153 Eu isotopic ratios of the samples are expected to be higher than those of standard sample.  The 149 Sm/147 Sm ratios are expected to be lower than that of standard materials because of extremely large neutron capture cross section of ¹ 49 Sm.  Stille et al 13 reported the isotopic variations of 143 Nd/146 Nd and 149 Sm/147 Sm in the BAX8 drill core that is another drill-core cross-cutting the reactor, and concluded that fissiogenic REE were not detected at a larger distance than 3 m from the reactor. Therefore, the attention for redistribution of REE in this study should be paid to the region within 1 m above or beneath the reactor. The most important difference in mineralogy between BAX3 and other drill-cores is the occurrence of uraninite, cof- finite, and phosphatian U-minerals.  The coffinite and phos- phatian minerals are considered to be formed by supergene weathering. 13 As shown in Figure 2, several kinds of second- arily formed U- and REE bearing minerals were found near the RZ. 8.14  Isotopic data of 140 Ce/142 Ce, 143 Nd/146 Nd, and 149 Sm/ 147 Sm show clear evidence of incorporation of fissiogenic isotopes in these minerals, while such evidence is not clearly identified from the 151 Eu/ 153 Eu ratios because of low fission product yields of ¹ ⁵ ¹ Eu and ¹ ⁵ ³ Eu.  The ¹ 40 Ce/¹ 42 Ce, ¹ 43 Nd/¹ 46 Nd, and 149 Sm/ 147 Sm isotopic data vary with the location distance from the RZ.  As one of example, Figure 3 shows depth-depen- dent variations of ¹ 40 Ce/¹ 42 Ce in the BAX3 drill core. 8.14  The reactor core mainly consisting of fissiogenic isotopes has the lowest 140 Ce/ 142 Ce value of 2.209.  The ratios gradually increase with the distance from the reactor in the sandstone layer (1215 to 1290), which reveals that supergene weathering led to the redistribution of fissiogenic REE.  However, the variation of 140Ce/142Ce ratios above the reactor (1050 to 1180) shows a dif- ferent trend from those under the reactor.  In particular, the Ce isotopic variation is rather sharp in the clay layer (depths between 1160 and 1180 cm in Figure 3), which indicates that migration of fissiogenic Ce was limited.  The variations of 143Nd/146Nd and 149Sm/147Sm ratios with depth also show the similar trend. 14  Most of fissiogenic Ce, Nd, and Sm were prob- ably adsorbed by clays.  This result provides a simple illustra- tion of the performance of a clay barrier in the uptake of fission products by adsorption onto clays and incorporation in second- arily formed U-bearing minerals. 3.3. Pb isotopic ratios of illite.  Radium has no stable iso- topes.  226Ra having the longest half-life ( T 1/2 = 1600 y) among radium radioisotopes exists as a radioactive precursor of final decay product from 238U in nature, and finally decays to stable isotope 206 Pb.  Long-lived nuclides such as 226 Ra may be frac- tionated from their parents in association with geochemical events in the terrestrial environments.  It is of particular inter- est to understand the geochemical behaviors of Ra from the viewpoint of long-term repository safety of radionuclear waste. Pb isotopic compositions of geological samples provide chronological information, if a radiochemical equilibrium is established between U and Pb.  The Pb isotopic ratios, 207 Pb/ 206 Pb, in geological samples normally show over 0.04604, because radiogenic 207 Pb and 206 Pb are decay products from 235U (T v 2 = 7.04 × 10 8 y) and 238U (T v 2 = 4.47 × 10 9 y), respec- tively.  The sample SD37.10 used in this study mainly consists of quartz, and contains calcite veins and illite grains.  The 207Pb/206Pb isotopic ratios of quartz, calcite, and illite in the sample are shown in Table 2.  The Pb isotopic data of quartz and calcite in the sample provide the geochronological infor- mation corresponding to the formation of 2.15 billion year-old basement rocks and the occurrence of later hydrothermal activ- ity in this region at 0.88 billion years, respectively. 15  On the other hand, illite shows high enrichment of 206 Pb, which cannot be explained simply from normal U decays. 15 In the case of the Oklo reactor samples, it is also possible to explain the low 207Pb/206Pb isotopic ratios as decay products from ²³³U-depleted reactor material.  Although reactor zone 13 is a small reactor (6 m wide and 10 m long), the vicinity of the reactor has a very high uranium content (up to 87 wt.% UO 2 ) with a high depletion of 235U (235U/238U = 0.0038) as the lowest value. 16 Depleted U can indeed produce lower 207Pb/206Pb iso- topic ratios, however, an extremely high depletion of ²³³U ( 235U/238U = 0.00212) is required to produce the low 207Pb/206Pb ratios (the lowest value = 0.0146) observed in the illite. Therefore, it is impossible to explain the low 207Pb/ 206 Pb ratios by the decay from depleted U even in the most depleted U region of the Oklo natural fission reactor.  Moreover, U isotopic compositions in the illite show normal U isotopic compositions ( 235U/238U = 0.00725) without depletion of ²³⁵U within analyti- cal uncertainties.  Therefore, another reason is required to explain the low 207Pb/206Pb ratios in illite. Chemical data also support that the low 207Pb/ 206Pb ratios were caused by selective adsorption of  226 Ra.  Ba has been often used as a chemical analogue of Ra. 17  Illite grains having low 207Pb/206Pb isotopic ratios also show a strong enrichment of Ba. The Ba concentrations of the illite grains are a few hundreds to a few thousands times higher than those in calcite and quartz coexisting with illite in the sample SD37.10. 15  Considering that Ba has been used as a chemical tracer of 226Ra because of the chemical similarities between Ba and Ra, the extremely low 207Pb/206Pb ratios strongly suggest the selective adsorption of 226Ra as a precursor of 206Pb in the illite grains.

4. Other Applications
Besides the geochemical studies of the migration behavior of fissiogenic isotopes, the isotopic data of RZs have relevance to other topics, geological repositories for the containment of radioactive waste and estimation of time-variation of funda- mental constants. 4.1. Role of organic matters.  Nagy et al. 18.19 studied organic matters in and around the Oklo RZs, and found that solid bitu- mens contain highly condensed aromatic hydrocarbons. Furthermore, they pointed out an important role of the bitu- mens to efficiently constrain mobilization and redistribution of uranium and fission products.  Liquid bitumen is considered to have been produced from organic matters by hydrothermal event during the reactor criticality, and soon became a solid to contain uraninite.  Mossman et al. 20,21 reported the results of in-situ analyses of trace elements in the bitumens by a laser ablation ICP-MS to find enrichments of fission products.  The various types of bitumen were distinguished by reflectance characteristics and δ 13 C values.  The results from the LA-ICP- MS analysis show that fissiogenic alkaline and alkaline earth elements have migrated from RZs either dissolved, or as fine particulate materials, in once liquid bitumen.  This fact has interesting implications for the evaluation of long-term con- tainment of radionuclides. 4.2. Particle physics.  In Physics field, there are fundamen- tal constants such as the speed of light c , the Dirac constant h , the Newtonian gravitational constant G , the Coulomb coupling constant α = e 2 /(4πc h ), and the strong-interaction counterpart of the fine-structure constant α = g 2 /(4πc h ).  Dirac firstly sug- gested the time variation of one of fundamental constants, G , based on his own "Large-numbers Hypothesis". 22  Several approaches have been attempted to set observational and exper- imental bounds on the time variation of fundamental con- stants. 23–25 In fact, there has been great interest by particle physicists in the constancy of the strong interaction coupling constant as a function of time.  Shlyakhter 26 exploited the calculation method of the bounds for α and α 2 from the isotopic variation of 149 Sm due to neutron capture reactions in an Oklo reactor.  After 20 years interval from Shlyakhter's first report, 26 there are several reports to estimate the upper bounds of the constants α and α 2 from the Oklo isotopic data. 3,27,28  It is still disputable to men- tion the conclusive decision about the time-variation of the fun- damental constants from the Oklo study, because there are some ambiguities for the estimation.  However, it is quite remarkable that isotopic studies of the Oklo reactors are of rel- evance to particle physics.
5. Conclusion
Since the discovery of the first natural reactor at the Oklo uranium ore in 1972, a number of systematic isotopic studies have been performed to investigate the geochemical behavior of elements and to characterize the operating conditions of the natural reactors.  In-situ isotopic analyses using a SHRIMP are effectively used to investigate the geochemical characteristics of fissiogenic and nucleogenic isotopes migrated from nuclear reactors in geosphere.  Apatite and clay minerals found near RZs show a strong isotopic evidence of selective adsorption of Pu and Ra, respectively.  REE isotopic data of secondary U- and REE-bearing minerals provide the redistribution of fissio- genic REE due to supergene weathering.  These results were firstly clarified using isotopic observations in micro- to sub- micro region of specific natural minerals.  The isotopic studies provide insights on the mechanism of mobilization and retarda- tion of radioisotopes in repository materials for radiowaste dis- posal.  In addition, it is quite interesting that the isotopic data are also of value in particle physics to examine time-variations of fundamental physical constants. Acknowledgement.  The author would like to acknowledge F. Gauthier-Lafaye, K. Horie, and M. Kikuchi for many helpful discussions and experimental assistances.  A part of this study was financially supported by a Grant-in-Aid for Scientific Research of Japan Society for the Promotion of Science (No. 17204051).