ANALYSIS OF FISSION PRODUCTS AND PU MIGRATION IN THE OKÉLOBONDO REACTOR ZONE USING SIMS

Approximately 2 Ga ago, natural nuclear fission reactors operated in the Oklo-Okélobondo uranium deposit in Gabon, Africa, resulting in the production of fission products, actinides and a decrease in the ²³⁵U/²³⁸U ratio to values as low as 0.0029.  The Oklo reactors have been studied extensively as a natural analogue to the mobility of radionuclides in a nuclear waste repository. Based on isotopic ratios of selected fission products, the reactor neutronics (e.g., neutron fluence, spectrum index, restitution factor and proportion of fission events due to ²³⁵U and ²³⁹Pu fission) can be calculated. Previously, chemical separation and TIMS analyses of bulk samples have been used to characterize the fissiogenic nuclides in uraninite (UO_2+x) to determine the average neutronic parameters of each reactor zone.  In this study, high spatial resolution secondary ionization mass spectrometry (SIMS) measurements of fissiogenic isotopic ratios ¹²⁵Te/¹³⁰Te, ¹²⁸Te/¹³⁰Te, ¹⁴⁵Nd/¹⁴⁶Nd and ²³⁵U/²³⁸U were measured in uraninite from the Okélobondo reactor zone. This reactor zone and periphery (~3 m by 1.5 m) were systematically sampled to reconstruct a two-dimensional distribution of isotopic variations, from which spatial variations in reactor neutronics were determined.  Within a single sample (µm to mm scale), the ²³⁵U/²³⁸U varies by as much as 2.5 %.  At the reactor zone scale (meters), the ²³⁵U/²³⁸U ratio varies by 17 % (0.00643 - 0.00776).  Depletion relative to the present day ²³⁵U/²³⁸U ratio (0.00725) is due to fission of ²³⁵U, which is a function of neutron flux that depends on the reactor zone geometry and composition.  Enrichment of ²³⁵U relative to the present day value occurs in 5 of 18 samples and results from the formation and decay of ²³⁹Pu to ²³⁵U (via neutron capture and b-decay reactions; followed by a-decay; t_1/2=24,100 yr), as opposed to the consumption of ²³⁹Pu that would further deplete ²³⁵U.   Enrichment suggests that water, a moderator in the natural reactors that was heated to temperatures as high as 380° C, caused the mobilization of Pu during criticality. Consistent with this hypothesis, the samples most enriched in ²³⁵U are in uraninite within the organic-rich clays adjacent to the reactor core.  The additional neutronic parameters calculated from the other isotopic pairs and their spatial variation will also be presented.