Analysis of Fission Products and Pu Migration in the Okelobondo Reactor Zone Using SIMS

Palenik, C.A., Fayek, M., Jensen, K.A., and Ewing, R.C. (2003) Analysis of Fission Products and Pu Migration in the Okelobondo Reactor Zone Using SIMS. Geological Society of America (Seattle, WA).

Presented on: 11/2/2003

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 (UO2+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; t1/2=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.

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