Long-term retention and chemical fractionation of fissionogenic Cs and Tc in Oklo natural nuclear reactor fuel

Highlights

• Fission product Cesium and Technetium retained in natural nuclear reactor fuel for billion years.

• Retention occurred during criticality; maintained despite local volcanism.

• Cesium and Technetium abundances anticorrelated; both found in metallic aggregates.

• Bismuth in Rhodium/Palladium alloys may be from decayed Neptunium.

• Lead mobility occurred as recently at 300 million years ago.

Abstract

We present the results of a coordinated NAUTILUS and NanoSIMS isotopic study of epsilon (ε) phase metallic aggregates from the Oklo natural nuclear reactor zone (RZ) 13. We observed that fissionogenic Tc and Cs were heterogeneously sequestered within the aggregates. Isotopes of these elements are relevant for improving the safety of spent nuclear fuel storage and reactor operation on generational timescales. Like the noble metals, nearly all of the Tc was retained within the reactor, though its abundance relative to Ru in the metallic aggregates varied by a factor of 10. The neutron fluence estimated from the production of ¹⁰⁰Ru from neutron capture on ⁹⁹Tc was estimated to be up to 1.2 × 10²¹ n·cm^-2. In contrast to Tc, nearly all of the fissionogenic Cs in the reactors was lost from the reactor fuel. The metallic aggregates contain the only phases yet identified to have sequestered radiocesium. Fissionogenic Cs isotopes decay over vastly different timescales, but were incorporated and retained within the ε-phase in proportions similar to stable ¹³³Cs. This indicates that retention began during criticality and sequestration lasted billions of years, despite local geologic activity and the presence of nearby magmatic dikes. Using fissionogenic Ba isotopes, we estimated that the metallic aggregates continually incorporated their radioactive Cs parents during criticality, though the majority of Cs was flushed out of the reactor on a characteristic timescale of 2.7 ± 0.6 years. We found that the abundance of Bi was correlated to Rh and Pd, and speculate that this may have been due to primary Np–Rh and Np–Pd alloys forming during or shortly after criticality. Using Pb–Pb data from uraninite and galena grains surrounding the metallic aggregates, we also inferred a final Pb mobility age of 298 Ma for RZ13, which is more recent than most estimates from other RZs.