Discovery of fissionogenic Cs and Ba capture five years after Oklo reactor shutdown

Understanding the release and sequestration of specific radioactive signatures into the environment is of extreme importance for long-term nuclear waste storage and reactor accident mitigation. Recent accidents at the Fukushima and Chernobyl nuclear reactors released radioactive ¹³⁷Cs and ¹³⁴Cs into the environment, the former of which is still live today. We have studied the migration of fission products in the Oklo natural nuclear reactor using an isotope imaging capability, the NAval Ultra-Trace Isotope Laboratory's Universal Spectrometer (NAUTILUS) at the US Naval Research Laboratory. In Oklo reactor zone (RZ) 13, we have identified the most depleted natural U of any known material with a ²³⁵U/²³⁸U ratio of 0.3655 ± 0.0007% (2σ). This sample contains the most extreme natural burnup in ¹⁴⁹Sm, ¹⁵¹Eu, ¹⁵⁵Gd, and ¹⁵⁷Gd, which demonstrates that it was sourced from the most active Oklo reactor region. We have discovered that fissionogenic Cs and Ba were captured by Ru metal/sulfide aggregates shortly following reactor shutdown. Isochrons from the Ru aggregates place their closure time at 4.98 ± 0.56 y after the end of criticality. Most fissionogenic ¹³⁵Ba and ¹³⁷Ba in the Ru migrated and was incorporated as Cs over this period. Excesses in ¹³⁴Ba in the Ru point to the burnup of ¹³³Cs. Cesium and Ba were retained in the Ru despite local volcanic activity since the reactor shutdown and the high level of activity during reactor operation.