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

Significance The Oklo natural nuclear reactors provide a wealth of information regarding the migration and retention of fission products in nuclear wastes. Radioactive volatile and gaseous elements easily escape from reactor fuel into the environment without proper containment. Cesium, in particular, represents a significant environmental and health hazard. Here, we used an isotope imaging system to identify the location of sequestered fissionogenic Cs and Ba in Oklo. Cesium and Ba were captured in Ru metal/sulfide aggregates shortly after reactor criticality ceased. These elements were otherwise nearly completely lost from the reactor. We have further discovered the most depleted natural U on Earth, indicating that these fission products were retained in the most active region of the reactor.