Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

Ba isotopic studies of the Oklo and Bangombé natural fission reactors in east Gabon provide information on the geochemical behavior of radioactive Cs (135Cs and 137Cs) in a geological medium. Large isotopic deviations derived from fissiogenic Ba were found in chemical leachates of the reactor uraninites. The fissiogenic Ba isotopic patterns calculated by subtracting the non-fissiogenic component are classified into three types that show different magnifications of chemical fractionation between Cs and Ba. In addition, the isotopic signatures of fissiogenic 135Ba, 137Ba and 138Ba suggest an early differentiation between Cs and Ba of less than 20 years after the production of fissiogenic Cs and Ba. On the other hand, only small excesses of 135Ba (ε < +1.8) and/or 137Ba (ε < +1.3) were identified in some clay samples, which might have resulted from selective adsorption of 135Cs and 137Cs that migrated from the reactors by differentiation.