Geochemical Controls on the Retention of Fission Products at the Oklo Natural Fission Reactors

Abstract

A unique example of the utility of isotope geochemistry may be found in studies of the isotopic abundances of elements in samples from the Oklo uranium mine in the African Republic of Gabon. These studies demonstrate that two billion years ago, in a span of a few hundred thousand years, nuclear fission and associated processes profoundly changed the isotopic and chemical composition of discrete lenses of highly uraniferous rock within the mine (1,2). These changes can be quantified by characterizing operating parameters of the natural nuclear fission reactors and interpreting them in the context of the systematics of the nuclear reactions. Comparisons between the present, measured, composition of the natural reactor remnants and their calculated nuclear-reaction induced composition provides a quantitative characterization of compositional changes in the rocks during the last two billion years.