Fission product retention in the Oklo natural fission reactors

Abstract

Eight samples from a planar cross section of one of the natural fission reactor zones at the Oklo U mine were analyzed to determine the abundances and isotopic composition of U, Mo, Ru, Pd, Ag, Cd, Sn, Te and Nd. Fission product concentrations were calculated from the isotopic compositions. The relative abundances of these fission products are different from those produced by fission i.e. portions have been lost or gained from all the samples. The proportions of Te, Ru, 99Ru, Pd and Mo in the eight samples are invariant. We attribute this regularity to retention at the site of production—primary retention. Based upon this interpretation, we suggest that fixed proportions of Ru, Tc, Pd and Mo have been removed from the reactor zone. In contrast, Nd and Sn have been depleted in some regions of the zone and enriched in others. Portions of Ag and virtually all the fissiogenic Cd have been removed from the reactor zone. By analogy with anthropogenic spent fuel, we suggest that the degree of primary retention was controlled by phase assemblages formed in the Oklo natural reactor fuel in response to microscale conditions of pressure, temperature and composition produced by the nuclear reactions. The ability of these putative minerals to retain nuclear products was a function of their stability under physiochemical conditions established by the geological environment over the last 2 Ga. An integrated study of the natural reactors would test our hypothesis and provide valuable information for evaluating the long-term effectiveness of anthropogenic spent fuel as a container of nuclear waste in the geochemical environment.