Radionuclide behavior at the Oklo nuclear reactor, Gabon

The Oklo natural reactor, Gabon, remains the best natural analog for assessing the behavior of fission products, actinides, and actinide daughters in rocks. The rocks at Oklo are porous and permeable, as well as fractured and contain abundant water, yet many of the fission products and actinides have remained in place or close to their formation sites. The actinides Th, U, Np, Pu, and Am are similar in their crystal chemical characteristics, and were all retained in the host pitchblende. Elements incompatible in the pitchblende structure were lost by diffusion or other processes into the rocks surrounding the high-grade reactor ore. Alkali and alkaline earth elements Rb, Sr, Cs, and Ba were fixed in and very close to the reactor ores in clay minerals and in some secondary carbonates and sulfates. Local oxidizing conditions in the reactor zones caused some loss of Tc, Mo, Cd, adn Ru, but Tc, Mo, and Ru, were fixed in sulfides formed under close-by chemically reducing conditions. Local migration for Ag and Sn has been documented for some samples. Probable overall retention is indicated for Rh, Ru, Te, Nb, Zr, and Sb. Deficiencies in fissiogenic I and Kr, Xe have been noted, and data are not available for fissiogenic Br. Also, Oklo remains a logical site for in-depth study of present- and paleohydrology-stable isotopy, as well as distribution of fission products and actinides in UO 2 and surrounding minerals.