Natural Fission Reactors of Oklo, Gabon, West Africa

There are two sources of information (laboratory studies, and the natural reactors of Oklo, Gabon) for projecting the long-term stability of UO2 in the spent nuclear fuel, and the mobility of fission products and transuranium elements, under different geologic settings. Uraninite, UO2+a, and UO2 in the spent nuclear fuel, have similar structures, and similar geochemical behaviours. When U(IV) gets oxidized into U(VI) under oxidizing conditions, similar alteration products come into existence. Thus,
laboratory investigations could provide insights regarding the behaviour of the spent nuclear fuel in the repositories (Bruno et al., 2002).

Remnants of spontaneous fission that took place in a uranium deposit in Oklo, Gabon, two billion years ago, provide a unique natural “window” to view the fission processes, spent fuel, and the dynamics of the fission products and actinide elements (Janeczek, 1999). These reactors are thus capable of indicating the shape of things to come for the spent fuel in the repository. The reactors were of small size (a few metres). Two billion years ago, the uranium ore in Oklo contained 235U at the con­centration level of 3.5%, which is comparable to the concentration of 235U in the nuclear fuel in the light water reactors. Water and in some cases, carbon, served as moderators. By chance, there was no vanadium to absorb the neutrons. Quartz in the surrounding sandstone served as reflector. The temperature of the reactors was 400-500°C in the peripheries, and about 1000°C in the centre. Small hydrothermal systems came into existence around the reactors. The reactor got turned on when water, the moderator, entered the pores of the rock. The reactor got turned off when the heat in the reactor, evaporated the water and drove it out. Later when the reactor cooled sufficiently to allow water to enter the pores, the process started once again. It has been estimated that these reactors fissioned about ten tonnes of 235U over a period of several hundred thousand years. Naudet (1991) gives a fascinating account of these fossil reactors.

There are a number of analogies between the laboratory observations of spent fuel and what has been found on the ground in the case of the Oklo reactors: capture and incorporation into apatite of 239Pu (which is now in the form of its decay product, 235U), and the presence of micrometer to nanometer-sized metal alloy inclusions of Mo, Ru, Pd, T c, and Rh – the epsilon phase. Thus, many of the processes of dissolu­tion, alteration and retardation processes observed in the case of UO2 in the spent nu­clear fuel could be found in Oklo, thus strengthening our confidence in predicting the behaviour of the spent fuel over hundreds of thousands of years (Bruno et al., 2002).