Lessons from the Oklo Reactor Waste [10]

11.58. At Oklo, in Gabon, near the equator in West Africa, a nuclear fission reactor located in part of a rich uranium ore vein started sponta­neously about 1.8 billion years ago and ran for several hundred thousand years before shutting down by itself. It is estimated that about 15,000 MW • у of fission energy was produced, yielding about 6 tons of fission products and 2.5 tons of plutonium. Since both uranium-235 and uranium — 238 are radioactive with different half-lives, the relative decay tends to shift the isotopic ratio. At the time of the reactor operation, natural ura­nium contained about 3 percent uranium-235. The existence of ground­water in the ore veins of this tropical area probably led to just the correct conditions for criticality. The reactor probably operated at “slow simmer” and was self-regulating since too much power would tend to boil off the moderating groundwater. Presumably, operation finally ceased when the ore became depleted.

11.59. The most interesting lesson from the Oklo reactor operation is that most of the radioactive waste has remained at the reactor site for over 1 billion years despite the abundance of groundwater. Metals having a valence of 1 or 2 are soluble in water, and were leached away. However, isotopes of these elements have short half-lives and thus would soon be harmless. The elements of concern, particularly plutonium, were retained in the geologic formation and effectively immobilized. Therefore, it would seem that a repository with additional barriers should perform at least as well.