Oklo Nuclear Reactor

In the 1970s, uranium samples from the Oklo uranium mine in Gabon, Africa, were discovered to have abnormally high concentrations of the isotope U-235, as high as 3 percent, when only about 0.72 percent of the isotope was expected in a natural source. Supposedly the high concentration of U-235 is explained by realizing that the uranium deposits at Oklo acted as a natural nuclear reactor. Could this natural reactor have been a breeder reactor making its own Pu and U-235?

Answer

The reaction sequence shows how to breed Pu from local U-238, which is the most abundant naturally occurring uranium isotope. Initially, neutrons come from the fission of U-235. However, the very high abundance of U-238 means that this isotope will absorb some of the neutrons to become U-239, decay by beta decay to neptunium 239, and then decay to Pu- 239. Some of the resulting Pu-239 undergoes fission.

U-238 + n → U-239 → Np-239 + e– + anti-ν

Np-239 → Pu-239 + e– + anti-ν

However, because the natural reactors at Oklo probably operated for such a long time, the Pu-239 had time to decay by alpha decay to U-235. Thus the Oklo natural reactors were true breeder reactors, fissioning more U-235 than originally existed in the reactors. The evidence for the breeder process remains in the reactor as more of the fission products than could possibly be produced by the amount of U-235 that has been lost from each of the reactor sites.

A second piece of evidence for Pu fission is the isotopic composition of the fission products in the mass range 100 to 110. To breed Pu and additional U-235, the reactors must have operated for periods significantly greater than the half-life of Pu 239, about 24,360 years.