The analysis of uranium ores shows that the ratio of 238U:235U isotopes in the natural uranium is constant (139:1), and the concentration of 235U is about 0.7%. There is just one uranium pitchblende, in Oklo (Gabon), in which the ratio of 238U:235U is higher than the usual value, the concentration of 235U is below 0.5% ( U: U > 200:1). Studies of this uranium mine have shown that the concentra­

tion of the rare earth elements is also higher, and they show similar ratios to the fission products of 235U. For example, natural neodymium contains 27% 142Nd, while the Oklo ores contain less than 5% 142Nd. The 143Nd content, however, typi­cally is 12%, while its concentration in the Oklo samples is 24%. Neodymium formed in the fission of 235U contains 29% 143Nd and no 142Nd isotope.

These values indicate that the fission of 235U could be taken place a very long time ago; that is, a natural nuclear reactor could have been present long ago. Natural water probably acted as the moderator. Based on the composition of the fis­sion products and the uranium content, the properties of the natural reactors are estimated to be as follows: the neutron flux was <109 neutron/cm2 s in the core of the reactor, and its power was less than 10 kW about 2 billion years ago. It con­sumed about 6 tons of 235U, and produced about 1 ton of 239Pu.