Here is an interesting problem for young earth creationists. In 1972 an area with unusual properties was discovered in the Oklo uranium mine in Gabon. Usually, the ore from this mine was very high grade, containing as much as 70% uranium. During routine assay of the ore it was discovered that one particular area had different properties. The percentage of uranium was lower than in surrounding ores. Further, the ratio of U235 to U238 was lower. Additionally, the ratio of Pb206 to Pb207 was also different from the surrounding ore, indicating lower levels of U235 in the initial ore body. Even more important was the fact that this particular area had all of the characteristics of spent nuclear reactor fuel after the daughter elements of a fission reaction had decayed.
Many of you probably need some background information about the above and why it is unusual. In order to start and maintain an fission reaction with uranium, the fuel must contain in excess of 3% U235. The percentage of U235 to U238 in the world today is less than 1%. Typical fission reactors use enriched fuel with about 3.3% U235 to 96.7% U238. Given a mass of uranium fuel placed in close enough proximity and surrounded by a moderator (to slow down fast neutrons that bounce off the nucleus of atoms), a fission reaction can begin and be maintained.
At first, some sort of sabotage to steal uranium was suspected. So another sample from the same site was extracted and assayed. The results were the same. The percentage of U238 was low, the percentage of U235 was low and the ratio between the two was way off. In addition, elements that could only be the product of a fission reaction or the daughter products of such radioactive elements were found in predicted percentages.
The conclusion was clear. Oklo was a natural nuclear reactor. At some time in the past the percentage of fissile elements was high enough to begin and maintain a nuclear reaction. The moderator seems to have been a aquifer that ran through the ore body.
Calculations on the decay rate of U235 vs U238 from present day and projected into the past indicate that the percentage of U235 would have been about 3%, enough to sustain a fission reaction, some 2 billion years ago. Further, there are no radioactive fission products present in the ore body. Instead, there are the decay products of those elements.
Moreover, any fission reaction in the presence of U238 will result in the production of Pu239. (U238 will capture a free neutron from the fission reaction and become U239. U239 will decay (1/2 life of 24 minutes) by beta decay to Neptunium [Np239], which decays by beta decay (1/2 life of 2 days) to Pu239. Pu239 is also fissile, just as is U235. In fact, about 1/3 of the power from a commercial nuclear reactor comes from the fission of Pu239 (Which of course, will produce more plutonium if U238 is present). The process of neutron capture continues so that Pu239 becomes Pu240 that becomes Pu241 that becomes Pu242. The fuel spent fuel from modern nuclear reactors contains about 0.52% Pu239, 0.21% Pu240, 0.10% Pu241, and 0.05% Pu242.
And yet, the ore from the 17 unquestionable natural nuclear reactors at Oklo do not contain any detectable plutonium. If the earth is less than 10,000 years old, then where did it go? (Pu239 = 1/2 life 2.411x10^4Y, Pu242 = 1/2 life 3.76x10^5Y)
I hope I have explained this clearly. I am willing to answer any question you may have.