The natural reactor at Oklo, west Africa
In what is now Gabon in west Africa in 1972, French researchers found a deposit of uranium which had only 0.44% U-235 compared to the normal 0.72%. Some samples were found with a U-235 concentration as low as 0.29%. This indicated that some of the U-235 had undergone nuclear fission reactions at some point in the past. Also, other elements were found which could only have been produced as by-products of nuclear fission.
Models of the process suggested sustained that fission reactions took place over a period of about a million years. The age estimates from cores in the reactor zones suggest a time frame between 1.7 and 1.9 billion years ago. It is presumed that ground water seeping through the ore served as a natural moderator to slow down the fission neutrons. The reason that this natural reactor could run on light water is that natural uranium two billion years ago had a higher proportion of U-235 than it does today. In fact, doing the calculations, the percentage of U-235 was about 3% - essentially meaning that it was equivalent to low-enriched reactor-grade uranium used today.
One of the interesting observations was that the bulk of the fission products seemed to be still in place in their geologic depository after nearly 2 billion years. This could be taken as a suggestion that geologic storage of radioactive waste is feasible.
Uranium forensics - where did it come from?
Surprisingly, there is sometimes a way to tell where a particular lot of uranium ore came from, even when it has been through several processing steps.
As well as the isotopes of uranium that are important for power generation (U-235 and U-238), uranium ore contains trace amounts of a lighter isotope U-234. The concentration of U-34 in a natural deposit is dependent on many factors. The age of the rock, amount of ground water, water permeability of rocks, and assorted geological factors all play a part in altering this concentration.
Ores from different locations therefore tend to have different 'fingerprints' of U-234 composition. The world average is about 54 parts per million (ppm) and the range is between 48 and 62 ppm.
Because U-234 is not very different in mass to U-235, it tends to come through the enrichment process with the U-235, and does so in a predictable way.
Unfortunately, this is not a fool-proof process. Given two samples of natural Uranium from different sources, combining them will produce a sample with a U-234 concentration that is somewhere between the two. For example, combining equal amounts of uranium with 50 and 60 ppm of U-234 respectively will produce a sample with a concentration of 55 ppm.
Of course, you need at least two sources of uranium to hide the origin like that, making it harder to do if you are trying to be covert about your procurement activities.