Here is something for you to ponder over.
A natural nuclear reactor
Eventual final disposal of nuclear waste means that the radioactive isotopes will be buried (with appropriate barriers - see "Final disposal" above) in an appropriate disposal site for a very long time. One of the serious questions we should consider is are there going to be any problems far in the future associated with the burial of the waste. The system for final waste disposal is usually based on naturally occurring phenomena. One such natural analogue is naturally occurring nuclear reactors in Gabon, West Africa in the Oklo mine called the Oklo nuclear reactors.
About 1.7 billion years ago, deep underground in Africa, favorable natural conditions prompted nuclear reactions to take place. These natural conditions were sufficient amounts of Uranium-235 and the evolution of plants which subsequently caused rainwater to filter down through cracks in rocks. The water was necessary to slow down the neutrons emitted via uranium decay so that they could interact with other particles and produce nuclear chain reactions. The reactors operated for about 1 million years. The reactions stopped because the uranium depleted to amounts that were too small to keep the reactions going.
It has been shown that the Oklo reactors fissioned Uranium-238, Uranium-235 and Plutonium-239. This is exactly the elements that are fissioned in todays man-made nuclear reactors. Note also that there was no Plutonium-239 on earth when the Oklo reactors formed. This means that the reactors themselves must have produced this isotope. This is also the case for man-made nuclear reactors.
Once the natural reactors burned out they left radioactive nuclear waste. This waste is very similar to the waste generated by nuclear power stations. The nuclear waste was held in place deep underground by granite, sandstone and clays surrounding the reactors' site. The important point is that the waste has not moved much over approximately 2 billion years (see the
fact sheet from Office of Civilian Radioactive Waste Management, USA and the article by Walton and Cowan from the "Proceedings of a Symposium on the Oklo Phenomenon", 1975 listed in the bibliography at the end of this section).
http://en.wikipedia.org/wiki/Oklo
http://nuclearinfo.net/Nuclearpower/WebHomeWasteFromNuclearPower