ABSTRACT
One of the more extraordinary consequences of the enormous age of the earth and the relatively short half-life of uranium- 235 is that at least one natural nuclear reactor sprang into spontaneous operation about 2 billion years ago. Imagine a brilliant cryptographer poring for years over an ancient piece of stone inscribed in some unknown language. Imagine his delight turning to amazement when he cracks the code and hesitatingly transcribes the following words: recipe for a home-reactor—take one bathtub full of uranium ore; add water and stir. The total estimated amount of missing uranium-235 indicated that the total energy produced by the Oklo reactor in its lifetime was about equal to that delivered in a year by about six modern power reactors. However, it should be remembered that this is only one illustration of the behaviour of plutonium over very long periods. It may be that the Oklo environment was particularly favourable for plutonium immobilization.
