The Uncertainty of Oclo

Uranium-238 is far more common. It makes up over 99% of all uranium. Oclo was a natural nuclear reactor that arose due to a special set of circumstances 1.7 billion years ago. The sites with the missing uranium had not been disturbed for over a billion years, which ruled out the idea that terrorists had dug up uranium. But then where had it gone? And what could have scorched the rocks?