This topic should probably should be moved to 'against the mainstream' section.

I don't really understand the nuclear engineering aspects of this very well, but there's certainly evidence that natural reactors existed in the past. (Ever'thin I know I learnt from the 'help' files...)

"At Oklo, as on the rest of the earth and solar system, 2000 million years ago the relative abundance of 235U was 3000 atoms per 100,000 atoms. This is one of the major reasons why nuclear fission started."

"Natural fission reactors cannot form today because there is insufficient 235U in natural U. There are also several other important factors which must be satisfied before natural fission reactions will commence."

Reference
from Dr Robert Loss's pages on the Oklo fossil reactors.

It's an interesting topic and I'll see if I can wrassle up some expert opinions. (Because my own is at this point uninformed and flabby... )

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Jim he allowed [the stars] was made, but I allowed they happened; I judged it would have took too long to make so many. Jim said the moon could 'a laid them... --Huckleberry Finn