Bill wrote:
> For the record there is a third type of natural uranium
>reactor that was experimented on in the 50's but never
>pursued. It is silicon moderated.

That phrase "natural uranium reactor" instantly brought this to mind
-an actual _natural_, fossil reactor. I know I've seen this somewhere
on Yahoo, but I don't think it's here yet.

In 1972 the very well preserved remains of several ancient natural
nuclear reactors were discovered in the middle of the Oklo Uranium
ore deposit.
... the Oklo fossil reactors have been radioactively dated to be
about 2000 million years old. The uranium in these reactors is
thought to have come from the tiny amounts of uranium orginally
scattered throughout the earth's crustal rocks during its formation.
The evolution of plants and the resulting oxidizing rainwaters
slowly dissolved and mobilized the uranium which was then slowly
concentrated by other chemical conditions into the resulting U
orebodies. This process took hundereds of millions of years...

http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/index.shtml

Another link that google turned up

http://www.alamut.com/proj/98/nuclearGarden/bookTexts/Lovelock_Oklo.ht
ml
Excerpt from James LoveLock: The Ages of Gaia (1988)
A bizarre consequence of the appearance of oxygen was the advent the
world's first nuclear reactors. Nuclear power from its inception has
rarely been described publicly except in hyperbole. The impression
has been given that to design and construct a nuclear reactor is a
feat unique to physical science and engineering creativity.
It is chastening to find that, in the Proterozoic, an unassertive
community of modest bacteria built a set of nuclear reactors that ran
for millions of years.
...
In the place that is now Oklo such a stream flowed into an algal mat
that included microorganisms with a strange capacity to collect and
concentrate uranium specifically.
...
Bacteria could not have debated the costs and benefits of nuclear
power. The fact that the reactors ran so long and that there was more
than one of them suggests that replenishment must have occured and
that the radiation and nuclear waste from the reactor was not a
deterrent to that ancient bacterial ecosystem. (The distribution of
stable fission products around the reactor site is also valuable
evidence to suggest that the problems of nuclear waste disposal now
are nowhere near so difficult and dangerous as the feverish
pronouncements of the antinuclear movement would suggest.)
The Oklo reactors are a splendid example of geophysical homeostasis.

The U at Oklo was more enriched then what we have as the U-235 hadn't decayed
as fully.

Back then a natural U reactor would use light water as the moderator (as Oklo
did).

--- John Frazer wrote: