Did you hear about the natural reactor?
The following is from page 181 of John Lamarsh's
Intro. to NE text (Addison-Wesley Publishing Company - Reading,
MA 1983)
... If a canister holding either a whole fuel assembly or solidified
waste should disintegrate, even soon after its emplacement in a
repository, there is good reason to believe that the fission products
and TRU nuclides would not diffuse far into the environment. Strong
support for this contention is furnished by what has become known
as the _Oklo phenomenon_. Oklo is the name of a uranium mine in
the African nation of Gabon, where France obtains much of the uranium
for her nuclear program. When uranium from this mine was introduced
into a French gaseous diffusion plant, it was discovered that the
feed uranium was already depleted below the 0.711 w% of ordinary
natural uranium. It was as if the uranium had already been used
to fuel some unknow reactor.
And so it had. French scientists found traces of fission products
and TRU waste at various locations within the mine. These observations
were puzzling at first, because it is not possible to make a reactor
go critical with natural uranium, except under very special circumstances
with a graphite or heavy water moderator, neither of which could
reasonably be expected to have ever been present in the vicinity
of Oklo. The explanation of the phenomenon is to be found in the
fact that the half-life of U235, 7.13E8 years, is considerably
shorter than the half-life of U238,4.51E9 years. Since the original
formation of the earth, more U235 has therefore decayed than U238.
This, in turn, means that the enrichment of natural uranium was
greater years ago than it is today. Indeed, it is easy to show
(see Prob. 2.37) that about 3 billion years ago this enrichment
was in the neighborhood of 3 w%, sufficiently high to form a critical
assembly with ordinary water, which is known to have been present
near Oklo at that time.
The relevance of the Oklo phenomenon to present-day disposal of
radioactive wastes is that neither the fission products (identified
by their stable daughters) nor the plutonium migrated from the
Oklo site in the billions of years since the reactor was critical.
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