Natural uranium generally has precisely the same proportion of readily
fissionable U-235 irrespective of where on Earth it is found. On day in 1972 a
French analyst discovered a discrepancy, a small deficiency in U-235. Tracing
down the source of the uranium, led investigators to the first of sixteen sites
of naturally occurring nuclear fission reactors, like the one pictured at right.
which had operated nearly 2,000 million years ago under conditions
Kuroda quite like had predicted in 1956. The discovery proved that sustained
nuclear fission chain reactions can and in fact did occur in nature.
To many scientists and engineers, the
discovery of the natural nuclear reactors at Oklo were of interest for
demonstrating that under certain conditions nuclear waste, the fission products,
could remain more-or-less immobile for long periods of time. For J. Marvin
Herndon, though, the Oklo discovery was like glimpsing for the first time an
entirely new aspect of natural behavior. Kuroda had predicted that natural
reactors could occur in the distant past in thick seams of uranium ore, and he
had predicted that neutrons would be slowed by ground water, leading to a slow
(thermal) nuclear reactor. Subsequent investigations proved that in the main
Kuroda was correct, but that to a small extent, the natural reactors at Oklo had
also functioned as fast neutron breeder reactors, producing plutonium. That
particular observation, to Herndon, seemed especially important, but nearly two
decades would elapse before the pieces began to fall into place. The consequence
was the beginning of a logical progression of understanding that would
eventually reshape geophysics, planetary physics, and astrophysics. There is
much, much more to this story.