Source : p.122-24 from
The Ages of Gaia
A bizarre consequence of the appearance of oxygen was the advent of the world's first nuclear reactors. Nuclear power from its inception has rarely been described publicly except in hyper-bole.
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.
During the 1970s, a shipment of uranium from Oklo was found to be depleted in the fissionable isotope 235U.
Natural uranium is always of the same isotopic composition:
Only the 235U isotope can take part in the chain reactions necessary for power production or for explosions. Naturally, the fissionable isotope is guarded carefully and its proportion in uranium subjected to thorough and repeated scrutiny.
Imagine the shock that must have passed through the French atomic energy agency when it was discovered that the shipment of uranium had a much smaller proportion of 235U than normal.
Whatever had happened, a sinister explanation seemed likely.
The truth, when it came, was not only a
fascinating piece of science but must also have been an immense
relief to minds troubled with images of tons of undiluted 235U in
the hands of fanatics.
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.
They performed their unconscious task so well that eventually enough uranium oxide was deposited in the pure state for a nuclear reaction to start. When more than a "critical mass" of uranium containing the fissionable isotope is gathered together in one place there is a self-sustaining chain reaction.
The fission of uranium atoms sets free neutrons that cause the fission of more uranium atoms and more neutrons and so on.
Provided that the number of neutrons produced balances those that escape, or are absorbed by other atoms, the reactor continues.
This kind of reactor is not explosive; indeed it
is self-regulating. The presence of water, through its ability to
slow and reflect neutrons, is an essential feature of the reactor.
When the power output increases, water boils away and the nuclear
reaction slows down.
isotope decays more rapidly than the common isotope 238U, and at beginning the proportion of fissile uranium was not 0.7
percent as now but 33 percent. Uranium so enriched could have been
the source of spectacular nuclear fireworks had any bacteria then
been unwise enough to concentrate it. This also suggests that the
atmosphere was not oxidizing in the early Archean.
(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 or dangerous as the feverish pronouncements of the antinuclear movement would suggest.)
reactors are a splendid example of geophysiological homeostasis.