About the Oklo Natural Uranium Reactor in Gabon

by James Lovelock

 

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.

This extraordinary event occurred 1.8 eons ago at a place now called Oklo in Gabon, Africa, and was discovered quite by accident. At Oklo, there is a mine that supplies uranium mainly for the French nuclear industry. 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 : 99.27 percent 238U, 0.72 percent of 235U, and traces of 234U.

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.

Had some clandestine group in Africa or France found a way to extract the potent fissionable isotope, and were they now storing this for use in terrorist nuclear weapons? Had someone stolen the uranium ore from the mine and substituted spent uranium from a nuclear industry elsewhere? 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.

The chemistry of the element uranium is such that it is insoluble in water under oxygen-free conditions, but readily soluble in water in the presence of oxygen. When enough oxygen ap- peared in the Proterozoic to render the ground water oxidizing, uranium in the rocks began to dissolve and, as the uranyl ion, became one of the many elements present in trace quantities in flowing streams.

The strength of the uranium solution would have been at most no more than a few parts per million, and uranium would have been but one of many ions in solution. 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.

A nuclear fission reaction is a perverse kind of fire; it burns better when well watered. The Oklo reactors ran gently at the kilowatt- power level for millions of years and used up a fair amount of the natural 235U in doing so.

The presence of the Oklo reactors confirms an oxidizing envi- ronment. In the absence of oxygen, uranium is not water soluble. It is just as well that it is not; when life started 3.6 eons back, uranium was much more enriched in the fissile isotope 235U. This isotope decays more rapidly than the common isotope 238U, and at lifeís 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.

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 occurred 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 or dangerous as the feverish pronouncements of the antinuclear movement would suggest.) The Oklo reactors are a splendid example of geophysiological homeostasis.

 

Source : p.122-24 from The Ages of Gaia

 

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Short biography of the Author

James Lovelock : independent scientist, environmentalist, author and researcher, Doctor Honoris Causa of several universities throughout the world, he is considered since several decades as a one of the main ideological leaders, if not the main one, in the history of the development of environmental awareness. James Lovelock is still today one of the main authors in the environmental field. He is the author of " The GaiaTheory ", and " The Ages of Gaia ", which consider the planet Earth as a self-regulated living being, as well as, more recently his "Homage to GAIA", an autobiography published in september 2000.

James Lovelock is in favor of the use of clean nuclear energy, respectful of the environment : read the introduction of James Lovelock to the book "Environmentalists For Nuclear Energy". He supports the Association of Environmentalists For Nuclear Energy (EFN).

James Ephraim Lovelock (born July 26, 1919) is an independent scientist, author, researcher and environmentalist who lives in England. He is most famous for proposing and popularizing the Gaia hypothesis, in which he postulates that the Earth functions as a kind of superorganism. He studied chemistry at Manchester University before taking up a Medical Research Council post at the Institute for Medical Research in London. In 1948 he received a Ph.D. in medicine at the London School of Hygiene and Tropical Medicine. Within the United States he has taught at Yale, Baylor University College of Medicine, and Harvard University.

A lifelong inventor, some of his inventions were adopted by NASA in their program of planetary exploration. It was while working for NASA that Lovelock developed the Gaia Hypothesis. Lovelock is currently president of the Marine Biological Association, was elected a FRS in 1974, and in 1990 was awarded the first Amsterdam Prize for the Environment by the Royal Netherlands Academy of Arts and Sciences.

Lovelock was among the first researchers to sound the alarm about the threat from the greenhouse effect. His opinion is that "Only nuclear power can now halt global warming."

Some of James Lovelock's books

Gaia: A New Look at Life on Earth (1979, 3rd ed. 2000) Oxford University Press. ISBN 0192862189
Gaia: The Practical Science of Planetary Medicine Gaia Books (1991 ed.) Oxford University Press (2001 ed.) ISBN 0195216741
Homage to Gaia: The Life of an Independent Scientist Oxford University Press (2000) ISBN 0198604297 (Lovelock's autobiography).

For more information on James Lovelock's fascinating life as an independent scientist & environmentalist, you may click here or read his autobiography "Homage to GAIA".