The Oklo Reactor
It has been seventy years since Otto Hahn, a German chemist working at the Kaiser-Wilhelm Institute of Chemistry, first detected barium after neutron bombardment of a uranium atom and since then fission has not left the spot-light. There is something about nuclear fission that captures the human imagination. "There's an element of magic in it," says the character of Neils Bohr in reference to fission in Michael Frayn's fictional play Copenhagen, "you fire a neutron at the nucleus of a uranium atom and it splits into two other elements. It's what the alchemists were trying to do-- to turn one element into another."
However, it is not only this chemical card-trick that impresses human minds; the awesome power unleashed is stunning, to put it mildly. Nuclear fission produces more energy per unit mass than the most terrible high explosives known to man. Only sixty kilograms of uranium-235 are necessary to destroy a city and are the equivalent to 13 thousand tons of TNT. For over 50 years this nuclear reaction has been used to harness energy and as of July 2008 there are 439 nuclear power plants in the world, operating in 31 countries, and producing over 14% of the world's electricity. Nuclear fission is thought of as one of the most incredible inventions of mankind. However, what most people don't know is that two billion years ago there existed a fission reactor under the soils of Africa which operated in a self-controlled and self-contained manner for hundreds of thousands of years.
It all stated in 1972 when French mining researchers in Gabon, Africa unearthed a strange find in a uranium deposit. Although the samples of uranium ore looked exactly like any other they had an incredibly odd isotope signature. Nearly half of the fissible isotope U-235 was gone. Since all natural uranium in our solar system has the same isotope signature this was odd, indeed. A few hypotheses were put forth, most dealing with human calculation errors and samples contamination, but when the strange uranium composition was, without a shadow of a doubt, traced back to the ore deposit the situation called for further investigation. An analysis of the elements present found 30 which are direct products of uranium fission. In fact, the type of uranium unearthed in Oklo was so similar in composition to that of spent fuel in reactor cores that only one conclusion was plausible: the uranium deposit in Oklo had at one point in time been home to a naturally occurring fission chain reaction.
What triggered the reaction was water which seeped in through the sandstone in which the deposit was found. The water's unique containment in the uranium ore was key to the reactor's initiation. During it's time of operation, 2 billion years ago, the Oklo reactor had the same uranium composition as modern fuel cells and thus the deposit (unlike deposits today) was viable nuclear fuel. The water's role in the process is termed "water-moderation" by modern nuclear engineers. Whenever a uranium atom would erupt in spontaneous fission the neutrons it released were reflected back into the mass of uranium by the surrounding water particles. In this manner, a chain-reaction was started and the uranium deposit began fissing large amounts of uranium to create heat (like modern reactors).
What's even more impressive is that the reactor was self-regulating. When large amounts of water seeped into the mass of uranium it began its fission cycle which released heat and energy. If the reactor began to overheat the water would evaporate and the reactor would shut down. The reactor would then cool down and ground water would begin to slowly seep back in to the ore initiating the process, once again. This ingenious system kept the reactor running for hundreds of thousands of years. The reactor would turn on and off (as often as 10 times a day by some recent estimates) and the special conditions made sure a meltdown never occurred.
Another amazing thing about the Oklo reactor is that the nuclear waste was incredibly well confined. The sandstone held the radioactive materials with almost zero loss of matter. Today, containing the nuclear waste produced by nuclear power plants is a question of great importance. Nuclear waste is highly water soluble and reacts with atmospheric oxygen so if any amount of nuclear waste were to be left uncontained it would quickly spread through the world. With over 400 commercial power plants running worldwide, the waste produced can have catastrophic consequences. Reactor waste is currently sitting in metal containers in storage yards and no long-term solutions have been devised.
Nuclear waste has an incredibly long life-span: nuclear elements have half-lives lasting billions of years, in some cases, so storing them is a particular challenge to engineers. Engineers build things to last decades, centuries, sometimes millennia, but never has an engineer built something to last an eon. The Oklo reactor sites are an encouraging discovery because they show that it can be done. Currently, the prospects of storing the nuclear waste in large mountains are being explored and hopes are high that a solution will soon be attainable. Still; a billion years is a long time.
One thing, however, is certain: with its self-regulating and self-containing reactor, the Oklo site is an amazing feat if engineering by any standards. We, as a species, have to keep in mind that when we encounter a technological problem the chances are high that nature has already solved it. Mother Nature is an engineer, a bio-chemist, a physicist, a geologist, and more. The wealth of knowledge contained on Earth is beyond our comprehension. We must look to nature in order to solve our modern problems because if we can learn anything from discoveries like the Oklo reactor it's that we have a lot to learn.