A Natural Nuclear Reactor

A Natural Nuclear Reactor

Almost forty years ago, analysts at a French nuclear fuel-processing plant stumbled on startling evidence from the Oklo uranium deposit in Gabon, West Africa. What they found, in effect, was a natural geological repository of spent reactor fuel.

In the modern world, uranium isotopes appear in very stable proportions, whether they're found in the Earth's crust, on the Moon, or in meteorites: we expect to see mostly U-238, a tiny trace of U-234, and a uniform 0.720 percent of U-235, the fissile material that will sustain a nuclear chain reaction. If the proportion of U-235 is short, alarms go off. The Gabon proportion was only 0.717 percent, which seems like a small discrepancy, but that made the total shortfall 200 kilograms (440 pounds), enough to make six or so bombs.

After some excitement, the analysts realized that conditions in the distant past had been just right to permit the uranium ore to undergo spontaneous self-sustained fission, which used up some of the U-235 that should have been there. One required condition is a uranium ore deposit at least several feet thick; this ensures that the emitted neutrons, which travel no more than a couple of feet on average, will be absorbed by other uranium atoms before escaping the vein of ore. Another requirement is the presence of groundwater, which acts as a neutron moderator (it slows the bouncing particles down). A third requirement is the absence of neutron-absorbing impurities such as boron or lithium.

Lest we panic at the notion of natural China Syndromes popping up all over the Earth, it's comforting to learn that one additional required condition no longer obtains anywhere we know of in the Solar System: the proportion of U-235 must be around 3 percent, as it is in the kind of enriched uranium that fuels power stations. These days the natural proportion of U-235 is always under 1 percent, but the same was not true two billion years ago.

Examination of the fissile products at Gabon shows that this ancient reactor was active for several hundred thousand years, during which time it produced more than two tons of plutonium, some of which itself underwent fission to form lighter elements. Intermediate fissile products included iodine and tellurium and, finally, stable xenon gas, which was trapped in nearby minerals and preserved throughout geological ages for study by fascinated physicists today. Not every bit of the heavier elements decayed, but what stayed behind stayed put to a surprising degree. The remaining plutonium, for instance, has moved less than ten feet in 2 billion years.

The energy rate of the natural reactor was not high -- perhaps 100 kilowatts, or about nine times the size of my household's emergency generator -- but it went on long enough to produce 15 gigawatt-years before winding down. There is no evidence of an explosion, only a long, slow simmer at perhaps 300 degrees Celsius (500 degrees Fahrenheit) until the proportion of fissile U-235 dropped too low to sustain any further reaction.

Sources: Scientific American; ecolo.org; Wikipedia