Driving around southern New Jersey, northern Delaware and southeastern Pennsylvania, you can occasionally glimpse the characteristic distorted hourglass shape of a nuclear cooling tower to the side of the road.

Regardless of how you feel about our use of nuclear power, you have to be impressed with the grand technology involved. Fuel rods, atomic fission, computer-controlled moderators, cooling towers, hundreds of megawatts of power. Surely only human beings are capable of assembling such a technological marvel!

It's possible, however, that natural nuclear reactors were operating two billion years ago, long before human beings walked the Earth.

Scientists have located 16 or more places in Equatorial Africa that appear to be fossil fission reactors.

To understand how this can be possible, we must think about what's necessary to have a self-sustaining nuclear reaction.

First, you need fuel. Uranium is the most likely candidate; pockets of uranium ore occur in many places on the Earth. This element has been a part of the Earth since its formation more than four billion years ago.

There are two main types, or isotopes, of uranium. One isotope is easier to split apart (fission) than the other. U235 (so-called because it has a total of 235 neutrons plus protons in its nucleus) undergoes fission much more readily than its heavier cousin, U238.

Unfortunately for power plant planners, there is a lot more U238 around than U235. Less than one per cent of today's uranium ore is made up of the good stuff. Generally you need U235 to make up at least three-percent of your fuel supply to keep a reaction running. Today we must do substantial work to raise the concentration of U235 to a useful level in our nuclear fuel.

Things were different in the past. Because U235 is much more radioactive than U238, more of it has disappeared (actually been converted into different elements) since the Earth's formation, so there was more of it around in past ages. Around two billion years ago, the lighter isotope did make up around three-percent of the total. If we could somehow gather uranium ore into concentrated packets, we'd have our “fuel rods.”

It's a nice coincidence that a process had come into existence that could do that gathering. Green plants were starting to release oxygen into the atmosphere about 3.5 billion years ago. As a result, water became acidic, giving it the power to dissolve trace amounts of minerals from crustal rocks, then deposit them in relatively concentrated chunks in river beds and deltas.

In Oklo, Gabon, in Africa, seams of rock a few inches thick and rich in uranium oxide were created in several locations. This satisfied the fuel requirement.

But one other important item was needed. When a uranium atom fissions, or splits apart, it releases one or more neutrons (and a big dollop of energy.) These subatomic particles can then be captured by neighboring uranium atoms, which makes them unstable and likely in turn to split in two, releasing more energy and more neutrons. The result is a “chain reaction” that provides the power output of the reactor.

It doesn't always happen, however. The neutron from the first split must encounter the new atom gently, at moderate speeds, or it won't be captured at all. Unfortunately, when the neutron is ejected from the first atom, it can be traveling at one-sixth the speed of light! We need a moderator, something to drastically slow down those fast neutrons.

Some modern reactors use graphite rods as moderators. But simple water works well, too, and is used in a majority of today's reactors. It also worked nicely in the Oklo reactors.

At some point, water covered the seams of uranium oxide exposed by natural geological processes. The slow neutrons could now be captured by U235 atoms, and a nuclear chain reaction could take place.

Of course, it wasn't as efficient as our power plants. For one thing, the reaction raised the temperature of the ore enough to boil away the water, which stopped the reaction altogether. Some scientists have suggested a reactor might work for half an hour before losing its water moderator, then shut down for a few hours until things had cooled off so the water could return. You might think of this as a nuclear geyser, though the water probably didn't shoot up into the air as it does in geothermal geysers.

How powerful were these reactors? One of the larger ones might have produced 100 kilowatts, enough to power a few typical homes if it could be harnessed. Of course, you'd only have power for half an hour out of every three hours, so you'd have to plan your usage well!

These natural nuclear power plants may have worked for 100,000 years. They aren't possible now. The level of natural U235 remaining in uranium ore today is no longer high enough to support a self-sustaining reaction. But long ago, we might have been able to find some nuclear hot spots to warm our cold toes, if we had been around at the time.

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Astronomical events.

The Rowan University Department of Physics and Astronomy will hold a public observatory open house on Wed., Sept. 22, to celebrate the autumnal equinox, the Harvest Moon, the closest approach of Jupiter in many years, and the conjunction of Jupiter and Uranus (whew!) The observatory and observing deck will be open by 8 p.m. Brief live presentations in the planetarium will be offered by yours truly at 7:30 and 9 p.m. This event is free and open to the public.

The Edelman Planetarium will open its doors to the public on Saturday, Sept. 25. We'll be bringing back a popular star show, “The Rowan Universe,” at 7 p.m. each Saturday night. If you are looking more for entertainment than education, we will offer a new laser show, “Laser Michael Jackson” at 8:15 on the same nights. More information is available the planetarium website,

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Keith Johnson is the director of the Edelman Planetarium at Rowan University, and wishes he had a copy of

Tom Swift in the Caves of Nuclear Fire

, the first popular mention of natural nuclear reactors. He'd be glad to hear of an available free copy — e-mail him regarding this or anything astronomical at

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