Why nuclear power is completely natural (Tom Hartsfield)

Link: http://www.realclearscience.com/blog/2016/02/earths_natural_nuclear_power_plant. html

Why Nuclear Power Is Completely Natural Posted by Tom Hartsfield

Roughly 40% of the American public opposes nuclear power, according to some surveys. We haven't commissioned a truly new plant in 40 years. You might say that for decades, nuclear power has "felt the Bern" of a generally distrustful public glare. Disaster movies,environmentalist nitwits, and eternal doomsayers beat us up about its "unnatural" dangers. page 1

But I wonder if many of these people know that our planet quietly ran its own naturally occurring nuclear fission reactors for more than 100,000 years?

These natural nuclear reactors were born in Earth's crust by pure chance, and yet, they ran for hundreds of thousands of years without exploding in a giant runaway chain reaction, or even melting down in a small-scale reaction, all with no oversight and no fuss. When they burned through their fuel supply, they simply died out and lay dormant until humans came along to discover them.

Scientists have discovered at least 16 natural reactors beneath the surface of the African country of Gabon. They lie in two large deposits of Uranium known as Oklo and Bangombe. Altogether, they produced very roughly 100,000 watts of power at any given time. During their lifetimes, this amounted to about the amount of power that 15 large US reactors produce in one year.

To read a highly detailed description of the Oklo system, written by a scientist involved in researching it, click here.

I'll give you an abridged version: The reactors ran off of the element Uranium isotope U-235. Nearly two billion years ago, when the reactors started up, this isotope was more common in Earth's crust than it is today. When enough of this material is brought in close enough proximity, a nuclear chain reaction takes off. A fissioning U-235 atom spits out neutrons. When these neutrons strike a second and a third U-235 atom, those atoms undergo fission as well, sending out even more neutrons.

The Oklo deposit contained a just-high-enough density of U-235 atoms to put them close enough together for a chain reaction to occur under the right circumstances.

To run continuously, a reactor has to reach a steady balance. For every uranium atom that fissions, one of its neutrons must be absorbed by another atom. If the neutrons produced by one fission cause, on average, more than one further fission, the chain reaction exponentially blooms into a runaway meltdown or explosion. If one fission's neutrons set off an average of less than one subsequent fission, the process withers and dies.

On their own, most of the neutrons produced by a fission are flying out too fast to attain a high probability of sparking the next fission. Getting close to steady balance requires help. This is achieved by a moderator. The moderator is a substance that surrounds the U atoms, forcing the neutrons they give off to pass through. In passage the neutrons are

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slowed down; this drastically increases the probability that they will be absorbed by another U-235 atom which will then fission to continue the chain reaction.

The Oklo reactors used a moderator that is extremely common in 20th century nuclear plants: water. According to research, water naturally flowed into these reactors and moderated for perhaps 30 minutes. Slowly, all of those passing neutrons heated the water until it boiled off. At this point the reaction petered out because the fission neutrons were no longer slowed down and absorbed by other uranium atoms. The reactor cooled down, gradually allowing liquid water to flow back in. After a few hours the reactor filled and fission proceeded again.

And so on. And on. For an eon.

As their natural supply of fissile uranium dwindled, the regulated fission gradually died down and the sites cooled off. The waste products of that reaction were sealed into the ground where they will likely never hurt a soul. The leftover uranium is actually not very useful for building nuclear weapons.

This natural design is at once fundamentally similar to our own reactor designs and totally different in its precise details. But its philosophical importance is very clear. We needn't fear that we are doing the unnatural by harnessing nuclear fission to produce power. Nature itself has run nuclear reactors for a thousand times longer than we have even known the secrets of their power.

Tom Hartsfield is a scientist and writer. He holds a PhD in physics from the University of Texas.

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