On Nov. 29, 1955, Experimental Breeder Reactor #1 (EBR-1) outside Arco, ID became the first reactor on the planet to suffer a meltdown, thereby inaugurating the era of nuclear reactor accidents. Now, to be fair, the meltdown was only partial, was due to operator error, and occurred at a time when there were only a handful of reactors operating in the world. Still, the fact that meltdowns go back at least 50 years demonstrates just how fine the line is between sustainable energy source and an outtake from The China Syndrome.

For a quick review, all you need for a nuclear chain reaction is a starter stock of some enriched fissile isotope (say, Uranium-235 or Plutonium-239) and a few free neutrons to start the ball rolling. Hit a nucleus of U235 with a neutron, and it will split into two lesser elements and release a bunch of energy (about 180 million electronvolts) and three neutrons.

This trio of neutrons will go on to hit three more U235 nuclei lying around. Thus, the process will spiral upward in an exponential manner, releasing a lot of energy — the kind that wipes cities off the map, if you have enough fissile material involved.

Of course, it’s not that easy, because the neutrons released by U235 fission (or that of any fissile material) are often thrown off too fast to impact other nuclei. To slow down the neutrons, reactors employ neutron moderators — usually deuterium or graphite — which keep the particles moving at reaction-friendly speeds.

Fair warning: Once you’ve encouraged a chain reaction with a neutron moderator, you need a counterbalance to keep the nuclear reaction under control. That’s where control rods made of cadmium, hafnium, or boron — all of which absorb neutrons — come into play, siphoning off the neutrons that keep the reaction going.

The trick to maintaining a healthy nuclear reactor is controlling the speed and production of your neutrons. Get either factor out of balance, and the reactor will either fizzle or meltdown.

If that sounds complicated, it is. That’s why meltdowns — and sustained fission chain reactions of any kind — are virtually unheard of in nature. Virtually, but not completely unheard of, as there’s at least one place where naturally occurring reactor-style nuclear fission is known to have taken place.


Get the answer.

Where is the world’s only known naturally occurring nuclear reactor located — a geological formation that induced and maintained a fission chain reaction without any human intervention?

In the Oklo region of the Central African republic of Gabon, you’ll find an exhausted uranium mine that, were you to go back in time about 1.5 billion years to the Precambrian, would have been chugging away as the world’s only nature-made nuclear fission reactor. (Reason No. 9,153 that having a time machine would rock.)

Now, don’t go picturing a big lightning-bolts-and-mushroom-clouds event where Mother Nature conjures up an Industrial Light and Magic-worthy meltdown of apocalyptic proportions. The Oklo reactor is really just a geological formation that had the fantastical luck of arranging itself in the perfect fashion to coax some sustained fission chain reactions out of its uranium deposits, none of which likely did much more than throw off some heat, electricity, and radiation that was probably utterly unnoticed by the primitive Mesoproterozoic microbes that were only just then developing the physical capabilities to accommodate sexual reproduction, let alone Geiger counters or Greenpeace demonstrations.

The formation in its most basic form was a vein of uranium ore sandwiched between two layers of sandstone. Groundwater filtered through the sandstone and acted as a neutron moderator, allowing for sustained fission reactions. As the reactions threw off heat, the groundwater evaporated, suspending the reactions and preventing meltdown. This process continued off and on for several hundred thousand years, depleting the U235 content of the ore, which is usually about 0.72 percent.

In 1972, when French nuclear energy officials noted that certain shipments of ore from the Oklo mine did not contain the predictable 0.72 percent of U235, they launched an investigation, fearing that someone had made off with fissile material. After extensive review, they arrived at the unlikely conclusion that uranium ore had been fissioned through natural geological processes — a possibility that had been predicted by University of Arkansas researcher Paul Kuroda in 1956 — but had never been observed before or since.

Despite a few hundred millennia of self-depletion, the Oklo mine still had enough fuel to support 40 years of extraction, 16 of which went by before anyone noticed they were dealing with a natural nuclear reactor. That’s not just an explosive geophysical surprise — it’s some radioactively radical Geek Trivia.

The Quibble of the Week

If you uncover a questionable fact or debatable aspect of this week’s Geek Trivia, just post it in the discussion area. Every week, yours truly will choose the best post from the assembled masses and discuss it in a future edition of Geek Trivia.

This week’s quibble comes from the October 24 edition of Geek Trivia, “Flying by the (ejection) seat of your pants.” TechRepublic member Gezelig dinged me for not having the most up-to-date info on the remaining flights of the space shuttle program.

“Not so much a quibble as perhaps an update. You say, ‘STS-125 will be Atlantis’ last spaceflight,’ yet here you find this: ‘The shuttle program is finalizing the approval of a manifest acceleration that will shake up the remaining flights of the shuttle to ease the Constellation schedule. NASA Associate Administrator for Space Operations Bill Gerstenmaier has been holding meetings on Wednesday to approve the changes, which include flying Atlantis for an additional two flights, adding STS-131 and STS-133 logistics flights to the confirmed schedule, and ending the shuttle program earlier — at the end of March 2010.'”

Never underestimate the power of a government enterprise to extend its operating life. Thanks for the update, and keep those quibbles coming.

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