Geek Trivia: (Melt)down to Earth

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?

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.


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