JUN 11, 2015 4:08 PM PDT

An Old (Much Better and Safer) Reactor Design Is New Again

WRITTEN BY: Andrew J. Dunlop
What if someone could create a reactor that not only didn't produce nuclear waste, but could actually used nuclear waste as a fuel. What if this reactor was safe by it's very design. What if when it lost power, instead of melting down, it would automatically shut down and cool down? Well, it turns out that there is a whole other way of making a nuclear reactor called a molten salt reactor that was abandoned in the 1950's that does all of these things. Leslie Dewan and Mark Massie, rediscovered this abandoned technology when they were students together at MIT. They are now forming a new company called Transatomic to research, design and build this alternative design of nuclear reactors commercially.



Okay, now before you go getting all freaked out, let's be clear: we are talking here about a totally different kind of nuclear reactor. The kind that you're used to is called a light water reactor. That's what Three Mile Island and Chernobyl and Fukushima were. It's the only kind of reactor that has ever been built for commercial power production. But, as you may have noticed, this design has some major downsides, most of which you know about: they can melt down (see above), they produce highly radioactive waste mostly in the form of spent fuel rods. Here's a downside you may not have known about: the reason why the spent fuel rods are still so highly radioactive is that they only use about four percent of their fuel by the end of their useful life.

In a light water reactor fuel rods filled uranium pellets are submerged in water which, in the process of slowing down the uranium's neutrons, heats up and creates steam. This process creates poisonous substances like xenon and krypton, (the element, not Kal-El's home planet) which accumulate in the rods and, within about four years, render them incapable of being used as fuel rods anymore. So you have these still highly radioactive spent fuel rods that have to be stored in pools that have to be refrigerated to keep them from starting fires. And guess where all of these spent fuel rods are stored? At the nuclear plants they used to power. But don't worry. Even though there are getting to more and more of these things, they won't be radioactive for ever, only a hundred thousand years or so.



Molten salt reactors are completely different. In them, there are no fuel rods. The uranium (which, by the way, could be from spent fuel rods from light water plants) is heated above 500°C at which point it turns to liquid. Then, instead of being submerged in water, it flows past zirconium hydride, which slows its neutrons and induces fission, creating heat. There are no rods to trap krypton and xenon, so it is, instead, continuously off-gassed. "You basically simmer the reactor like a Crock-Pot for decades," says Dewan. "That's how we achieve a 96 percent burn rate. We're able to leave the uranium in and constantly remove the poisons that would otherwise shut it down." And, get this: the fuel salt stays in a loop with a drain that's blocked by something called a freeze plug, a chunk of electrically cooled frozen salt. Astoundingly, if the reactor loses electricity, instead of melting down the way a light water reactor would, the freeze plug melts, and the fuel drains into a tank where it cools turns into a solid mass. Once power is restored, the fuel can be re-heated and power production can resume.


(Source: Popular Science)
About the Author
Andrew J. Dunlop lives and writes in a little town near Boston. He's interested in space, the Earth, and the way that humans and other species live on it.
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