Nuclear Waste to Energy


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A disaster-proof power plant with low emissions, which runs on nuclear waste, is cheap to build, and is scalable to boot: it sounds like a utopian fantasy. But the designs for just such a plant are ready to go. Only the minor issues of funding and regulatory approval keep the world from slaking its thirst for energy with Transatomic’s Waste-Annihilating Molten-Salt Reactor (WAMSR).

Where today’s typical light water reactor uses highly enriched solid fuel, the WAMSR dissolves a much lower percentage of uranium-235 into the molten salt coolant. When the molten salt enters the reactor and encounters the moderator, which slows neutrons down to the right speed for fission, the temperatures run higher than in a light water reactor, but at atmospheric pressure. This makes for a simpler and safer plant.

The general design has long been proven. In the 1960s a molten-salt reactor at Oak Ridge National Laboratory proved that it was effectively meltdown proof and could shut down without operator interaction. But, thanks to the type of moderator it used, the reactor was huge, and the salt it used had to be enriched with high levels of uranium-235.

Leslie Dewan, chief science officer at Transatomic, Cambridge, MA, made two major changes to the Oak Ridge design. To shrink the reactor, she changed the moderator from graphite to zirconium hydride, an alloy used as a moderator in TRIGA and SNAP reactors for decades.

A top down view of the molten-salt reactor experiment. Image: Oak Ridge National Laboratory

Molten Salt

The reactor at Oak Ridge used molten lithium fluoride, which meant the uranium had to be enriched to 33%. Using a mix of lithium fluoride and uranium fluoride, Dewan is able to keep the percentage of U-235 down to 1.8%. That means she can power the reactor with nuclear waste. “Everyone’s really excited about the possibility of finding a use for the waste—not just getting rid of it, but turning it into electricity,” she says.

High-level nuclear waste comes in two components. Actinides, like plutonium and neptunium, have extremely long half-lives. Fission byproducts, like xenon and krypton, have half-lives in the hundreds of years. “What we’re able to do is take the long-lived actinide component of waste, burn it into electricity, and convert it into the shorter lived component,” says Dewan. That means there are no proliferation issues to speak of.

A lower percentage of enriched uranium and a lower operating pressure make for a great reduction in risk. But what makes the reactor failsafe is a trick as simple as a seesaw. Below the reactor is a freeze valve, a plug of salt electrically frozen. In the event of a loss of electricity, the valve would melt and the molten salt would flow into a tank below to cool. “In the worst case scenario it drains into the tank and freezes itself solid,” says Dewan. “In some way, it’s the opposite of a conventional reactor, where you start with a solid form and if there’s an accident it melts.”

Technical Hurdles

There are other new, waste-consuming reactors in the works, but they have larger technical hurdles to face. Bill Gates’ TerraPower, for instance, would use large amounts of waste as well, but it still needs a highly enriched core, thus negating the safety benefits. They also have some materials issues to work out with the cladding to their fuel. China recently allotted $500 million to building two test reactors, which Dewan calls a “cousin technology.”

By Dewan’s estimate, if she could get her hands on all the world’s spent nuclear fuel—all 270 metric tons of it—she could power the world for the next 70 years. “Technology-wise there aren’t a lot of big hurdles,” she says. The only thing keeping the world from switching on the A.C. to full blast free of all guilt is the regulatory hill Dewan has to climb.

She’s up for that challenge as well. “I didn’t at all realize it would be like this when I started out,” she says. “It’s still an interesting set of problems to solve.”

Michael Abrams is an independent writer.

In some way, it’s the opposite of a conventional reactor, where you start with a solid form and if there’s an accident it melts.

Dr. Leslie Dewan,
chief science officer,
Transatomic

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January 2014

by Michael Abrams, ASME.org