One of the lesser-noted facts of the Fukushima nuclear disaster—where loss of coolant in spent-fuel pools has resulted in massive radiation releases—is that some fuel at the plant was stored in so-called dry casks, and these casks survived the March 11 earthquake and tsunami intact.
This fact is likely to result in new calls to move some spent fuel out of water pools at reactor sites in the United States—where it is packed more densely than the fuel in the stricken Japanese pools—and into outdoor dry casks, experts say.
“What will likely happen very quickly is that the [Nuclear Regulatory Commission] and utilities will arrive at a consensus that moving fuel to dry storage needs to be accelerated to get as much spent fuel out of the pools as fast as possible,” says Ron Ballinger, an MIT nuclear engineer. In Japan, he says, “the dry storage casks weathered the earthquake and tsunami with zero problems.”
Until now, U.S. regulators have decided that keeping fuel in pools—and even allowing the fuel to be more densely packed—is safe. Most U.S. nuclear reactors have air-cooled, dry-cask storage for some reactor waste, but generally this is only because the pools cannot fit any more. Older waste that has had a chance to cool for a few years in pools can be moved to dry casks.
The U.S. is home to at least 65,000 tons of nuclear reactor waste, more than in any other nation, and this figure grows by about 2,200 tons each year.
“In general, U.S. reactors have a great deal more fuel in their spent-fuel-pools than the reactors at Fukushima,” says Richard Lester, who heads the Department of Nuclear Science and Engineering at MIT. If a Fukushima-scale event were to strike a typical U.S. nuclear plant fuel pool, he says, “I think you would potentially have a worse situation simply by virtue of there being more fuel—a lot more fuel in the cases of the pools at the U.S. reactors.”
Spent uranium reactor fuel generates great quantities of heat even after it is removed from the core of a reactor. For that reason, spent rods must be immersed in deep pools of circulating water for several years in order to cool them enough. But after several years, dry casks become a feasible storage option. The casks—generally barrel-shaped steel-and-concrete structures that stand 20 feet high and sit outdoors—only need passive air cooling.
In a pool, by contrast, the proximity of fuel rods to one another causes heat buildup that requires water to be circulated continually. As Fukushima has demonstrated, pumps and their backup systems can fail, and water in spent fuel pools can leak out or boil away.
Over the past three decades, delays in opening a permanent repository for spent nuclear fuel in the United States has led the U.S. Nuclear Regulatory Commission to allow existing spent fuel pools to be “reracked” to increase the density of rods inside them.
Of 84 current or former U.S. reactor sites holding spent fuel—a figure that includes some sites with more than one power plant—63 already have dry casks, 10 are applying to build them, and 11 haven’t yet announced plans, according to Nuclear Regulatory Commission data.