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.
“If there is a loss-of-coolant accident, you are going to be in big trouble, especially with these high-density racks and the pools being heavily loaded—and even more so if there happens to be freshly discharged fuel in the pool,” says Allison Macfarlane, a geologist and associate professor of environmental science and policy at George Mason University, who was one of several coauthors of a 2003 report warning of the danger posed by dense reracking. “A lot of these pools are in upper stories at the power plant,” meaning breaches or cracks could let water run out. “If there is a loss of water, you can have a release of radioactivity much larger than Chernobyl, because there is a lot more fuel in the pool than in the core of the reactor.”
Last year, President Obama canceled plans to open the Yucca Mountain underground fuel repository 90 miles northwest of Las Vegas, and appointed a commission to come up with alternatives. The commission, due to issue its report in June, has not made any statements about Fukushima. Macfarlane, a commission member, says she could not discuss its possible suggestions. However, the body is scheduled to meet in a public session May 13 in Washington.
The 2003 report said that in the event of coolant loss in a densely packed pool, air cooling would not suffice. Temperatures could rise to 600 °C within an hour, causing the zirconium fuel cladding to rupture, and then increase to 900 °C, whereupon the cladding would burn, resulting in huge quantities of released radioactive material, the report said.
The report proposed immediate reversion to lower-density pool configurations, with more cooled fuel put in dry casks and moved to central sites. In looser-packed pools, the report said, airflow alone could be enough to prevent fire in the event of coolant loss. It said this could be done for no more than $7 billion nationally, which would work out to a wholesale electricity price increase of 0.06 cents per kilowatt-hour generated from the fuel.
These steps were not carried out. A subsequent National Research Council report also said the fire scenarios required more study, and suggested other measures while leaving dense configurations intact. “It appears to be feasible to reduce the likelihood of a zirconium cladding fire by rearranging spent fuel assemblies in the pool and making provision for water-spray systems that would be able to cool the fuel, even if the pool or overlying building were severely damaged,” the report said. Fuel rearranging and backup cooling of pools are being implemented, a Nuclear Regulatory Commission spokesman says.
If the U.S. government had followed through on its 1982 commitment to open a spent-fuel repository— and its subsequent contracts with utilities to begin removing the fuel in 1998—the pressure on U.S. spent fuel pools would have been relieved, Lester says. “There were schedules that described how the DOE [Department of Energy] was going to move the fuel, and which fuel would be moved,” he says. “I think we can say, on the basis of all of that, that the pools would not be nearly as full as they are now.”
He says it was crucial to begin establishing central sites for dry-cask storage as part of a comprehensive plan for waste storage and disposal, which he says should not rule out Yucca Mountain. “One possible use for the site is for temporary storage,” Lester says.
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