WASHINGTON — U.S. nuclear plants use the same sort of pools to cool spent nuclear-fuel rods as the ones now in danger of spewing radiation at Japan's Fukushima Daiichi plant, only the U.S. pools hold much more nuclear material. That's raising the question of whether more spent fuel should be taken out of the pools at U.S. power plants to reduce risks.
Workers in Japan have been struggling for days to get water into the spent-fuel pools at the plant, so that the fuel rods won't be exposed to the air, burst into flames and set off a large radiological release.
Experts are debating whether America's spent fuel pools would fare as badly or worse in an accident, and whether they could be made safer.
Edwin Lyman, a physicist and nuclear expert at the Union of Concerned Scientists, said he has long been concerned that U.S. spent-fuel pools are too full. Lyman said that his group, which doesn't take a position for or against nuclear power, recommends reducing risk at the spent-fuel pools by transferring some of the fuel rods to dry casks.
"I think that's being borne out by what we're seeing in Japan," he said Thursday.
The Japanese plant's pools are far from capacity, but still contain an enormous amount of radioactivity, Lyman said. A typical U.S. nuclear plant would have about 10 times as much fuel in its pools, he said.
The Nuclear Regulatory Commission reaffirmed its position that the U.S. pools are operated safely. Pools are necessary to cool spent fuel for five years after it's removed from a reactor. After that, either keeping it in the pool or moving it into dry casks, steel and concrete containers filled with an inert gas, is a safe method for temporary storage, said Scott Burnell, a Nuclear Regulatory Commission spokesman.
Fission — the splitting of uranium atoms in a chain reaction — produces the heat energy that boils water into steam to drive turbine generators to produce electricity. Every 18 months to 24 months, the plant is shut down and the oldest bundles of fuel rods are removed and replaced.
The spent fuel no longer produces enough energy to sustain a nuclear reaction. However, it still generates large amounts of radiation and heat. The pools contain the radioactivity and dissipate the heat.
Dangerous heating begins when the water drops below the top of the rods because it has boiled away or evaporated. As the rods balloon and rupture and the fuel pellets inside melt, radioactive material gets released.
Once a pool reaches capacity, some fuel rods must be transferred to dry casks, Burnell said. Until then, the plant operator can decide whether to keep the material in the water or move it. "Under our regulations, there's no safety reason to move the fuel more rapidly," he said.
The NRC closely regulates the use of spent fuel pools, Burnell said. "We allow spent fuel to be stored in ways that analysis has shown are acceptable and safe."
Burnell said that research after the Sept. 11, 2001, attacks and extensive reviews of plant operations show "that it is possible even under emergency conditions to maintain an appropriate level of water in a spent fuel pool using very simple techniques" even under conditions where radiation levels would be high.
High radiation levels have deterred efforts in Japan to dump water into the pools.
Energy Secretary Steven Chu said at a Capitol Hill hearing on Wednesday that after spent-fuel rods dissipate heat in a pool of water, "The next stage is that you can put them in dry cask storage, which is much safer."
Chu said that dry-cask storage at nuclear power plants could be used for about 50 to 60 years. He said that gives the nation time to figure out a plan for a permanent repository.
In 2004, Congress asked the National Academy of Scientists, the government's main science advisory group, to look at how spent-fuel pools would withstand a terrorist attack.
An NAS committee wrote that an attack that drained a power plant's spent-fuel pool might start a fire that would release large amounts of radiation. One recommendation was a water-spray system to cool the fuel if the facility was damaged.
The report also said that dry cask storage has two advantages over storage in pools: It relies only on air circulation for cooling, and it splits the spent fuel into multiple containers. Congress didn't ask the committee to recommend whether the country should speed the transfer to dry casks, and the NAS panel didn't address the issue.
Lyman and others have long called on the NRC to require plant owners to move spent fuel to dry casks. Lyman was part of a group that wrote in a separate 2004 study published in the journal Science and Global Security that a large radiation release from a fire in a storage pool could result in thousands of cancer deaths and require billions of dollars for decontamination. Princeton professor Frank von Hippel, another author, said in an e-mail on Thursday that the utilities objected to spending $8 billion on casks and the NRC didn't require them to do so.
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