WASHINGTON — Most members of the Nuclear Regulatory Commission indicated on Monday that they considered it safe to continue storing most spent nuclear fuel in pools, even though concerns remain about potential accidents and terrorist attacks.
At a commission meeting, four of the five members indicated that storage in the pools was safe enough, at least compared with moving some of the fuel into giant steel and concrete casks, where they can be stored dry, with no reliance on water, pumps or filters to keep them cool.
Concerns about the safety of spent fuel pools grew after the disaster at the Fukushima Daiichi nuclear power plant in Japan nearly three years ago, when the possibility of damage to a pool there led the State Department to advise Americans to stay more than 50 miles from the plant. Before then, the National Academy of Sciences, in a study ordered by a worried Congress after the attacks of Sept. 11, 2001, found that a successful terrorist attack on a spent fuel pool was plausible and that regulators should consider reducing the loading in spent fuel pools, which hold far more radioactive materials than nuclear reactors do.
"It's legitimate to describe spent fuel pools at reactors in the United States as pre-emplaced radiological weapons," Gordon R. Thompson, the executive director of the Institute for Resource and Security Studies in Cambridge, Mass., and a research professor at Clark University in Worcester, Mass., told the commissioners. The spent fuel pools are a magnet for terrorists, he said.
But David A. Heacock, the chief nuclear officer of Dominion Nuclear, which operates reactors in Virginia and elsewhere, said that the probability of an event that would damage a spent fuel pool was "effectively zero" and that the steps needed to mitigate such an accident were simple, with preparations already made.
"This is not a complicated mitigation, nor is it difficult," he said. "It's basically, just add water."
Nuclear fuel is barely radioactive when it is put into a reactor, but when uranium is split, the fragments, materials like strontium and cesium, are unstable and seek to return to stability by giving off energy, or subatomic particles, for as long as centuries. The particles continue to produce heat so they must be stored under water for a few years.
When the reactors now in service were built, the plan was to remove the fuel after a few years, but because the government has failed to provide a place to bury the spent fuel, utility companies have had to squeeze more and more of it into the spent fuel pools. At Fukushima Daiichi unit 4, which suffered a hydrogen explosion, the Nuclear Regulatory Commission came to believe that the pool was dry or almost dry, and engineers feared that a fire would spread the radioactive materials widely.
As it turned out, the pool at Fukushima Daiichi was full all the time.
But Mr. Thompson said that if water was drained from a spent fuel pool, the radiation field would be so intense that it would deliver a fatal dose to a worker within minutes, making "mitigating action" impossible. The nuclear industry, however, disagreed.
The nuclear commission's chairwoman, Allison M. Macfarlane, was the sole commissioner of the five whose questions indicated she might be open to moving more fuel to dry casks, which are already in wide use. Before she was named to the commission, Ms. Macfarlane spoke favorably of that idea, as did her predecessor, Gregory Jaczko, before he was named chairman. But both backed off when they assumed leadership of the commission.
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