ROCKVILLE, Md. — Dragged by a federal appeals court into a rare public discussion of the risks that terrorists could attack a nuclear plant, the Nuclear Regulatory Commission heard arguments on Tuesday from a California group that the commission’s staff had overlooked one category of potentially serious attacks.
The commission, determined to dispose of the issue promptly, heard the arguments directly instead of delegating them to administrative law judges, the first time since 1989 that the sitting commissioners have heard such oral arguments.
But the three-hour session was not a revealing one, largely because the lawyer for the commission staff said there were major issues that could not be described in open session without compromising national security.
The commission’s ruling could be important because the spent fuel storage system proposed for the Diablo Canyon nuclear power plant, near Avila Beach, Calif., is being adopted at scores of other reactor sites around the country because of the Energy Department’s failure to establish a national burial site for used fuel. At issue was whether storage casks that the Pacific Gas and Electric Company wants to build at the Diablo Canyon plant could be hit with incendiary missiles, piercing the steel and concrete shell and lighting the metal cladding of the fuel. If that happened, plant opponents contend, the fire could turn radioactive cesium into a gas, which would float widely with the wind and then resolidify.
“I cannot discuss anything that concerns what scenarios the staff considers credible,” said Lisa B. Clark, the lawyer for the commission staff.
Ms. Clark added that the staff was aware of the mode of attack raised by the California group, San Luis Obispo Mothers for Peace. “It does not alter the staff’s conclusion that there would not be any significant environmental consequences of a terrorist attack,” she said.
But the lawyer for the mothers’ group, Diane Curran, said that the commission staff had provided a list of the background documents it relied on, and that these did not cover the threat described by her group’s technical consultants.
“The most obvious thing wasn’t even on the table, not even remotely,” Ms. Curran said.
In calculating the threat of accident, the commission takes into account the probability of the event, and its consequences, but the commission has long argued that it is impossible to calculate the probability of a terrorist attack and thus it does not need to take that threat into account when approving installations like the cask storage.
But the mothers’ group sued and demanded an analysis of that risk, and in June 2006 won a favorable ruling from the United States Court of Appeals for the Ninth Circuit, in San Francisco. The commission staff then performed an environmental assessment, which is an abbreviated version of an environmental impact statement, and concluded that there would be no significant impact from the threat of terrorism against the casks.
The details of how the staff reached that conclusion were evidently murky even to one of the four commissioners who heard the case on Tuesday. The commissioner, Gregory B. Jaczko, asked how the staff could assume that the risk was low if it could not assign a numerical value to the likelihood of an attack.
“Well, you have to use your judgment,” Ms. Clark said.
For accidents, she said, “we’re very comfortable, and we understand how to deal with probability, how to evaluate it in quantitative terms.” But the threat of terrorism “is going to take us outside of that familiar space,” she continued.
Still, she asserted, “the staff’s judgment, based on their experience,” indicated that this was not a threat to the environment. The casks, she said, were “robust.”
Mr. Jaczko responded, “So we’re down to the staff’s belief that this probably isn’t going to happen?”
The chairman of the commission, Dale E. Klein, tried through questions to make the case that even if an attack were successful, people would be exposed to doses of radiation that were quite small.
The mothers’ group was advised by Gordon D. Thompson, a physicist, who said that the chimneylike design of the casks, intended to keep the fuel from overheating, could help fan a fire. Ms. Clark argued that Mr. Thompson had not seen the intelligence reports on the capabilities of terrorists, but Ms. Curran said equipment to do the job was available to “subnational groups.”
“It is clear that weapons are available that can penetrate a cask and start a fire,” Ms. Curran said. “U.S. Army-shaped charges are more than capable of penetrating concrete and armor plating.”