After cooling for about five years, the rods can be moved to dry storage -- heavy casks of lead and steel. But the casks are expensive, and commercial reactors have elected to leave the rods in the pools until the pools fill up. Lochbaum said some pools hold 800 to 1,000 tons of rods. In the event of a terrorist strike, Lochbaum said, the dry casks would be much safer, because explosions could drain the pools and set off fire and radiation hazards.
The nuclear industry wants the fuel moved to a storage site in Nevada, but that project has long been plagued by delays and opposition. Steven Kraft, director of waste management at the Nuclear Energy Institute, an industry group, said studies had shown that the pools are as safe as the dry casks -- the same position adopted by the Nuclear Regulatory Commission.
Kraft said that the risk of catastrophic attacks is minuscule and that modeling analyses have shown that even plane crashes are unlikely to affect the pools' integrity. And even if they did cause damage, he added, there would not be catastrophic consequences because of safety systems already in place.
"If the pool is safe and the casks are safe and they both meet the requirements, there is no justification for going through what is a huge amount of expense and worker exposure" to move the rods to dry storage, he said.
In his letter to Congress, Diaz said the academy's recommendation to move fuel to dry storage was based on "scenarios that were unreasonable."
But Arjun Makhijani, a nuclear engineer with the Institute for Energy and Environmental Research, a nonprofit research and advocacy organization that supports underground dry storage of the rods, said the commission had been lax.
"There is no question that any terrorist who wants to know about spent fuel has plenty of information already," he said of the withheld report. "Publication of a report on security will not help terrorists. The only thing it is hindering is discussion of public safety."
Diaz's letter to Congress shows that the academy recommended that the government conduct additional analyses to evaluate "the vulnerabilities and consequences" to storage pools of "attacks using large aircraft or large explosives." The academy also called for a review and upgrade of security measures to prevent theft of spent fuel rods by insiders and an assessment of security by "an independent organization."
The commission letter defended measures it has in place and said that "the likelihood an adversary could steal spent fuel . . . is extremely low." The letter said the additional analysis demanded by the academy study was "more than is needed" and rejected the call for an independent security analysis, saying the commission's own assessments were "sound and realistic."
To keep the report secret, the federal agency used a classification called "Safeguards Information" that it applies to data that are unclassified but reveal sensitive details about nuclear facilities and security procedures. Brenner, the spokesman, emphasized that the academy's report and the commission's response had been seen by the Department of Homeland Security and members of Congress charged with oversight. "The full report is there with those with the appropriate clearances," he said.
The academy's Colglazier said the science organization had produced many classified reports but had never encountered such hurdles in creating a public version.
"We don't want to provide information in our report that could be used by terrorists to exploit vulnerabilities," he said. "But we also want the public and decision makers to know what things need to be addressed."
The scientist also rejected Brenner's reassurance that the classified report had been seen by relevant decision makers. Governors of states with nuclear plants need to see the report, he said, and the public had an important role as well.
"The way our political system works, when politicians hear from their constituents, they are motivated to take action that they don't when the public is unaware," he said.