


An attack would be difficult but ''certainly no more difficult than the Sept. 11 attacks,'' Kevin Crowley, the study director at the National Academy, said in a conference call with reporters.

Louis J. Lanzerotti, the chairman of the committee that issued the report and a professor of physics at the New Jersey Institute of Technology, said the study group ''identified several scenarios that could have serious consequences at some plants.'' The report was finished last summer as a classified document, but release of an unrestricted version was delayed until Wednesday by arguments between the academy and the Nuclear Regulatory Commission over what could be made public.

The public version of the study did not lay out which plants were the most vulnerable, although experts say those are the older boiling-water reactors, where the fuel pool is high above the ground and the building is not designed to withstand strong internal pressures.

The pools are made of reinforced concrete with stainless-steel linings. They are meant to withstand natural hazards like earthquakes but were not designed with terrorism in mind.

The scientists also questioned the ability of the commission to issue findings that the public would find credible. They called for independent reviewers to assess security measures for spent fuel. They also called for the installation of water-spray systems at the pools to keep fuel from catching fire, even if the water were drained, as feared in an attack.

Congress asked for the study in 2003, after a group of experts who are generally critical of the nuclear industry published a paper in a Princeton scientific journal saying that terrorists could attack a pool and that the resulting release of radiation could have consequences ''significantly worse than Chernobyl.'' The academy's recommendations fell far short of what the Princeton group called for, but its report did find that ''there were indeed credible concerns about the safety and security of spent nuclear fuel storage in the current threat environment.''

The academy is an advisory body, but it is more influential than the private organizations that have raised questions about terrorism at nuclear sites; many of those groups have a long history of opposing nuclear power. The scientists said they hoped their findings would prompt the government to provide money for cask storage at sites found vulnerable.

Nuclear reactors create vast amounts of radioactive materials as they fission uranium to make heat. Spent fuel in the storage pools contains far more radioactive materials than the reactors do.

Most of the plants now operating were designed to store fuel for only a few years, because engineers expected that it would either be recycled or buried. The Energy Department was supposed to begin accepting fuel for burial in 1998 but has not yet done so. As a result, the reactor operators have squeezed spent fuel more tightly into the pools, raising the heat load and, according to some analyses, raising the risk of fire if the pools were ever drained.

Building enough casks to keep up with waste creation would cost millions of dollars a year at every reactor.

The Nuclear Regulatory Commission has repeatedly said that cask storage and pool storage are equally safe. On March 14, the commission's chairman, Nils J. Diaz, told reporters that the pools ''are not easily breached structures.''

After an attack, they would be very easy to cool, he said. ''You get a couple of fire hoses, and spray them, and you have many, many hours,'' he said, before there could be any radiological release, giving officials time to contain the problem.