Since the Sept. 11 attacks on New York and Washington, lawmakers and activist groups have raised repeated concerns that a similar strike against a nuclear power plant could spew radioactive material that would kill or sicken thousands of nearby residents.

A study commissioned by the Nuclear Energy Institute (NEI) concluded such an attack could damage the thick concrete walls that surround nuclear reactors, but would not breach them. Reactors are housed in structures designed to contain the equivalent of a small nuclear explosion.

The study was not sanctioned by the Nuclear Regulatory Commission, the federal agency charged with protecting nuclear plants. It has its own study underway, but has not set a release date, an NRC spokesman said.

The $1 million industry study, funded by the Energy Department, used computer modeling to simulate a strike by an airliner flying low to the ground at 350 miles per hour, similar to the speed of the commercial jetliner that struck the Pentagon more than a year ago, said the NEI, the nuclear industry's main lobbying group.

The study showed that such a strike would damage a plant's ability to generate electricity, but "public health and safety would be protected," NEI President Joe Colvin said.

Containment structures, used fuel storage pools and containers used to hold radioactive byproducts would withstand the impact "despite some concrete crushing and bent steel," NEI said in a statement.

The results "validate the industry's confidence that nuclear power plants are robust and protect the fuel from impacts of a large commercial aircraft," Colvin said.

The industry group did not release the full report, citing security concerns.

Rep. Edward Markey, a Massachusetts Democrat and long-time critic of the nuclear industry, criticized the report's reliance on virtual computer models.

"It is less than comforting to know that the nuclear energy industry has computer models predicting that a virtual airplane carrying a virtual load of fuel ... would be unlikely to result in a virtual release of radioactivity," Markey said in a statement.

CRITICS SAY PLANTS VULNERABLE

Activist groups said they were not convinced that nuclear plants are safe from aircraft attack.

"There are a lot of safety equipment outside of the containment that would disable and potentially lead to a meltdown," said Jim Riccio, a nuclear expert with Greenpeace. The greatest chance of causing a plant meltdown comes from a loss of off-site power, Riccio said.

"If an airplane strikes one of these nuclear sites it's going to be in a world of hurt. And only the grace of God is going to prevent a meltdown," he said.

Critics say that in addition to the nuclear core, another security risk is the large amount of spent fuel waste stored at most of the nation's nuclear power plants. Some 2,000 metric tonnes of nuclear waste is produced every year by the power plants, which must store the waste on site until a federal, underground repository in Nevada is completed.

Several Democrats unsuccessfully pushed earlier this year to federalize the privately employed security guards at nuclear power plants as another safety precaution. Utilities and the NRC opposed the proposal.

Some U.S. nuclear power plants, such as the Indian Point plant about 25 miles north of New York City, have had National Guard troops and other armed patrols keeping watch.

STUDY ANALYZES ENGINE IMPACT

The new study's results also apply to the damage done by an aircraft engine, which packs more punch than the fuselage because of its density. The 9,500-pound engine size assumed in the study is typical of most commercial airlines in service, NEI said.