ASHINGTON, Nov. 2 — As they survey the industrial landscape for objects that terrorists could turn into weapons, members of Congress, governors and others are showing growing anxiety about the vulnerability of nuclear reactors, and especially their spent fuel.

The Coast Guard and the National Guard are already patrolling many plants, and the Nuclear Regulatory Commission says improvements have been made since Sept. 11 to make reactors less susceptible to sabotage. The industry emphasizes that many design features intended to protect plants against accident result in "robust" structures that are also resistant to military attack.

But studies that were available until recently on the Internet are being cited by a variety of others as reason to worry. One, done 20 years ago for the Nuclear Regulatory Commission, raises the possibility of an airplane crashing into a containment dome or some less-hardened part of a reactor and causing a meltdown. Another, dated September 2000, suggests that breaching a cask used to store spent fuel would create a lethal radiation dose in an area many times larger than that caused by a 10- kiloton nuclear weapon.

Other experts note that the spent fuel pools can contain 20 to 30 times as much radioactive material as the reactor core does. And the pools are in buildings not nearly as strong as those that house the reactors.

"I'm not so worried about the core; I'm worried about the spent fuel pool," said Gov. Howard Dean of Vermont, who has asked for the establishment of a permanent five- mile no-flight zone around the Vermont Yankee nuclear plant in the southeastern corner of his state. "There's basically no protection there," he said in a telephone interview.

Experts disagree about the extent of the vulnerability, and the Nuclear Regulatory Commission and the industry say there is no cause for alarm. But the House Energy and Commerce Committee voted Thursday to require the commission to review the potential for attacks on nuclear plants , specifically to identify a new "design basis threat," or threat around which the plant's defenses are geared. The commission had opposed the amendment.

The provision's author, Representative Edward J. Markey, Democrat of Massachusetts, is a longtime opponent of the industry. Still, he won the near-unanimous agreement of his colleagues. "The Nuclear Regulatory Commission is refusing to take up the question at all," Mr. Markey said. "We're mandating that they take it up."

His amendment would also guarantee the continued existence of the office within the N.R.C. that evaluates physical protection at reactors. Before Sept. 11, the agency had a plan to turn that function over to an industry group, which it said could run tests more frequently.

The details of the design basis threat against which the plants are tested are classified, but the threat is known to be a commando-type attack. Paul Leventhal, president of the Nuclear Control Institute, a nonproliferation group, suggested today that the basis should be "19 suicidal terrorists, technically sophisticated, coming at you from different directions." That would describe the groups that hijacked four airliners on Sept. 11.

Some arguments are revised versions of the case that opponents have made against nuclear power for years. "We've never heard of a terrorist taking aim at a wind turbine," said Anna Aurelio, legislative director of the U.S. Public Interest Research Group, which favors ending the use of nuclear power.

The Nuclear Regulatory Commission's chairman, Richard Meserve, said that various improvements had been made since Sept. 11, but he added that reactors were smaller than either the World Trade Center towers or the Pentagon and, thus, more difficult to crash into. "It would not be a trivial thing to have a kamikaze attack," Mr. Meserve said. "It's a lot harder to hit than the World Trade Center."

"We have all kinds of infrastructure in this country that is vulnerable to aircraft," he added. "You think about dams, chemical plants, refineries, skyscrapers, pipelines, any number of things.

"I don't particularly lose any sleep over collisions with spent fuel pools, as compared to those other things."

But threats to the nation's nuclear power industry have new resonance with some elected officials since the hijackings. "The risk assessment that existed prior to Sept. 11 is clearly inadequate," Representative Peter Deutsch, a Florida Democrat who is another member of the Energy and Commerce committee, said at the committee's meeting on Thursday. He said that a reassessment was urgently needed because some threats were clearly beyond what a private company could defend against and would require government action. In a telephone interview, he added that it was clear that the reactor containment would not be the only possible target.

While the most obvious area of concern at a nuclear plant is the reactor, which operates under high temperatures and pressures and could vent radioactive steam in an accident, the bulk of the radioactive material at most plants is in the spent fuel pool.

The radioisotopes, like cesium and strontium, are created in the reactor by splitting uranium. Since the fuel is moved from the reactor after about three years, it begins to accumulate in the spent fuel pool. While there, it sits under about 25 feet of water, which shields the radiation and carries off the heat that continues to emanate from the fuel.

The industry estimates that even if all cooling stopped, the water would not begin boiling for 20 to 40 hours, and that even if it boiled, all that would be needed to end the problem is to add more water through something as simple as a fire hose. "These are huge structures, with a lot of inertia," said Lynnette Hendricks, director of licensing at the Nuclear Energy Institute, the industry's trade association.

Critics say that if the fuel were allowed to get too hot, it could ignite the cladding — a metal called zirconium — that holds the uranium fuel in place. The metal was selected primarily because it can be easily penetrated by neutrons, the sub- atomic particles that sustain a chain reaction.

But a petition filed earlier this week with the N.R.C. by a nuclear safety group argued that the zirconium could provide the chemical energy to fuel a fire that would disperse the radioactive materials. The group was seeking to prevent the owners of the Millstone nuclear plant, in Waterford, Conn., from storing more fuel in a pool there.

Until recently, the commission's staff said that zirconium would not burn once the fuel was a few years old, and its heat production was reduced as some of the radiation died off. But earlier this year, the staff retreated from that position.

Still, Ms. Hendricks said that to set up a situation in which such a fire could occur, "you need to hook up a lot of `what-ifs.' "

The other way to store fuel is to put it in dry casks, massive concrete and steel boxes filled with inert gas. Before Sept. 11, safety advocates and nuclear engineers described this as safer, at least for older fuel, because it used no water for fuel to leak into and no pumps to fail.

But the casks sit outside the plant buildings, sometimes in sight from roads or nearby hillsides. They have been tested for transit accidents, but their security against attack with an antitank weapon or other armament is less certain.

A draft study by the National Council on Radiation Protection and Measurements discussed the risk of shipping spent fuel and calculated that breaching a cask could produce a lethal radiation dose in an area of 2,700 square kilometers. In comparison, the study said, a 10-kiloton nuclear blast would produce those doses in 47 square kilometers.

Government officials note, though, that creating a hole in a cask is not the same as dispersing its contents; dispersion would depend on the size of the breach and the energy available to break up the fuel.

The federal government was supposed to take responsibility for disposing of civilian reactor fuel in 1998, but the plan is now more than 10 years behind schedule. The Energy Department is trying to demonstrate that Yucca Mountain, near Las Vegas, is a suitable spot for deep burial, but has encountered a variety of problems.

So the spent fuel risk, however great it turns out to be, will stay with the plants for years to come. In Wiscasset, Maine, where the Maine Yankee nuclear plant used to operate, the state is demanding the fuel be hauled out. Otherwise, the site could become, in the words of Paula Craighead, the state's nuclear safety adviser, "Yucca Mountain without the mountain."