ASHINGTON, Sept. 11 — Security guards at 24 nuclear reactors at 13 sites, interviewed by a nonprofit watchdog group, said they feared being outnumbered and outgunned by terrorists in an attack. They also said that rules on when they might use deadly force were ambiguous and could allow terrorists to succeed.

In the survey, by the Project on Government Oversight, many also complained that they had been forced to work so many hours since security was tightened after the attacks on the World Trade Center and the Pentagon that they found it difficult to function.

One guard, speaking on condition that his name and his place of work not be identified, said in a telephone interview: "You got people carrying rifles, guns, and they're tired. Supervisors are coming around to see if we're awake, instead of resolving the problem."

The guard said a fellow guard at his plant had recently fallen asleep behind the wheel on her drive home and crashed her car. Guards have been working 12 hours a day, five to six days a week for the last year, he said.

Asked about the survey, Richard A. Meserve, the chairman of the Nuclear Regulatory Commission, said that there were no limits on the number of hours a guard could be ordered to work but that "they have to be capable to perform their function." Since the terrorist attacks, Mr. Meserve said, the number of guards at reactors nationwide had gone to about 6,000 from about 5,000.

Most are contract employees; some work directly for the reactor operators. "These are not rent-a-cops," Mr. Meserve said. "They are people who have serious weapons and training."

But the report found that at some plants, the guards were paid less than the custodians and that turnover at the plants was high. By some accounts, turnover has increased lately as experienced guards have found better security jobs at airports.

Training is also quite limited, some guards say.

Mr. Meserve said he had not read the report, but he agreed that Congress should set national standards governing the authority of nuclear reactor guards to use deadly force.

In the survey, and in telephone interviews, guards said that in the states where they worked they were allowed to use their guns if intruders threatened their safety. But they said that if someone jumped a fence with a backpack or other item that could arguably contain a bomb or other weapon, but did not clearly threaten the guard, they could not fire their guns.

Mr. Meserve, speaking at a three-day seminar called the Nuclear Renaissance, organized by Infocast, a commercial conference organizer, said today that the commission had decided that security drills would be held at each nuclear plant every three years, rather than every eight years, as was done before Sept. 11.

In the past, those drills included force-on-force exercises, with testers playing the role of terrorists, using simulated weapons that resemble laser-tag toys. Those were suspended after the attacks but will resume late this year or early next year, he said.

Mr. Meserve also said that his agency was considering establishing "cradle to grave" licensing of radioactive materials that could be used by terrorists in "dirty bombs," and reforming the system by which the import and export of such materials is licensed.

Mr. Meserve, in his speech and in remarks afterward, emphasized that nuclear plants, because of their design, were inherently tough targets, and that even before the terrorist attacks, they had significant security protections.

He also said that the number of guards interviewed by the organization — "more than 20," according to the report — was too small for drawing conclusions.

Nonetheless, the guards, at various plants, repeated common themes. One was a lack of training. At a Tennessee Valley Authority plant, one guard said his training consisted of firing about 40 rounds of ammunition, which took about three hours, to requalify to carry a gun. "Any training after that, that's up to you," he said in a telephone interview.

Guards questioned other areas of security. Some said new lighting around the plants illuminated them but not potential infiltrators. Others said they felt vulnerable in new guard towers.

The T.V.A. guard, who would not allow his name to be used, also complained about the ambiguity of the rules on deadly force. If there is no clear threat to the guard, he said, "you may hesitate."

Mr. Meserve, the regulatory commission chairman, said his agency had repeatedly asked Congress over the years to set a national standard for use of deadly force by power plant guards, but to no avail.