or the first time since the terrorist attacks of Sept. 11, 2001, the Nuclear Regulatory Commission will resume mock armed assaults on nuclear plants, including an invasion of the Indian Point nuclear power plant in Westchester County this spring or summer.

The commission had suspended the drills after the 9/11 attacks to develop new exercises that better reflected terrorist threats. But commission members told Congress yesterday that revamped drills would resume this year, starting at four plants — including an exercise in June or July, or possibly earlier, at Indian Point, 35 miles north of Midtown Manhattan, in Buchanan.

"I think we agree that that is clearly a priority," Jeffrey S. Merrifield, a member of the commission, said under questioning by Senator Hillary Rodham Clinton, who has pressed for the drills in light of heightened anxiety over safety and security at the plant.

The so-called force-on-force drills will also be conducted more frequently, every three years instead of every eight years. The last such drill at Indian Point was in March 1994.

Word of the drills came as the commission weathered controversy over remarks by its chairman, Richard A. Meserve, who a day earlier criticized a state-commissioned report on emergency plans for apparently giving "undue weight" to the impact of a terrorist attack on the plant.

Mr. Meserve last month began notifying members of Congress about the new drills, without identifying the plants. Mrs. Clinton had urged the Nuclear Regulatory Commission to include Indian Point among the first to be tested. She also encouraged the Entergy Corporation, which owns the plant, to volunteer to be among the first, and it did so, a commission spokeswoman said.

Antinuclear activists and other critics have questioned, particularly after Sept. 11, how well the nation's 103 active reactors are guarded, pointing to reports of fatigued and unprepared security officers.

In a report written for Entergy last year, a majority of the guards at the Indian Point plant said they doubted that they could thwart terrorists, and several were found to be out of shape and lacking training. Entergy has said that since the report was delivered it has improved the training of the security force and has put in place a variety of measures to foil terrorists.

In past drills, a small team of intruders would enter a plant's premises and, with simulated weapons, try to outmaneuver the security guards to get to vulnerable areas.

Critics have derided the drills for involving only a handful of intruders and for the fact that the nuclear commission tells the plant in advance when the drill will take place. Commission officials have said the mock intruders in half of the cases reached vulnerable areas, potentially causing a release of radiation.

Diane Screnci, a spokeswoman for the commission, said it would have new regulations governing the exercises in place next month. But she said the commission still planned to notify the plants in advance because they must assign certain guards for the exercise while the others continue guarding the plant.

Mrs. Clinton, who questioned members of the commission appearing before the Senate Subcommittee on Clean Air, Climate Change and Nuclear Safety, said afterward, "This will help us determine where we stand when it comes to security at Indian Point."

Jim Steets, a spokesman for Entergy, said the company welcomed the drills as an opportunity to demonstrate the strength of its security force, which he said had been bolstered since 9/11. Last month Entergy announced that it would merge the two security forces at the plant, a legacy from a time when different owners operated the two active reactors. Entergy bought Indian Point 3 from the New York Power Authority in 2000 and Indian Point 1, which is defunct, and Indian Point 2 from Con Edison in 2001.

"We are willing participants," Mr. Steets said. "We would be more than happy to be one of the early places to be tested."

Alex Matthiessen, executive director of Riverkeeper, an environmental organization that has raised concerns about safety and security at the plant, said he was elated that Indian Point would be among the first plants tested.

"As the nuclear plant in the most densely populated area, I think it is fitting and appropriate to subject it to the force-on-force testing this year," Mr. Matthiessen said.

The Nuclear Regulatory Commission declined to discuss how the drills would be conducted.