Marty Virgilio, deputy executive director of the agency, told the commissioners that the problems had been fixed but more work was needed. Mr. Virgilio discussed the findings at a briefing on the vulnerability of American reactors to severe natural disasters like the earthquake and tsunami that hit the Fukushima Daiichi plant in Japan on March 11.

The N.R.C. engineers said they had found no glaring lapses so far, but many potential problems. One is that many of the preparations the industry took after 9/11 for “severe accident mitigation” were taken voluntarily, and thus are not routinely evaluated by commission inspectors.

Mr. Virgilio’s boss, Bill Borchardt, the commission’s chief staff official, said that some of the preparations for severe accidents, including training, procedures and hardware, “don’t have the same kind of regulatory pedigree” as the equipment in the original plant design.

Ever since the Fukushima accident began, the N.R.C. and American nuclear power plant operators have argued that steps taken in this country to respond to terror attacks would also be helpful in case of severe accidents started by natural disasters.

The five-member commission received a two-hour briefing on Thursday from the leaders of a task force that is supposed to conduct a 90-day review of the vulnerability of American reactors to such disasters. Charlie Miller, the staff member leading the effort, characterized the changes under consideration as “enhancements,” not fundamental changes.

But as laid out by the staff, some of the changes could be far-reaching. For example, current planning is focuses on handling a problem at just one reactor, even if there are multiple reactors at a single plant, which is quite common. “You have to take a step back and consider what would happen if you had multiple units affected by some beyond-design-basis events,” Mr. Miller said. At Fukushima Daiichi, there are six reactors, with significant problems at four of them.

Another problem, N.R.C. staff members acknowledged, is that they have never paid much attention to the problems of handling an emergency at a time of widespread damage to surrounding roads, power systems and communications links — something that might very well happen in a major natural disaster such as an earthquake. In the past, the commission has explicitly rejected the idea of combined events.

As the hearing opened, Representative Edward J. Markey, a Massachusetts Democrat who is frequently critical of the commission, released a report arguing that there are a variety of other shortcomings at American plants, including the frequent failure of emergency diesel generators, which are essential to plant safety if the power grid goes down.

Mr. Markey also criticized the commission for not having a requirement for a backup power source for spent fuel pools while the reactor is shut for maintenance or refueling. The Fukushima accident has cast new attention on spent fuel pools; the United States recommended that Americans stay 50 miles from the Fukushima plant because of damage to the spent fuel pool of the Unit 4 reactor, which had been shut down at the time of the March 11 earthquake and tsunami.

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