The Nuclear Regulatory Commission said it has decided that "certain security information" formerly included in the reactor oversight process will no longer be publicly available.

"The commission has a responsibility for public health and safety, and that responsibility is evaluated in considering which information should be made public," NRC Chairman Nils Diaz said in a brief statement.

He pointed out the regulatory panel, which oversees all of the country's 103 civilian nuclear reactors, had deliberated for months on finding the balance between its "commitment to openness and the concern that sensitive information might be misused by those who wish us harm."

As a result, the commission's assessment of "physical security" at nuclear facilities will be classified and exempt from public request under the Freedom of Information Act, officials said.

Often criticized in the past for their secretiveness, US nuclear regulators became more open in the wake of the March 28, 1979, accident at the Three Mile Island nuclear power plant near Middletown, Pennsylvania, considered the most serious since the United States began using nuclear power to generate electricity.

A failure of the cooling system at the plant resulted in a partial meltdown of the reactor's core and led to the release of radioactive gases into the atmosphere.

But then-Pennsylvania governor Richard Thornburgh advised a partial evacuation of surrounding areas only two days later, largely because operators of the plant tried to play down the seriousness of the situation, according to industry analysts.

An upshot of the incident was the commission's decision to make some of its safety inspection findings public in a periodic report.

It was not immediately clear whether the new gag order would cover only security arrangements around the plants, or include safety flaws that can be exploited by terrorists.

Industry officials insist US nuclear power plants have dramatically increased their security in the aftermath of September 11.

A total of 370 million dollars have been spent since the attacks on hiring some 2,000 new security guards, erecting concrete barriers to protect the plants from car bombs and on buying security systems, according to the Nuclear Energy Institute, the industry's main lobby group.

But in a letter to Diaz sent Tuesday, Danielle Brian, executive director of the Project on Government Oversight, a non-profit watchdog group, said some of these guards have minimal training.

There have been instances of guards reporting for duty drunk -- and of staged security drills, in which organizers make sure that mock attackers are repelled, Brian said.

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