The Nuclear Regulatory Commission says controlling the airspace over atomic power plants is the best available way to prevent a terrorist attack.

The agency also again acknowledged that the plants were not built to withstand a fully fueled jetliner crashing into them.

"The commission believes that the nation's efforts associated with protecting against terrorist attacks by air should be directed toward enhancing security at airports and on airplanes," the director of the NRC's Office of Nuclear Reactor Regulation wrote a whistle-blower group.

Samuel J. Collins said the NRC, the Federal Aviation Administration and Defense Department have discussed the idea of protecting air space over the power reactors and other sensitive sites.

In the meantime, the FAA's post-Sept. 11 warning to pilots not to circle or loiter over those areas remains in effect, he wrote in an Aug. 10 response to a petition by the National Whistleblower Center for security upgrades.

NRC officials have concluded the probability of terrorists using a large airliner to damage a nuclear power plant "remains acceptably low," Collins said. He said there have been "no specific credible threats against any NRC-licensed facility since Sept. 11" and Congress is moving to strengthen aviation security.

In January, the NRC alerted nuclear power plants that terrorists could be planning an attack on a power reactor using a hijacked commercial airliner. But the information from an Al Qaeda operative turned out to be months old and was eventually deemed not to be credible.

"No existing nuclear facilities were specifically designed to withstand the deliberate high-velocity direct impact of a large commercial airliner such as a Boeing 757 or 767," Collins wrote. "Prior to Sept. 11, such a scenario was not considered to be a credible threat."

Constructed of thick exterior walls and interior barriers of reinforced concrete, nuclear power plants "afford a measure of protection against deliberate aircraft impacts" — mostly smaller planes — and can withstand tornadoes, hurricanes, fires, floods and earthquakes, Collins said.

The NRC also has ordered nuclear power plant operators to come up with other new lines of defense.

Michael Kohn, an attorney for the whistle-blower group, said the commission has not identified the true level of risk posed by a terrorist threat.