A Nuclear Regulatory Commission model simulating the effects of a catastrophic accident at one U.S. nuclear power reactor found that the plant, based on a design similar to Japan's crippled Fukushima Daiichi, came close to a suffering a partial meltdown, Rep. Henry Waxman (D-Calif.) said today.

The Peach Bottom Atomic Power Station, located in York County, Pa., would come within hours of suffering core damage in the event of a highly unlikely, sustained loss of primary and backup power, the previously unreleased NRC model found.

"When a simulation purporting to determine the realistic consequences of a severe accident nearly results in a partial meltdown, Congress should be asking tough questions," Waxman said.

Details of the NRC models were obtained by the Union of Concerned Scientists (UCS), a nuclear safety watchdog, through freedom-of-information requests. UCS released excerpts from the draft assessment in conjunction with a nuclear-safety hearing held by a House Energy and Commerce subcommittee.

The NRC models represent an extreme worst-case scenario, including simulations that do not incorporate safety measures added since the Sept. 11, 2001, terrorist attacks, NRC officials said. NRC prepared the models, which did not address spent fuel pools, as part of its State-of-the-Art Reactor Consequences Analyses, a program initiated in 2006 to game out disaster scenarios regardless of improbability.

"[This] is a study that's done without full respect of risk involved," Martin Virgilio, the NRC's deputy executive director for reactor and preparedness programs, testified today. "Risk is what can happen and how likely can it happen, and how likely are these consequences."

Politicians should be careful about extrapolating the worst-case scenarios seen in Japan to U.S. facilities, said Rep. Cliff Stearns (R-Fla.), chairman of the energy panel's oversight committee.

"We should not confuse what is happening in Japan with our own preparedness and assume they are one and the same," he said. "We should not make unsupported assumptions about risks or response measures or get ahead of the facts."

The NRC has seen evidence that the Japanese did not keep pace with the United States in the safety upgrades made to their older reactors, Virgilio said. In particular, the Fukushima Daiichi reactors appeared to either lack or not use the type of "hardened vents" necessary to release buildups of hydrogen gas, a common feature added to older U.S. plants. Excess hydrogen caused the explosions early on at Fukushima, deeply complicating recovery efforts.

Rep. Ed Markey (D-Mass.), however, contended that NRC officials told his office that the Fukushima plant did have hardened vents. If they did, Virgilio responded, "I don't believe they used them."

The NRC models simulated two days of blackout at the Pennsylvania plant, driven by an unspecified catastrophic disaster. Without safety features added since Sept. 11, these loss-of-power scenarios resulted in core damage and partial meltdown, similar to the events seen at Three Mile Island. However, in the post-Sept. 11 scenario, steam-driven cooling avoided even partial core damage.

The two-day limit used by NRC seems arbitrarily short, Waxman said.

"The assumption was that response efforts would only get more numerous and more effective after two days," Waxman said. "There is still a lot we don't know about what went wrong at the Fukushima plant. But we can safely conclude two days is not enough time to know whether a reactor will melt down."

The UCS and Democratic lawmakers focused on discussions between the NRC's national office and a regional branch over whether the Sept. 11 safety features could be included in the modeling scenarios, debating their seismic resistance. The agency's disaster models are not final, Virgilio noted, and the concerns raised by staff scientists will be considered in completing the agency's final report.

"What you see is the NRC in operation," he said.

The NRC also updated legislators on Fukushima Daiichi. The agency believes cooling at the reactors is under control. However, risks remain that the reactors could again begin overheating. The agency has received conflicting information that the core is covered by water, and is at times uncovered, Virgilio said.

"I'd be pressed to say that [Fukushima] is stable today," he added.

Currently, the Fukushima reactors are not releasing dangerous levels of radioactive particles into the air, said Donald Cool, a senior health physics adviser at the NRC. The current atmospheric conditions do not represent a health threat to the Japanese or American people.

"The current conditions are stable and should remain stable," Cool said.

Markey, however, railed against the desperate, ad hoc measures that Japanese officials, caught unprepared, have had to use to plug leaking radioactive water, using straw and other materials. It echoes the pantyhose and golf balls used in attempts to plug last year's Gulf of Mexico oil spill, he said.

"The Japanese have been compelled to try a 'nuclear junk shot,'" he said.

Markey also contended that NRC officials had told him that the second reactor at Fukushima had likely gotten so hot that it had melted through its pressure vessel, the steel container that holds the nuclear reactor, though it remains within the concrete containment building. Virgilio, however, said it was far from clear to the agency that such drastic melting had occurred.