UPDATE: Modeling: US Reactor Comes Within Hour Of Core Damage

An analysis of hypothetical nuclear accidents conducted by U.S. nuclear officials shows that a reactor in Pennsylvania, known as Peach Bottom, comes close to suffering core damage in situations where all power is lost at the plant, House Democrats said.

The study, known as State-of-the-Art Reactor Consequence Analyses and conducted by the U.S. Nuclear Regulatory Commission, analyzed the effects of severe accidents at two nuclear reactors: the Peach Bottom plant, which has a design similar to the damaged Fukushima Daiichi plant in Japan, and the Surry reactor in Virginia, Democrats said.

Under one scenario, in which a severe station blackout takes out all power, the simulation analysis showed that the Peach Bottom reactor "came within one hour of core damage," according to a memo House Democrats released Wednesday.

"A simulated meltdown was narrowly averted through the manual turning of steam valves" to activate cooling systems, the memo said.

In a less-severe scenario, in which the plant has access to backup battery power for four hours, the operator was able to prevent core damage.

In a House hearing Wednesday, a top nuclear official responded to the report and said the analysis of nuclear accidents looked at "very unrealistic events."

"We ignore all probabilities" of the events actually taking place, said Martin Virgilio, deputy executive director for reactor and preparedness programs at the Nuclear Regulatory Commission. "We're testing the envelope."

In conducting its hypothetical simulations, the NRC assumed the Peach Bottom nuclear plant used new equipment and procedures introduced since the Sept. 11 terrorist attacks, the memo said. Without that equipment, the simulation showed the plant would suffer core damage and would release radioactive contamination in two days.

There appeared to be some disagreement among U.S. nuclear officials, however, regarding the effectiveness of the new equipment and procedures. According to July 28 email among NRC officials, referenced in the memo, senior analysts at the commission said the new equipment and procedures "have really not been reviewed to ensure that they will work to mitigate severe accidents."

The email identified specific concerns about operating the Reactor Core Isolation Cooling system without battery power, the Democrats said in the memo. This is the same system that allowed the plant to avert core damage in the simulations.

NRC's Virgilio said the email reflect the "healthy debate" that occurs among commission staff.

"Our staff is encouraged to challenge various issues as they're being evaluated," Virgilio said.

The purpose of Wednesday's hearing, held by a House Energy and Commerce subcommittee, was to analyze events at Tokyo Electric Power Co.'s (9501.TO) Fukushima Daiichi plant in Japan. Following a March 11 earthquake and tsunami, the plant lost crucial cooling systems, causing damage that has led to leaked radiation.

Rep. Ed Markey (D., Mass.), a critic of nuclear power, said in a statement that the NRC believes the core of Unit 2 at the Fukushima plant has "gotten so hot that part of it has probably melted through the reactor pressure vessel." Markey also said that at least one other reactor core has been severely damaged.

The Peach Bottom Atomic Power Station plant is jointly owned by Exelon Corp. (EXC) and Public Service Enterprise Group Inc. (PEG) and operated by Exelon. The Surrey Power Station is owned by Dominion Resources Inc. (D).

-By Tennille Tracy, Dow Jones Newswires; 202-862-6619; tennille.tracy@dowjones.com

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