FirstEnergy's Beaver Valley Unit 1 nuclear plant shut down suddenly Monday afternoon, releasing a small amount of radiation into the atmosphere.

FirstEnergy now has two of its four nuclear reactors off-line. The other is Davis-Besse, which has been out for more than a year. A Nuclear Regulatory Commission report released Tuesday said FirstEnergy's troubled Davis-Besse plant was just a year or two away from a serious accident when a safety inspection last year unearthed unexpected reactor damage.

The Beaver Valley complex, which holds two nuclear reactors, is on the Ohio River near Shippingport, Pa., not far from the West Virginia and Ohio lines.

At Beaver Valley, tritium, a radioactive byproduct of the reactor, was vented along with steam into the atmosphere, FirstEnergy and the NRC said. The amount of radiation released, well under a millirem, was less than 1 percent of the amount of radiation the plant is allowed to put out into the environment in a year's time, the Akron utility said. The tiny amount of radiation was detectable while it was inside the plant; the amounts were too small to be detected in the atmosphere outside the plant because it dissipated, FirstEnergy spokesman Todd Schneider said.

Workers bumped scaffolding into a steam line valve, forcing the valve to close and shut off a steam line that leads to the plant's power turbine, Schneider said. A diaphragm in the valve needs to be replaced, he said.

The workers were getting the plant ready for a refueling outage that is scheduled to start next month, Schneider said.

The NRC said the gaseous tritium would have little to no effect on the public. Most people are exposed to 300 to 360 millirems of radiation a year from natural and man-made sources, NRC spokesman Neil Sheehan said.

The Beaver Valley release amounted to 0.000000512 millirems, he said.

Repairs are under way at Unit 1, which likely will be back online by the end of the week, Schneider said. ``The plant shut down as designed. All of the safety systems worked.''

Beaver Valley Unit 1 automatically shut down at 3:48 p.m. Monday after it detected low steam line pressure. The plant shift manager declared the shutdown an ``unusual event'' at 4 p.m. while also saying the unit was stable. An unusual event is the lowest of the NRC's four emergency classifications. The unusual event designation was lifted at 5:35 p.m. Monday, according to the NRC.

``To be conservative, they declared it an unusual event,'' Sheehan said.

Davis-Besse details

The NRC report on Davis- Besse said that under a worst-case scenario, in which boric acid continued to quickly chew out a larger cavity into the six-inch-thick carbon steel reactor head, a loss of coolant accident could have happened this year or in 2004.

As a result of the boric acid damage, the NRC report classified the Davis-Besse incident as ``highly safety significant'' and gave it the agency's worst color-coded safety rating, red. The report and safety rating followed a nearly yearlong investigation into what caused boric acid to eat a large cavity nearly all the way through the top of the nuclear reactor and start a second, much smaller cavity.

The red designation will not affect FirstEnergy's efforts to get Davis-Besse ready to restart by mid-April, Schneider said. But the designation will be a factor in deciding the size of a civil penalty the utility expects to pay. The NRC will determine when Davis-Besse will be allowed to restart.

FirstEnergy will not contest the red designation, Schneider said.

David Lochbaum, nuclear expert for the Union of Concerned Scientists, said the NRC's red designation ``is more historical than it is instructive.''

Surprising damage

The NRC took nearly a year to determine the red finding because it never expected the kind of damage found March 5 at Davis-Besse and had to revamp methodologies and formulas in trying to figure out how boric acid corroded the top of the reactor, Lochbaum said.

``That complication messed up their mathematical models,'' he said. ``They had to sit down and essentially start with a clean slate.''

The NRC report said that the creation of one large boric acid cavity significantly increased the chances for a loss of coolant accident, in which high pressure reactor coolant jets out of the reactor and into the massive containment building.

There was no loss of coolant accident or release of radiation at Davis-Besse, which is in Oak Harbor along the Lake Erie shoreline. A FirstEnergy report released earlier this year said that an emergency sump system, designed to keep the nuclear fuel from melting in a loss of coolant accident, was defective and could have been clogged by debris and rendered unusable.

By luck only, a thin interior lining of stainless steel held coolant inside the reactor vessel and prevented an accident, the NRC report said.

The report also reveals that the NRC staff has problems understanding exactly what damage the boric acid corrosion would have caused at the reactor.

At the time the damage was found, it did not appear that the plant was in imminent danger of a loss of coolant accident, the report said.

Other problem plants

This is the third time since 1999 that the NRC has determined a plant merits a red designation, NRC spokesman Jan Strasma said. The NRC gave similar designations to the Indian Point II plant in New York and Point Beach plant in Wisconsin, he said.

The NRC expands its inspections and oversight at troubled plants after they get a red designation, Strasma said.

In Davis-Besse's case, those expanded efforts were under way shortly after the boric acid corrosion was first found.