Equipment designed to prevent a hydrogen gas explosion similar to what happened during the Three Mile Island partial nuclear meltdown has been inoperable at FirstEnergy Corp.'sDavis-Besse plant since it opened in 1977, a report shows.

For all those years, Davis-Besse technicians had followed procedures to turn knobs on the equipment that indicated interior valves were working, but the valves actually had frozen closed.

Davis-Besse workers discovered the broken equipment during a safety systems check in early May, according to the report given to the Nuclear Regulatory Commission. The Union of Concerned Scientists on Tuesday released copies of the report, which was filed June 30 with the NRC.

The broken equipment is the latest in a long list of other safety problems discovered at the troubled plant in Oak Harbor, about 25 miles east of Toledo.

FirstEnergy said backup systems likely would have detected a hydrogen gas buildup if there was a major accident inside the containment chamber, warding off the likelihood of an interior explosion. The massive concrete and steel containment chamber is the last line of defense between a malfunctioning or damaged reactor and the environment.

The Akron utility will permanently replace the faulty valves with open piping, spokesman Todd Schneider said. The problem should not slow downFirstEnergy's efforts to get the plant ready to restart by the end of summer, he said.

The Davis-Besse ``licensee event report'' said valves that allow containment gas analyzers to detect a hydrogen gas buildup inside the containment chamber ``have been in the closed position since plant startup in 1977.''

The stuck valves were found after tests May 2 showed that there was no flow of gas through a device called a heat exchanger, according to the NRC document. The exchanger is part of a system designed to control the amount of hydrogen released during what is called a loss of coolant accident, where the nuclear reactor coolant spills into the containment chamber.

Davis-Besse workers over the years conducted monthly tests that involved turning the knobs that controlled the valves; but while the knobs turned, the interior parts of the valve did not, Schneider said.

``They (plant workers) were following procedures to make sure the valves were open,'' he said. ``It appeared to be working satisfactorily.''

The Nuclear Regulatory Commission will look into the matter, spokesman Jan Strasma said.

It does not appear that the stuck valves is an industrywide issue, he said.

``It would appear to be a plant-specific-type problem,'' he said.

Davis-Besse and NRC inspectors are conducting thorough reviews of the plant's safety systems, Strasma said.

David Lochbaum, the Union of Concerned Scientists' nuclear expert, said in an e-mail that when nuclear fuel overheats, hydrogen is created when moist air interacts with the nuclear fuel rods. The containment gas analyzer is supposed to alert plant staff about hydrogen gas buildup so they can take action.

At Three Mile Island, large amounts of hydrogen were created when the Pennsylvania plants' nuclear fuel overheated on March 28, 1979, Lochbaum said. About 10 hours after the accident started, the hydrogen blew up inside the containment chamber, causing the interior pressure to jump from about three pounds per square inch to 28 pounds per square inch, he said. That's the equivalent of putting 36 tons of pressure on a containment door that is 3 feet by 6 feet, he said.

But even after Three Mile Island, Davis-Besse's malfunctioning hydrogen gas-detecting equipment went undiscovered.

Schneider said in the unlikely event of a loss-of-coolant accident, Davis-Besse workers had other means to detect a hydrogen buildup and deal with it.

If hydrogen gas builds up, workers can pump air inside the containment chamber to dilute it, he said. A fall-back option is to purge the hydrogen gas through filters into the environment, he said.

Davis-Besse has been kept shut down ever since a large corrosion hole was found in March 2002 nearly all the way through the reactor vessel head that covers the nuclear fuel. The damage will cost FirstEnergy hundreds of millions of dollars and has sparked technical and criminal investigations.

In September, Davis-Besse workers found that the plant's former sump strainer system, designed to keep the nuclear fuel from melting during a loss-of-coolant accident, may have been faulty ever since the plant was built and made the plant more susceptible to a worst-case meltdown. FirstEnergy since has spent about $2.3 million to build a state-of-the-art sump.