Davis-Besse is expected to remain idle for days as officials look for the source of a problem that caused the nuclear plant's reactor to automatically shut itself down yesterday morning.
FirstEnergy Corp. has all but ruled out faulty procedures or human error, saying the problem likely is equipment-related.
But the Nuclear Regulatory Commission has not ruled out anything.
"We're not at that point yet," Jan Strasma, NRC spokesman, said when asked if procedures or human error could be dismissed.
Such unexpected shutdowns - known as reactor "trips" - occur when computerized safety systems detect something out of kilter. The systems automatically insert boron-filled control rods into the reactor core, shutting down the plant.
Davis-Besse generates 7 percent of the electricity FirstEnergy provides to its 4.4 million customers in Ohio, Pennsylvania, and New Jersey. The company lists itself on promotional literature as the nation's fifth-largest electric utility.
Richard Wilkins, a company spokesman, said FirstEnergy hopes the anticipated cooler temperatures of the next few days will allow it to meet all needs for power by itself. It has suppliers contracted to provide electricity whenever it can't, he said.
Davis-Besse generates 935 megawatts of electricity, of which 883 are put out on the electric grid. The rest is used to power the plant.
The automatic shutdown occurred without incident at 10:24 a.m. as workers were performing routine tests on reactor trip circuit breakers. Such tests are done once every three weeks in some part of the plant. The particular valve tested yesterday likely was last checked about three months ago, Mr. Strasma said.
FirstEnergy officials emerged from a lengthy meeting yesterday afternoon convinced that workers doing the test had not strayed from the established procedure and that the procedure itself was adequate, Mr. Wilkins said.
"The initial investigation seems to have eliminated human error as the cause, so they're looking at equipment issues," Mr. Wilkins said.
All equipment potentially involved in an automatic shutdown, from control drive motors to circuit breakers themselves, will be the first examined, he said.
The utility, figuring the task of identifying and fixing the problem will take several days, has scheduled some routine maintenance activities that can only be done when the plant is off-line, Mr. Wilkins said.
The company is keeping the plant in "hot standby" to save time on the restart process, he said.
Barring a major complication, the NRC is unlikely to put restart authorization on hold, Mr. Strasma said.
Two of the agency's three residents inspectors assigned to Davis-Besse are monitoring the company's investigation at the plant.
The third resident inspector was at the agency's Midwest regional office near Chicago yesterday, he said.
Davis-Besse was idled for more than two years after its former reactor head nearly burst. The degrading of the material was caused by the worst corrosion build-up of its kind in U.S. nuclear history, a condition the NRC blamed on apathy and neglect.
The plant has had a fairly smooth run after addressing numerous equipment, management, and performance issues cited by the agency.
It remained online for 131 days after working through its initial kinks after gaining the NRC's restart authorization March 8.
It had to temporarily scale back from full power for a few days in early July because of an unexplained "burp" of radioactive gases as employees replaced one of two reactor-coolant filters.
The NRC has said the March 8 restart ended the nuclear industry's worst safety failure since one of the reactors at the Three Mile Island nuclear complex in Pennsylvania experienced a partial meltdown in 1979.
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