The Calvert Cliffs nuclear power plant was shut down unexpectedly yesterday after a possible flaw in the emergency power system was revealed during a federal safety inspection.
Baltimore Gas and Electric Co. declared an "unusual event," the least serious level of nuclear emergency, at 5:25 a.m. at its twin-reactor plant on the Chesapeake Bay in Calvert County. The utility then gradually took its two 825-megawatt units out of service over the next five hours.
BG&E decided to take the action after determining that the plant's emergency power system might not be able to supply enough electricity to prevent a reactor meltdown in case of a very unlikely chain of mishaps.
The Nuclear Regulatory Commission recently has directed plant operators to look more closely than before at prevention of even highly improbable accidents, a BG&E spokesman said.
The possible flaw was identified during a routine NRC inspection, which is continuing, NRC officials said.
The shutdown was ordered after the plant's operators decided that, in certain circumstances, the three backup diesel generators might be unable to supply enough electricity to run safety systems, such as pumps that spray cooling water into an overheating reactor.
The emergency power system should work as planned in the event of most conceivable accidents, BG&E spokesmen said. But plant operators were unable to prove the system would not fail should the plant lose off-site power and a small leak devel
op in a reactor's cooling pipes, leading to a gradual buildup of temperature and steam pressure inside the reactor.
Under those circumstances, there would be a possibility that two or more reactor safety systems would kick in simultaneously, and the backup generators could not provide sufficient electricity to run them, said Peter Wilson, the NRC's senior inspector at Calvert Cliffs.
"They just don't know whether the plant would respond in an accident. That's why they're shutting down," said Mr. Wilson.
"We have to assume that, unless we can prove otherwise, something would go wrong," said Karl Neddenien, a BG&E spokesman.
The plant's operators first recognized the possibility of such an electrical-supply failure "a few years ago," according to Mr. Neddenien.
But utility officials "felt we could live with it," he added, because they determined that the odds of it happening were no greater than 1 in 10 million.
Federal regulators, however, have told utilities recently to take a closer look at nuclear safety systems, Mr. Neddenien said.
Given such guidance, the BG&E spokesman said he did not know why the utility waited until this week, when the NRC inspection team showed up, to decide to shut down Calvert Cliffs.
"That's one of the strongest things we'll look at right now," the BG&E spokesman said. "Could we have caught this, and how could we do a better job?"
Unit One was scheduled to begin a three-month refueling outage tonight anyway.Unit Two will remain shut down until concerns about its emergency electrical supply system are resolved.
The shutdown should have no effect on the electric bills paid by BG&E's Central Maryland customers, said Arthur Slusark, another utility spokesman.
Demand for electricity is relatively low at this time of year, he explained, and the company can generate enough power at its other plants for the time being to make up for the loss of Calvert Cliffs Unit Two.
This is the second "unusual event" at Calvert Cliffs in the past year, said Diane Screnci, an NRC spokeswoman. A low-level emergency was declared last June when the air conditioning failed inside the reactors' control room.
The NRC just last month removed Calvert Cliffs from its "watch list" of plants requiring extra regulatory oversight.
One or both reactors had been shut down for various mechanical and safety problems for nearly 2 1/2 years, until last May.
A 1988 NRC report criticized BG&E for putting power production ahead of safety in its operation of the plant, and the utility was fined more than $650,000.