The power plant had been operating at less than half its usual capacity since Unit 3, the plant's largest generator, shut down on August 9. Ordinarily, the plant produces 2,100 megawatts but just over half of that power comes from Unit 3's 1,233 megawatt pressurized water reactor.
The shut down was caused by the unexpected loss of an electrical distribution panel that supplies power to some instrumentation that monitors feed water. Feed water is the water that goes into the steam generators that turns into steam. When the water levels dropped in the steam generator, the plant's safety systems automatically shut the reactor down.
The shutdown was not considered an emergency, as the safety systems worked exactly as they were supposed to, but the plant did notify the Nuclear Regulatory Commission (NRC). Unit 2 was not affected and continued to operate as usual.
Dominion has been in touch with the NRC a second time since then to notify the regulatory agency that it would be conducting scheduled preventative maintenance on the radiation monitor inside the Millstone stack this past Monday.
The plant has numerous monitors around the site that track radiation levels. This one, however, is the only monitor in the stack and it failed a surveillance test on April 16.
"There was a pump associated with the radiation monitor that circulates air that wasn't working," said Dominion's Millstone spokesman, Ken Holt. "We didn’t think we needed to notify the NRC for that but after discussion with our resident inspectors we decided we did. The radiation monitor was restored to service April 21."
On Monday, the radiation monitor was out of service again from 1 p.m. to 3 p.m. for maintenance. "We notified the NRC and state and local officials that needed to be notified," Holt said.
When the radiation monitor is offline, Holt said, "There are compensatory actions we can take but they don’t provide the same assessment capabilities as the radiation monitor does. The reason we do planned maintenance on them is to ensure they are working."