The Pilgrim Nuclear Power Station in Plymouth was shut down early yesterday after a leak was detected in one of four safety relief-valve systems.
The steam that leaked was slightly radioactive and was contained inside the plant. It posed no threat to the public or to plant workers, said a spokesman for the Nuclear Regulatory Commission.
The leak occurred in a small valve that triggers a larger safety valve to open to relieve pressure, said Rob Williams, a spokesman for Entergy Nuclear, which owns the power station. There are four such safety valve systems, which are used in the case of pressure buildup to vent steam from the reactor to the “suppression pool,’’ a doughnut-shaped reservoir of water at the base of the reactor building.
Williams said the problem was detected by temperature sensors Monday, and the shutdown occurred at 1:30 a.m. yesterday. He said he could not speculate about how long it would take to repair the valve and power the plant back up. “In past instances such as this,’’ similar repairs have taken about a day, he said.
An NRC inspector was at the site Monday to monitor the problem and the plant’s shutdown and will be present during repairs and when the plant restarts.
Neil Sheehan, a spokesman for the federal agency, said controlled shutdowns happen from time to time, and the event at Pilgrim had not triggered the need for closer oversight. He said there would need to be more than six such shutdowns within 7,000 hours of plant operation .
Through September, the plant had zero such events this year, but Pilgrim was shut down last month because of a problem with a different type of valve.
According to records from last year, the nationwide industry average for such nonemergency events, in which a nuclear plant is powered down, was less than 1 per 7,000 hours of operation.
Last spring, an emergency shutdown at Pilgrim occurred amid safety issues. Last month, the NRC officially required a yearlong review of safety procedures because of the problems, which occurred as control room operators were restarting the reactor after it had been refueled.
Sheehan said the new problem was not discovered because of the safety review, which has yet to begin.
“This type of work is not uncommon in steam-generating plants,’’ Williams, the Entergy spokesman, said.
“It’s an instance where the detected leakage means that the safety valve system needs work, so it can’t be done online, so come off line, make the repairs,’’ and then power up the plant again, he said.