The Pilgrim Nuclear Power Station in Plymouth was shut down early this morning because of a leak detected in one of four safety relief valve systems. The steam leak was slightly radioactive and contained inside the plant. It posed no threat to the public or to plant workers, according to 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, according to Rob Williams, a spokesman for Entergy Nuclear, which owns the power station. There are four such safety valve systems that are used in the case of pressure buildup, to vent steam from the reactor to the “suppression pool,” a donut-shaped reservoir of water at the bottom of the reactor building.

Williams said that the problem was detected on Monday, and that the shutdown occurred at 1:30 a.m. on Tuesday. The problem was detected by temperature sensors. He said he could not speculate about how long it would take to repair the valve and power the plant back up, but that “in past instances such as this,” similar repairs have taken on the order of a day to fix.

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An NRC inspector was at the site Monday to monitor the shutdown and will be present during repairs and when the plant restarts. Neil Sheehan, NRC spokesman, said that such controlled shutdowns do happen from time to time, and that at this juncture, Pilgrim has not triggered the need for oversight or safety concerns for the issue—there would need to be more than six such shutdowns over 7,000 hours of plant operation. Through the end of the third quarter of this year, the plant had zero such events, but in mid-November, the station was shut down because of a problem with a different type of valve. According to federal data from last year, the average number of such non-emergency shutdowns at nuclear plants was less than one per 7,000 hours of plant operation.

Last spring, safety issues and an emergency shutdown at Pilgrim occurred. In November, the NRC officially required a yearlong review of safety procedures because of the problems, which occurred as control room operators as were restarting the reactor after it had been refueled.

Sheehan said that the new problem was not discovered because of that 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 offline, make the repairs,” and then power it up again.