Waterford — The Unit 2 reactor at the Millstone Power Station shut down early Friday morning because of an issue with the electrical supply for the heaters in its water pressurizer, the Nuclear Regulatory Commission announced.

The unit, a pressurized water reactor, experienced an electrical short in a breaker that supplies power that enables some of the heaters to open, NRC spokesman Neil Sheehan said. An emergency diesel generator that could have provided backup power was out of service, so the shutdown was required according to the plant’s technical specifications, Sheehan said.

Millstone spokesman Ken Holt said the emergency generator was undergoing routine maintenance. Plant operators began shutting the plant down manually at about 3:30 a.m., and the faulty heater was removed from service.

“All systems operated normally,” Holt said.

The NRC inspector at the plan was notified of the shutdown. Unit 3 is unaffected.

The last unplanned shutdown of Unit 2, the smaller of the two operating reactors at the power station, occurred in November. Built in 1975, it is the older of the two units and generates 869.5 megawatts of electricity. Unit 3, built in 1986, generates 1,210 megawatts.

j.benson@theday.com