WATERFORD — Officials at the Millstone Nuclear Power Plant shut down one of two generating units Friday morning after a feed water pump failed, according to a spokesman for Dominion Energy, the owner of the facility.

Unit 2 was shut down by plant operators at 7 a.m. Friday, said Ken Holt, a spokesman for Virginia-based Dominion Energy said. Holt said the company would not comment on when Unit 2 would be put back online and restored to full power for competitive reasons.

This is the second time this month one of Millstone’s two nuclear power generating units has been taken offline. Unit 3, Millstone’s other operating nuclear generator, was taken off line by plant operators on Dec. 17.

Holt said Unit 3 came back online on Monday and was operating at full capacity a day later.

The last time both nuclear power generating units were offline at the same time was earlier this year, he said. Unit 3 was offline for a scheduled refueling and Unit 2 had to be shut down because of a problem with a heating unit. The unit is used to heat water that is then fed into the reactor.

When both generating unit are operational, Holt said Millstone produces 2,100 megawatts of electricity, which is enough to power 2.1 million homes. With Unit 2 offline, Millstone is producing 1,200 megawatts of electricity, enough to power 1.2 million homes, he said.

Millstone is one of just two nuclear power plants currently operating in New England. The Seabrook Nuclear power plant in New Hampshire is the other.

Unit 2 began operating in 1975, according to the federal Nuclear Regulatory Commission, while Unit 3 came online in 1986.

Unit 1 at Millstone began operating in 1970 and was shut down in 1995 after a leaking valve was found.

Both Millstone generating units now operating are pressurized water reactors.

This type of reactor uses fission, a process in which uranium atoms are split, to heat water in the reactor. The water is kept under pressure so it doesn’t boil, and then is pumped into tubes in a heat exchanger.

Those tubes heat water from a separate source to create steam. The steam then turns turbines in a generator to produce electricity.