HARTFORD, Conn. (AP) — Connecticut’s nuclear power plant shut down one of two units on Sunday because seawater used to cool down the plant is too warm.

Unit 2 of Millstone Power Station has occasionally shut for maintenance or other issues, but in its 37-year history it has never gone down due to excessively warm water, spokesman Ken Holt said on Monday.

Water from Long Island Sound is used to cool key components of the plant and is discharged back into the sound. The water may not be warmer than 75 degrees and following the hottest July on record has been averaging 1.7 degrees above the limit, the Nuclear Regulatory Commission said.

The federal agency issued an “emergency license amendment” last week, allowing Millstone, a subsidiary of Dominion Resources Inc., to use an average temperature of several readings.

“It wasn’t enough to prevent us from shutting down,” Holt said.

Richmond, Va.-based Dominion does not have an estimate of when the unit will restart, Holt said.

Millstone provides half of all power used in Connecticut and 12 percent in New England. Its two units produce 2,100 megawatts of electricity, which is reduced by 40 percent with Unit 2 down, Holt said.

Marcia Blomberg, a spokeswoman for regional grid operator ISO-New England, said the loss of electricity will not be a major problem. The Holyoke, Mass.-based agency generally operates with a margin of reserve and plans for the possibility of lost resources, she said.

“Generators are big machines,” she said. “It happens frequently that resources are unable to start up or have to power down.”

In addition to the extreme heat last month, the mild winter didn’t help because it kept Long Island Sound water unusually mild, Holt said.

Neil Sheehan, a spokesman for the Nuclear Regulatory Commission, said Millstone can do little to correct the problem. Cooling millions of gallons of water before circulating it in the plant is not an option, he said.

“Just hope for a cooling,” he said.