The shutdown of the plant’s 1,200-megawatt Unit 3 was triggered when an instrument that measures the flow of water into a steam generator lost power, and the water was not diverted correctly, Millstone spokesman Ken Holt said. He said the company was investigating the cause of the problem.
“We’ll determine the cause of the power loss and bring the unit back online,” Holt said.
It was the first automatic shutdown at Millstone since last summer, when its other operating unit went off-line because of unusually warm water in Long Island Sound. The unit that shut down Friday is the larger of its two operating units.
A spokesman for the Nuclear Regulatory Commission, Neil Sheehan, said the reactor tripped as designed.
“The shutdown was uncomplicated, with all safety systems performing as they should,” he said.
An NRC inspector assigned to Millstone went to the plant Friday night to observe the company’s response.
Sheehan said inspectors will follow up on the company’s efforts to diagnose the cause of the malfunction, and the agency will note the unplanned shutdown in its tracking of forced outages.
Last summer, Unit 2 was forced to shut down for nearly two weeks because the water in the Long Island Sound was warmer than the limit of 75 degrees that’s in place to keep the plant operating safely.
The NRC issued an “emergency license amendment” allowing Millstone, a subsidiary of Richmond, Va.-based Dominion Resources Inc., to use an average temperature of several readings.
Holt said the incident Friday was the first automatic shutdown at Unit 3 in more than 3 ½ years.