Two Dominion (NYSE:D) nuclear units in Connecticut went out of service during the Memorial Day weekend due to a loss of offsite power, according to an event notification report from the Nuclear Regulatory Commission (NRC).
Shortly after 7 a.m. on Sunday, May 25, Millstone Units 2 and 3 experienced a loss of offsite power and subsequent trip of both units, NRC said in the report posted on its website May 27.
“The loss of offsite power was due to one main line coming into the plant shorting to ground,” NRC said in an update several hours after the incident. “Emergency plan procedures have been implemented, natural circulation has been established, and the plant is stable. Offsite electrical power has been restored to both Unit 2 and Unit 3. Emergency diesel generators have been secured, and plant equipment has been restored to a normal line up,” the NRC said.
“Extended loss of instrument air following the loss of offsite power complicated the recovery actions,” NRC went on to say. “The normal reactor coolant system letdown flow path was not able to be established. This resulted in the rupture of the pressurizer relief tank rupture disk inside containment. The relief valve on the volume control tank lifted when restoring normal letdown. This caused the primary drains transfer tank to overflow into the auxiliary building. This overflow path has been isolated,” NRC said.
Radiation monitors are “indicating normal levels” in the affected area, NRC said. Both Millstone 2 and 3 were still listed at zero percent (0%) power generation in the latest NRC reactor status report.
Millstone 2 had recently returned to service following a regularly-scheduled refueling outage that ended earlier in May. Millstone 2 and 3 are pressurized water reactor (PWR) units that together have a generating capacity in excess of 2,000 MW.
Both the company and NRC are investigating the incident.
“Millstone is working with ISO-NE [ISO-New England], the local grid operator, and Connecticut Light & Power, the local utility, to implement a solution to prevent it from happening again,” said a plant spokesperson. The plant spokesperson declined to say when the units might return to service.
Only nine U.S. reactors not currently producing power
Despite the weekend incident involving the Millstone units, NRC data showed that only nine of the nation’s 100 commercial power reactors were generating no power at all early May 27.
The figure stood at 11 shortly before the Memorial Day holiday and was at 20 in late April. Nuclear reactors typically conducted scheduled refueling or maintenance-only outages in the low demand shoulder seasons of spring and fall.
NRC data also indicates that one of the units returning to service in recent days was the Xcel Energy (NYSE:XEL) Prairie Island 2 nuclear unit in Minnesota. It had been taken offline May 18 to remove an obstruction in drain piping from a pump.