Waterford – One of the two nuclear reactors at the Millstone Power Station shut down automatically on Saturday afternoon due to a loss of condenser vacuum used in the cooling of steam that passes through the plant turbine, the Nuclear Regulatory Commission announced this morning.

Operators at Unit Two, a pressurized water reactor, were in the process of a routine flushing out of the condenser system when the condenser vacuum was lost, resulting in the automatic shutdown of the turbine and reactor, the NRC said. An NRC resident inspector assigned to the plant did not identify any safety concerns, the agency said. The inspectors are continuing to follow up on the event.

Troubleshooting performed by plant operators identified a failed electrical relay in the circuitry in one of the water pumps as the cause of the problem that led to the unplanned shutdown, the NRC said.

The electrical relay has been replaced and the plant was restarted on Monday. The shutdown lasted about 48 hours, Millstone spokesman Ken Holt said. As of this morning, Unit 2 was at 47 percent power. Holt said the plant would return to full power over the next couple of days. Other than the failed electrical relay, “all plant systems performed as they should,” he added.

Unit 3, the other operating reactor at the power station, was not affected by the incident.