Waterford – Dominion, the owner of Millstone Power Station, is investigating the unexpected shutdown of a water pump in the Unit 2 reactor Sunday that triggered an automatic shut down of the reactor itself.

“We’re investigating the cause,” said Dominion Spokesman Ken Holt. “We’re troubleshooting and testing while at the same time making preparations to restart.”

The shutdown occurred at 3:15 p.m. Sunday as plant workers were backwashing the reactor’s condenser, which is used to cool down and condense into water the steam that has already passed through the turbine, said Neil Sheehan, a spokesman for the Nuclear Regulatory Commission, in an email.

One of two water pumps was placed out of service while the work was being performed, as is customary, but the second pump suddenly and independently “tripped,” or shut itself off, Holt and Sheehan said.

The plant safely shut down, there were no complications and the cause of the pump trip is being investigated, Sheehan said.

In a pressurized water reactor like this one, the reactor keeps water under pressure so that it heats, but does not boil, according to the NRC. The reactor coolant system transfers heat produced in the reactor core to the steam generators. At that point, heat is produced for the turbine-generator, which in turn produces electricity.