Waterford - Dominion, the owner of Millstone Power Station, spent Monday analyzing what went wrong Sunday to keep operators from restarting the Unit 3 reactor, which had been shut down for refueling.

On Sunday, the reactor was at about 2 percent power just before 6 p.m. when operators halted the startup after an auxiliary steam boiler shut off unexpectedly, causing a loss of vacuum conditions used to run a condenser, said Dominion spokesman Ken Holt and Neil Sheehan, a spokesman for the Nuclear Regulatory Commission.

"They've had a number of issues trying to restart ... equipment issues," Sheehan said. "There were no safety issues associated with the shutdown, but we would expect Dominion to fully understand what happened before they go back online."

The reactor had been shut down for several weeks, not only to replenish fuel in the reactor but to repair other equipment problems revealed during outage maintenance reviews, said Holt. The goal is to ensure operators have full confidence in the equipment so the reactor functions properly for the next 18 months, he said.

"We're looking at possible causes and re-establishing conditions for restarting the reactor," said Holt.

In a nuclear reactor, the condenser turns steam from the turbine, which generates electricity, back into water and cycles it back to the steam generator, where the water is turned into steam again. When a reactor is at low power, the auxiliary boilers are needed to provide extra steam and maintain a vacuum in the condenser so the reactor can start up again.

"We were at very low reactor power when this happened," Holt said. "Once the reactor is at sufficient power, the auxiliary boilers aren't needed."

The company is continuing to isolate the cause of the problem while preparing to restart, Holt said.

Other unanticipated equipment problems during the refueling outage included having to send faulty valves offsite for repairs; fixing the machine that helps take fuel out of the reactor and move spent fuel rods so they can be put in wet storage for cooling; and taking longer than expected to inspect and repair piping used to cool reactor system components, Holt said.

p.daddona@theday.com