Waterford — Millstone Power Station remained off-line Tuesday as operators worked to complete cleanup and repairs of two pressurized tanks for cooling water that ruptured after the unexpected shutdown of both reactors Sunday due to a power failure.

Millstone spokesman Ken Holt said this is the first time both reactors have had to shut down simultaneously. The outage occurred about 7 a.m. Sunday after a short in the transmission lines that supply power to operate plant systems even as the reactors produce power and send it out onto the grid.

Holt said that the problem originated in a relay switch on the Connecticut Light & Power transmission lines and that power was restored by noon Sunday. But Millstone 2 and 3 remain offline as crews repair damage that occurred in the aftermath of the outage and analyze exactly what happened and any lessons that can be learned, he said. The Nuclear Regulatory Commission was notified of an unusual event, the lowest of four levels of emergency classification.

"We want to understand why we lost offsite power," Holt said. "We called in people over the holiday weekend to come in. Our equipment and operators responded as they were trained to."

During the shutdown, emergency diesel generators automatically supplied power to keep reactor safety systems operating.

Neil Sheehan, spokesman for the Nuclear Regulatory Commission, said Millstone crews will have to clean up contaminated water that leaked from the ruptured tanks, as well as repair the pressurized tanks.

"We are continuing to review the company's actions and are considering whether the event warrants a more detailed inspection," he said.

In a Preliminary Notification issued Tuesday on the shutdown, the NRC said the loss of offsite power caused an "extended loss of instrument air" and "complicated the recovery actions." Monitors at the plant indicate normal levels of radiation at the site of the leaks, the notification said.

David Lochbaum, director of the Nuclear Safety Project for the Union of Concerned Scientists, said the water that leaked had low levels of radioactivity and is "not much danger to workers or the public." Still, he said, the fact that two leaks occurred in two different buildings in Unit 3 during the shutdown is cause for concern.

"A relatively uncomplicated event became relatively complicated," Lochbaum said. "There were things that occurred that shouldn't have happened."

He said it is not yet clear whether the problems were the result of equipment failures, design flaws, gaps in operator training or from operators not following proper procedures. While this is the first time both reactors have had to shut down at the same time at Millstone, multiple shutdowns have occurred four or five times this year at other nuclear power stations across the country, he said.

Essentially, he said, the incident revealed that there are "holes" in Millstone's safety net systems.

Dominion has a reputation for doing thorough analysis of shutdown and safety incidents and learning from any mistakes that are uncovered, Lochbaum said.

"That's something Dominion does well," he said.

j.benson@theday.com