Waterford - Dominion is investigating how and why two sets of vents at the Millstone Unit 3 reactor were left open for two weeks in a leak collection release system that only operates during accidents.

There were no accidents or incidents between May 11, when Unit 3 was preparing to exit a refueling outage, and Friday, when the condition was discovered, said Ken Holt, a Dominion spokesman. The vent positions were immediately corrected when found, Holt said.

However, operators want to know whether the system, which prevents radiation from being released outside the containment building where the reactor is housed, would have still been able to function during an accident with the vents open, said Holt and Neil Sheehan, a spokesman for the Nuclear Regulatory Commission.

The Supplementary Leak Collection System, or SLCRS, is in standby unless there's an accident, Holt said. It appears that testing was completed without the vents being returned to their proper position, he said.

"Since there was no accident, there was no release of radioactivity as a result of the system being misaligned," said Sheehan. "The question is: Why was it misaligned for two weeks or so? And that's something our inspectors will be looking at further."

Dominion is the parent company of Dominion Nuclear Connecticut, which owns Millstone Power Station. The nuclear power complex operates two reactors, Units 2 and 3, and houses one closed reactor, Unit 1.

The vents, called dampers, are part of the SLCRS used in the event of an accident to control the release of radiation to the public. The vents are in a tunnel that connects the containment building and an auxiliary building. Suction draws in any air that may leak past containment to adjacent buildings so it is filtered before it is released to the atmosphere, Sheehan and Holt said.

"We're looking … (at) procedures we can change to make sure it doesn't happen again," Holt said.

The NRC resident inspectors assigned to Millstone are following up as well and will document any deficiencies, Sheehan said.

Holt also noted that Dominion verified the positions of nearly 100 other vents at Unit 3 to make sure they were properly configured.

p.daddona@theday.com