The fire was caused by a piece of an expansion joint that broke off in a duct carrying electricity to Vermont Yankee's transformer, according to the NRC.
"Part of an expansion joint just basically peeled off and, rattling around in the duct, it caused (electrical) shorts and faults," NRC spokesman Neil Sheehan said. "Because of the violence, because of the resulting effects of this, an oil pipe came loose and because of very high temperatures associated with the raw electricity, a fire started."
Sheehan said the expansion joint, which was part of the duct or pipe that conducted electricity from the plant's generator to its transformer, was original equipment on the 32-year-old reactor.
Entergy Nuclear, the owner of Vermont Yankee, has proposed increasing power output at the plant by 20 percent, a process known as the "uprate."
Sheehan said Entergy had told the NRC it believed the fire was not connected to plant modifications during the plant's regular refueling outage in April.
But Sheehan stressed that the NRC did not yet agree with this conclusion.
"We're still looking at it," he said, noting that the NRC had sent a special team to Vermont to help with the investigation.
Sheehan said Entergy's investigators found the broken piece of expansion joint in the duct, leading them to their fire theory.
The plant has been off-line since the June 18 fire, forcing utilities that usually buy power from the facility to look elsewhere for energy.
According to an agreement with the state Public Service Board Entergy agreed to pay the difference in increased power costs if the plant closed because of uprate-related problems.
Sheehan said two weeks of investigation and repair work had revealed very little damage to the transformer itself, which was installed two years ago by Entergy, shortly after it bought the plant.
The new transformer is needed to handle the proposed power increase.
Entergy spokesman Robert Williams declined comment on the root cause of the fire. But he said the plant could be "reconnecting to the grid next week."
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