BRATTLEBORO -- During testing of a ventilation system at Vermont Yankee nuclear power plant in Vernon exhaust fans failed to come on, resulting in an overpressure of the reactor building.
Rob Williams, spokesman for Yankee, said technicians had started up the heating, ventilation and air conditioning system in the reactor building, which normally is negatively pressured. When the exhaust fans failed to kick on, a six-by-ten-foot aluminum "blowout" panel detached from the reactor building and landed on the roof of the plant's turbine building.
"The panels worked as designed," said Rob Williams, spokesman for Yankee. "But we are looking into why the exhaust fans didn't operate."
The panels at Yankee are there to protect against tornado wind loading that can cause overpressurization conditions in the reactor building, said Neil Sheehan, spokesman for the Nuclear Regulatory Commission.
"If the reactor building is threatened by tornado-force winds, the panels can blow out and provide near instantaneous pressure equalization in the building," said Sheehan.
The ventilation test was being conducted on Monday while the plant has been powered down for a refueling outage. Williams said neither workers at the plant nor the public was put in danger when the panel blew off the building.
The reactor building is negatively pressurized, meaning the pressure inside the building is less than outside, keeping potential leaks of radioactive materials inside.
Because the reactor had been powered down for refueling, there was no danger of radioactive materials escaping to the atmosphere, said Sheehan.
"Our resident inspectors at Vermont Yankee, with support from Region I Office specialists, will continue to review the activities until the issue is fully resolved, including the development of a root cause evaluation and corrective actions," said Sheehan.
A temporary cover has been put in place pending the installation of a permanent fix.
In a memo to the Vermont State Nuclear Advisory Panel, Uldis Vanags, the nuclear engineer for the Vermont Department of Public Service, noted the panels are secured with a wire rope to prevent them from falling to the ground when they release.
"However, in this case the panel that released fell onto the turbine building roof," wrote Vanags.
He stated the reactor building serves as secondary containment that is at negative pressure so all air in the building is vented up the plant stack.
"Vermont Yankee is currently in a refueling outage, and secondary containment was not required at the time the event occurred," wrote Vanags. "Within approximately a half hour, Vermont Yankee restarted reactor building ventilation to reestablish a negative pressure and exhaust up the plant stack."
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