Vermont Yankee is seen here from the Hinsdale, N.H., side of the Conneticut river. Zachary P. Stephens/Reformer
Wednesday June 20, 2012

VERNON -- Officials say a fault in a drive motor forced the Vermont Yankee nuclear power plant to reduce its power output by nearly two-thirds on Monday.

According to Yankee spokesman Rob Williams, at about 1 p.m. there was a reported electrical failure in one of the plant's two motor generator sets, which control the reactor's power by varying the flow of the reactor cooling water.

Williams said the plant is operating at 38 percent power output and will be maintained at that level until the repairs are made to the "Alpha" set.

"We're making preparations to lift the motor generator set out and have it refurbished," he said Tuesday.

No timeline was given as to when the reactor could be back to full capacity, Williams said.

This is the second time in the past nine months the nuclear reactor has had to reduce its power output because of faulty generator sets.

In September, the "Beta" motor generator malfunctioned, which caused the Vernon reactor to reduce its output to 46 percent.

Neil Sheehan, a spokesman for the Nuclear Regulatory Commission, told the Reformer that "it appears likely the motor will need to be replaced."

At the time the motor stopped worked, operators observed "some smoking" near the motor generator sets' lube oil pump, but no fire was detected, Sheehan said.

"Our resident inspectors assigned to Vermont Yankee were immediately made aware of the issue and have been

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monitoring the company's actions in response," Sheehan said. "They have not identified any immediate safety concerns, but we do expect plant personnel to get to the bottom of the problem involving the motor generator sets as soon as possible."

David Lochbaum, director of the Nuclear Safety Project with the Union of Concerned Scientists, said control rods are needed to further reduce the plant's power level and doing so can cause instability.

Each recirculation loop draws water from the reactor vessel and sends it through the motor pump before returning it through jet pumps, he said.

"Varying the rate at which water flows through the reactor core changes the rate at which atoms fission, and thereby the reactor's power level," he said. "The component that caused a problem at VY was in the control circuit for one of the two recirculation pump motors."

Although the plant can operate at about 36 percent with one recirculation pump running, it's not without concern.

"The response is expected," Lochbaum said. "The failure is not."

Josh Stilts can be reached at jstilts@reformer.com, or 802-254-2311 ext. 273.