Broken 'chiller' valve threatens VY shutdown
BRATTLEBORO -- A faulty cooling system threatened to shut down Vermont Yankee nuclear power plant over the weekend.
But in a little more than 24 hours, technicians at the plant repaired a valve in a unit meant to keep safety-related equipment cool.
"It was an air-conditioning unit, a chiller, used to keep plant equipment cool," said Larry Smith, manager of communications for Yankee.
According to Neil Sheehan, spokesman for the Nuclear Regulatory Commission, the defective unit was discovered during routine surveillance.
"They were unable to adjust the flow to the required range, which they later found out was because the discharge valve disk had separated from the stem, so no amount of opening the stem was going to move the disk and allow more flow," he said.
With the cooler out of service, the "A" trains of the low-pressure coolant injection and core spray systems were declared inoperable, said Sheehan.
"Per the plant's technical specifications, they were required to start shutting down," he said. "They only got down to 95-percent power because they were able to open equipment hatches in the floor to allow air flow into the rooms, which provided sufficient cooling. At that point, they no longer had to shut down and came back to 98-percent power."
The inoperable unit was discovered at 1:40 a.m. on Saturday. The bad discharge valve was replaced and the unit was tested and declared operable at 4:45 a.m. on Sunday.
The chiller was one of four in a room where mechanisms for the residual heat removal and core spray systems are located, said Smith.
"If we had to use that residual heat removal pump or core spray system it could have been a problem," he said.
The residual heat removal system is needed when a reactor is shut down, said Ray Shadis, technical consultant for the New England Coalition, which opposes the continued operation of Yankee.
While a reactor is running, about 95 percent of the heat is produced due to the splitting of atoms. When it is not running the fuel emits heat. Though it's a small amount of heat, it could still cause a meltdown if not properly cooled, said Shadis.
"It's essential that there be systems in place to take that heat away," he said. "The residual heat removal system is one of those systems."
Mechanisms related to the core spray system and low-pressure injection system could also have been inoperable if the room had overheated, said Shadis.
"This (chiller) valve is part of a system that affects both of them," he said.
Sheehan said there are multiple redundancies in Yankee's cooling system that serve as backups.
"Even if they did not get it fixed in time, they have two ‘trains' of residual heat removal," said Sheehan, meaning the plant would still have been able to shutdown safely with one train out of operation.
Apart from the safety implications, said Shadis, the failure of the valve indicates a weakness in Yankee's maintenance and in-service inspection programs.
Shadis said the fault should have been caught prior to a routine "walk down."
"This is something that slipped through the cracks," he said.
It's also a sign of something much more important, insisted Shadis.
"This is a sign of an aging plant, that components are falling apart," he said.
This is not the first time there have been problems with cooling systems at Yankee, said Shadis, including with the high-pressure coolant injection system that sprung a leak in Sept. 2010.
Bob Audette can be reached at email@example.com, or at 802-254-2311, ext. 160.
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