Entergy says blown fuses problem still a mystery
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By Susan Smallheer
Staff Writer - Published: September 1, 2010
VERNON – The source of what caused seven fuses to fail and darken two panels of warning lights in the control room of Vermont Yankee on Sunday night remains a mystery.
The blown fuses prompted the nuclear plant to declare a low-level emergency, which last 90 minutes until the fuses were replaced.
Entergy Nuclear spokesman Larry Smith said that the company has ordered a root-cause analysis report on the issue, to get to the bottom of what caused the fuses to blow.
Smith said the plant and its emergency warning lights were completely operable and the plant didn't have to take any new precautions because of the problem.
Uldis Vanags, the nuclear engineer with the Department of Public Service, visited the plant on Tuesday and said Vermont Yankee personnel were still unsure what had happened.
He said two out of the control room's five panels had been affected by the blown fuses. The two that went out include warning lights for such safety systems as reactor water levels, main steam line indicators, recirculation pump indicators, oil levels, and the reactor coolant injection system, among others.
“There a lot of things here,” he said. “These are safety parameters. They are important and that's why it fell into a ‘unusual event,'” the Nuclear Regulatory Commission's term for the lowest level of emergency.
If the warning lights blackout had occurred along with a plant transient or emergency shutdown, “you've lost a lot of information and it would be more difficult to operate the plant,” he said.
Vanags said that Entergy engineers and technical workers were working at an “elevated level” to try and get to the bottom of the cause of the problem. He said it was “highly unlikely” that a fuse failure was a cause of the emergency.
He said the electrical circuit controlling the warning lights in the control room was designed to keep working even if just one of the fuses was still working. Seven fuses were rendered inoperable, he said.
“I hate to use analogies with cars, but here we go. On the dash, there's a temperature gauge for the engine, but when the temperature gets really high, a light will go on to catch your attention,” said Vanags, explaining the role of the warning lights in the control room. “That warning light is designed to catch your attention,” he said.
He said he was “very familiar” with the electrical circuit design in the control room, saying its design dated back 30 years or so, and used cylindrical, one-inch long glass fuses. He called it a “buss fuse.”
Entergy had originally reported to the NRC on Sunday night that only one fuse had blown, but further investigation showed that seven fuses were affected.
Neil Sheehan, a spokesman for the NRC, said Entergy would have to file an updated report on the incident, in addition to conducting the root cause analysis.
@Body tagline:susan.smallheer@rutland herald.com
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