Though the cause for the failure of one of the plant's 22 cooling towers has not yet been determined, said a spokesman for the power plant, he assured the public that shutting down 11 of the fans doesn't affect safety.
"Vermont Yankee is coming down to 50 percent to investigate and repair damage to one of the 11 sections in one of our two river water cooling towers," said Rob Williams, spokesman for Vermont Yankee. "The river water cooling tower is constructed with wooden beams and some of these beams and the river piping in that section failed."
There are two banks of 11 fans -- or cells -- that comprise the alternate cooling system at the plant. Main cooling is achieved by pushing river water through the system and returning it to the river.
The cooling tower banks are 50 feet high, 40 feet wide and 300 feet long, said Neil Sheehan, spokesman for the Nuclear Regulatory Commission.
Even though air temperatures have been less than seasonable recently, river temperatures have remained warm, meaning the plant has to rely on the massive fans to cool water before returning it to the river.
"We must stay within our river water temperature discharge limit," said Williams.
"During warmer weather, when the
"This type of tower has a history of failing," said Arnie Gundersen, an former industry insider and Burlington high school teacher who wrote his master's thesis on cooling towers and has testified for the New England Coalition on Nuclear Pollution which has questioned the integrity of the cooling towers.
He called the towers "a wounded knee design," which, under very high stresses, "have been known to fail catastrophically, toppling like dominoes."
Increasing the load on the towers with an uprate of 540MWe to 650MWe added to the already stressed-out structures, said Gundersen.
"They knew they were old," he said, and that's what the recent discussion in Vermont Environmental Court about raising the overall temperature of the Connecticut River by 1 degree was all about. "They don't want to stress those towers out."
"They were concerned about the structural integrity of the towers," agreed Ray Shadis, a technical consultant for NEC, who added "there are safety implications. To have one part fail and not assume that the entire structural integrity is at issue is really pushing your luck."
A study conducted by a consultant hired by Entergy that cleared the cooling towers for operation under uprate conditions was seriously flawed, said Diana Sidebotham, the president of NEC.
Some of the faults of the study, she said, included not conducting a physical examination of tower cells, inadequate documentation on the breaking strength of tie rods and did not adequately address the effects of aging, moisture or cooling system chemicals on the wood.
"This has been one of the our issues from way back," she said, as it has been for the Connecticut River Watershed Council.
David Deen, a river steward for the watershed council, said the failure was "a little bit of I told you so."
"We brought this up at the uprate," he said. "We asked that they be required to do a system analysis on the cooling towers. That was rejected by the state's public services board."
The tower cooling system is made up of 22 fans, a deep water basin, pumps and pump motors and heat exchangers. A header pipe carries heated water to the top of the cooling towers where it is sprayed upon "fill," corrugated metal that acts like a radiator. The fan blows down on top of the fill. The whole assembly is supported by a wooden structure.
The NRC's Sheehan said the problem came to light late last week, when plant technicians heard rubbing sounds coming from one of the fans. A visual inspection revealed some of the wood structure housing the fan had fallen to the ground, said Williams, resulting in broken beams and what appears to be a hole in the side of the assembly.
"They owe it to themselves to thoroughly examine these structures," said Gundersen, adding that wood is actually better than metal "but you have to have a program in place to replace the wood."
Williams said the fans get "periodic surveillance," but Gundersen said internal bracing makes the bank of fans impossible to walk through so inspections are done with remote cameras.
"There are no visual inspections," he said Gundersen. "You can only see where the cameras can go."
One of the cells, which is part of the plant's safety system, has not been affected by the cooling fan shutdown, said Rob Williams, spokesman for Vermont Yankee. This cell is considered "seismically qualified" to withstand an earthquake or other natural phenomena. Its function is to cool the plant down during an emergency.
With a reduction in power production, Vermont utilities, such as Central Vermont Power Supply and Green Mountain Power, which buy electricity from Vermont Yankee, might have to buy power from the spot market to meet the state's demands. But until the state has had a chance to evaluate the effects of the power output decrease, no one really knows what the effect will be on ratepayers' pocketbooks.
"Whatever the condition of the plant, we are getting real time information," said David O'Brien, the commissioner of the state's Department of Public Services.
Right now, with Central Vermont Power being "long on power," or having more than is needed, reducing Yankee's power output "may or may not" end up costing ratepayers more money, he said. "We need to get a full sense of what the ramifications are."
Bob Audette can be reached at email@example.com or 802-254-2311, ext. 273.