Saturday, August 25, 2007
Close-up of Vermont Yankee cooling tower collapse
Investigation begins at VY
By BOB AUDETTE, Reformer Staff
Saturday, August 25
BRATTLEBORO -- Hard-hatted workers clambered over and around the wreckage that used to be the wall of one of Vermont Yankee's external cooling fans Friday, looking for clues as to why the structure gave way.
Tuesday afternoon, the plant had to ratchet back on its power output after the wall gave way, spilling plastic slats, asbestos panels and wood beams onto the ground between the two banks of 11 cooling fans each.
Normally, a visitor to the site would see water vapor rising from the cowling around each of the 22 huge fans and hear them spinning, sucking heat from the water and moving it into the atmosphere.
A visitor would also hear the more than 350,000 gallons of water a minute that flow through the system, falling through thousands of plastic slats to cool the water before returning it to reactor, where it is used to cool and convert steam produced by the reactor back into water.
Near the top of the cooling tower, a 52-inch pipe that runs along a platform abruptly ends where a section of the pipe fell to the ground during the collapse. The pipe carried the water from the plant and distributed it to the 11 fans in the bank creating what John Dreyfuss, director of nuclear safety assurance at Vermont Yankee, called "a little rainstorm inside there."
But on Friday, the fans were turned off. With the reactor running at 50 percent its thermal output is reduced, so the cooling towers are not needed. Though its power output has fluctuated since the failure, it is producing on average about 265MWe.
Over the next few days, workers will go through the tangle of debris piece by piece, trying to find the reason for the failure.
"Not the kind of equipment performance to expect," said Dreyfuss. "As we remove timbers, we'll also be performing our investigation to find out what happened," a process he called "very deliberative."
Operators will not crank the plant back up to full power until workers can confirm the structure of the cooling fans is sound, he said.
Prior to the wall failure, technicians had shut down the fan because employees had reported hearing noises coming from it, which they were unable to reproduce.
"There was no evidence of structural issues (at that time)," said Dreyfuss.
As for the debris that collapsed out from the wall, "most the material you see is cosmetic," he said.
On hand to assist in the investigation were state inspectors, two resident inspectors from the Nuclear Regulatory Commission and officials from the state's Office of Homeland Security.
Dreyfuss said homeland security had determined the failure wasn't caused by sabotage, and discounted the notion that it was related to ongoing contract negotiations between Entergy, the owners of the power plant, and the International Brotherhood of Electrical Workers, which represents 157 workers there.
"We've ruled that out as well," he said.
Entergy recently received permission to increase Vermont Yankee's power output by 20 percent, from 540MWe to 650MWe, but Dreyfuss said there was nothing to indicate the power uprate had anything to do with the failure.
But not everyone is convinced.
"Did increased water flow rate through the cooling towers, or higher cooling tower fan turning rates, cause the collapse?" asked David Lochbaum, nuclear safety engineer with the Union of Concerned Scientists, in an e-mail to the media. "If so, why didn't the company's and the NRC's pre-uprate reviews identify this threat and prevent it from happening?"
If they did miss the problem, he wrote, "what other misses have not yet revealed themselves?"
Nuclear reactors such as that at Vermont Yankee are 33 percent efficient, wrote Lochbaum.
"While the reactor core may produce three units of energy, only one unit of electricity is generated," he wrote. "The remaining two units of energy must be discharged to the environment as waste heat. Nuclear plants rely on large amounts of water to carry away this waste heat."
Vermont Yankee has a single-unit boiling water reactor that had a maximum reactor core power level output of 1593 Megawatts thermal before its uprate approval. At 120 percent, the plant produces 1912MWt.
Plant cooling is provided by either an open-cycle system, a closed-cycle system or a hybrid of the two. How the plant cools depends on various factors, including river and air temperatures, but its cooling systems are meant to minimize its impact on the river.
When the plant uses its closed-cycle system, the cooling towers dissipate heat to the atmosphere. When cooling towers are used, water from the river is used to replace vapor blown out by the fans. Most of the rest of the water is sent back to the plant's main condenser, thousands of metal tubes that carry cooling water which are used to cool steam from plant turbines and convert it back into water. That water is sent back to the reactor to produce more steam, and the water in the condenser is sent back to the towers for cooling.
In the open-cycle mode, no water passes through the cooling towers. Water is taken from the river and discharged south of the facility.
Bob Audette can be reached at firstname.lastname@example.org or 802-254-2311, ext. 273.
Isabel Vinson contributed to http://www.theenvironmentals.com/ last year. Her nude picture in front of the Vermont Yankee nuclear power plant ended up on the cover of the region's largest newspaper, The Brattleboro Reformer... I didn't know until a few minutes ago that she followed it up with a radio interview... where she shines, as usual!
She has it blogged at: "radio interview oooh"