VERNON - An inside portion of one of 22 cooling towers at Vermont Yankee failed Tuesday, forcing the plant to cut power production by more than 50 percent.

Robert Williams, spokesman for plant owner Entergy Nuclear, said the plant would cut power even more in order to investigate and repair the problem.

"The river water piping and the series of screens and supports failed ... and fell to the ground," Williams said.

Williams refused to say whether the damage to the wooden cooling tower was related to the plant's 2006 power uprate, when the plant boosted power production by 20 percent to 610 megawatts.

Neither Entergy nor the Nuclear Regulatory Commission had any estimate on how long the repairs would take or how long the plant would be working at reduced power.

If the problems are traced back to the additional stresses from the power uprate, it comes too late for a Vermont ratepayer-protection agreement, which expired earlier this month, Williams said.

Vermont regulators had negotiated the ratepayer-protection agreement to cover unforeseen problems associated with the uprate, but the plant has been operating without incident for almost 18 months at the higher level.

Vermont Yankee provides about one-third of all the electricity used by Vermont consumers or about half of its normal production. The rest of it is sold by Entergy on the open market.

At the time of state hearings about the proposed power uprate the New England Coalition raised concerns about the effects the additional stresses would place on the cooling towers. The coalition predicted problems ahead for the original 1972 structures.

"I hate to say, 'I told you so, I told you so, I told you so,' but I told you so," said Raymond Shadis, senior technical advisor for the anti-nuclear environmental group. Shadis said the Vermont Yankee tower collapse came despite an in-depth inspection by the Nuclear Regulatory Commission in 2005, which gave the towers a clean bill of health.

One of the coalition's expert witnesses at that time, nuclear industry critic Arnold Gundersen of Burlington, a former nuclear engineer who wrote his master's thesis on cooling towers, said that similar cooling towers at other types of power plants and refineries had collapsed.

Gundersen said he had been told that Entergy had "heard noises for a week" before Tuesday's collapse.

"This would not have happened for another 10 years if not for the uprate," Shadis said.

Diane Screnci, a spokeswoman for the Nuclear Regulatory Commission, said she didn't know why the problem with the cooling tower wasn't discovered before the collapse.

"That's one thing we will investigate," she said. "We need to understand what happened."

Scenci said that Entergy's review of the problem was already in progress, and she said that the plant was stable.

The affected tower is one of 11 in two banks of cooling towers that are in operation at the plant from May to October, when the temperature of the nearby Connecticut River rises and is unable to provide all the cooling necessary for the reactor. The towers, which send plumes of steam high into the air some summer days, are made of Douglas fir, according to Shadis.

Each of the two banks of towers, or cells, are 50 feet tall, 40 feet wide, and stretches 300 feet long. They are located on the southern edge of the plant, near the Connecticut River.