MONTPELIER, Vt. (AP) – Vermont Yankee’s on-again, off-again power increase is on again.

Technicians at the 34-year-old nuclear plant in Vernon began the process of increasing the plant’s power output by 20 percent, from a rated capacity of 540 megawatts to 650 megawatts in early March, after receiving final regulatory approvals in late February.

But they’ve twice halted the increase, which is being done in stages equal to 5 percent of the original power level, to check out sounds that instruments are picking up that may indicate undue strain on plant components.

The second unplanned hiatus in the power increase process ended Friday evening, when officials at the Nuclear Regulatory Commission told Vermont Yankee that it could increase its power output by another notch, from 112.5 percent to 115 percent of its original output.

Neil Sheehan, spokesman for the NRC’s Northeast regional office in King of Prussia, Penn., said in an e-mail that “NRC staff, with support from its consultants at Argonne National Laboratory, completed its review of the information. The NRC staff had no objections, based on that review, to Vermont Yankee continuing with the power ascension to the next plateau, 115-percent power.”

Vermont Yankee spokesman Robert Williams said the 115 percent level was reached at 11 a.m. Saturday. “Since March 4, Vermont Yankee has completed power increases totaling 81 megawatts for the New England electrical grid,” Williams said in a statement released by e-mail. “The 81 megawatts is enough to power approximately 81,000 homes.”

Sheehan and Williams said when the plant hit 115 percent, it will pause for 96 hours, or four days, to collect data on how it is performing at the new power level.

The four-day pauses were to occur at 105, 110 and 115 percent of original power before the plant was given the green light to go to 120 percent. But twice, once at the 105 percent level and again halfway between the 110 and 115 percent levels, instruments have picked up sounds indicating possible strain on the plant’s steam dryer.

The steam dryer is a large unit at the top of the reactor that removes moisture from the steam made by the boiling water reactor before it is sent to spin the plant’s turbines. Officials with Vermont Yankee’s owner, Entergy Nuclear, and the NRC have expressed concern about the steam dryer because steam dryers have cracked at some other nuclear plants around the country that have increased power.

Sonic signals picked up when the plant hit 105 percent of original power in early March caused it to stop the power increase process for four weeks, while Vermont Yankee officials joined their counterparts from General Electric, which built the plant, and the NRC in reviewing the data and determining the plant could proceed to the next power level.

Vermont Yankee appeared to clear the 110 percent hurdle without any significant problems, but officials said on April 6 that the power increase had been halted again at 112.5 percent because of a new sonic signal – this one at a slightly higher frequency than the one picked up at 105 percent. That caused the most recent pause in the power increase process, this one lasting about two weeks.

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