Beaver Valley corrosion may mean wider problem
FirstEnergy Nuclear Operating Co. executives proposed an inspection plan for Beaver Valley-1’s containment liner that entails performing “non-random,” or selective, volumetric examinations through December 31, 2010.
Fenoc presented the plan last week at a meeting of NRC’s Advisory Committee on Reactor Safeguards, ACRS, as part of the license renewal application process for Beaver Valley-1 and -2 in Pennsylvania. The inspection plan is aimed at addressing corrosion found in April 2009 in unit 1’s containment liner (Inside NRC, 31 Aug., 1). Fenoc in August 2007 applied for a 20-year license renewal for both Beaver Valley units.
Fenoc executives said during the meeting that the corrosion, which penetrated through unit 1’s containment steel liner plate, was caused by a piece of wood accidentally left behind during plant construction in the 1970s. Fenoc has since repaired and tested the liner, which has not shown evidence of uniform corrosion, company executives said.
Fenoc will, however, continue to monitor the containment liner for corrosion, executives said at the hearing. The “non-random” examinations entail performing ultrasonic testing, or UT, along at least eight locations on the liner that are potentially susceptible to corrosion.
In addition to carrying out a 100% visual inspection of the liner as planned in the 2012 refueling outage, Fenoc would perform an additional visual inspection in next year’s refueling outage, Pete Sena, site vice president, said in the presentation.
Fenoc also would perform UT sampling along 75 randomly selected, 1-foot by 1-foot samples of the liner over the next three refueling outages, Sena said in the presentation. After that, the company would evaluate a statistical method to analyze the data, completing the entire random sample plan before the unit’s current operating license expires in January 2016.
Fenoc executives said wood was used less frequently during construction in unit 2 than in unit 1. As a result, the proposed inspection timeframe is longer for unit 2. The utility proposed completing non-random and random volumetric examinations at unit 2 prior to the expiration of its current license in May 2027. Under the proposal, Fenoc would perform visual inspections during refueling outages in 2009 and 2011.
ACRS board members focused many of their questions during the meeting on the root cause of the corrosion, asking how a piece of wood could corrode steel. Fenoc engineers responded that that it’s plausible over time the wood’s low pH levels and high humidity could have caused the corrosion.
ACRS member Said I. Abel-Khalik asked whether the unit’s subatmospheric containment could have caused the steel liner to “buckle.” That, he said, could create a gap between the liner and its concrete shield.
Tom Westbrook, lead civil structural engineer at Beaver Valley, said buckling did not occur because the liner is designed with weld stud fasteners in the back that are attached to the concrete shield. “Part of the design criteria was to make sure that that liner is anchored to the concrete structure during the subatmospheric operations.,” he said. “So, no, there was no separation or gap created between the liner and the concrete.”
But Arnold Gundersen, a nuclear engineer retained by interest group Citizen Power, has said subatmospheric operating conditions could in fact create buckling. Citizen Power has been critical of Fenoc’s inspection plan and in an August 27 letter to ACRS said UT testing should cover more of the liner’s surface area and start “immediately.”
Gundersen said in an August interview that until recently engineers operated the containment buildings at Beaver Valley-1 and -2 at subatmospheric pressures for at least two decades. He said the inside of the containment buildings were kept at roughly two-thirds of normal atmospheric pressure as a safety measure. He said the operating conditions could have increased the gap between the metal containment liner and its concrete shield, potentially providing space for oxygen and moisture to corrode the liner over time.Paul Gunter, director of reactor oversight for antinuclear group Beyond Nuclear, said at the hearing that visual inspections don’t provide “reasonable assurance” of unit 1’s safety and that the random sample size is too small. “We do not share confidence that a patch as you go [inspection plan] for the 20-year extension should provide this committee or the public with any confidence,” he said.
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