Regulators on Monday encouraged U.S. nuclear plants to review their replacement schedules for capacitors, outlining problems that degraded material within them have caused at a number of reactors in recent years.While not mentioned in the Nuclear Regulatory Commission notice, the NRC disclosed on July 9 that a capacitor is believed to have caused a brief fire within an annunciator panel in the control room of the Browns Ferry 3 reactor in January.The notice is advisory only and does not require action on the part of plant owners. It encourages diligence in replacing capacitors that can degrade with age and exposure to environmental conditions. Two examples were cited specifically in the notice.The first involves a small control room fire following a reactor trip at Surry unit 1 in June, 2010. According to the notice (available below), one hour following the inadvertent trip, "failure of a resistor/capacitor (RC) suppressor in a nuclear instrument (NI) cabinet resulted in a small control room fire, which was extinguished by the use of manual carbon dioxide fire extinguishers. Approximately 3 hours later, another RC suppressor failed in a second NI cabinet, causing a control power fuse to blow and the source range NIs to become deenergized. The source-range NIs were restored in about 5 minutes. (No power failures or blown fuses resulted from the RC suppressor failures in the first NI cabinet.)"In the second example, the NRC makes note of the failures of three loading timers associated with service water pumps for emergency diesel generators at Hatch unit 2 between March 2005 and February 2009. A root-cause report by the license holder determined age-related electrolytic capacitor degradation was to blame. "Specifically, the power supplies for the [loss-of-offsite-power/loss-of-coolant-accident] circuitry were exhibiting excessive voltage ripple on their outputs. The root cause team attributed this to degradation of electrolytic capacitors in the power supply circuits, which tend to exhibit increased noise toward the end of life. These capacitors had been installed for 20 years, which was beyond their vendor-recommended service life of 10 years," the NRC report read.In a separate document, the agency also listed five other plants reporting similar capacitor problems dating back to 2006.-- Download the NRC Information Notice here.-- Download the list of additional degradation instances here.
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