Evaluation of Air-Operated Valves at U.S. Light-Water Reactors (NUREG-1275, Volume 13)
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Manuscript Completed: February 2000
Date Published: February 2000
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
This report and its companion document, Idaho National Engineering and Environmental Laboratory report, "NUREG/CR-6654, A Study of Air-Operated Valves in U.S. Nuclear Power Plants," present the results of a comprehensive review of air-operated valve (AOV) operating experience and visits to 7 U.S. light-water reactor sites at which there are 11 operating reactors. The study described in this report was conducted by the NRC's Office of Nuclear Regulatory Research to collect information to form the basis for determining if additional regulatory attention is needed to address AOVs.
The major safety concern of this study from a risk perspective is the simultaneous commoncause failure of AOVs, which disable redundant trains of a safety system. The scenario of most concern is that during an accident or transient, AOVs in redundant trains of a safety system fail when subjected to pressure, temperature, and flow conditions different from those seen during normal operation or testing. Normal testing or routine operation of these valves, if performed under pressure, temperature, flow conditions different from those expected during an accident or transient, may not reflect the actual capability of the valve to perform during an accident or transient.
Several instances from operating experience are noted in this study where AOVs were shown to be unable to operate under the conditions expected during an accident or transient. These were usually found through diagnostic testing methods similar to those utilized to verify MOV operability in response to Generic Letter 89-10, "Safety Related Motor-Operated Valve Testing and Surveillance – 10 CFR 50.54(f)," June 28, 1989, and its supplements. Some failed to operate in real events. Current inservice testing and technical specification operability tests may not assure AOV capability for pressure and flow conditions during an accident or transient.
Another concern is the potential for simultaneous common-cause failure of two or more AOVs in important safety systems due to contamination from the pneumatic system or from fabrication and maintenance activities.
Some of the licensees visited found that certain AOVs had high risk achievement worth and Fussell Vesely risk rankings. The study presents the results of licensee calculations which showed that risk achievement worth for common-cause AOV failures at three plants visited ranged from slightly over 1 to 202.
This study notes that the implementation of an effective AOV program, incorporating the use of analysis, diagnostic testing, and lessons learned from operating experience, can minimize the likelihood of AOV failures resulting in risk significant events. It also notes that cooperation between the NRC and industry to develop the guidance for effective AOV programs would facilitate and optimize their implementation.