A Study of Air-Operated Valves in U.S. Nuclear Power Plants (NUREG/CR-6654)
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Manuscript Completed: December 1999
Date Published: February 2000
0. Rothberg, S. Khericha, J. Watkins, M. Holbrook
Idaho National Engineering and Environmental Laboratory
Bechtel BWXT Idaho, LLC
Idaho Falls, ID 83415-3129
H. Ornstein, NRC Project Manager
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code E8238
A study of air-operated valves in nuclear power plant applications was conducted for the NRC Office of Research (the project was initiated by NRC/AEOD). The results of the study were based on visits to seven nuclear power plant sites, literature studies, and examinations of event records in databases available to the NRC. The purpose is to provide information to the NRC staff concerning capabilities and performance of air-operated valves (AOVs).
Descriptions of air systems and AOVs were studied along with the support systems and equipment. Systems and equipment that contain AOVs and SOVs were studied to determine their dependencies. Applications of AOVs and SOVs were listed along with current NRC requirements.
Observations and conclusions included:
- Licensees may not know if the design basis loads or environmental conditions can be met with acceptable margins for the AOVs in their plants. AOVs may have reduced operating margins caused by such factors as aging, load mechanisms not understood or considered in the original design, or previously contaminated air. Calculations or valve descriptions were found that included mistakes or inaccurate information.
- Air systems and solenoid-operated valves (SOVs) have been and continue to be sources of common-cause failures of AOVs. Accumulators are potential sources of AOV failures. Air-operated dampers have been and continue to be potential sources of safety system failures.
- Accident sequence precursor (ASP) analyses performed by the NRC indicated that there have been a number of risk significant events involving AOVs.
- Generic probabilistic risk assessments performed by LNEEL indicated that changes in the failure probabilities of AOVs and SOVs can result in proportional changes in system unreliability and that AOVs and SOVs can have an important role in system reliability. It was also noted that some plants were using low generic probability values to estimate AOV failure probabilities in their PRA calculations.
Examples were observed during the plant visits of events and conditions, including common-cause events and conditions, involving AOVs and SOVs which were under-reported or not reported to the NRC. This may have resulted in computing lower estimates of the risk and safety significance associated with AOV failures.
- Many of the licensees' individualized plant reviews for Maintenance Rule evaluations and for the plants' AOV programs included reviews of PRA findings by expert panels. The licensees found that by using their plants' operating experience and plant-specific probabilistic risk assessments, certain AOVs had high risk importance, high risk achievement worth, or were important to preventing large early releases.
- The nuclear industry and several licensees are preparing or have prepared AOV program plans which include design basis reviews, margin calculations, and use of diagnostic systems to ensure the operability of AOVs.
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