NUREG/CR-6654 INEEL/EXT-98-00383 A Study of Air-Operated Valves in U.S. Nuclear Power Plants Idaho National Engineering and Environmental Laboratory Bechtel BWXT Idaho, LLC Gp,JAR#EG'u U.S. Nuclear Regulatory Commission do4 Office of Nuclear Regulatory Research eoIEJ' Washington, DC 20555-0001 AVAILABILITY NOTICE Availability of Reference Materials Cited in NRC Publications NRC publications in the NUREG series, NRC regu <http://www.nrc.gov> lations, and Title 10, Energy,of the Code of Federal Regulations, may be purchased from one of the fol AfterJanuary 1, 2000, the public may electronically lowing sources: access NUREG-series publications and other NRC records in NRC's Agencywide Document Access 1. The Superintendent of Documents and Management System (ADAMS), through the U.S. Government Printing Office Public Electronic Reading Room (PERR), link RO. Box 37082 <http://www.nrc.gov/NRC/ADAMS/index.html>. 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Neither the United results of such use, of any information, apparatus, product, or States Government nor any agency thereof, nor any of their em process disclosed in this report, or represents that its use by ployees, makes any warranty, expressed or implied, or assumes such third party would not infringe privately owned rights. NUREG/CR-6654 INEEL/IEXT-98-00383 A Study of Air-Operated Valves in U.S. Nuclear Power Plants Manuscript Completed: December 1999 Date Published: February 2000 Prepared by 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 Prepared for 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 ABSTRACT A study of air-operated valves in nuclear power plant applications was con ducted 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 data bases 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 ii° NUREG/CR-6654 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. NUREG/CR-6654 iv CONTENTS A B STR A C T ....................................................................................................................................... i.. EX ECUTIV E SUM M A RY ................................................................................................................ xi ACKN OW LED GMENTS .................................................................................................................. xiii 1. INTRODUCTION AND PURPOSE ..................................................................................... 1 2. B A C K G RO U N D ..................................................................................................................... 3 3. O B JECT IV E S .......................................................................................................................... 5 4 . SC O PE ..................................................................................................................................... 7 5. DESCRIPTION OF AIR SYSTEMS AND AOVS ............................................................... 9 6. APPLICATIONS OF AOVS IN NUCLEAR POWER PLANTS ........................................... 13 7. CURRENT NRC EQUIPMENT QUALIFICATION, MAINTENANCE, AND TESTING REQUIREMENTS FOR AOVS IN NUCLEAR POWER PLANTS ................... 15 8. AOV OPERATING EXPERIENCE AT THE PLANTS VISITED ....................................... 17 8.1 AOV Operating Experience at Palo Verde 1, 2, and 3 (PVNGS), Trip No. 1............... 17 8.1.1 Atmospheric Dump Valve Failures ................................................................. 17 8.1.2 Letdown Containment Isolation Valve Leakage .............................................. 18 8.1.3 Downcomer Feedwater Isolation Valve Leakage ............................................ 18 8.1.4 Failure of a Vacuum Breaker Solenoid and Subsequent Investigations Involving SO V s ............................................................................................... 20 8.1.5 AOV M argin Calculations ................................................................................ 20 8.2 AOV Operating Experience at Enrico Fermi 2, Trip No. 2 .......................................... 21 8.2.1