IE Circular No. 81-14, Main Steam Isolation Valve Failures to Close
SSINS No.: 6835 Accession No.: 8107230037 IEC 81-14 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 November 5, 1981 IE CIRCULAR NO. 81-14: MAIN STEAM ISOLATION VALVE FAILURES TO CLOSE Description of Circumstances: Based on data available in Licensee Event Report (LER) files, the failure rate of main steam isolation valves (MSIVs) closing has ranged from a ten-year low of three to fourteen in 1980. Recent failures, similar to those occurring over the last ten years, are primarily related to the following two causes: (1) poor quality control air to the pilot valves and (2) binding of the MSIV valve stems with the valve stem packing. (Refer to IE Information Notice 80-16 for additional details.) These two failure modes contributed to about 85% of the MSIV failure to close events. Both causes also represent common-mode failure mechanisms of which two examples occurred in 1980: (1) two valves failed to close at Nine Mile Point Unit 1 because of rust in the pilot solenoid valves, and (2) three valves failed to close at Trojan Unit 1 because of stem binding. These two failure modes are significant in that; (1) they identify mechanisms by which more than one MSIV may fail to close at the same time thus leading to conditions which have not been considered in the plant's safety analyses,* and (2) they are continuing to occur even though corrective actions reported in the LERs indicate that the technology is available to prevent such failures. It was noted that more than half of the current operating reactor units have not reported any failure of an MSIV to close in ten years, whereas the 28 reporting units experienced at least 90 failures (from all causes). This indicates that major differences may exist in MSIV reliability and in the quality of support systems and operational controls that affect the MSIV reliability. Data from the LER files is attached to show trends, recent history, and detailed data showing licensee experience: Table 1 - MSIV Failures To Close Per Year, shows the ten-year history of all reported failures by three categories: solenoid or pilot valve related, stem binding, and other. The first two categories account for 92% of the failures. (The 85% for the two causes stated in the first paragraph of this circular was obtained by disregarding what appeared to be truly isolated solenoid or solenoid installation failures not related to control fluid quality.) *Note: The potential for poor quality air leading to multiple valve control failures in other safety-related systems is currently under evaluation by NRC. . IEC 81-14 November 5, 1981 Page 2 of 2 Table 2 - MSIV Failures To Close, by Plant, for 1979 and 1980, shows failure experiences by plant for the last two years. Table 3 - LER Data, by Plant, on MSIV Failures To Close (1970-1980), is an extraction of all the LER data from 1970 to 1980 that show details of each plants experience. Tables 1 and 2 were developed from data in this table. Recommended Actions for Holders of Operating Licenses: 1. Review MSIV operating experience for problems that are causing failure to close events or are causing equipment degradation that requires other than routine maintenance to prevent a failure to close. 2. Evaluate corrective action identified in maintenance records, LERs, etc., for adequacy in addressing the root cause of problems and develop plans for additional corrective action as necessary. 3. Where control air quality is suspected of contributing to problems: a. Review the air system(s) to ensure that measures have or will be taken to prevent air quality degradation in the future. b. Consider monitoring and/or alarms (such as dew point alarms) to warn of air quality degradation. 4. When stem binding is contributing to problems, maintenance procedures should be reviewed to ensure that they: a. Include precautions against detrimental affects such as over tightening packing glands or using inappropriate lubricants. b. Require tests to demonstrate that the valves will perform under operating conditions before being placed in service. Recommended Action for Holders of Construction Permits: 1. Evaluate MSIV control air system designs in light of both successful and unsuccessful industry experience. 2. Consider design changes where appropriate to ensure high reliability and to minimize or eliminate the common-mode failure potential present in current designs. No written response to this circular is required. If you desire additional information regarding these matters, please contact the Director of the appropriate NRC Regional Office. Attachments: 1. Tables I, II, III 2. Recently issued IE Circulars
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021