United States Nuclear Regulatory Commission - Protecting People and the Environment


                                                          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  

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