United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-54: Common Mode Failure of Main Steam Isolation Nonreturn Check Valves

                                                            SSINS No.: 6835 
                                                            IN 83-54       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                               August 11, 1983

Information Notice No. 83-54:   COMMON MODE FAILURE OF MAIN STEAM 
                                   ISOLATION NONRETURN CHECK VALVES 

Addressees: 

All nuclear power reactor facilities holding an operation license (OL) or 
construction permit (CP). 

Purpose: 

This information notice is provided as a notification of a potentially 
significant problem pertaining to common mode failure of main steam 
isolation nonreturn check valves at a nuclear power facility. The apparent 
cause of the failure was increased friction due to over tightening of the 
packing gland to preclude steam leakage. The increased friction was 
sufficient to prevent valve closing under no flow conditions and the 
licensee was unable to conclude that the valves would have operated as 
designed in the event of a steam line break with reverse flow. No specific 
action is required in response to this information notice, but it is 
expected that recipients will review the information for applicability to 
their facilities. 

Description of Circumstances: 

On June 8, 1983, Portland General Electric Company reported (LER 83-06) 
finding all four main steam nonreturn check valves stuck open after the 
Trojan Nuclear Plant had shut down and steam flow had been stopped. 

Each of the four main steam lines is equipped with a main steam isolation 
valve and a main steam isolation nonreturn check valve. One of the plant's 
design-basis accidents is the instantaneous rupture of the main steam line 
in any location and the failure of one of the main steam isolation valves to
close. For this postulated event, no more than one steam generator will blow
down if the nonreturn check valves operate properly. Thus, this failure of 
the four nonreturn check valves caused a reduction in the defense-in-depth 
of protection which could have led to a blowdown of more than one steam 
generator if a break were to occur upstream of the main steam isolation 
nonreturn check valve in one loop and the main steam isolation valve failed 
in another loop. All main steam isolation valves were operable during the 
period in question as shown by the periodic operating test records. 

The apparent cause of the failure was packing-induced friction which 
occurred when the packing gland was tightened to preclude steam leakage past 
the packing and valve stem. The packing was tightened following the last 
refueling outage and subsequent startup in July 1982. The valves were 
previously repacked in the Spring of 1981. The two-year interval between 
repacking (Spring of 1981 

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to June 1983) seems to be too long as the packing removed from the valves 
was dried and brittle. This condition contributed to the frictional forces 
that prevented the valves from closing. 

As part of their evaluation of the event, the utility performed an analysis 
to determine if the main steam nonreturn check valves would shut during a 
design basis event. Based on this analysis, the utility was unable to 
conclude that the reverse steam flow would provide sufficient torque to 
close the valves. 

A contributing factor to the valve failure was the fact that the check 
valves had been inadvertently omitted from the Inservice Testing program and 
hence, were not routinely tested for operability. 

To preclude reoccurrence, the utility is planning on performing the 
following corrective actions prior to reaching full power following the 1983 
refueling outage: 

1.   The check valve packing will be replaced each refueling outage. 

2.   Possible gland follower binding will be eliminated by increasing the 
     inside diameter of the gland follower to allow easier movement along 
     the shaft. 

3.   A testing program will be provided to adjust packing before and after 
     heatup to  ensure free movement of the disk while minimizing steam 
     leakage through the packing. 

4.   An engineering evaluation will be performed to determine alternative , 
     corrective action should this condition persist, including reevaluation
     of valve operation with reverse steam flow. 

Additionally, the main steam isolation check valves have been added to the 
Inservice Testing program. 

No written response to this notice is required. If you have any questions 
regarding this matter, please contact the appropriate NRC Regional Office, 
or this office. 


                                  Edward L. Jordan Director 
                                  Division of Emergency Preparedness 
                                    and Engineering Response 
                                  Office of Inspection and Enforcement 

Technical Contact:  R. J. Kiessel, IE
                    (301) 492-8119

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