Headquarters Daily Report

                         AUGUST 31, 1998

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                    REPORT             NEGATIVE            NO INPUT
                    ATTACHED           INPUT RECEIVED      RECEIVED

HEADQUARTERS        û                                      
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PRIORITY ATTENTION REQUIRED  MORNING REPORT - HEADQUARTERS AUGUST 31, 1998

Licensee/Facility:                     Notification:

                                       MR Number: H-98-0134
Upward Setpoint Drift Of Mssv          Date: 08/31/98
                                                                          

Subject: OBSERVATION OF UPWARD SETPOINT DRIFT OF MAIN STEAM SAFETY VALVES
         (MSSVs) IN PRESSURIZED WATER REACTORS (PWRs)                    


Discussion:

Based on information contained in several Licensee Event Reports and     
discussions with a few licensees, the staff is aware of a potential      
generic trend involving an upward setpoint drift in certain MSSVs at     
PWRs.  In some cases, most or all of the MSSVs failed to meet the        
technical specification criteria of +/-l percent (or +/-3 percent for    
some plants), with some valves drifting as much as approximately 10      
percent above nominal setpoints.  The cause of the drift is believed to  
be a disk-to-seat sticking phenomenon which appears to involve           
differential thermal growth, galling, and subsequent bonding between     
valve disks and seats.  Licensees have attempted to correct the situation
by either modifying the valve internals or by modifying the seat         
maintenance procedure.  In addition, an augmented ASME test frequency has
been used to compensate for the sticking phenomenon.                     
                                                                         
Licensees have had significant interaction with Dresser, the MSSV vendor,
who has not, as yet, been able to definitively identify a root cause. The
above reported problems have all involved Dresser model 3707R MSSVS.     
                                                                         
Contact:  G. Hammer, NRR/DE                                              
          301-415-2791                                                   
          E-mail:  cgh@nrc.gov                                           
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