United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-56: Reliability of Main Steam Safety Valves

                                                            SSINS No.: 6835 
                                                            IN 86-56       

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

                                July 10, 1986

Information Notice No. 86-56:   RELIABILITY OF MAIN STEAM SAFETY VALVES 

Addressees: 

All pressurized-water reactor facilities holding an operating license or 
construction permit. 

Purpose: 

This information notice (IN) is provided as additional notification of NRC's
concern for the reliability of spring-actuated main steam safety valves 
following reports of multiple failures during testing and problems during 
power operations and scram recovery. IN 86-05, "Main Steam Safety Valve Test
Failures and Ring Setting Adjustments," previously addressed the problem of 
inadequate flow capacity of these valves. 

It is expected that recipients will review the information for applicability
to their facilities and consider actions, if appropriate, to preclude 
similar problems from occurring at their facilities. However, suggestions 
contained in this notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

While researching IN 86-05, the following problems with main steam safety 
valves (MSSVs) that had occurred during testing, power operations, or scram 
recovery were tabulated from the licensee event report files. 

           MAIN STEAM SAFETY VALVE PROBLEMS (01/01/81-03/01/86) 

     PROBLEM        NUMBER    NUMBER    TESTING   POWER     POST-SCRAM
                    VALVES    PLANTS    (EVENTS)  (EVENTS)  (EVENTS) 

FAILURE TO OPEN       13        6          5         0         1 
FAILURE TO RECLOSE    15        9          2         1         8 
SPURIOUS OPENING      6         4          2         3         1 
LEAKING               7         4          1         3         0 
SET POINT DRIFT 
    HIGH              44        10         11        0         1 
    LOW               97        14         18        3         3 
    UNSPECIFIED       75        11         14        0         0





8607030267 
.

                                                              IN 86-56 
                                                              July 10, 1986 
                                                              Page 2 of 3  

A number of reports have been received concerning events involving MSSVs. 
Four of the more significant reports are summarized in this and the 
following paragraphs. At North Anna 2, 8 of 15 valves would not lift at the 
maximum pressure available to the testing device at the site (1147 to 1161 
psig). All 15 valves were subsequently sent to Wyle Laboratories for further 
testing. The as-found setpoints varied from 1105 to 1223 psig compared with 
the specified 1085 +/- 11 to 1135 +/- 11 psig setpoints. All valves were 
refurbished and setpoints were adjusted to be within technical specification
limits (LER 50/339-86/001). 

Salem 2 reported one valve leaking during heatup following a refueling 
outage. After the spindle nut was backed away from the forked lever, the 
valve reseated, but later it lifted and did not immediately reseat. It was 
gagged shut. Subsequently two other valves prematurely lifted. They too were
gagged shut. Later, when a fourth valve lifted, it was declared inoperable. 
All 20 MSSVs were then tested and reset to the appropriate setpoints. The 
licensee was unable to determine the cause for the setpoint drift (LER 
50/311-85-007). 

Oconee 2 reported that 2 MSSVs had failed to reseat promptly. They reseated 
at 900 psig instead of 1010 psig following a transient that included a 
reactor scram (LER 50/270-85/006). 

On October 19, 1985, the 16 MSSVs of Calvert Cliffs Unit 2 were tested to 
check and, if required, to adjust the relief pressure setpoints. Unit 2 
contains 2 steam generators, each with 8 MSSVs on a steam header. Eleven of 
the 16 valves were determined to be out of specification, with the as-found 
setpoints between 22 to 69 psi higher than their nominal setpoints, compared
with the technical specification requirement of +/- 10 psi. The Licensee 
Event Report is included in this notice as Attachment 1 to give an example 
of the problems found during testing, and because it is an especially 
thorough treatment of corrective actions that may be of value to other 
facilities. 

Discussion: 

The safety significance of failure of the MSSVs to open on a PWR is a 
potential for over-pressurizing the secondary system with a possibility, of 
a loss of its pressure boundary integrity. Failure to reclose has led to 
overcooling transients and lower-than-normal water levels in the steam 
generator. Spurious opening, usually at power, has led to reactor scrams. 
Leaking valves tend to have more problems than properly functioning valves. 
Setpoint drift-low may cause spurious opening of the valves and may interact
synergistically with a steam generator tube rupture to cause relief through 
the faulted steam generator in the case where the MSSVs on the faulted steam
generator have lower-than-required setpoints. Setpoint drift-high can cause 
secondary pressure to rise above the American Society of Mechanical 
Engineers (ASME) Boiler and Pressure Vessel Code-specified system pressure 
limit. 
.

                                                              IN 86-56     
                                                              July 10, 1986 
                                                              Page 3 of 3  

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 


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

Technical Contact:  Mary S. Wegner, IE
                    (301) 492-4511

Attachments:
1.   LER 50/312-85/011
2.   Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013