Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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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.
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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
