ML18153C864
| ML18153C864 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 01/03/1992 |
| From: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 91-680, NUDOCS 9201090001 | |
| Download: ML18153C864 (3) | |
Text
e VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 January 3, l99l United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, D. C. 20555 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATION UNITS 1 AND 2 PRESSURIZER SAFETY VALVE OPERABILITY Serial No.
NO/ETS/RCS Docket Nos.
License Nos.91-680 50-280 50-281 DPR-32 DPR-37 In October of 1989, Virginia Electric and Power Company was notified by their Nuclear Steam System Supplier (Westinghouse) of a potential shift in the operating set pressure of pressurizer safety valves. The potential shift existed for safety valves that were tested and reset on a steam medium and then operated with a loop seal (water medium). At the time of the notification, Unit 1 was operating at full power and Unit 2 was shutdown. The Unit 2 safety relief valves were removed from the RCS, shipped to an offsite test facility, and tested on both steam and water mediums to determine any set pressure shift. An average set pressure shift of +4.8% was observed between tests on a steam medium and on a water medium.
To assess the impact of the set pressure shift on the safety analysis results, an evaluation of the transients that challenge the pressurizer safety valves and a reanalysis of the most limiting transients were performed. Compensatory measures were established to ensure the Reactor Coolant System pressure remained at an acceptable level throughout design basis pressure transient events assuming a greater than 5.4% set pressure shift were to occur.
The required compensatory measures included maintaining one PORV and the direct reactor trip on turbine trip operable.
At that time, a decision was made to set the safety valves on a steam medium to eliminate any possibility of a set pressure shift in the downward direction if a loop seal was lost during operation. This action was taken based on the potential that a loss of loop seal with the valves set on a water medium could cause a downward set pressure shift. This shift would increase the probability of a challenge to the safety valve and a small break LOCA (stuck open valve). Due to the noted set pressure shift when tested on steam and operated on a water medium, a Technical Specification change was requested and granted to revise the pressurizer safety valves lift set pressure tolerance from + 1 %, -1 % to +5%, -1 % for Cycle 10 operation for both Units. To support the increased set pressure limit of +5%, administrative controls were put in place to maintain the compensatory measures identified above.
In early 1990, the Westinghouse Owners Group (WOG) formed a working group to evaluate and resolve the potential generic issue of safety valve set pressure shift.
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Over the next year, in conjunction with an ASME Code Committee, the working group investigated the phenomenon and concluded that the set pressure shift, when measured at the first valve spindle movement, does not exceed + 1 %. However, the loop seal purge time affected the maximum RCS pressure during transients and was the major contributor to the apparent set pressure shift. The results of this evaluation are documented in WCAP-12910, "Pressurizer Safety Valve Set Pressure Shift." The WCAP discusses the methodology to calculate a conservative loop seal purge time.
WCAP-12910 was submitted to the NRC in March 1991, for review and comment.
During the WOG's ongoing evaluation, a second Technical Specification change was requested and granted in May of 1990, to allow +5%, -1 % set pressure limits for Cycle 11 operation for both Surry Units. Because of the uncertainty associated with the resolution of the set pressure shift issue and the potential for additional modification of the safety valves and the discharge piping, Surry postponed separately planned modifications for the installation of pipe supports on the safety valve discharge lines until final resolution of the set pressure issue.
Using the WCAP-1291 O methodology, a plant specific loop seal purge time was calculated for both Surry Units 1 and 2. The calculated purge time was determined to be less than the maximum allowable purge time given in the WCAP. With the set point shift expected to be less than 1 % and acceptable loop seal purge times, the temporary expanded Technical Specification set pressure limits of +5% and -1 % will no longer be necessary and can expire at the end of Cycle 11 for both Units 1 and 2, as noted in the Technical Specification footnote. Although the plant specific loop purge time is calculated to be acceptable, only a small amount of operating margin was identified.
Therefore, to provide additional operating margin, and eliminate the loop purge time issue, modifications (i.e., steam trim package for the valves, loop seal drains, and the appropriate piping supports) will be completed on each unit.
Since the WCAP methodology is still under review by your staff, the compensatory measures noted above will be maintained in place until the WCAP methodology is approved by the staff or the modifications are completed.
Because the safety valve discharge piping support requirements for drained loop seals are significantly different from the requirements for water-filled loop seals, the original safety valve piping support project is being modified and the appropriate piping supports will be installed in conjunction with the installation of the steam trim and loop seal drain lines. Currently, these modifications are scheduled for completion during the Spring 1994 refueling outage for Unit 1 and the Fall 1994 refueling outage for Unit 2.
If you have any questions, please contact us.
- VJiZrnrs, W. L. Stewart Senior Vice President - Nuclear
.J cc:
U. S. Nuclear Regulatory Commission Region II 101 Marietta Street, N. W.
Suite 2900 Atlanta, Georgia 30323 Mr. M. W. Branch NRC Senior Resident Inspector Surry Power Station