ML18153C445
| ML18153C445 | |
| Person / Time | |
|---|---|
| Site: | Surry, North Anna  |
| Issue date: | 11/16/1990 |
| From: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 88-737D, GL-88-17, NUDOCS 9011260196 | |
| Download: ML18153C445 (3) | |
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VIRGINIA ELECTRIC AND PowER COMPANY RICHMOND, VIRGINIA 23261 November 16, 1990 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATION UNITS 1 AND 2 NORTH ANNA POWER STATION UNITS 1 AND 2 SUPPLEMENTAL RESPONSE TO Serial No.
88-737D NAPS/PAK:bgp:R4 Docket Nos. 50-280 50-281 50-338 50-339 License Nos. DPR-32 DPR-37 NPF-4 NPF-7 GENERIC LETTER 88-17 LOSS OF DECAY HEAT REMOVAL Virginia Electric and Power Company responded to Generic Letter (GL) 88-17, Loss of Decay Heat Removal, expeditious actions by letter (Serial No.88-737) on January 6, 1989.
This response provid-ed plans to implement the eight expeditious recommendations listed in the generic letter. Based on discussions with the NRC resident inspectors, we are providing additional information to clarify our response concerning hot leg vent path configuration.
Expeditious Action 8 required each licensee to 'implement procedures and administrative controls that reasonably assure that all hot legs are not blocked simultaneously by closed loop stop valves unless a vent path is provided that is large enough to prevent pressurization of the reactor vessel (RV) upper plenum or unless the reactor coolant system (RCS) configuration prevents RV water loss if pressurization should occur.'
Our response to Expeditious Action 8 stated that procedures are being revised to require that a hot leg vent path be provided if maintenance activities require an opening on the cold leg.
The procedures also require the cold leg isolation valve to be closed first when a loop is being isolated and the hot leg isolation valve to be opened first when returning the loop to service.
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-Docket Nos. 50-280&281 50-338&339 Serial No.
88-737D The hot leg vent required by GL 88-17, when all loops are isolated, is provided by removal of one or more pressurizer safety valves or removal of the pressurizer manway. The vent size requirements are determined by engineering calculations and are dependent on core operating history and the time since shutdown.
NRC letters, dated February 13 and March i 0, 1989 provided comments on the proposed Virginia Electric and Power Company expeditious actions. One specific comment was that calculations should be performed to verify the effectiveness of the hot leg vent path. To address this concern, Virginia Electric and Power Company requested that Westinghouse perform vent path calculations for Surry and North Anna.
A potential concern was identified with the effectiveness of using the pressurizer surge line and pressurizer as a hot leg vent path because the surge line is connected at the RCS mid-loop on both Surry units and on North Anna Unit 2. With the water level in the reactor vessel raised above mid-loop elevation to prevent the formation of a vortex in the suction piping of the residual heat removal (RHR) pumps, the opening of the pressurizer surge line would be covered with fluid. Initially establishing a vent path could be inhibited. (This is not applicable to North Anna Unit 1 because that surge line is connected to the top of the RCS loop piping.)
In the event of a loss of decay heat removal capability, the initial heatup of the core will result in boiling of the hot leg fluid. Subsequently, a pressure increase of several psi in the RV head will occur which will cause some water to spill from the cold leg opening until the water level is depressed to near the mid-loop elevation and the vent path would be established. Once the level is near mid-loop, the flow through the pressurizer surge line will be essentially only vapor and any additional significant loss of liquid from the reactor vessel due to the pressurization would be terminated.
Discussions with Westinghouse confirmed that the piping configurations of Surry 1 & 2 and North Anna 2 were considered in the Westinghouse computer cotle used to perform the vent path calculations. The Westinghouse calculations for time to core uncovery conservatively begins after this volume in the hot leg has been displaced to the cold leg. Based on this evaluation, the concern of a mid-loop versus a top-of-loop connection for the pressurizer surge line as a hot leg vent path, is not considered to be significant.
In addition, we provided by letter (Serial No.88-732) dated October 31, 1989 the results of our final evaluation for programmatic instrumentation enhancements. During the RHR instrumentation study, various means of providing a second independent level indication were evaluated. As a result of the study we committed to install an ultrasonic level detector system on the "C" loop hot leg to measure the vessel water level. During development and installation of the design change package, it was determined that the ultrasonic detector system should actually be installed on "B" loop hot leg. The combination of the currently installed "C" loop cold leg standpipe system and the "B" loop hot leg ultrasonic system will provide reliable and diverse RCS level information.
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&ocket Nos. 50-280&281 50-338&339 Serial No.
88-737D The ultrasonic detector system was installed on North Anna Unit 2 during the 1990 refueling outage and is being installed on Surry Unit 1 during the current outage. A similar system will be installed on North Anna Unit 1 and Surry Unit 2 during each unit's next scheduled refueling outage.
If you have any questions or require additional information, please contact us.
Very truly yours,
~
W. L. Stewart Senior Vice President - Nuclear cc:
U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, N.W.
Suite 2900 Atlanta, Georgia 30323 Mr. W. E. Holland NRC Senior Resident Inspector Surry Power Station Mr. M. S. Lesser NRC Senior Resident Inspector North Anna Power Station page 3 of 3