ML13330B481
| ML13330B481 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 02/25/1989 |
| From: | Nandy F SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 8903020658 | |
| Download: ML13330B481 (2) | |
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Southern California Edison Company P 0. BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD. CALIFORNIA 91770 F. R. NANDY TELEPHONE MANAGER OF NUCLEAR LICENSING (818) 302-1896 February 25, 1989 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D. C. 20555 Gentlemen:
Subject:
Docket 50-206 Single Failure of 480 V Switchgear 3 San Onofre Nuclear Generating Station, Unit 1 During a review of the Unit 1 electrical power distribution system for a proposed Technical Specifications change, a design deficiency was identified in the automatic controls of the 480 Volt Switchgear system. The design deficiency would have resulted in the potential loss of both trains of safeguards in the event of postulated single failure of Train B with a Safety Injection Signal with concurrent Loss of Offsite Power (SISLOP), or a Safety Injection Signal without Loss of Offsite Power for steamline breaks outside containment only. A 30-day written Licensee Event Report 88-019 was submitted to the NRC on January 12, 1989, to describe in detail the design deficiency and its consequences. The purpose of this letter is to provide detailed information regarding the corrective actions to eliminate loss of electrical power to equipment required for post-LOCA ECCS recirculation with an assumed single failure and the acceptability of isolated safety-related loads on Switchgear 3.
The design deficiency consists of the absence of a SIS/SISLOP automatic trip signal from Sequencer 1 on Train A to normal feeder breaker llCll or 480V breaker 1303 for Switchgear 3. Only Sequencer 2 on Train B provides a SISLOP signal to the Switchgear 3 lockout relays to the normal feeder breaker 1303.
Switchgear 3 provides power to Motor Control Center (MCC) 3 which is not environmentally qualified for a steam line break outside the containment. In the event of a main steam line break outside the containment, MCC 3 could cause spurious operations of load equipment. In the event of a coincident single failure of Train B and the environmental failure of MCC 3, the emergency diesel generators could have been overloaded beyond the Technical Specification load limit. This single failure is conservatively assumed to result in the loss of both trains of safeguards.
89030:20658 890225 00 DR ADOCK 050002406 PDC:
Document Control Desk
-2 Previous plant modifications have removed safety-related equipment MOV 358 and MOV-1100D, charging System recirculation realignment valves and MOV-850C Uninterruptible Power Supply (UPS) from this bus.
The remaining safety-related loads on Switchgear 3 and MCC 3 are:
Boric Acid Injection Pump G-12; South Component Cooling Water (CCW) Pump G-15C; MOV-883, which isolates the charging and refueling water pump from the RWST; and the Battery Charger for the MOV-850C and MOV-358 UPS. The South CCW Pump G-15C is not credited to meet the Technical Specification Limiting Condition for Operation as a result of the November 1987 single failure analysis findings; hence, it is not considered a post-accident load. The Boric Acid Injection System, including Pump G-12, is not required for mitigation of any accidents. MOV-883 is redundant to check valve CRS-301, which isolates the recirculated coolant from the RWST, and is assumed closed after 30 minutes by a local manual action in the event of MCC-3 failure. The inservice testing requirements for CRS-301 are under review to assure its capability to perform as a backup to MOV-883.
This review will be completed prior to restart from the current outage. The UPS for MOV-850C and MOV-358 has a conservative duty cycle of 30 minutes during which these valves can be cycled to desired positions for safety injection and recirculation. After this, manual actions outside the control room can be credited. Therefore, the isolation of safety-related loads on Switchgear 3 is acceptable in the current configuration.
As a corrective action in LER 88-019 on the Switchgear 3 single failure issue, SCE committed to identifying other automatic swing bus arrangements. A similar swing bus configuration was identified on the 120 VAC Vital Buses 1, 2, 3, and 4. The vital buses are powered from inverters, with automatic transfer switches, from DC Bus 1. In the event of DC Bus 1 failure, these vital buses would be automatically powered from 37.5 kVA and 7.5 kVA transformers fed from MCC-2. An electrical fuse/breaker coordination analysis was performed which demonstrated that there was a proper fuse/breaker coordination of the vital buses from MCC-2. Therefore, no other single failure issue was identified.
If you have any question regarding this matter, please call me.
Very truly yours, cc: 3. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3