ML13330B476
| ML13330B476 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 02/14/1989 |
| From: | Nandy F SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 8902160422 | |
| Download: ML13330B476 (2) | |
Text
Southern California Edison Company P o.
BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD. CALIFORNIA 91770 F. R. NANDY TELEPHONE MANAGER OF NUCLEAR LICENSING February 14, 1989 (a 18 302-1896 U. S. Nuclear Regulatory Commission Attention:
Document Control Desk Washington, D. C. 20555 Gentlemen:
Subject:
Docket 50-206 Amendment Application No. 157 Setpoint Analysis for Steam/Feedwater Flow Mismatch Trip San Onofre Nuclear Generating Station, Unit 1 By letter dated November 11, 1988, SCE submitted Amendment Application No. 157 to include steam/feedwater flow mismatch trip in Section 2.1, "Reactor Core Limiting Combination of Power, Pressure, and Temperature," and Section 3.5.1, "Reactor Trip System Instrumentation."
In ensuing discussions, the NRC staff requested the basis for the setpoint of the mismatch trip. The purpose of this letter is to provide the basis for the high and low steam/feedwater flow mismatch trip setpoints to support the staff review of the subject amendment request.
A reactor trip will occur on steam/feedwater flow mismatch if the feedwater flow falls below or exceeds the steam flow by more than a fixed offset, i.e.,
the trip setpoints, in feedwater flow perturbation events.
The mismatch trip is not credited for steam flow perturbation events.
The high steam/feedwater flow mismatch trip setpoint is necessary to satisfy the potential single failure of one low mismatch trip channel in the two out of three trip logic for design basis feedwater line break (FWLB) events.
The high mismatch trip setpoint is determined by the limiting value of either:
the increase in the main feedwater flow through the ruptured line required to decrease the flow in the remaining intact lines to the low mismatch trip setpoint; or the feedwater flow instrument range limit.
The setpoint is conservatively determined by the feedwater flow instrument range limit.
An additional single failure, such as the failure of PT-459, is not assumed in calculating the high mismatch trip setpoint since the high mismatch trip is required only when one low mismatch trip channel is assumed failed.
The safety limit for the low feedwater flow setpoint is determined by the largest partial loss of normal feedwater (LONF) or FWLB event which does not require a mismatch trip. The limiting cases at 50% and 100% power were evaluated to determine the bounding event.
The events considered were a complete LONF to one steam generator (equivalent to a 33% LONF to three steam generators or a FWLB which causes a 33% mismatch) at 100% power and complete LONF to three steam generators (or large FWLB) at 50% power. The limiting 8 902160 422 89021 4 FER ADOCK 05000206 1
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Document Control Desk
-2 case was determined.to be the LONF at 50% power due to higher instrument uncertainties at the lower power level.
The single failure of PT-459, which compensates for the change in the steam density as a function of power in the three mismatch channels, was assumed in the most adverse direction.
The applicable transient analyses model the limiting events as a step change in normal feedwater flow for LONF or FWLB. The reactor trip as a result of the mismatch is modeled simply as a trip after a predetermined, very conservative time delay following the step change in flow. The mismatch trip instrumentation and logic response is, thus, not modeled in the transient analyses.
For both setpoints, the reduced Tavg program was determined bounding.
The steam density input error in the event of the single failure of PT-459 is maximized. In addition, other process parameter effects, such as the feedwater density, are maximized relative to the instrument range limit.
For the limiting LONF and FWLB upstream of the steam generator feedwater line check valves, steam flow remains constant with the decreasing feedwater flow during the assumed trip response time. For the FWLB between the steam generator and feedwater line check valves, the transient analyses show that steam flow approaches the applicable mismatch trip setpoints, i.e., steam flow decreases for the affected steam generator and increases for the intact steam generators. Therefore, the assumption of constant steam flow is conservative for the mismatch setpoint calculation.
If you have any questions 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 Unit 1, 2 and 3 J. H. Hickman, California Department of Health Services