ML19256E591
| ML19256E591 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 09/18/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19256E589 | List: |
| References | |
| NUDOCS 7911080102 | |
| Download: ML19256E591 (3) | |
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o UNITED STATES 8'
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NUCLEAR REGULATORY COMMISSION g
c WASHINGTON, D. C. 20666
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 14 TO FACILITY OPERATING LICENSE NO. NPF-2 ALABAMA POWER C0ffANY JOSEPH M. FARLEY NUCLEAR PLANT, UNIT N0. 1 DOCKET NO. 50-348 Introduction The single rod drop in a Pressurized Water Reactor (PWR) is a Departure from Nucleate Boiling (DNB) limited transient. Westinghouse has informed the NRC and licensees that there is a deficiency in the analysis of the single rod drop transient.
In three-loop Westinghouse plants, the reactor control system obtains its reactor power signal from a dedicated excore detector. Recent spatial analyses by Westingho.
indicated that when the reactor is in the automatic mode a droppe rod in the core quadrant adjacent to the dedicated detector would 'esult in a power overshoot greater than the value calculated by the uethods used in the Final Safety Analysis Report (FSAR). Without a reactor trip this could lead to exceeding the DNB limit. No credit is taken in the analysis for the negative flux rate trip. Westinghouse recommended adjustment of the negative flux rate trip constants to all owners of Westinghouse reactors having plants without a turbine runback feature. The adjustment would result in a reactor trip on any single c'ropped rod, which would preclude a DNB problem. By letter dated June 20, 1979 Alabama Power Company (APC) proposed system modifications and Technical Specification changes to implement the Westinghouse design changes for Farley Nuclear Plant Unit No. 1 (FNP-1).
In the interim, pending NRC approval, APC committed to operate with the reactor in the manual control mode.
Discussion and Evaluation At FNP-1 the high positive and negative flux rate trip circuits use an dutioneered (high) excore detector signal. The auctioneered detector signal feeds a rate-lag processing circuit whose output is fed to the high positive and negative flux rate trip bistables. To ensure that the drop of any rod will cause a reactor trip regardless of rod worth or location, two changes were proposed:
(1) lowering the Technical Specification rate-lag circuit time constant from two seconds to one second, and (2) lowering the negative flux rate trip value from -5% to -3%.
The limiting safety 1294 243 hh 7911080
2 system (LSS) setpoint is >l second for the time constant, and is f3.5%
for the negative flux rate trip value. The new LSS setpoints result in reactor trips for negative flux rates 1% to 2% per second slower than would have occurred with the original setpoints. The new setpoints are designed to ensure that a reactor trip will occur for any dropped rod. Therefore, the potential for the automatic control system causing power overshoots as a result of a dropped rod would be eliminated.
The neutron flux rate-ldg circuit output is a direct function of the time constant and is used in the high positive flux rate trip circuit (whose trip setpoint is not being changed). The net result in lowering the time constant from two seconds to one second is that fewer positive flux ramp reactor trips will-occur now. However, the flux ramps (permitted by the new setpoints) are relatively low rates and are generally in the range of those produced by the automatic control system (i.e., not rod ejections). The FSAR states that protection for rod ejection accidents is provided by the Ngh flux (high and low setpoints) signal, and the high positive rate trip function is a " complementary" trip. Changing the rate-lag circuit time constant will not alter the role of the high positive flux rate trip in affording reactor protection during rod accidents.
Since we expect the new setpoints to ensure' that all rod drops will result in reactor trips, which will eliminate the possibility of auto-matic rod control system induced power overshoots, and since the positive flux rate trip is still available as a complementary trip for rod ejection accidents, the proposed setpoints are acceptable. We will continue to mdintdin CognIZdfWe of reactor operating data to ensure that all actual Cdses of dropped control rods indeed result in reactor trips in the power pidnts with negative flux rate trips. Should any cases occur where this is not the case, we would require further readjustment of the trip setpoints, or other corrective action.
Environmental Consideration We have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact. Having made this determination, we have further concluded that the amendment involves dn action which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR p51.5(d)(4), that an environmental impact statement or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
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1 Conclusion We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered
- ,? does not involve a significant decrease in a safety margin, the
- :;dment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance < f this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Date: September 18, 1979 1294 245