ML19208B503
| ML19208B503 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 08/16/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19208B500 | List: |
| References | |
| NUDOCS 7909200451 | |
| Download: ML19208B503 (3) | |
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SAFETY EVALUATION REPORT ELECTRICAL, INSTRUMENTATION AND CONTROL DESIGN ASPECTS OF THE LOW TEMPERATURE OVERPRESSURE MITIGATING SYSTEM FOR THE MAINE YANKEE NUCLEAR POWER PLANT TAC 6741 Prepared by:
Plant Systems Branch Division of Operatir.g Reactors 87'd255 7909200@S"/
l.0 INTRODUCTION By letter to the Maine Yankee Atomic Power Company (MYAPCO) dated August 11, 1976, the U.S. Nuclear Regulatory Commission (NRC) requested an evaluation of system designs to determine susceptibility to overpressurization events and an analysis of these possible events, and proposed interim and penna-nent modifications to the systems and procedures to reduce the likelihood and consequences of such events. By letter dated December 2, 1976 and subsequent letters (refer to the Appendix), the Maine Yankee Atomic Power Company submitted the additional infonnation requested by the NRC staff, including the administrative operating procedures and the proposed low temperature overpressure mitigating system. The system hardware includes sensors, actuating mechanisms, alanns, and valves to prevent a reactor coolant system transient from exceedin3 the pressure and temperature limits of the Technical Specifications for Maine Yankee as required by the Code of Federal Regulations, Title 10, Part 50 (10 CFR 50), Appendix G.
2.0 EVALUATION The enclosed Technical Evaluation Report was prepared for us by the Lawrence Livermore Laboratory, as part of our D0R technical assistance program.
3.0 SYSTEM DESCRIPTION The MYAPC0 design for the Maine Yankee OMS is based on the use of two pressurizer solenoid-operated relief valves (SORV's) along with two passive spring-loaded safety valves (SV's) which are located on the suction line of the RHR system piping. These valves, in conjunction with specific procedural controls, form the bases for the following conditions:
(1) Each pressurizer SORV will provide sufficient and redundant relief capacity to ensure that the reactor coolant system (RCS) pressure remains below 589 psig when the RCS tempera-ture is below 2200F. The pressurizer SORV low pressure setpoint is 500 psig.
(2) The two RHR SV's together will provide sufficient and redundant relief capacity to ensure that the RCS pressure remains below 589 psig when the RCS temperature is below 2200F. The RHR SV low pressure setpoint is 400 psig.
(3) A 2250F temperature switch will be provided to ensure that the RHR SV isolation valves red the pressurizer-SORV isolation 0
valves are open at temperatures below 225 F.
Valves that are not open will be annunciated on the main control board.
(4) Additional assurance of preventing inadvertent blowdown at 0
RCS temperatures above 300 F is provided by the inclusion of a 3000F temperature switch. This switch will ensure that the RHR SV isolation valves are closed and that the pressurizer SCRV's have been reset to the high setpoints required for nonnal plant operation.
Failure of the RHR SV isolation valves to close or failure of the pressurizer SORV's to be reset to the high setpoint will be annunciated on the main control board.
O.E2c
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' The two pressurizer SORV's are equipped with a low pressure setpoint feature. This f,eature, when enabled by the operator by means of a key lock switch, causes each pressurizer SORV to open when the pressurizer pressure reaches a setpoint of 500 psig.
In accordance with plant cooldown procedures, the pressurizer SORV's are set to the low pressure setpoint when the system pressure is less than 450 psig. This low pressure setpoint is to be established prior to decreasing the RCS temperature below 2200F. Plant operating proce-dures provide that nomally the RHR system suction isolation valves are open below 2200F thereby making available the relief capacities of these two safety valves as reactor vessel overpressure protection devices. We find these design features acceptable.
4.0 CONCLUSION
S The design of the Maine Yankee low temperature overpressure mitigating system in the areas of electrical, instrumentation and control (EI&C) is in accordance with those design criteria originally prescribed by us and later expanded during subsequent discussions with the licensee.
We find the EI&C aspects of the modifications acceptable on the basis that: (1) the system is redundant and satisfies the single failure criterion; (2) the system is testable in a periodic basis, and (3) the recommended technical specifications reduce the probability of overpressurization events to an acceptable level.
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