ML20206F029
| ML20206F029 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 05/03/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20206F014 | List: |
| References | |
| NUDOCS 9905050314 | |
| Download: ML20206F029 (3) | |
Text
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UNITED STATES '
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NUCLEAR REGULATORY COMMIS810N WASHINGTON, D.C. 3000H001 "f
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION EXEMPTION FROM ESTABLISHING LOW TEMPERATURE OVERPRESSURE SETPOINTS IN ACCORDANCE WITH 10 CFR 50.60 AND 10 CFR PART 50. APPENDIX G PACIFIC GAS AND FI FCTRIC COMPANY
, DIABLO CANYYON NUCI FAR POWER PLANT. UNITS 1 AND 2 DOCKET NOS. 50-275 AND 50-323
1.0 INTRODUCTION
By letter dated September 3,1998, as supplemented by letters dated January 22,1999, February 5,1999, and March 17,1999, Pacific Gas and Electric Company (PG&E or the licensee) submitted a license amendment request to revise TS 3.4.9.1, Figures 3.4-2 and 3.4-3 regarding the Appendix G pressure temperature (PR) limits to extend the applicability up to 16 effective full power years (EFPY). The licensee also updated the controlling materials for the P/T curves to reflect the current analysis for generating the new curves. The licensee also requested an exemption from 10 CFR 50.60 and Section IV.A.2 to Appendix G to 10 CFR Part 50, for use of the American Society of Mechanical Engineers (ASME) Code Case N-514 in determining the acceptable low temperature overpressure protection (LTOP) system setpoints.
The use of ASME Code Case N-514 would compensate for the more restrictive Appendix G P/T limits in the proposed TS affecting LTOP setpoints. The licensee has provided the justification to support its determination that the current LTOP setpoints are unchanged.
2.0 EVALUATION
- The NRC has established requirements in 10 CFR Part 50 to protect the integrity of the reactor coolant pressure boundary (RCPB)in nuclear power plants. As part of these requirements, 10 CFR Part 50, Appendix G, requires that P/T limits be established for reactor pressure vessels (RPVs) during normal operating and hydrostatic or leak rate testing conditions.
Specifically,10 CFR Part 50, Appendix G, states that "The appropriate requirements on... the pressure-temperature limits ed minimum permissible temperature must be met for all conditions." Pressurized wsw/ reactor (PWR) licensees have installed cold overpressure mitigation systems (COMS)/LTOP systems in order to protect the RCPBs from being operated outside of the boundaries established by the P/T limit curves and to provide pressure relief of the RCPBs during low temperature overpressurization events.
. In the submittal of September 3,1998, PG&E requested an exemption for staff approval to use Code Case N-514 as an attemative method for establishing the setpoints for the LTOP systems that have been installed for overpressure protection of the DCPP RCPBs. PG&E determined 9905050314 990503 PDR ADOCK 05000275 P
4 that the exemption request from the provisions of 10 CFR 50.60 and 10 CFR Part 50, Appendix G, was necessary because these regulations, establish a relatively restrictive " operating window," that could contribute to inadvertent pressure relief actuation or damage to the RCP sosis.
PG&E has noted in its submittal of September 3,1998, that the underlying purpose of tne subject regulations is to establish limits to protect the RPVs from brittle failure during low temperature operation and that the LTOP provides a physical means of protecting these limits.
PG&E proposed that establishing the LTOP pressure setpoints in accordance with the provisions in Code Case N-514 would provide an acceptable level of safety against overpressurization events of the DCPP RPVs. The Code Casa dictates that when the LTOP system is enabled, the peak pressure resulting from an LTOP design basis transient will not exceed 110 percent of the pressure limits established by the P/T limit curves for the plant, as required by 10 CFR Part 50, Appendix G, and by Appendix G to the Code. The Code Case also requires that the LTOP system be enabled at a temperature of 200'F or at a temperature value equivalent to the sum of the limiting ART + 50*F, whichever is greater. The staff has previously found that Code Case N-514 provides an " acceptable level of safety" based on the amount of conservatism that has been explicitly incorporated into the methodologies for generating PR limit curves, as prescribed in 10 CFR Part 50, Appendix G; Appendix G to the Code; and Regulatory Guide (RG) 1.99, " Radiation Embrittlement of Reactor Vessel Materials l' Revision 2.
The conservatism includes: (1) a safety factor of 2 on the pressure stresses; (2) a margin factor applied to the calculation of ART values in accordance with the methodology of RG 1.99, Revision 2; (3) an assumed 1/4T flaw with a 6:1 aspect ratio; and (4) a limiting materia!
toughness based on dynamic crack arrest data.
The sta# agrees with PG&E's determination that an exemption would be required to approve the use of Code Case N-514. The staff examined PG&E's rationale to support the exemption request and concurred that the use of Code Case N-514 would also meet the underlying intent of the regulations. Based upon a consideration of the conservatism that is explicitly incorporated ints the methodologies of 10 CFR Part 50, Appendix G; Appendix G to the Code; and RG 1.99, Revision 2; the sta# concluded that permitting the LTOP setpoints to be established at the level specified in the Code Case (e.g., - 110 percent of the limit defined by the P/T limit curves) would provide an adequate margin of safety against brittle failure of the RPVs. This is also consistent with the determination that the sts# has reached for other plants which operate under similar operating conditions. Therefore, the staff concludes that requesting the exemption under the special circumstances of 10 CFR 50.12(a)(2)(ii) is appropriate and that the methodology of Code Case N-514 may be used to establish the LTOP setpoints for the DCPP RCS. In addition, Code Case N514 has been adopted by many other licensees experiencing like circumstances.
3.0 CONCLUSION
PG&E requested an exemption for staff approval to use Code Cese N-514 as an attemative method for establishing the setpoints for the LTOP systems at DCPP Units 1 and 2. Based the conservatism that is incorporated into the methods of Appendix G of the Code for calculating P6 limit curves, the staff concludes that permitting the LTOP setpoints to be established in
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o accordance with the Code Case (e.g., at a level s 110 percent of the limit defined by the P/T limit curves) would provide an adequate margin of safety against brittle failure of the DCPP reactor vessels. Therefore, the staff concludes that requesting the exemption under the special circumstances of 10 CFR 50.12(a)(2)(ii) is appropriate and that the methodology of Code Case N-514 may be used as the basis for establishing the LTOP setpoints for the DCPP RCPBs.
Principal Contributor: James Medoff Date:
May 3, 1999
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