ML20206H222

Exemption from Requirements of 10CFR50.60 That Would Allow STP Nuclear Operating Co to Apply ASME Code Case N-514 for Determining Plant Cold Overpressurization Mitigation Sys Pressure Setpoint.Commission Grants Exemption

ML20206H222
Person / Time
Site:	South Texas
Issue date:	05/04/1999
From:	Zwolinski J NRC (Affiliation Not Assigned)
To:	HOUSTON LIGHTING & POWER CO.
Shared Package
ML20206H204	List: ML20206H200 ML20206H222
References
NUDOCS 9905110094
Download: ML20206H222 (5)

Text

e 7590-01 P UNITED STATES OF AMERICA NUCLEAR REGULATORY COMMISSION In the Matter of

)

)

STP NUCLEAR OPERATING COMPANY

) Docket Nos. 50-498 and 50-499

)

(South Texas Project Electric Generating

)

Station, Units 1 and 2)

)

EXEMPTION 1.

STP Nuclear Operating Company is the holder of Facility Operating License No. NPF-76 and Facility Operating License No. NPF-80, which authorizes operation of the South Texas Project (STP), Units 1 and 2. The licenses provide, among other things, that the licensee is subject to all rules, regulations, and orders of the Commission now or hereafter in effect.

These facilities consist of two pressurized water reactors at the licensee's site located in Matagorda County, Texas.

II.

Section 50.60(a) to Title 10 of the C.gde of Federal Reaulation (10 CFR) Part 50 requires, in part, that except as provided in Section 50.60(b), all light-water nuclear power reactors, other than reactor facilities for which the certifications required under Section 50.82(a)(1) have been submitted, must meet the fracture toughness requirements for the reactor coolant pressure boundary set forth in Appendix G of 10 CFR Part 50. Sect!on 50.60(b) of 10 CFR Part 50 states that proposed alternatives to the described requirements of Appendix G of Part 50 or 9905110094 990504 PDR ADOCK 05000498 p,

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' portions thereof may be used when an exemption is granted by the Commission under 10 CFR 50.12.

Ill.

. By letter dated March 18,1999, STP Nuclear Operating Company requested that the NRC exempt STP, Units 1 and 2, from the application of specific requirements of 10 CFR 50.60 and Appendix G to 10 CFR 50. Specifically, STP Nuclear Operating Company proposes to use American Society of Mechanical Engineers (ASME) Code Case N-514 to permit setting the 1

pressure setpoint of STP's cold overpressure mitigation system (COMS) such that the pressure-temperature (P-T) limits required by Appendix G of 10 CFR Part 50 could be j

exceeded by 10 percent during a low temperature pressure transient.

The Commission has established requirements in 10 CFR Part 50 to protect the Integrity of the reactor coolant system pressure boundary. As a part of these, Appendix G of 10 CFR Part 50 requires that P-T limits be established for reactor pressure vessels during normal operation anu vessel hydrostatic testing. As stated in Appendix G,"The appropriate requirements on...the pressure-temperature limits...must be met for all conditions." In order to avoid approaching these P-T limit curves and provide pressure relief during low temperature overpressurization (LTOP) events, pressurized water reactor licensees have installed protection j

systems (COMS/LTOP) as part of the reactor coolant system (RCS) pressure boundary. STP Nuclear Operating Company is req'ilred, as part of the STP Technical Specifications, to develop, update, and submit reactor vessel P-T limits and COMS setpoints for NRC review and approval.

STP Nuclear Operating Company determined that the exemption request from the l

provisions of 10 CFR 50.60 and Appendix G was necessary since these regulations require, as previously noted, that reactor vessel conditions not exceed the P-T limits established by

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Appendix G. In referring to 10 CFR 50.12 cn specific exemptions, STP Nuclear Operating Company cited special circumstances regarding achievement of the underlying purpose of the regulation as its basis for requesting this exemption [10 CFR 50.12(a)(2)(ii)].

STP Nuclear Operating Company noted in support of the 10 CFR 50.12(a)(2)(ii) criteria that the underlying purpose of the subject regulation is to establish limits to protect the reactor vessel from brittle failure during low temperature operation and that the COMS provides a physical means of assuring that operation remains within these limits. STP Nuclear Operating Company proposed that establishing the COMS pressure setpoint in accordance with the N 514 provisions, such that the vessel pressure would not exceed 110 porcent of the P-T limit allowables, would still provide an acceptable level of safety and mitigate the potential for an inadvertent actuation of the COMS. The use of N-514 was based on tt's conservatisms that have been explicitly incorporated into the procedure for developing the P-T limit curves. This procedure, referenced from Appendix G to Section XI of the ASME Code, includes the following conservatisms: (1) a safety factor of 2 on the pressure stresses; (2) a margin factor applied to RT using Regulatory Guide 1.99, Revision 2," Radiation Embrittlement of Reactor Vessel Matnials"; (3) an assumed 1/4 thickness flaw with a 6:1 aspect ratio; and (4) a limiting material toughness based on dynamic and crack arrest data.

In addition, STP Nuclear Operating Company stated that a COMS pressure setpoint must be sufficiently high to prevent the inadvertent actuation of the COMS as a result of normal operating pressure surges. STP Nuclear Operating Company requests use of Code Case N-514 to incorporate pressure instrumentation uncertainty in P-T limit calculations, while providing an operating band that permits system makeup and pressure control. Such an inadvertent actuation could lead to the unnecessary release of reactor coolant inside containment and could introduce undesirable thermal transients in the RCS.

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4 The Commission has determined that the application of 10 CFR 50.60 in these particular circumstances is not necessary to achieve the underlying purpose of that rule and that the use of Code Case N-514 would meet the underlying intent of the regulation. Based upon a consideration of the conservatisms, which are explicitly defined in the Appendix G methodology, 4

it was concluded that permitting the COMS setpoint to be established such that the vessel pressure would not exceed 110 percent of the limit defined by the P-T limit curves would provide an adequate margin of safety against brittle failure of the reactor vessel. This is also consistent with the determination that has been reached for other licensees under similar conditions based on the same conditions. Therefore, the exemption requested under the special circumstances of 10 CFR 50.12(a)(2)(ll) was found to be acceptable. The staff also agreen that limiting the potential for inadvertent COMS actuation may improve plant safety.

IV.

t The Commission has determined that, pursuant to 10 CFR 50.12, this exemption is authorized by law, will not present an undue risk to the public heaith and safety, is consistent l

with the common defense and security, and is otherwise in the public interest. Therefore, the Commission hereby grants STP Nuclear Operating Company an exemption from the l

l requirements of 10 CFR 50.60 in order to apply ASME Code Case N-514 for determining j

f STP's cold overpressurization mitigation system pressure setpoint.

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Pursuant to 10 CFR 51.32, the Commission has determined that the granting of this exemption will have no significant effect on the quality of the human environment (64 FR 23689).

This exemption is effective upon issuance.

Dated at Rockville, Maryland, this 4th day of May 1999.

FOR THE NUCLEAR REGULATORY COMMISSION l

ORIGINAL SIGNED BY John A. Zwolinski, Director Division of Licensing Project Management Office of Nuclear Reactor Regulation OFC PD(WVPM PDIVA/PM PDIV-1/LA EMOS'n OGC PDIV.1/SC NAME_ _

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