ML20206H335
| ML20206H335 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 04/30/1999 |
| From: | Zwolinski J NRC (Affiliation Not Assigned) |
| To: | WOLF CREEK NUCLEAR OPERATING CORP. |
| Shared Package | |
| ML20206H291 | List: |
| References | |
| NUDOCS 9905110169 | |
| Download: ML20206H335 (5) | |
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l 7590-01-P UNITED STATES OF AMERICA NUCLEAR REGULATORY COMMISSION in the Matter of
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WOLF CREEK NUCLEAR OPERATING
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Docket No. 50-482 CORPORATION
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(Wolf Creek Generating Station)
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EXEMPTION 1.
Wolf Creek Nuclear Operating Corporation (WCNOC or licensee) is the holder of Facility Operating License No. NPF42, which authorizes operation of the Wolf Creek Generating Station (WCGS). The license provides, among other things, that the licensee is subject to all ruies, regulations, and orders of the Commission now and hereafter in effect.
The facility is a pressurized water reactor located at the licensee's site in Coffey County, Kansas ll.
Section 50.60(a) to 10 CFR Part 50 requires 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 and material surveillance program requirements for the reactor coolant pressure boundary set forth in Appendices G and H of 10 CFR Part 50. Section 50.60(b) of 10 CFR Part 50 states that proposed alternatives to the described requirements of Appendices G and H of Part 50 or portions thereof may be used when an exemption is granted by the Commission under 10 CFR 50.12.
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Ill.
. By letter dated December 29,1998, WCNOC requested that the NRC exempt WCGS
. from the application of specific requirements of 10 CFR 50.60 and Appendix G to 10 CFR Part 50. SpecificallylWCNOC proposes to use Arnerican Society for Mechanical Engineers (ASME) Code Case N-514 to permit setting the pressure setpoint of WCGS'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 exceeded by ten 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' l
Part 50 requires that P-T limits be established for reactor pressure vessels (RPVs) during normal operation and 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 l
avoid approaching these P-T limit curves and provide pressure relief during low temperature overpressurization events, pressurized water reactor licensees have installed protection l
systems (COMS) as part of the reactor coolant system pressure boundary. WCNOC is required as part of the WCGS Technical Specifications (TS) to develop, update, and submit l
reactor vessel P-T limits and COMS setpoints for NRC review and approval l
WCNOC determined that the exemption request from the provisions of 10 CFR 50.60 l
and Appendix G was necessary since these regulations require, as noted above, that reactor vessel conditions not exceed the P-T limits established by Appendix G. In referring to 10 CFR I
L 50.12 on specific exemptions, WCNOC cited special circumstances regarding achievement of the underlying purpose of the regulations as their basis for requesting this exemption [10 CFR 50.12(a)(2)(ii)].
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3-WCNOC noted in support of the 10 CFR 50.12(a)(2)(ii) criteria that the underlying purpose of the subject regulation is to establish fracture toughness requirements for ferritic materials of pressure retaining components of the reactor coolant pressure boundary. To accomplish this, Appendix G of Section XI of the ASME Code requires the P-T limits be calculated: (a) using a safety factor of 2 on the principal membrane (pressure) stresses, (b) with margin added to the reactor vessel nil ductility reference temperature (RTer) in accordance with Regulatory Guide 1.99, Rev. 2, " Radiation Embrittlement of Reactor Vessel Materials," (c) assuming a flaw at the surface with a depth of 1/4 of the vessel wall thickness and a length of 6 times its depth, and (d) using a conservative fracture toughness curve that is based on the lower bound of static, dynamic, and crack arrest fracture toughness tests on I
material similar to the reactor vessel material. Code Case N-514 provides for normal operation g
1 within the P-T limits determined in accordance with ASME Section XI, Appendix G, but allows determination of setpoints for COMS events such that the maximum pressure in the vessel
. would riot exceed 110 percent of the Appendix G limits.
WCNOC proposed that establishing the COMS pressure setpoint in accordance with the N-514 provisions, such that the vessel pressure would not exceed 110 percent of the P-T limit allowances, would still provide an acceptable level of safety and mitigate the potential for an inadvertent actuation of the COMS. The safety margins provided by application of Code Case N-514 result in a safety factor of 1.8 on the principal membrane (pressure) stresses, with all other factors, including assumed flaw size and fracture toughness, remaining the same as ASME Section XI, Appendix G, methodology. Due to the isothermai nature of the COMS events, the margin with respect to toughness for a COMS transient is within the range provided by ASME Section XI, Appendix G, for normal heatup and cooldown in the low temperature 4
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e range. Thus, applying Code Case N-514 will satisfy the intent of 10 CFR 50.60 for fracture toughness requirements. Further, application of Code Case N-514 will relieve operational restrictions for WCGS; it will reduce the potential for inadvertent RCS pressure relief events, thereby improving plant safety, and will reduce unnecessary burdens on operators during important plant evolutions.
The Commission has determined that application of 10 CFR 50.60 in these particular I
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, 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 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 c6nditions based on the same considerations. Therefore, the exemption requested under the special circumstances of 10 CFR 50.12(a)(2)(ii) was found to be acceptable. The staff also agrees that limiting the potential for inadvertent COMS actuation r,1ay improve plant safety.
IV.
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 health and safety, is consistent with the common defense and security, and is otherwise in the public interest. Therefore, the Commission hereby grants Wolf Creek Nuclear Operating Corporation an exemption from the requirements of 10 CFR 50.60 in order to apply ASME Code Case N-514 for determining the WCGS cold overpressurization mitigation system pressure setpoint.
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5-Pursuant to 10 CFR 51.32, the Commission has determined that the granting of this exemption will have no significant impact on the environment (64 FR 23136 ).
This exemption is effective upon issuance.
FOR THE NUCLEAR REGUMTORY COMMISSION
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John A. Zwolinski, Director Division of Licensing Project Management Office of Nuclear Reactor Regulation Dated at Rockvilla, Maryland this 30th dayof April 1999
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