01-5217. Calvert Cliffs Nuclear Power Plant, Inc; Calvert Cliffs Nuclear Power Plant, Unit Nos. 1 and 2; Exemption

1.0 Backround

Calvert Cliffs Nuclear Power Plant, Inc. (CCNPPI, the licensee) is the holder of Renewed Facility Operating License Nos. DPR-53 and DPR-69 which authorize operation of the Calvert Cliffs Nuclear Power Plant, Unit Nos. 1 and 2 (CCNPP). The licenses provide, among other things, that the facility is subject to all rules, regulations, and orders of the U.S. Nuclear Regulatory Commission (NRC, the Commission) now or hereafter in effect.

The facility consists of two pressurized-water reactors located in Calvert County, Maryland.

2.0 Purpose

Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix G, requires that pressure-temperature (P-T) limits be established for reactor pressure vessels (RPVs) for normal operating and hydrostatic or leak rate testing conditions. Specifically, 10 CFR part 50, Appendix G states, “The appropriate requirements on both the pressure-temperature limits and the minimum permissible temperature must be met for all conditions.” Appendix G of 10 CFR Part 50 also states that the “P-T limits identified as “ASME [American Society of Mechanical Engineers] Appendix G limits” in Table 1 require that the limits must be at least as conservative as limits obtained by following the methods of analysis and the margins of safety of Appendix G of Section XI of the ASME Code.” Section XI of the ASME Code, Appendix G, Figure G-2210-1 specifies a K_la fracture toughness curve for reactor vessel materials in determining P-T limits.

To address provisions of a proposed license amendment to the Technical Specification P-T limits for CCNPP, the licensee requested, in its submittal of September 14, 2000, that the NRC staff exempt CCNPP from application of specific requirements of 10 CFR Part 50, Section 50.60(a) and Appendix G, and substitute use of ASME Code Case N-640. Code Case N-640 permits the use of an alternate reference fracture toughness (K_lc fracture toughness curve instead of K_la fracture toughness curve) for reactor vessel materials in determining the P-T limits. Since the K_lc fracture toughness curve shown in ASME Section XI, Appendix A, Figure A-2200-1 (the K_lc fracture toughness curve, K_lc curve) provides greater allowable fracture toughness than the corresponding K_la fracture toughness curve of ASME Section XI, Appendix G, Figure G-2210-1 (the K_la fracture toughness curve, K_la curve), using Code Case N-640 for establishing the P-T limits would be less conservative than the methodology currently endorsed by 10 CFR Part 50, Appendix G, and, therefore, an exemption to apply the Code Case would also be required by 10 CFR 50.60.

Code Case N-640 (Formerly Code Case N-626)

The licensee has proposed an exemption to allow use of ASME Code Case N-640 in conjunction with ASME Section XI, 10 CFR 50.60(a) and 10 CFR part 50, Appendix G, to determine P-T limits. These revised P-T limits have been developed using the K_lc fracture toughness curve, in lieu of the K_la fracture toughness curve, as the lower bound for fracture toughness.

Use of the K_lc curve in determining the lower bound fracture toughness in the development of P-T operating limits curve is more technically correct than use of the K_la curve since the rate of loading during a heatup or cooldown is slow and is more representative of a static condition than a dynamic condition. The K_lc curve appropriately implements the use of static initiation fracture toughness behavior to evaluate the controlled heatup and cooldown process of a reactor vessel. The NRC staff has required use of the initial conservatism of the K_la curve since 1974 when the curve was codified. This initial conservatism was necessary due to the limited knowledge of RPV materials. Since 1974, additional knowledge has been gained about RPV materials which demonstrates that the lower bound on fracture toughness provided by the K_la curve is well beyond the margin of safety required to protect the public health and safety from potential RPV failure. In addition, P-T curves based on the K_lc curve will enhance overall plant safety by opening the P-T operating window with the greatest safety benefit in the region of low temperature operations.

3.0 Discussion

Pursuant to 10 CFR 50.12, the Commission may, upon application by any interested person or upon its own initiative, grant exemptions from the requirements of 10 CFR part 50, when (1) the exemptions are authorized by law, will not present an undue risk to public health or safety, and are consistent with the common defense and security; and (2) when special circumstances are present. These circumstances include the special circumstances that “Application of the regulation in the particular circumstances would not serve the underlying purpose of the rule or is not necessary to achieve the underlying purpose of the rule; * * * ”

The underlying purpose of 10 CFR part 50, Appendix G is to provide an adequate margin of safety against brittle failure of the RPV. Use of a P-T limit that is at least as conservative as the limits obtained by following the methods of analysis and margin of safety of the ASME Code, Section XI, Appendix G is not necessary, in this case, to achieve the underlying purpose of the rule. Specifically, substitution of the K_lc fracture toughness curve for the K_la fracture toughness curve for establishing the P-T limits provides a more technically correct outcome in that it accounts for the rate of loading during heatup or cooldown and is more representative of a static condition. In addition, the staff has determined that improved knowledge regarding the RPV materials justifies elimination of unnecessary conservatisms, such as that brought about by the use of the K_la curve. Use of the less conservative K_lc curve would provide an adequate margin of safety against brittle failure of the RPV in this case, due in part to the remaining conservatisms incorporated into the methodologies of 10 CFR part 50, Appendix G and Regulatory Guide 1.99 which would still be applicable. Therefore, use of a P-T limit that is at least as conservative as the limits obtained by following the methods and margins of safety of the ASME Code, Section XI, Appendix G, is not necessary in this case to achieve the underlying purpose of the rule, i.e., to provide sufficient margin of RPV fracture toughness to ensure structural integrity of the RPV.

Therefore, the staff concludes that granting an exemption under the special circumstances of 10 CFR 50.12(a)(2)(ii) is appropriate and that the methodology of Code Case N-640 may be used to revise the P-T limits for CCNPP.

4.0 Conclusion

In summary, the ASME Section XI, Appendix G, procedure was conservatively developed based on the level of knowledge existing in 1974 concerning RPV materials and the estimated effects of operation. Since 1974, the level of knowledge about these topics has been greatly expanded. The NRC staff concurs that this increased knowledge permits relaxation of the ASME Section XI, Appendix G, requirements by application of ASME Code Case N-640. Implementation of the proposed P-T limits, as allowed by ASME Code Case N-640, are sufficient to ensure the structural integrity of RPVs during plant operations. Thus, pursuant to 10 CFR 50.12(a)(2)(ii), the underlying purpose of the regulation will continue to be served.

Accordingly, the Commission has determined that, pursuant to 10 CFR 50.12(a), the exemption is authorized by law, will not endanger life or property or common defense and security, and is, otherwise, in the public interest. Also, special circumstances are present in that application of the regulation is not necessary to achieve the underlying purpose of the rule. Therefore, the Commission hereby grants CCNPPI an exemption from the requirements of 10 CFR Part 50, Section 50.60(a) and 10 CFR Part 50, Appendix G, for CCNPP.

Pursuant to 10 CFR 51.32, the Commission has determined that the granting of this exemption will not have a significant effect on the quality of the human environment (66 FR 9729).

This exemption is effective upon issuance.

Dated at Rockville, Maryland, this 26th day of February 2001.

John A. Zwolinski,

Director, Division of Licensing Project Management, Office of Nuclear Reactor Regulation.