In the Matter of Commonwealth Edison Company (LaSalle County Station, Units 1 and 2); Exemption

I.

Commonwealth Edison Company (ComEd, the licensee) is the holder of Facility Operating Licenses Nos. NPF-11 and NPF-18 for operation of LaSalle County Station, Units 1 and 2, located in LaSalle County, Illinois. The licenses state, among other things, that the facility is subject to all of the rules, regulations, and orders of the U.S. Nuclear Regulatory Commission (the Commission) now or hereafter in effect.

II.

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) during 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 specifies that the P-T limits must meet the safety margin requirements specified in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code), Section XI, Appendix G. ASME Code specifies use of K_1A fracture toughness curve.

To address provisions of the proposed amendments to the technical specification (TS) P-T limits, in its submittal of February 29, 2000, the licensee requested that the staff exempt LaSalle from application of specific requirements of 10 CFR 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_1c fracture toughness curve instead of K_1a fracture toughness curve) for reactor vessel materials in determining the P-T limits. Since the K_1c fracture toughness curve shown in ASME Code, Section XI, Appendix A, Figure A-2200-1 provides greater allowable fracture toughness than the corresponding K_1a fracture toughness curve of ASME Code, Section XI, Appendix G, Figure G-2210-1 (the K_1a fracture toughness 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 be required.

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

The licensee has proposed an exemption to allow the use of ASME Code Case N-640 in conjunction with ASME Code, Section XI; 10 CFR 50.60(a); and 10 CFR Part 50, Appendix G, to determine P-T limits.

The proposed amendments to revise the P-T limits for LaSalle rely in part on the requested exemption. These revised P-T limits have been developed using the K_1c fracture toughness curve, in lieu of the K_1a fracture toughness curve, as the lower bound for fracture toughness.

Use of the K_1c 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_1a 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_1c curve appropriately implements the use of static initiation fracture toughness behavior to evaluate the controlled heatup and cooldown process of a reactor vessel. The staff has required use of the initial conservatism of the K_1a 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_1a 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_1c curve would enhance overall plant safety by opening the P-T operating window with the greatest safety benefit in the region of low temperature operations.

Since the reactor coolant system (RCS) P-T operating window is defined by the P-T operating and test limit curves developed in accordance with the ASME Code, Section XI, Appendix G, continued operation of LaSalle with these P-T curves without the relief provided by ASME Code Case N-640 would unnecessarily require that the RPV maintain a temperature exceeding 212 degrees Fahrenheit in a limited operating window during pressure tests. Consequently, steam vapor hazards would continue to be one of the safety concerns for personnel conducting inspections in primary containment. Implementation of the proposed P-T curves, as allowed by ASME Code Case N-640, does not significantly reduce the margin of safety and would eliminate steam vapor hazards by allowing inspections in primary containment to be conducted at lower coolant temperature. Thus, pursuant to 10 CFR 50.12(a)(2)(ii), the underlying purpose of the regulation will continue to be served.

In summary, the ASME Code, 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 Code, Section XI, Appendix G, requirements by application of ASME Code Case N-640, while maintaining, pursuant to 10 CFR 50.12(a)(2)(ii), the underlying purpose of the ASME Code and the NRC regulations to ensure an acceptable margin of safety.

III.

Pursuant to 10 CFR 50.12(a), 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. Special circumstances are present whenever, according to 10 CFR 50.12(a)(2)(ii), “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 the requirement to use the K_1a curve to develop P-T limits is to provide an adequate margin of safety against brittle failure of the RPV. Code Case N-640 permits application of the lower bound static initiation fracture toughness value (K_1a) equation as the basis for establishing the curves in lieu of using the lower bound crack arrest fracture toughness value equation (i.e., the K_1a equation, which is based on conditions needed to arrest a dynamically propagating crack, and which is the method invoked by Appendix G to Section XI of the ASME Code). Use of the K_1c equation in determining the lower bound fracture toughness in the development of the P-T operating limits curve is more technically correct than the use of the K_1a equation 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_1c equation appropriately implements the use of the static initiation fracture toughness behavior to evaluate the controlled heatup and cooldown process of a reactor vessel. Therefore, use of the K_1c curve in developing P-T limits provides an adequate margin against brittle failure of the RPV. As a result, the application of the regulation in the particular circumstances is not necessary to achieve the underlying purpose of the rule.

Therefore, the staff concludes that requesting 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 LaSalle County Station, Units 1 and 2.

IV.

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, and that special circumstances are present. Therefore, the Commission hereby grants Commonwealth Edison Company an exemption from the requirements of 10 CFR 50.60(a) and 10 CFR Part 50, Appendix G, for LaSalle County Station, Units 1 and 2.

Pursuant to 10 CFR 51.32, an environmental assessment and finding of no significant impact has been prepared and published in the Federal Register (65 FR 60986). Accordingly, based upon the environmental assessment, the Commission has determined that the granting of this exemption will not result in any significant effect on the quality of the human environment.

This exemption is effective upon issuance.