04-16532. Tennessee Valley Authority, Sequoyah Nuclear Plant, Unit Nos. 1 and 2; Exemption

1.0 Background

The Tennessee Valley Authority (TVA, the licensee) is the holder of Facility Operating License Nos. DPR-77 and DPR-79, which authorize operation of the Sequoyah Nuclear Plant (facility or SQN), Unit Nos. 1 and 2, respectively. The licenses provide, among other things, that the facility is subject to all rules, regulations, and orders of the Nuclear Regulatory Commission (NRC, the Commission) now or hereafter in effect.

The facility consists of two pressurized water reactors located in Hamilton County, Tennessee.

2.0 Request/Action

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. TVA requested that they be able to use Westinghouse Report WCAP-15315, “Reactor Vessel Closure Head/Vessel Flange Requirements Evaluation for Operating PWR [Pressurized-Water Reactor] and BWR [Boiling-Water Reactor] Plants” in lieu of 10 CFR, Appendix G, Footnote 2 to Table 1.

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. Therefore, in determining the acceptability of the licensee's exemption request, the staff has performed the following regulatory, technical, and legal evaluations to satisfy the requirements of 10 CFR 50.12 for granting the exemption.

3.1 Regulatory Evaluation

It is stated in 10 CFR part 50, Appendix G that “[t]he minimum temperature requirements * * * pertain to the controlling material, which is either the material in the closure flange or the material in the beltline region with the highest reference temperature * * * the minimum temperature requirements and the controlling material depend on the operating condition (i.e., hydrostatic pressure and leak tests, or normal operation including anticipated normal operational occurrences), the vessel pressure, whether fuel is in the vessel, and whether the core is critical. The metal temperature of the controlling material, in the region of the controlling material which has the least favorable combination of stress and temperature, must exceed the appropriate minimum temperature requirement for the condition and pressure of the vessel specified in Table 1 [of 10 CFR Part 50, Appendix G].” Footnote 2 to Table 1 in 10 CFR part 50, Appendix G specifies that RPV minimum temperature requirements related to RPV closure flange considerations shall be based on “[t]he highest reference temperature of the material in the closure flange region that is highly stressed by bolt preload.”

In order to address provisions of amendments to modify SQN Units 1 and 2 Technical Specifications (TSs) to implement a pressure-temperature limits report (PTLR) for each unit, TVA requested in its submittal dated September 6, 2002, that the staff exempt SQN Units 1 and 2 from the application of specific requirements of 10 CFR part 50, Appendix G, as they pertain to the establishment of minimum temperature requirements, for all modes of operation addressed by 10 CFR part 50, Appendix G, based on the material properties of the material of the RPV closure flange region that is highly stressed by the bolt preload. The licensee's initial technical basis for this exemption request was submitted on December 19, 2002. The requirements from which TVA requested that SQN Units 1 and 2 be exempted shall be referred to for the purpose of this exemption as “those requirements related to the application of Footnote 2 to Table 1 of 10 CFR Part 50, Appendix G.” The proposed action is in accordance with the licensee's application for exemption contained in its September 6, 2002, submittal, and is needed to support the TS amendments that are contained in the same submittal. The proposed amendments will revise the SQN Units 1 and 2 TSs to permit the implementation of a PTLR for each unit.

TVA's final, complete technical basis for the requested exemption was submitted to the NRC by letters dated June 24, 2003, and December 18, 2003. The licensee's June 24, 2003, letter included as an attachment Westinghouse report WCAP-15984-P, Revision 1, “Reactor Closure Head/Vessel Flange Requirements Evaluation for SQN Units 1 and 2.” This revision of WCAP-15984 updated information provided in WCAP-15984-P, Revision 0, which had been submitted to the staff on December 19, 2002. The licensee's December 18, 2003, letter provided responses to specific questions raised by the NRC staff to clarify information in WCAP-15984-P, Revision 1.

3.2 Technical Evaluation

WCAP-15984-P, Revision 1 included a fracture mechanics analysis of postulated flaws in SQN Units 1 and 2 RPV closure flange regions under boltup, 100 degrees Fahrenheit per hour (°F/hr) heatup, 100 °F/hr cooldown, and steady-state conditions, with the heatup and cooldown transients being modeled in accordance with what would be permissible using P-T limit curves based on SQN Units 1 and 2 beltline materials. Westinghouse performed finite element modeling to calculate the stresses present at critical locations within the flange region and determined that the 100 °F/hr heatup transient was the most severe condition with the upper head-to-flange weld being the most limiting location. With these stresses, Westinghouse calculated the applied stress intensity (K_{I applied}) for semi-elliptical, outside diameter initiated, surface breaking flaws with an aspect ratio (length vs. depth) of 6:1, and with depths ranging from 0 to 90 percent of the thickness of the component wall. The K_{I applied} values were calculated in accordance with the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) Section XI, Appendix G, subparagraph G-2220 requirements for the analysis of flange locations. Westinghouse then compared these K_{I applied} values to ASME Code lower bound static crack initiation fracture toughness (K_IC) values determined from the nil-ductility transition reference temperature (RT_NDT) values for the SQN Units 1 and 2 RPV closure flange materials. Westinghouse also provided an assessment of the potential for changes in the material RT_NDT values for the SQN Units 1 and 2 RPV closure flange materials due to thermal aging resulting from exposure to the RPV operating environment.

The use of ASME Code K_IC as the material property for the fracture mechanics analysis represents the most significant change between the analysis provided in WCAP-15984-P, Revision 1 and the analysis which was performed as the basis for establishing the minimum temperature requirements in 10 CFR part 50, Appendix G. The minimum temperature requirements related to Footnote 2 to Table 1 of 10 CFR part 50, Appendix G were incorporated into the Code of Federal Regulations in the early 1980s and were based on analyses which used ASME Code lower bound crack arrest/dynamic test fracture toughness (K_IA) as the parameter for characterizing a material's ability to resist crack initiation and propagation. The use of ASME Code K_IA is always conservative with respect to the use of ASME Code K_IC for fracture mechanics evaluations, and its use in the evaluations which established the requirements in 10 CFR part 50, Appendix G was justified based on the more limited knowledge of RPV material behavior that was available in the early eighties. However, the use of ASME Code K_IC, not ASME Code K_IA, is consistent with the actual physical processes that would govern flaw initiation under conditions of normal RPV operation, including RPV heatup, cooldown, and hydrostatic and leak testing. Based on our current understanding of the behavior of RPV materials, the NRC staff has routinely approved licensees utilization of ASME Code K_IC as the basis for evaluating RPV beltline materials to demonstrate compliance with the intent of 10 CFR part 50, Appendix G through the licensees use of ASME Code Cases N-640 and N-641.

The minimum K_IC value given in ASME Code for a RPV steel, regardless of material RT_NDT value or temperature, is 33.2 ksi√in. This value represents the “lower shelf” of the ASME Code K_IC curve. Based on information in WCAP-15984-P, Revision 1 and the licensee's December 18, 2003, response to NRC staff questions, it is apparent that the K_Iapplied for any flaw up to 1/4 of the wall thickness (1/4 T) at the limiting location (refer to WCAP-15984-P, Revision 1, Figure 4-2), would not exceed 33.2 ksi√in (including staff consideration of ASME Code structural factors) until between 1 and 2 hours into the 100°F/hr heatup transient. The temperature at the tip of postulated flaws up to 1/4 T size would be adequate at that point in time to ensure that the limiting SQN flange materials would exhibit fracture toughness properties in excess of ASME Code “lower shelf” behavior.

Hence, the analysis provided in WCAP-15984-P, Revision 1 has demonstrated that, for the most limiting transient addressed by 10 CFR Part 50, Appendix G, the combination of factors which would have to exist (high stresses in the RPV flange region along with the metal of the flange region being at low temperature) cannot exist simultaneously, and the structural integrity of the SQN Units 1 and 2 RPV closure flange materials will not be challenged by facility operation in accordance with P-T limit curves based consideration of SQN Units 1 and 2 beltline materials. Therefore, the more conservative minimum temperature requirements related to Footnote 2 to Table 1 of 10 CFR part 50, Appendix G are not necessary to meet the underlying intent of 10 CFR part 50, Appendix G, to protect SQN Units 1 and 2 RPVs from brittle failure during normal operation under both core critical and core non-critical conditions and RPV hydrostatic and leak test conditions.

3.3 Legal Basis for Exemption

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. The staff accepts the licensee's determination that an exemption would be required to permit TVA to not meet those requirements related to the application of Footnote 2 to Table 1 of 10 CFR part 50, Appendix G. The staff examined the licensee's rationale to support the exemption request and agrees that based on the information provided in WCAP-15984-P, Revision 1 and TVA's December 18, 2003, letter, an acceptable technical basis has been established to exempt SQN Units 1 and 2 from requirements related to the application of Footnote 2 to Table 1 of 10 CFR part 50, Appendix G. The technical basis provided by TVA has established that an adequate margin of safety against brittle failure would continue to be maintained for SQN Units 1 and 2 RPVs without the application of those requirements related to the application of Footnote 2 to Table 1 of 10 CFR part 50, Appendix G, for normal operation under both core critical and core non-critical conditions and RPV hydrostatic and leak test conditions. Hence, the staff concludes that, pursuant to 10 CFR 50.12(a)(2)(ii), the underlying purpose of 10 CFR part 50, Appendix G will be achieved without the application of those requirements related to the application of Footnote 2 to Table 1 of 10 CFR part 50, Appendix G. Therefore, the staff concludes that requesting the exemption under the special circumstances of 10 CFR 50.12(a)(2)(ii) is appropriate, and should be granted to TVA such that those requirements related to the application of Footnote 2 to Table 1 of 10 CFR part 50, Appendix G need not be applied to SQN Units 1 and 2.

4.0 Conclusion

Accordingly, the Commission has determined that, pursuant to 10 CFR 50.12(a), the exemption is authorized by law, will not present an undue risk to the public health and safety, and is consistent with the common defense and security. Also, special circumstances are present. Therefore, the Commission hereby grants TVA an exemption from those requirements related to the application of Footnote 2 to Table 1 of 10 CFR part 50, Appendix G, for SQN Units 1 and 2.

Pursuant to 10 CFR 51.32, the Commission has determined that the granting of this exemption will not result in any significant effect on the quality of the human environment (69 FR 32372).

This exemption is effective upon issuance.

Dated in Rockville, Maryland, this 7th day of July, 2004.

For the Nuclear Regulatory Commission.

Ledyard B. Marsh,

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