03-24670. Exelon Generation Company, LLC, Byron Station, Units 1 and 2; Exemption

1.0 Background

The Exelon Generation Company, LLC (the licensee) is the holder of Facility Operating License Nos. NPF-37 and NPF-66 which authorizes operation of the Byron Station, Units 1 and 2. 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 Ogle County in Illinois.

2.0 Request/Action

Title 10 of the Code of Federal Regulations (10 CFR), part 50, requires, among other items, that each boiling or pressurized light-water nuclear power reactor fueled with oxide pellets within cylindrical zircaloy or ZIRLO cladding, must, as provided in paragraphs (b) through (d) of 10 CFR 50.44, include means for control of hydrogen gas that may be generated, following a postulated loss-of-coolant accident (LOCA) by:

(1) Metal-water reaction involving the fuel cladding and the reactor coolant.

(2) Radiolytic decomposition of the reactor coolant, and

(3) Corrosion of metals.

Section 50.46 of Title 10 of the Code of Federal Regulations, “Acceptance Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors,” requires, among other items, that each boiling or pressurized light-water nuclear power reactor fueled with uranium oxide pellets within cylindrical zircaloy or ZIRLO cladding, must be provided with an emergency core cooling system (ECCS) that must be designed so that its calculated cooling performance following postulated LOCAs conforms to the criteria set forth in paragraph (b) of 10 CFR 50.46. Section 50.46 also requires that ECCS cooling performance must be calculated in accordance with an acceptable evaluation model and must be calculated for a number of postulated LOCAs of different sizes, locations, and other properties sufficient to provide assurance that the most severe postulated LOCAs are calculated. Section 50.46 provides further that an acceptable evaluation model may be developed in conformance with the features of 10 CFR part 50, appendix K models.

Appendix K to part 50 of Title 10 of the Code of Federal Regulations, “ECCS Evaluation Models,” requires, among other items, that the rate of energy release, hydrogen generation, and cladding oxidation from the metal/water reaction shall be calculated using the Baker-Just equation.

In summary, 10 CFR 50.44, 10 CFR 50.46, and 10 CFR part 50, appendix K, make no provisions for use of fuel rods clad in a material other than Zircaloy or ZIRLO. The licensee has requested the use of a limited number of “lower tin” ZIRLO clad replacement fuel rods in one lead test assembly (LTA) with a tin composition that is less than the licensing basis for ZIRLO tin composition, as defined in Westinghouse design specifications.

3.0 Discussion

Pursuant to 10 CFR 50.12, the Commission may, upon application by any interested person or upon its own initiative, grant an exemption from the requirements of 10 CFR part 50 only if (1) the exemption is authorized by law, will not present an undue risk to public health or safety, and is consistent with the common defense and security; and (2) special circumstances are present. Special circumstances are present if application of the regulation is not necessary to achieve the underlying purpose of the rule.

The licensee here requests an exemption in order to use newly developed low tin ZIRLO that is an improved version of the ZIRLO material, and is not described in WCAP-12610-P-A, “VANTAGE+ Fuel Assembly Reference Core Report,” which describes the use of ZIRLO clad fuel. The staff examined the licensee's rationale to support the exemption request(s) and, for the reasons set forth below, concludes that the licensee would meet the underlying purpose of 10 CFR 50.44, 50.46 and part 50, appendix K.

The underlying purpose of 10 CFR 50.44 is to ensure that means are provided for the control of hydrogen gas that may be generated following a LOCA. The licensee has provided means for controlling hydrogen gas and has previously considered the potential for hydrogen gas generation stemming from a metal-water reaction. The LTA rods containing the lower tin ZIRLO (LT-2) cladding are similar in chemical composition to zircaloy cladding. Accordingly, previous calculations of hydrogen production resulting from a metal-water reaction will not be significantly changed. As such, application of 10 CFR 50.44 is not necessary for the licensee to achieve its underlying purpose in these circumstances.

The underlying purpose of 10 CFR 50.46, and 10 CFR part 50, appendix K, is to establish requirements for the calculation of ECCS performance. The ECCS performance requirements include peak cladding temperature, maximum cladding oxidation, hydrogen generation, and coolable geometry. With respect to 10 CFR 50.46, the licensee has previously performed a LOCA safety analysis using the approved Westinghouse methodology including the Byron Station ECCS Model Safety Analysis of Record for LTAs of lower tin ZIRLO (LT-1) cladding. The unique features of the LTAs were evaluated for effects on the LOCA analysis. The result showed that the Byron Station ECCS Model Safety Analysis of Record remained bounding for those LTAs. The staff recognizes that the current LTAs will be located at non-limiting core locations, and the ZIRLO (LT-2) is very similar to the approved ZIRLO and the previously exempted ZIRLO (LT-1) in chemical composition and mechanical behavior. Accordingly, the currently approved methodology for analyzing ECCS performance is acceptable to apply for the low tin ZIRLO (LT-2) LTAs, as is further discussed below with respect to the Baker-Just equation. As also discussed below, results of comparative LOCA calculations with the same plant operating parameters will be performed in the reload analysis for Cycle 13 to verify that the current ECCS Model Safety Analysis of Record remains bounding for these four LTAs for Byron Station Unit 1 Cycle 13.

Paragraph I.A.5 of appendix K to 10 CFR part 50 states that the rates of energy, hydrogen concentration, and cladding oxidation from the metal-water reaction shall be calculated using the Baker-Just equation. Since the Baker-Just equation presumes the use of zircaloy clad fuel, strict application of the rule would not permit use of the equation for the ZIRLO (LT-2) cladding for determining acceptable fuel performance. The underlying intent of this portion of the Appendix K, however, is to ensure that analysis of fuel response to LOCAs is conservatively calculated. Due to the similarities in the chemical composition of the ZIRLO (LT-2) and zircaloy, the application of the Baker-Just equation in the analysis of the improved ZIRLO (LT-2) clad fuel will conservatively bound all post-LOCA scenarios. Thus, application of appendix K, paragraph I.A.5, is not necessary for the licensee to achieve the underlying purpose of the rule in these circumstance.

According to the submittal, the licensee will perform reload analysis to demonstrate adequate ECCS performance, and show that the LTAs do not have a significant impact upon the analysis for Byron Station Unit 1. In the Cycle 13 reload analyses, the licensee will verify that the predicted peak cladding temperature of the LTAs are significantly lower than that predicted for the resident fuel. The licensee will also verify that the Baker-Just equation conservatively predicts local cladding oxidation of the LTAs of only a few percent. Also, the licensee will verify that maximum hydrogen generation is unchanged with the inclusion of the LTAs, and the coolable geometry is maintained following a LOCA. As such, application of 10 CFR 50.46 and 10 CFR part 50, appendix K is not necessary to achieve their underlying purposes in these circumstances.

To summarize, based on the previously acceptable performance of the LTAs in Byron Station Units 1 and 2, the staff concludes that the licensee has demonstrated that the LTAs will perform adequately under LOCA conditions, and thus the LTAs are acceptable for operation in Byron Station Unit 1 Cycle 13.

Therefore, application of 10 CFR 50.44, 10 CFR 50.46, and appendix K to 10 CFR part 50 is not necessary to achieve their underlying purposes and that special circumstances are present. Thus, it is acceptable to grant an exemption, pursuant to 10 CFR 50.12(a)(2), from the requirements of 10 CFR 50.44, 10 CFR 50.46, and Appendix K to 10 CFR Part 50 for the irradiation of the lower tin ZIRLO (LT-2) clad fuel rods in Byron Station Unit 1 during Cycle 13.

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 Exelon Generation Company, LLC, an exemption from the requirements of 10 CFR 50.44, 10 CFR 50.46, and 10 CFR part 50, appendix K, for irradiation of low tin ZIRLO (LT-2) clad fuel rods in Byron Station Unit 1 during Cycle 13.

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 (68 FR 54246).

This exemption is effective upon issuance.

Dated at Rockville, Maryland, this 22nd day of September, 2003.

For the Nuclear Regulatory Commission.

Eric J. Leeds,

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