[Federal Register Volume 66, Number 57 (Friday, March 23, 2001)]
[Notices]
[Pages 16302-16303]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-7350]


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NUCLEAR REGULATORY COMMISSION

[Docket No. 50-353]


Exelon Generation Company, Limerick Generating Station, Unit 2

1.0  Background

    The Exelon Generation Company (Exelon, the licensee) is the holder 
of Facility Operating License No. NPF-85 which authorizes operation of 
the Limerick Generating Station, Unit 2 (Limerick Unit 2). The license 
provides, among others 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 a boiling water reactor located in 
Montgomery and Chester Counties in Pennsylvania.

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, section IV.A.2.a, 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 identified as ``ASME 
[American Society of Mechanical Engineering Pressure and Vessel Code 
(ASME Code)] Appendix G limits'' in Table 1 require that the limits 
must be at least as conservative as the limits obtained by following 
the methods of analysis and the margins of safety of Appendix G of 
section XI of the ASME Code.
    To address provisions of a proposed license amendment to the 
technical specification P-T limits for the Limerick facility, the 
licensee requested in its submittal of November 20, 2000, as 
supplemented December 20, 2000, that the staff exempt Limerick Unit 2 
from application of specific requirements of Appendix G to 10 CFR Part 
50, and substitute use of ASME Code Case N-640. Code Case N-640 permits 
the use of an alternate reference fracture toughness (Klc 
fracture toughness curve instead of Kla fracture toughness 
curve) for reactor vessel materials in determining the P-T limits. 
Since the Klc fracture toughness curve of ASME Section XI, 
Appendix A, Figure A-2200-1 (the Klc fracture toughness 
curve, Klc curve) provides greater allowable fracture 
toughness than the corresponding Kla fracture toughness 
curve of ASME Section XI, Appendix G, Figure G-2210-1 (the 
Kla 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. The regulations (10 CFR 
50.60(b)) state that proposed alternatives to the requirements in 
Appendix G to 10 CFR Part 50 may be used when an exemption is granted 
by the Commission under 10 CFR 50.12.

3.0  Discussion

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.
    The proposed license amendment to revise the P-T limits for 
Limerick Unit 2 relies in part on the requested exemption. These 
revised P-T limits have been developed using the Klc 
fracture toughness curve, in lieu of the Kla fracture 
toughness curve, as the lower bound for fracture toughness.
    Use of the Klc 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 Kla 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 
Klc curve appropriately

[[Page 16303]]

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 Kla 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 Kla 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 Klc 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.
    Since the reactor coolant system P-T operating window is defined by 
the P-T operating and test limit curves developed in accordance with 
ASME section XI, Appendix G, continued operation of Limerick Unit 2 
with these P-T curves without the relief provided by ASME Code Case N-
640 would unnecessarily require the licensee to maintain the RPV at a 
temperature exceeding 212  deg.F 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, continues to maintain an adequate 
margin of safety and would eliminate steam vapor hazards by allowing 
inspections in primary containment to be conducted at a lower coolant 
temperature. Thus, pursuant to 10 CFR 50.12(a)(2)(ii), the underlying 
purpose of the regulation will continue to be served.
    Pursuant to 10 CFR 50.12(a), the Commission may, upon application 
by an 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. As stated in 10 CFR 
50.12(a)(2)(ii), these special circumstances include situations in 
which ``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 staff 
examined the licensee's rationale to support the exemption request and 
determined that the use of the code case would meet the underlying 
purpose of 10 CFR Part 50, Appendix G; therefore, application of the 
assumed flaw types and the Kla equation in Appendix G to 
section XI of the ASME Code, as invoked by the rule, is not necessary 
to meet the underlying purpose of the regulation, and thus meets the 
special circumstance criterion of 10 CFR 50.12(a)(2)(ii) for granting 
the exemption request. Based upon a consideration of the conservatism 
that is explicitly incorporated into the methodologies of 10 CFR Part 
50, Appendix G; Appendix G of the ASME Code; and Regulatory Guide 1.99, 
Revision 2; the staff concludes that application of the code case as 
described would provide an adequate margin of safety against brittle 
failure of the RPV. This is also consistent with the determination that 
the NRC staff has reached for other licensees under similar conditions 
based on the same considerations, including Quad Cities Nuclear Power 
Station, Units 1 and 2, exemption dated February 4, 2000. Therefore, 
the staff concludes that granting an exemption under the special 
circumstances provision of 10 CFR 50.12(a)(2)(ii) is appropriate, and 
that the methodology contained in Code Case N-640 would serve the 
underlying purpose of the rule for Limerick Unit 2.

4.0  Conclusion

    Accordingly, the Commission has determined that, pursuant to 10 CFR 
50.12(a): (1) 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 (2) special circumstances are present. 
Therefore, the Commission hereby grants Exelon Generation Company an 
exemption from the requirements of 10 CFR Part 50, Appendix G, for 
Limerick Unit 2.
    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 15913).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 21st day of March 2001.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 01-7350 Filed 3-22-01; 8:45 am]
BILLING CODE 7590-01-P