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[Federal Register Volume 62, Number 249 (Tuesday, December 30, 1997)]
[Notices]
[Pages 67922-67923]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-33846]

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NUCLEAR REGULATORY COMMISSION

[Docket No. STN 50-457]

Commonwealth Edison Company (Braidwood Nuclear Station, Unit No. 
2); Exemption

I.

    Commonwealth Edison Company (ComEd, the licensee) is the holder of 
Facility Operating License No. NPF-77, which authorizes operation of 
the Braidwood Nuclear Station, Unit 2. The license provides, among 
other things, that the licensee is subject to all rules, regulations, 
and orders of the Commission now or hereafter in effect.

II.

    In its letter dated November 30, 1994, as supplemented on May 11, 
1995, the licensee requested an exemption from the Commission's 
regulations. Title 10 of the Code of Federal Regulations, Part 50, 
Section 60 (10 CFR 50.60), ``Acceptance Criteria for Fracture 
Prevention Measures for Lightwater Nuclear Power Reactors for Normal 
Operation,'' states that all lightwater nuclear power reactors must 
comply with the fracture toughness and material surveillance program 
requirements for the reactor coolant pressure boundary as stated in 
Appendices G and H to 10 CFR Part 50. Appendix G to 10 CFR Part 50 
defines pressure-temperature (P-T) limits during any condition of 
normal operation, including anticipated operational occurrences and 
system hydrostatic tests to which the pressure boundary may be 
subjected over its

[[Page 67923]]

service lifetime, and that are obtained by conforming to the methods of 
analysis and the margins of safety in the American Society of 
Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code), 
Section XI, Appendix G. It is required in 10 CFR 50.55a that any 
reference to the ASME Code, Section XI, in 10 CFR Part 50 refers to the 
addenda through the 1988 Addenda and editions through the 1989 Edition 
of the Code unless otherwise noted. It is specified in 10 CFR 50.60(b) 
that alternatives to the described 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.
    To mitigate low-temperature overpressure transients that would 
produce pressure excursions exceeding the required limits while the 
reactor is operating at low temperatures, the licensee installed a low-
temperature overpressure protection (LTOP) system. The system contains 
pressure-relieving devices called power-operated relief valves (PORVs). 
The PORVs are set at a low enough pressure so that if an LTOP transient 
occurred, the mitigation system would prevent the pressure in the 
reactor vessel from exceeding the required limits. To prevent the PORVs 
from lifting as a result of normal operating pressure surges, some 
margin is needed between the PORV setpoint and the normal operating 
pressure. In addition, normal operating pressure must be high enough to 
prevent damage to reactor coolant pumps that may result from cavitation 
or inadequate differential pressure across the pump seals. Hence, the 
licensee must operate the plant within a pressure window that is 
defined as the difference between the minimum pressure required for 
reactor coolant pumps and the operating margin to keep the PORVs from 
lifting. When instrument uncertainty is considered, the operating 
window is small and presents difficulties for plant operation.
    The licensee has requested the use of the ASME Code Case N-514, 
``Low Temperature Overpressure Protection,'' for determining the LTOP 
system setpoint. Code Case N-514 allows use of an LTOP system setpoint 
so that system pressure does not exceed 110 percent of the P-T limits 
during an LTOP event. Code Case N-514 is consistent with guidelines 
developed by the ASME Working Group on Operating Plant Criteria to 
define pressure limits during LTOP events that avoid certain 
unnecessary operational restrictions, provide adequate margins against 
failure of the reactor pressure vessel, and reduce the potential for 
unnecessary activation of pressure-relieving devices used for LTOP. The 
content of this code case has been incorporated into the ASME Code, 
Section XI, Appendix G, and was published in the 1993 Addenda to 
Section XI.

III.

    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 (1) when 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 10 CFR 50.60 and 10 CFR Part 50, Appendix 
G, is to establish fracture toughness requirements for ferritic 
materials of pressure-retaining components of the reactor coolant 
pressure boundary to provide adequate margins of safety during any 
condition of normal operation, including anticipated operational 
occurrences, to which the pressure boundary may be subjected over its 
service lifetime. Section IV.A.2 of Appendix G to 10 CFR Part 50 
requires that the reactor vessel be operated with P-T limits at least 
as conservative as those obtained from following the methods of 
analysis and the required margins of safety of Appendix G of Section XI 
of the ASME Code.
    Appendix G of the ASME Code requires that the P-T limits be 
calculated (1) Using a safety factor of 2 on the principal membrane 
(pressure) stresses, (2) assuming a flaw at the surface with a depth of 
one-fourth of the vessel wall thickness and a length of 6 times its 
depth, and (3) using a conservative fracture toughness curve that is 
based on the lower bound of static, dynamic, and crack arrest fracture 
toughness tests on material similar to the Braidwood reactor vessel 
material.
    For determining the LTOP system setpoint, the licensee proposed to 
use safety margins based on an alternate methodology consistent with 
ASME Code Case N-514. The code case allows the setpoint for mitigating 
LTOP events to be so determined that the maximum pressure in the vessel 
would not exceed 110 percent of the Appendix G P-T limits. This results 
in a safety factor of 1.8 on the principal membrane stresses. All other 
factors, including assumed flaw size and fracture toughness, remain the 
same. Although this methodology would reduce the safety factor on the 
principal membrane stresses, the proposed criteria will produce 
adequate margins of safety for the reactor vessel during LTOP 
transients and, thus, will satisfy the underlying purpose of 10 CFR 
50.60 for fracture toughness requirements. Further, by relieving the 
operational restrictions, the potential for undesirable lifting of the 
PORVs would be reduced, thereby making the plant safer.

IV.

    For the foregoing reasons, the NRC staff has concluded that the 
licensee's proposed use of the alternate methodology in determining the 
acceptable setpoint for LTOP events will not present an undue risk to 
public health and safety and is consistent with the common defense and 
security. The NRC staff has determined that there are special 
circumstances present, as specified in 10 CFR 50.12(a)(2)(ii), in that 
application of 10 CFR 50.60 is not necessary in order to achieve the 
underlying purpose of this regulation which is to provide adequate 
fracture toughness of the reactor pressure boundary.
    Accordingly, the Commission has determined that, pursuant to 10 CFR 
50.12(a), an exemption is authorized by law, will not endanger life or 
property or the common defense and security, and is, otherwise, in the 
public interest. Therefore, the Commission hereby grants an exemption 
from the requirements of 10 CFR 50.60; in accordance with ASME Code 
Case N-514, the LTOP system setpoint may be determined so that system 
pressure does not exceed 110 percent of the Appendix G P-T limits in 
order to be in compliance with these regulations. This exemption is 
applicable only to LTOP conditions during normal operation.
    Pursuant to 10 CFR 51.32, the Commission has determined that 
granting this exemption will not have a significant effect on the 
quality of the human environment (62 FR 59008).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 12th day of December 1997.

    For the Nuclear Regulatory Commission.
Frank J. Miraglia,
Acting Director, Office of Nuclear Reactor Regulation.
[FR Doc. 97-33846 Filed 12-29-97; 8:45 am]
BILLING CODE 7590-01-P
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