[Federal Register Volume 60, Number 140 (Friday, July 21, 1995)]
[Notices]
[Pages 37692-37694]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-17976]



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NUCLEAR REGULATORY COMMISSION
[Docket No. STN 50-456]


In the Matter: Commonwealth Edison Company (Braidwood Station, 
Unit 1); Exemption

I

    Commonwealth Edison Company (ComEd, the licensee) is the holder of 
Facility operating License No. NPF-72, which authorizes operation of 
Braidwood Station, Unit 1. The facility is a pressurized water reactor 
located at the licensee's site in Will County, Illinois. 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 10 CFR 50.60, ``Acceptance Criteria for Fracture Prevention 
Measures for Light-Water Nuclear Power Reactors for

[[Page 37693]]
Normal Operation,'' it states that all light-water nuclear power 
reactors must meet the fracture toughness and material surveillance 
program requirements for the reactor coolant pressure boundary as set 
forth in Appendices G and H to 10 CFR Part 50. Appendix G to 10 CFR 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 service lifetime. In 10 CFR 50.60(b) it specifies 
that alternatives to the described requirements in Appendices G and H 
to 10 CFR Part 50 may be used when an exemption is granted by the 
Commission under 10 CFR 50.12.
    To prevent low temperature overpressure transients that would 
produce pressure excursions exceeding the Appendix G P/T limits while 
the reactor is operating at low temperatures, the licensee installed a 
low temperature overpressure (LTOP) system. The system includes 
pressure-relieving devices called Power-Operated Relief Valves (PORVs). 
The PORVs are set at a pressure low enough so that if an LTOP transient 
occurred, the mitigation system would prevent the pressure in the 
reactor vessel from exceeding the Appendix G P/T limits. To prevent the 
PORVs from lifting as a result of normal operating pressure surges 
(e.g., reactor coolant pump starting, and shifting operating charging 
pumps) with the reactor coolant system in a water solid condition, the 
operating pressure must be maintained below the PORV setpoint. In 
addition, in order to prevent cavitation of a reactor coolant pump, the 
operator must maintain a differential pressure across the reactor 
coolant pump seals. Hence, the licensee must operate the plant in a 
pressure window that is defined as the difference between the minimum 
required pressure to start a reactor coolant pump and the operating 
margin to prevent lifting of the PORVs due to normal operating pressure 
surges. Braidwood, Unit 1, is expected to exceed the 5.37 effective 
full power years on August 2, 1995; therefore, operating with the 
current LTOP limits may result in encroachment of the P/T limit curves 
of the reactor vessel during normal operation of the plant after August 
2, 1995.
    The licensee proposed that in determining the design setpoint for 
LTOP events for Braidwood Unit 1, the allowable pressure be determined 
using the safety margins developed in an alternate methodology in lieu 
of the safety margins currently required by 10 CFR Part 50, Appendix G. 
The proposed alternate methodology, Code Case N-514, is consistent with 
guidelines developed by the American Society of Mechanical Engineers 
(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. Code Case N-514, ``Low 
Temperature Overpressure Protection,'' has been approved by the ASME 
Code Committee. The content of this code case has been incorporated 
into Appendix G of Section XI of the ASME Code and published in the 
1993 Addenda to Section XI. The NRC staff is revising 10 CFR 50.55a, 
which will endorse the 1993 Addenda and Appendix G of Section XI into 
the regulations.
    An exemption from 10 CFR 50.60 is required to use the alternate 
methodology for calculating the maximum allowable pressure for the LTOP 
setpoint. By application dated November 30, 1994, as supplemented on 
May 11, 1995, the licensee requested an exemption from 10 CFR 50.60 for 
this purpose.

III

    Pursuant to 10 CFR 50.12, the Commission may, upon application by 
any interested person or upon is 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 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 this Appendix requires that the reactor vessel be operated 
with P/T limits at least as conservative as those obtained by following 
the methods of analysis and the required margins of safety of Appendix 
G of the ASME Code.
    Appendix G of the ASME Code requires that the P/T limits be 
calculated: (a) using a safety factor of two on the principal membrane 
(pressure) stresses, (b) assuming a flaw at the surface with a depth of 
one-quarter (\1/4\) of the vessel wall thickness and a length of six 
(6) times its depth, and (c) 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.
    In determining the setpoint for LTOP events, the licensee proposed 
to use safety margins based on an alternate methodology consistent with 
the proposed ASME Code Case N-514 guidelines. ASME Code Case N-514 
allows determination of the setpoint for LTOP events such that the 
maximum pressure in the vessel would not exceed 110 percent of the P/T 
limits of the existing ASME Appendix G. 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 provide adequate margins 
of safety to the reactor vessel during LTOP transients and will satisfy 
the underlying purpose of 10 CFR 50.60 for fracture toughness 
requirements.
    Using the licensee's proposed safety factors instead of Appendix G 
safety factors to calculate the LTOP setpoint will permit a higher LTOP 
setpoint than would otherwise be required and will provide added margin 
to prevent normal operating surges from lifting the PORVs or cavitating 
the reactor coolant pumps.

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 
pubic 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), such that 
application of 10 CFR 50.60 is not necessary in order to achieve the 
underlying purpose of this regulation.
    Accordingly, the Commission has determined that, pursuant to 10 CFR 
50.12(a), this exemption is authorized by law, will not endanger life 
or property or common defense and

[[Page 37694]]
security, and is, otherwise, in the public interest. Therefore, The 
Commission hereby grants Commonwealth Edison Company an exemption from 
the requirements of 10 CFR 50.60 such that in determining the setpoint 
for LTOP events, the Appendix G curves for P/T limits are not exceeded 
by more than 10 percent 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 impact on the human 
environment (60 FR 35570).

    Dated at Rockville, Maryland, this 13th day of July 1995.

    For the Nuclear Regulatory Commission.
Jack W. Roe,
Director, Division of Reactor Projects--III/IV, Office of Nuclear 
Reactor Regulation.
[FR Doc. 95-17976 Filed 7-20-95; 8:45 am]
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