[Federal Register Volume 61, Number 151 (Monday, August 5, 1996)]
[Notices]
[Pages 40670-40671]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-19849]


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NUCLEAR REGULATORY COMMISSION

[Docket Nos. 50-295 and 50-304]


Commonwealth Edison Company (Zion Nuclear Power Station, Unit 
Nos. 1 and 2); Exemption

I

    Commonwealth Edison Company (ComEd, the licensee) is the holder of 
Facility Operating License Nos. DPR-39 and DPR-48, which authorize 
operation of the Zion Nuclear Power Station, Units 1 and 2, 
respectively. The licenses provide, among other things, that the 
licensee is subject to all rules, regulations, and orders of the 
Commission now or hereafter in effect.
    The facility consists of two pressurized water reactors located at 
the licensee's site in Lake County, Illinois.

II

    In its letter dated March 19, 1996, 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 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 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 service lifetime. It also states that the 
ASME Code edition and addenda specified in 10 CFR 50.55a are 
applicable. It is specified in 10 CFR 50.60(b) 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 10 CFR Part 50, Appendix G, 
P/T limits while the reactor is operating at low temperatures, the 
licensee installed a low temperature overpressure protection (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 10 
CFR Part 50, 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 solid water condition, the operating pressure must 
be maintained below the PORV setpoint. Applying the LTOP instrument 
uncertainties required by the staff's approved methodology results in 
an LTOP setpoint with an operating window between the LTOP setpoint and 
the minimum pressure required for reactor coolant pump seals which is 
too narrow to permit continued operation.
    To allow itself a wider operating pressure band, the licensee has 
requested the use of the American Society of Mechanical Engineers 
(ASME) Boiler and Pressure Vessel Code (Code) Case N-514, ``Low 
Temperature Overpressure Protection,'' which allows exceeding the 10 
CFR Part 50, Appendix G, safety limits by 10 percent. ASME 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 Appendix G of Section XI of the ASME Code and 
published in the 1993 Addenda to Section XI. However, 10 CFR 50.55a, 
``Codes and Standards,'' only authorizes addenda through the 1988 
Addenda.

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 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 50.60, 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 Zion 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 ASME Code Case N-514 guidelines. The ASME Code Case N-514 allows 
determination

[[Page 40671]]

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, 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 PORV would be reduced, thereby improving 
plant safety.

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), in 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), an exemption is authorized by law, will not endanger life or 
property or 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 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. This exemption permits using the 
safety margins recommended in the American Society of Mechanical 
Engineers (ASME) Boiler and Pressure Vessel Code (Code) Case N-514, 
``Low Temperature Overpressure Protection'' in lieu of the safety 
margins required by 10 CFR Part 50, Appendix G. This exemption is 
applicable only to LTOP conditions during normal operation.
    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 (61 FR 25921).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 30th day of July 1996.

    For the Nuclear Regulatory Commission.
William T. Russell,
Director, Office of Nuclear Reactor Regulation.
[FR Doc. 96-19849 Filed 8-2-96; 8:45 am]
BILLING CODE 7590-01-P