[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
