[Federal Register Volume 60, Number 46 (Thursday, March 9, 1995)]
[Notices]
[Pages 12988-12990]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-5774]
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NUCLEAR REGULATORY COMMISSION
[Docket No. 50-255]
Exemption
In the Matter of Consumers Power Co. (Palisades Plant).
[[Page 12989]]
I
The Consumers Power Company (the licensee) is the holder of
Facility Operating License No. DPR-20, which authorizes operation of
the Palisades Plant at a steady-state reactor power level not in excess
of 2530 megawatts thermal. This facility consists of one pressurized
water reactor located at the licensee's site in Van Buren County,
Michigan. 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
The regulation 10 CFR 50.60, ``Acceptance Criteria for Fracture
Prevention Measures for Light-water Nuclear Power Reactors for Normal
Operation,'' 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 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. 10 CFR 50.60(b) 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 ASME 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 maintain seal integrity of the 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. The licensee LTOP analysis indicates that
using the ASME Appendix G safety margins to determine the PORV setpoint
would result in a pressure setpoint within its operating window, but
there would be no margin for normal operating pressure surges.
Therefore, operating with these limits could result in the lifting of
the PORVs and cavitation of the rector coolant pumps during normal
operation.
The licensee proposed in a letter dated February 10, 1995, that in
determining the design setpoint for LTOP events for the Palisades
Plant, the allowable pressure be determined using the safety margins
developed in an alternate methodology in lieu of the safety margins
currently required by Appendix G to 10 CFR part 50. Designated Code
Case N-514, the proposed alternate methodology, 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 but not yet approved for use in Regulatory Guide 1.147.
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 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 February 10, 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 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 2 on the principal membrane
(pressure) stresses, (b) assuming a flaw at the surface with a depth of
one-quarter of the vessel wall thickness and a length of 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 Palisades 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. The 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% 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. Using the
licensee's proposed safety factors instead of ASME Appendix G safety
factors to calculate the LTOP setpoint will permit a higher LTOP
setpoint than would otherwise be required, but will provide added
margin to prevent normal operating surges from lifting the PORVs or
cavitation of the reactor coolant pumps. Although this methodology
would reduce the safety factor on the principal membrane stresses, the
proposed criteria will [[Page 12990]] provide adequate margins of
safety to the reactor vessel during LTOP transients, thus providing an
acceptable level of quality and safety. Accordingly, the use of the
Code case will satisfy the underlying purpose of 10 CFR 50.60 for
fracture toughness requirements for normal operation and anticipated
operational occurrences.
IV
For the foregoing reason, 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), 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), 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 the Consumers
Power Company an exemption from the requirements of 10 CFR 50.60 such
that in determining the setpoint for LTOP events, the ASME Appendix G
curves for P/T limits are not exceeded by more than 10% 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 prepared an
environmental assessment and determined that the granting of this
exemption will not have a significant effect on the quality of the
human environment (February 27, 1995, 60 FR 10615).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 2nd day of March 1995.
For the Nuclear Regulatory Commission.
Elinor G. Adensam,
Acting Director, Division of Reactor Projects--III/IV, Office of
Nuclear Reactor Regulation.
[FR Doc. 95-5774 Filed 3-8-95; 8:45 am]
BILLING CODE 7590-01-M
