[Federal Register Volume 62, Number 22 (Monday, February 3, 1997)]
[Notices]
[Pages 5061-5062]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-2537]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-266 and 50-301]
Wisconsin Electric Power Company (Point Beach Nuclear Plant, Unit
Nos. 1 and 2); Exemption
I
Wisconsin Electric Power Company (the licensee) is the holder of
Facility Operating License Nos. DRP-24 and DRP-27, which authorize
operation of the Point Beach Nuclear Plant, 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 Manitowoc County, Wisconsin.
II.
In its letter dated July 1, 1996, as supplemented November 18,
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 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 P/T limits of Appendix G to
10 CFR Part 50 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 P/T
limits of Appendix G to 10 CFR Part 50. To prevent the PORVs from
lifting as a result of normal operating pressure surges (e.g., reactor
coolant pumps starting or stopping) with the reactor coolant system in
a water solid condition, the operating pressure must be maintained
below the PORV setpoint. The maximum LTOP setpoint of 425 psig was
approved May 20, 1980, with the issuance of Amendments 45 (DPR-24) and
60 (DPR-27) to the Point Beach operating licenses. This LTOP system
received pressure input from the sensing taps located in the reactor
coolant system hot leg and at the pressurizer. Subsequent evaluation
determined that the methodology used to determine the LTOP system
setpoint did not account for the differential pressure across the core
during reactor coolant pump operation. A recent Westinghouse
calculation (NSAL 93-005) indicated that with both reactor coolant
pumps operating, the pressure at core midplane may be as much as 63
psig higher than at the pressure sensing points. To account for this
differential pressure, which could cause the reactor vessel midplane
pressure to exceed the ASME Section XI, Appendix G limits, the licensee
implemented an administrative requirement in 1993 allowing only one
reactor coolant pump in operation when reactor coolant temperature is
below 160 oF. Plant operation with this restriction places an
[[Page 5062]]
unnecessary burden on plant operators to ensure safety limits are
maintained.
The licensee has requested the use of the American Society of
Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) Case
N-514, ``Low Temperature Overpressure Protection,'' which allows
exceeding the pressure of the P/T limits of 10 CFR Part 50, Appendix G,
by 10 percent. ASME Code Case N-514, the proposed alternate
methodology, 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. ASME Code Case N-514 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.
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, Section XI.
Appendix G of Section XI 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 (\1/4\) 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 Point Beach
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 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 Code, Section XI, Appendix G. This approach
results in a safety factor of 1.8 on pressure. All other factors,
including assumed flaw size and fracture toughness, remain the same.
Although this methodology would reduce the safety factor on pressure,
it was demonstrated in the Bases of the ASME Code Case N-514 that due
to the isothermal nature of LTOP events, the margin with respect to
toughness for LTOP transients is within the range provided by ASME,
Section XI, Appendix G for normal heatup and cooldown in the low
temperature range. Thus, applying Code Case N-514 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)(ii), 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 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 66062).
This exemption is effective upon issuance.
For the Nuclear Regulatory Commission.
Dated at Rockville, Maryland, this 27th day of January 1997.
Frank J. Miraglia
Acting Director, Office of Nuclear Reactor Regulation.
[FR Doc. 97-2537 Filed 1-31-97; 8:45 am]
BILLING CODE 7590-01-P
