[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]
-----------------------------------------------------------------------
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]
BILLING CODE 7590-01-M
