[Federal Register Volume 62, Number 21 (Friday, January 31, 1997)]
[Notices]
[Pages 4818-4819]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-2377]
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NUCLEAR REGULATORY COMMISSION
[Docket No. 50-368]
Entergy Operations, Inc., (Arkansas Nuclear One, Unit 2);
Exemption
I
Entergy Operations, Inc. (the licensee) is the holder of Facility
Operating License No. NPF-6, which authorizes operation of Arkansas
Nuclear One, Unit 2. 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.
The facility consists of two pressurized water reactors, Arkansas
Nuclear One, Units 1 and 2, located at the licensee's site in Pope
County, Arkansas.
II
In its letter dated April 11, 1996, the licensee requested an
exemption from the Commission's regulations for Arkansas Nuclear One,
Unit 2. 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 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 two relief valves to limit high system pressure. The relief
valves 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
relief valves 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 relief
valve setpoint. However, the reactor coolant system pressure/
temperature operating window at low temperatures is defined by the LTOP
setpoint. Implementation of a LTOP setpoint without the additional
margin allowed by American Society of Mechanical Engineers (ASME) Code
[[Page 4819]]
Case N-514 would restrict the pressure/temperature operating window and
would potentially result in undesired actuation of the LTOP system.
This constitutes an unnecessary burden that can be alleviated by the
application of ASME Code Case N-514. Implementation of an LTOP setpoint
as allowed by ASME Code Case N-514 does not significantly reduce the
margin of safety associated with normal operational heatup and cooldown
limits. Further, the LTOP guidelines will reduce the potential for an
undesired lift of the LTOP valves.
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 Appendix G safety limits 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.
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.
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 ANO-2
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 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. The slight reduction in the membrane stress safety
factor, as proposed by Code Case N-514, is compensated by increased
safety from the standpoint of increased operational flexibility and the
reduced potential for unnecessary opening of the LTOP relief valves. In
summary, the use of Code Case N-514 is likely to improve overall safety
when evaluated as part of the complete plant safety concern.
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 allowing the use of alternate
criteria as described by Code Case N-514, which permits exceeding the
Appendix G safety factor by 10 percent during low temperature
operations.
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 20846).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 28th day of January 1997.
For the Nuclear Regulatory Commission.
Frank J. Miraglia,
Acting Director, Office of Nuclear Reactor Regulation.
[FR Doc. 97-2377 Filed 1-30-97; 8:45 am]
BILLING CODE 7590-01-P