[Federal Register: July 11, 2003 (Volume 68, Number 133)]
[Notices]
[Page 41405-41407]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr11jy03-93]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-369 and 50-370]
Duke Power Company, McGuire Nuclear Station, Units 1 and 2;
Exemption
1.0 Background
Duke Power Company (the licensee) is the holder of Facility
Operating License Nos. NPF-9 and NPF-17 that authorizes operation of
the McGuire Nuclear Station, Units 1 and 2 (McGuire). The license
provides, among other things, that the facility is subject to all
rules, regulations, and orders of the U.S. Nuclear Regulatory
Commission (NRC, the Commission) now or hereafter in effect.
The facility consists of two pressurized water reactors located in
Mecklenburg County, North Carolina.
2.0 Request/Action
Title 10 of the Code of Federal Regulations (10 CFR) section
50.60(a), requires that the fracture toughness and material
surveillance requirements of Appendix G to part 50 must be met for the
reactor coolant pressure boundary. Appendix G to part 50 requires that
pressure and temperature (P/T) limits be established for reactor
pressure vessels (RPVs) during normal operating and hydrostatic or leak
rate testing conditions. Specifically, section IV.A.2.a of Appendix G
to 10 CFR part 50 states that ``The appropriate requirements on both
the pressure-temperature limits and the minimum permissible temperature
must be met for all conditions.'' Further, section IV.A.2.b of Appendix
G to 10 CFR part 50 requires that these P/T limits must be at least as
conservative as limits obtained by following the methods of analysis
and the margins of safety of Appendix G to section XI of the American
Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code
(Code). The current ASME Code of Record for McGuire is the 1995 edition
through 1996 addenda of the ASME Code. The McGuire Code of Record does
not incorporate the provisions of ASME Code Case N-641. Although the
provisions of ASME Code Case N-641 were incorporated into Appendix G to
section XI of the ASME Code in the 1998 edition through 2000 addenda,
which is the latest edition and addenda codified in 10 CFR 50.55a,
McGuire has not adopted this edition and consequently must meet its
Code of Record to comply with Appendix G to part 50. Therefore, in this
case, the licensee is still required to obtain an exemption to apply
Code Case N-641.
In order to address provisions of amendments to the McGuire
Technical Specification (TS) P/T limit curves, the licensee requested
in its submittal dated December 12, 2002, as supplemented by letters
dated March 27 and April 23, 2003, that the NRC staff exempt McGuire
from application of specific requirements of 10 CFR 50.60 and Appendix
G to 10 CFR part 50, and substitute the use of ASME Code Case N-641.
ASME Code Case N-641 permits the use of an alternate reference fracture
toughness curve for RPV materials and permits the postulation of a
circumferentially-oriented flaw for the evaluation of circumferential
RPV welds when determining the P/T limits. The proposed exemption
request is consistent with, and is needed to support, the McGuire TS
amendment that was contained in the same submittal. The proposed
McGuire TS amendment will revise the P/T limits for heatup, cooldown,
and inservice test limitations for the reactor coolant system (RCS)
through 34 effective full power years of operation.
Code Case N-641
The licensee has proposed an exemption to allow the use of ASME
Code Case N-641 in conjunction with Appendix G to ASME section XI, 10
CFR 50.60(a) and 10 CFR part 50, Appendix G, to establish the P/T
limits for the McGuire, Units 1 and 2 RPVs.
The proposed TS amendment to revise the P/T limits for McGuire,
Units 1 and 2, relies in part, on the requested exemption. These
revised P/T limits have been developed using the lower bound
KIC fracture toughness curve shown in ASME, section XI,
Appendix A, Figure A-2200-1, in lieu of the lower bound KIA
fracture toughness curve of ASME, section XI, Appendix G, Figure G-
2210-1, as the basis fracture toughness curve for defining the McGuire
P/T limits. In addition, the revised P/T limits have been developed
based on the use of a postulated circumferentially-oriented flaw for
the evaluation of RPV circumferential welds in lieu of the axially-
oriented flaw that would be required by Appendix G to section XI of the
ASME Code. The other margins involved with the ASME section XI,
Appendix G, process of determining P/T limit curves remain unchanged.
Use of the KIC curve as the basis fracture toughness
curve for the development of P/T operating limits is technically
correct. The KIC curve appropriately implements the use of a
relationship based on static initiation fracture toughness behavior to
evaluate the controlled heatup and cooldown process of a RPV, whereas
the KIA fracture toughness curve codified into Appendix G to
section XI of the ASME Code was developed from more conservative crack
arrest and dynamic fracture toughness test data. The application of the
KIA fracture toughness curve was initially codified in
Appendix G to section XI of the ASME Code in 1974 to provide a
conservative representation of RPV material fracture toughness. This
initial conservatism was necessary due to the limited knowledge of RPV
material behavior in 1974. However, additional knowledge has been
gained about RPV materials that demonstrates the lower bound on
fracture toughness provided by the KIA fracture toughness
curve is well beyond the margin of safety required to protect the
public health and safety from potential RPV failure.
Likewise, the use of a postulated circumferentially-oriented flaw
in lieu of an axially-oriented one for the evaluation of a
circumferential RPV weld is more technically correct. The size of flaw
required to be postulated for P/T limit determination has a depth of
one-quarter of the RPV wall thickness and a length six times the depth.
Based on the direction of welding during the fabrication process, the
only technically reasonable orientation for such a large flaw is for
the plane of the flaw to be circumferentially-oriented (i.e., parallel
to the direction of welding). Prior to the development of ASME Code
Case N-641 (and the similar ASME Code Case N-588), the required
postulation of an axially-oriented flaw for the evaluation of a
circumferential RPV weld has provided an additional, unnecessary level
of conservatism to the overall evaluation.
In addition, P/T limit curves based on the KIC fracture
toughness curve and postulation of a circumferentially-oriented flaw
for the evaluation of RPV circumferential welds will enhance overall
plant safety by expanding the P/T operating window with the greatest
safety benefit being in the region of low temperature operations. The
operating window through which the operator heats up and cools down the
RCS is determined by the difference between the maximum allowable
pressure determined by Appendix G of ASME section XI, and the minimum
required pressure for the reactor coolant pump seals adjusted for
instrument uncertainties. A narrow operating window could potentially
have an adverse safety impact by increasing the possibility of
inadvertent overpressure protection system actuation due to
[[Page 41406]]
pressure surges associated with normal plant evolutions such as RCS
pump starts and swapping operating charging pumps with the RCS in a
water-solid condition.
Since application of ASME Code Case N-641 provides appropriate
procedures to establish maximum postulated defects and to evaluate
those defects in the context of establishing RPV P/T limits, this
application of the Code Case maintains an adequate margin of safety for
protecting RPV materials from brittle failure. The NRC staff has
reviewed the exemption request submitted by the licensee and has
concluded that an exemption should be granted from the requirements of
10 CFR 50.60 and section IV.A.2.b of Appendix G to 10 CFR part 50 to
permit the licensee to use the provisions of ASME Code Case N-641 for
the purpose of developing the McGuire Units 1 and 2 RPV P/T limit
curves. However, the NRC staff does not agree with the special
circumstances cited by the licensee in its December 12, 2002,
application regarding the basis for granting the exemption. The NRC
staff did not conclude that the circumstances cited above constitute
``undue hardship or other costs that are significantly in excess of
those contemplated when the regulation was adopted, or that are
significantly in excess of those incurred by others similarly
situated,'' pursuant to 10 CFR 50.12(a)(2)(iii). Rather, the NRC staff
concluded that the application of the technical provisions of ASME Code
Case N-641 provided sufficient margin in the development of RPV P/T
limit curves such that the underlying purpose of the regulations,
Appendix G to 10 CFR part 50, will continue to be met and that the
specific conditions required by the regulations (i.e., use of all
provisions in Appendix G to section XI of the ASME Code) were not
necessary. Therefore, the NRC staff grants the requested exemption to
the licensee based on the special circumstances of 10 CFR
50.12(a)(2)(ii), ``[a]pplication 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.''
In summary, the ASME section XI, Appendix G, procedure was
conservatively developed based on the level of knowledge existing in
1974 concerning reactor coolant pressure boundary materials and the
estimated effects of operation. Since 1974, the level of knowledge
about the fracture mechanics behavior of RCS materials has been greatly
expanded, especially regarding the effects of radiation embrittlement
and the understanding of fracture toughness properties under static and
dynamic loading conditions. The NRC staff concurs that this increased
knowledge permits relaxation of the ASME section XI, Appendix G
requirements by application of ASME Code Case N-641, while maintaining,
pursuant to 10 CFR 50.12(a)(2)(ii), the underlying purpose of the ASME
Code and the NRC regulations to ensure an acceptable margin of safety
against brittle failure of the RPV.
The NRC staff has reviewed the exemption request submitted by the
licensee and has concluded that an exemption should be granted from the
requirements of 10 CFR 50.60(a) and section IV.A.2.b of Appendix G to
10 CFR part 50 to permit the licensee to utilize the provisions of ASME
Code Case N-641 for the purpose of developing McGuire Units 1 and 2 RPV
P/T limit curves.
3.0 Discussion
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, pursuant to 10 CFR 50.12(a)(2)(ii), are
present in that continued operation of McGuire, Units 1 and 2, pursuant
to the requirements of 10 CFR 50.60 and section IV.A.2.b of Appendix G
to 10 CFR part 50, using P/T curves developed in accordance with ASME
section XI, Appendix G, without the relief provided by ASME Code Case
N-641, is not necessary to achieve the underlying purpose of 10 CFR
50.60 and Appendix G to 10 CFR part 50. Application of ASME Code Case
N-641 in lieu of the requirements of ASME Code section XI, Appendix G,
provides an acceptable alternate methodology that will continue to meet
the underlying purpose of 10 CFR 50.60 and Appendix G to 10 CFR part
50. The underlying purpose of the regulations in 10 CFR 50.60 and
Appendix G to 10 CFR part 50 is to provide an acceptable margin of
safety against brittle failure of the RCS during any condition of
normal operation to which the pressure boundary may be subjected over
its service lifetime.
The NRC staff examined the licensee's rationale to support the
exemption request, and accepts the licensee's determination that an
exemption would be required to approve the use of Code Case N-641. The
NRC staff agrees that the use of ASME Code Case N-641 would meet the
underlying intent of 10 CFR 50.60 and Appendix G to 10 CFR part 50. The
NRC staff concludes that the application of the technical provisions of
ASME Code Case N-641 provides sufficient margin in the development of
RPV P/T limit curves such that the underlying purpose of the
regulations (10 CFR 50.60 and Appendix G to 10 CFR part 50) continues
to be met and that the specific conditions required by the regulations
(i.e., use of all provisions in Appendix G to section XI of the ASME
Code) were not necessary. Therefore, the NRC staff concludes that the
exemption requested by the licensee is justified based on the special
circumstances of 10 CFR part 50(a)(2)(ii), ``[a]pplication 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.''
Based upon a consideration of the conservatism that is explicitly
incorporated into the methodologies of Appendix G to 10 CFR part 50;
Appendix G to section XI of the ASME Code; and Regulatory Guide 1.99,
Revision 2; the NRC staff concludes that application of ASME Code Case
N-641, as described, will provide an adequate margin of safety against
brittle failure of the RPV. This conclusion is also consistent with the
determination that the NRC staff has reached for other licensees under
similar conditions based on the same considerations. Therefore, the NRC
staff concludes that granting the exemption under the special
circumstances of 10 CFR 50.12(a)(2)(ii) is appropriate, and that the
methodology of Code Case N-641 may be used to revise the P/T limits for
the McGuire, Unit 1 and 2 RPVs.
4.0 Conclusion
Accordingly, the Commission has determined that, pursuant to 10 CFR
50.12(a), the exemption is authorized by law, will not present an undue
risk to the public health and safety, and is consistent with the common
defense and security. Also, special circumstances are present.
Therefore, the Commission hereby grants the licensee an exemption from
the requirements of 10 CFR 50.60(a), and 10 CFR part 50, Appendix G,
section IV.A.2.b, to allow application of ASME Code Case N-641 in
establishing TS requirements for the RPV limits for McGuire, Units 1
and 2.
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 (68 FR 31735).
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This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 3rd day of July 2003.
For the Nuclear Regulatory Commission.
Ledyard B. Marsh,
Acting Director, Division of Licensing Project Management, Office of
Nuclear Reactor Regulation.
[FR Doc. 03-17580 Filed 7-10-03; 8:45 am]
BILLING CODE 7590-01-P