[Federal Register Volume 67, Number 76 (Friday, April 19, 2002)]
[Notices]
[Pages 19456-19458]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 02-9621]


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NUCLEAR REGULATORY COMMISSION

[Docket No. 50-368]


Entergy Operations, Inc.; Arkansas Nuclear One, Unit 2; Exemption

1.0  Background

    Entergy Operations, Inc. (the licensee) is the holder of Facility 
Operating License No. NPF-6 which authorizes operation of the Arkansas 
Nuclear One, Unit 2 (ANO-2) nuclear power plant. 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 a pressurized water reactor located in 
Pope County, Arkansas.

2.0  Request/Action

    Title 10 of the Code of Federal Regulations (10 CFR), part 50, 
Appendix G requires that pressure-temperature (P-T) limits be 
established for reactor pressure vessels (RPVs) during normal operating 
and hydrostatic or leak rate testing conditions. Specifically, Appendix 
G to 10 CFR Part 50 states that ``[t]he appropriate requirements on 
both the pressure-temperature limits and the minimum permissible 
temperature must be met for all conditions.'' Further, Appendix G of 10 
CFR Part 50 specifies that the requirements for these limits are based 
on the application of evaluation procedures given in Appendix G to 
Section XI of the American Society of Mechanical Engineers (ASME) 
Boiler and Pressure Vessel Code (Code). In this exemption, consistent 
with the current provisions of 10 CFR 50.55(a), all references to the 
ASME Code denote the 1995 Edition through the 1996 Addenda of the ASME 
Code.
    In order to address provisions of amendments to the ANO-2 Technical 
Specification (TS) P-T limit curves, the licensee requested in its 
submittal dated October 30, 2001, that the staff exempt ANO-2 from 
application of specific requirements of Appendix G to 10 CFR Part 50, 
and substitute 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 ANO-2 TS amendment that 
was contained in the same submittal. The proposed ANO-2 TS amendment 
will revise the P-T limits for heatup, cooldown, and inservice test 
limitations for the reactor coolant system (RCS) through 32 effective 
full power years of operation.

Code Case N-641

    The licensee has proposed an exemption to allow 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 P-T limits for 
the ANO-2 RPV.
    The proposed TS amendment to revise the P-T limits for ANO-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 ANO-2 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 which 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 more technically 
correct than use of the KIA curve. 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

[[Page 19457]]

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 which demonstrates that 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 opening the P-T operating window with the greatest 
safety benefit 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 
inadvertant overpressure protection system actuation due to 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 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. Therefore, the licensee concluded 
that these considerations were special circumstances pursuant to 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 to 
permit the licensee to utilize the provisions of ASME Code Case N-641 
for the purpose of developing ANO-2 RPV P-T limit curves.

3.0  Dicsussion

    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 ANO-2 with the 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 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 alternative evaluational procedure 
which will continue to meet the underlying purpose of Appendix G to 10 
CFR part 50. The underlying purpose of the regulations in 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 
staff finds that the use of ASME Code Case N-641 would meet the 
underlying intent of Appendix G to 10 CFR part 50. Therefore, 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) continue to be met such 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. The NRC staff further concluded that the exemption requested 
by the licensee is justified based on the special circumstances of 10 
CFR 50(a)(2)(ii), that ``[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 staff concluded that application of ASME Code Case N-
641, as described, would provide an adequate margin of safety against 
brittle failure of the RPV. This is also consistent with the 
determination that the staff has reached for other licensees under 
similar conditions, based on the same considerations. Therefore, the 
staff concludes that requesting 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 ANO-2 RPV.

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

[[Page 19458]]

and security. Also, special circumstances are present. Therefore, the 
Commission hereby grants Entergy Operations, Inc. an exemption from the 
requirements of 10 CFR 50.60 and 10 CFR part 50, Appendix G, to allow 
application of ASME Code Case N-641 in establishing TS requirements for 
the reactor vessel pressure limits at low temperatures for ANO-2.
    Pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental 
assessment and finding of no significant impact was published in the 
Federal Register on April 8, 2002 (67 FR 16769). Accordingly, based 
upon the environmental assessment, the Commission has determined that 
the granting of this exemption will not have a significant effect on 
the quality of the human environment.
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 15th day of April 2002.

    For the Nuclear Regulatory Commission
Ledyard B. Marsh,
Acting Director, Division of Licensing Project Management, Office of 
Nuclear Reactor Regulation.
[FR Doc. 02-9621 Filed 4-18-02; 8:45 am]
BILLING CODE 7590-01-P