[Federal Register Volume 68, Number 59 (Thursday, March 27, 2003)]
[Notices]
[Pages 15006-15007]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-7340]



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NUCLEAR REGULATORY COMMISSION

[Docket No. 50-316]


Indiana Michigan Power Co., Donald C. Cook Nuclear Plant, Unit 2; 
Exemption

1.0 Background

    Indiana Michigan Power Company (the licensee) is the holder of 
Facility Operating License No. DPR-74 which authorizes operation of the 
Donald C. Cook (D. C. Cook) Nuclear Plant, Unit 2. The licensee 
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 
Stevensville, Michigan.

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 * * 
* the pressure-temperature limits and 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 D. C. Cook Unit 
2 Technical Specification (TS) P-T limit curves, the licensee requested 
in its submittal dated July 23, 2002, that the NRC staff exempt D. C. 
Cook Unit 2 from application of specific requirements of 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 D. C. Cook 
Unit 2 TS amendment that was contained in the same submittal. The 
proposed D. C. Cook Unit 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 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 D. C. Cook 2 RPV.
    The proposed TS amendment to revise the P-T limits for D. C. Cook 
Unit 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 D. C. Cook Unit 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 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 information 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 flaw size 
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 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 
inadvertent 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 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. 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.''

[[Page 15007]]

    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 D. C. Cook Unit 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 D. C. Cook Unit 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 
methodology 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 agrees within the licensee's determination that 
an exemption would be required to approve the use of Code Case N-641. 
The NRC staff agree that the use of ASME Code Case N-641 would meet the 
underlying intent of Appendix G to 10 CFR part 50. The NRC staff 
concludes 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) continued 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. 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 staff concludes 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 NRC 
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 D. C. Cook Unit 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 and security. Also, special circumstances are present. 
Therefore, the Commission hereby grants the licensee 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 D. C. Cook Unit 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 13336).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 19th day of March 2003.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 03-7340 Filed 3-26-03; 8:45 am]
BILLING CODE 7590-01-P