[Federal Register Volume 66, Number 162 (Tuesday, August 21, 2001)]
[Notices]
[Pages 43918-43920]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-20991]


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NUCLEAR REGULATORY COMMISSION

[Docket Nos. STN 50-454, STN 50-455, STN 50-456 AND STN 50-457]


Exelon Generation Company, LLC, Byron Station, Units 1 and 2, 
Braidwood Station, Units 1 and 2; Exemption

1.0 Background

    The Exelon Generation Company, LLC, (the licensee) is the holder of 
Facility Operating License Nos. NPF-37, NPF-66, NPF-72 and NPF-77, 
which authorize operation of the Byron Station, Units 1 and 2, and 
Braidwood Station, Units 1 and 2. The licenses provide, among other 
things, that the facilities are subject to all rules, regulations, and 
orders of the U.S. Nuclear Regulatory Commission (NRC, the Commission) 
now or hereafter in effect.
    Each of the above facilities consists of two pressurized water 
reactors. The Byron units are located in Ogle County in Illinois and 
the Braidwood units are located in Will County in Illinois. This 
exemption refers to all four units.

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, 10 CFR 
part 50, appendix G states that ``The appropriate requirements on both 
the pressure-temperature limits and minimum permissible temperature 
must be met for all conditions.'' Appendix G of 10 CFR part 50 
specifies that the requirements for these limits are the American 
Society of Mechanical Engineers (ASME) Code, Section XI, Appendix G 
Limits.
    To address provisions of amendments to the technical specifications 
(TS) P-T limits in the uprating submittal, the licensee requested in 
its supplement dated December 8, 2000, to its original submittal for 
power uprates for Byron and Braidwood, dated July 5, 2000, that the 
staff exempt Byron and Braidwood units from application of specific 
requirements of 10 CFR part 50, Sec. 50.60(a) and appendix G, and 
substitute use of ASME Code Cases N-588 and N-640. This request, to 
apply these code cases to the proposed P-T limits, was later withdrawn 
by the licensee for application with the power uprate in a letter dated 
February 20, 2001. However, the licensee requested the NRC to complete 
its review of the exemption request for future P-T limit applications. 
Code Case N-588 permits the postulation of a circumferentially-oriented 
flaw (in lieu of an axially-oriented flaw) for the evaluation of the 
circumferential welds in RPV P-T limit curves, whereas, Code Case N-640 
permits the use of alternate reference fracture toughness 
(KIC fracture toughness curve instead of KIa 
fracture toughness curve) for reactor vessel materials in determining 
the P-T limits. Since the pressure stresses on a circumferentially-
oriented flaw are lower than the pressure stresses on an axially-
oriented flaw by a factor of 2, postulating a circumferentially-
oriented flaw for the evaluation of the circumferential welds, as 
permitted by Code Case N-588, in establishing the P-T limits would be 
less conservative than the methodology currently endorsed by 10 CFR 
Part 50, Appendix G. Further, since the KIC fracture 
toughness curve shown in ASME Section XI, Appendix A, Figure A-2200-1 
provides greater allowable fracture toughness than the

[[Page 43919]]

corresponding KIa fracture toughness curve of ASME Section 
XI, Appendix G, Figure G-2210-1, using the KIC fracture 
toughness, as permitted by Code Case N-640, in establishing the P-T 
limits would be less conservative than the methodology currently 
endorsed by 10 CFR part 50, appendix G. Considering both, an exemption 
to apply the Code Cases would be required by 10 CFR 50.60.

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 
when (2) special circumstances are present.

Postulations of Circumferential Flaws in Circumferential Welds (Code 
Case N-588)

    The licensee proposed to revise future P-T limits in the pressure 
temperature limits report (PTLR) for Byron and Braidwood units using 
the postulation of a circumferentially-oriented reference flaw as the 
limiting flaw in an RPV circumferential weld in lieu of an axially-
oriented flaw required by the 1995 Edition (1996 Addenda) of ASME 
Section XI, Appendix G.
    Postulating the Appendix G reference flaw (an axially-oriented 
flaw) in a circumferential weld is physically unrealistic and overly 
conservative because the length of the flaw is 1.5 times the vessel 
thickness, which is much longer than the width of the reactor vessel 
girth weld. Industry experience with the repair of weld indications 
found during preservice inspection and data taken from destructive 
examination of actual vessel welds, confirms that all detected flaws 
are small, laminar in nature, and do not transverse the weld bead 
orientation. Therefore, any potential defects introduced during the 
fabrication process and not detected during subsequent nondestructive 
examinations, would only be expected to be oriented in the direction of 
weld fabrication. For circumferential welds, this indicates a 
postulated defect with a circumferential orientation.
    An analysis provided to the ASME Code's Working Group on Operating 
Plant Criteria (WGOPC) (in which Code Case N-588 was developed) 
indicated that if an axial flaw is postulated on a circumferential 
weld, then based on the stress magnification factors (Mm) 
given in the Code Case for the inside diameter circumferential (0.443) 
and axial (0.926) flaw orientations, it is equivalent to applying a 
safety factor of 4.18 on the pressure loading under normal operating 
conditions. Appendix G requires a safety factor of 2 on the 
contribution of the pressure load in the case of an axially-oriented 
flaw in an axial weld, shell plate, or forging. By postulating a 
circumferentially-oriented flaw on a circumferential weld and using the 
appropriate stress magnification factor, the margin of 2 is maintained 
for the contribution of the pressure load to the integrity calculation 
of the circumferential weld. Consequently, the staff determined that 
the postulation of an axially-oriented flaw on a circumferential RPV 
weld is a level of conservatism that is not required to establish P-T 
limits to protect the RCS pressure boundary from failure during 
hydrostatic testing, heatup, and cooldown.
    In summary, the ASME Section XI, Appendix G, procedure was 
developed for axially-oriented flaws, which is physically unrealistic 
and overly conservative for postulating flaws of this orientation to 
exist in circumferential welds. Hence, the NRC staff concurs that 
relaxation of the ASME Section XI, Appendix G, requirement by 
postulating a circumferentially-oriented flaw for the evaluation of the 
circumferential welds, as permitted by Code Case N-588, is acceptable 
and would maintain, 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.

Using the KIc Fracture Toughness Curve (Code Case N-640)

    The licensee proposed to revise future P-T limits in the PTLR for 
Byron and Braidwood units using the KIc fracture toughness 
curve, in lieu of the KIa fracture toughness curve, as the 
lower bound for fracture toughness.
    Use of the KIc curve in determining the lower bound 
fracture toughness in the development of P-T operating limits curve is 
more technically correct than the KIa curve since the rate 
of loading during a heatup or cooldown is slow and is more 
representative of a static condition than a dynamic condition. The 
KIc curve appropriately implements the use of static 
initiation fracture toughness behavior to evaluate the controlled 
heatup and cooldown process of a reactor vessel. The staff has required 
use of the initial conservatism of the KIa curve since 1974 
when the curve was codified. This initial conservatism was necessary 
due to the limited knowledge of RPV materials. Since 1974, additional 
knowledge has been gained about RPV materials, which demonstrates that 
the lower bound on fracture toughness provided by the KIa 
curve is well beyond the margin of safety required to protect the 
public health and safety from potential RPV failure. In addition, P-T 
curves based on the KIc curve will enhance overall plant 
safety by opening the P-T operating window with the greatest safety 
benefit in the region of low temperature operations.
    In summary, the ASME Section XI, Appendix G, procedure was 
conservatively developed based on the level of knowledge existing in 
1974 concerning RPV materials and the estimated effects of operation. 
Since 1974, the level of knowledge about these topics has been greatly 
expanded. The NRC staff concurs that this increased knowledge permits 
relaxation of the ASME Section XI, Appendix G, requirements by applying 
the KIc fracture toughness, as permitted by Code Case N-640, 
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.
    The staff concurs with the licensee's determination that an 
exemption would be required to approve the use of Code Cases N-588 and 
N-640. The staff examined the licensee's rationale to support the 
exemption request and concurred that the use of the Code Cases would 
meet the underlying intent of these regulations. Based upon a 
consideration of the conservatism that is explicitly incorporated into 
the methodologies of 10 CFR part 50, appendix G, of the ASME Code; and 
Regulatory Guide 1.99, Revision 2, the staff concludes that application 
of Code Cases N-588 and N-640, 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 pursuant to 10 CFR 50.12(a)(2)(ii) 
special circumstances are present and that an exemption may be granted 
to allow use of the methodology of Code Cases N-588 and N-640 to revise 
future P-T limits in the PTLR for Byron and Braidwood units.

4.0 Conclusion

    Accordingly, the Commission has determined that, pursuant to 10 CFR 
50.12(a), the exemption as authorized by law, will not endanger life or 
property or common defense and security, and is, otherwise, in the 
public interest.

[[Page 43920]]

Therefore, the Commission hereby grants Exelon Generation Company, LLC, 
exemption from the requirements of 10 CFR part 50, Sec. 50.60(a) and 10 
CFR part 50, appendix G, for Byron Units 1 and 2 and Braidwood 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. The environmental assessment is 
published in the Federal Register (66 FR 38755).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 8th day of August 2001.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 01-20991 Filed 8-20-01; 8:45 am]
BILLING CODE 7590-01-P