Exelon Generation Company, LLC Byron Station, Unit Nos. 1 and 2; Braidwood Station, Unit Nos. 1 and 2; Exemption, 69598-69600 [E6-20319]

Agencies

• NUCLEAR REGULATORY COMMISSION

[Federal Register Volume 71, Number 231 (Friday, December 1, 2006)]
[Notices]
[Pages 69598-69600]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: E6-20319]


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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, Unit Nos. 1 and 2; 
Braidwood Station, Unit Nos. 1 and 2; Exemption

1.0 Background

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

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, ``[t]he minimum temperature requirements * 
* * pertain to the controlling material, which is either the material 
in the closure flange or the material in the beltline region with the 
highest reference temperature * * * [T]he minimum temperature 
requirements and the controlling material depend on the operating 
condition (i.e., hydrostatic pressure and leak tests, or normal 
operation including anticipated operational occurrences), the vessel 
pressure, whether fuel is in the vessel, and whether the core is 
critical. The metal temperature of the controlling material, in the 
region of the controlling material which has the least favorable 
combination of stress and temperature, must exceed the appropriate 
minimum temperature requirement for the condition and pressure of the 
vessel

[[Page 69599]]

specified in Table 1 [of 10 CFR Part 50, Appendix G].'' Footnote 2 to 
Table 1 of 10 CFR Part 50, Appendix G, specifies that RPV minimum 
temperature requirements related to RPV closure flange considerations 
shall be based on ``[t]he highest reference temperature of the material 
in the closure flange region that is highly stressed by bolt preload.''
    In order to conform to certain provisions of proposed amendments 
that would modify the Byron and Braidwood Technical Specifications (TS) 
to revise the pressure-temperature limits report (PTLR) methodology for 
each unit, EGC requested in its application dated October 3, 2005, that 
the NRC staff exempt Byron and Braidwood from the specific requirements 
of 10 CFR Part 50, Appendix G, as they pertain to the establishment of 
minimum temperature requirements, for all modes of operation addressed 
by 10 CFR Part 50, Appendix G, based on the material properties of the 
material of the RPV closure flange region that is highly stressed by 
the bolt preload. The requirements from which EGC requested that Byron 
and Braidwood be exempted shall be referred to, for the purpose of this 
exemption as, ``those requirements related to the application of 
footnote (2) to Table 1 of 10 CFR Part 50, Appendix G.''
    EGC's technical basis was submitted to the NRC by letter dated 
October 3, 2005, which included as an attachment Westinghouse Report 
WCAP-16143-P, ``Reactor Closure Head/Vessel Flange Requirements 
Evaluation for Byron/Braidwood Units 1 and 2.'' WCAP-16143-P included a 
fracture mechanics analysis of postulated flaws in the Byron and 
Braidwood RPV closure flange regions under boltup, 100 [deg]F/hr 
heatup, 100 [deg]F/hr cooldown, and steady-state conditions, with the 
heatup and cooldown transients being modeled in accordance with what 
would be permissible using P-T limit curves based on Byron and 
Braidwood beltline materials. Westinghouse performed finite element 
modeling to calculate the stresses present at critical locations within 
the flange region and determined that the 100 [deg]F/hr heatup 
transient was the most severe condition, with the upper head-to-flange 
weld being the most limiting location. With these stresses, 
Westinghouse calculated the applied stress intensity 
(KI applied) for semi-elliptical, outside diameter 
initiated, surface breaking flaws with an aspect ratio (length vs. 
depth) of 6:1, and with depths ranging from 0 to 90 percent of the 
thickness of the component wall. The KI applied values were 
calculated in accordance with the American Society of Mechanical 
Engineers Boiler and Pressure Vessel Code (ASME Code) Section XI, 
Appendix G, Subparagraph G-2220 requirements for the analysis of flange 
locations. Westinghouse then compared these KI applied 
values to ASME Code lower-bound static crack initiation fracture 
toughness (KIC) values determined from the nil-ductility 
transition reference temperature (RTNDT) values for the 
Byron and Braidwood RPV closure flange materials. Westinghouse also 
provided an assessment of the potential for changes in the material 
RTNDT values for the Byron and Braidwood RPV closure flange 
materials due to thermal aging resulting from exposure to the RPV 
operating environment.
    The use of ASME Code KIC as the material property for 
the fracture mechanics analysis represents the most significant change 
between the analysis provided in WCAP-16143-P and the analysis that was 
performed as the basis for establishing the minimum temperature 
requirements in 10 CFR Part 50, Appendix G. The minimum temperature 
requirements related to footnote 2 to Table 1 of 10 CFR Part 50, 
Appendix G were incorporated into the Code of Federal Regulations in 
the early 1980s and were based on analyses that used ASME Code lower-
bound crack arrest/dynamic test fracture toughness (KIA) as 
the parameter for characterizing a material's ability to resist crack 
initiation and propagation. The use of ASME Code KIA is 
always conservative with respect to the use of ASME Code KIC 
for fracture mechanics evaluations, and its use in the evaluations that 
established the requirements in 10 CFR Part 50, Appendix G was 
justified based on the limited knowledge of RPV material behavior that 
was available in the early 1980s. However, the use of ASME Code 
KIC is more consistent with the actual physical processes 
that would govern flaw initiation under conditions of normal RPV 
operation, including RPV heatup, cooldown, and hydrostatic and leak 
testing. Based on its current understanding of the behavior of RPV 
materials, the NRC staff has routinely approved the use of ASME Code 
KIC by licensees as the basis for evaluating RPV beltline 
materials; licensees have previously demonstrated compliance with the 
intent of 10 CFR Part 50, Appendix G through the use of the ASME Code, 
first as Code Cases N-640 and N-641, and now via ASME Code, Section XI, 
Appendix G, which has been revised to use KIC in lieu of 
KIA.
    The minimum KIC value given in the ASME Code for RPV 
steel, regardless of the material RTNDT value or 
temperature, is 33.2 ksi[radic] in. This value represents the ``lower 
shelf'' of the ASME Code KIC curve. Based on information in 
WCAP-16143-P, it is apparent that the KI applied for any 
flaw up to \1/4\ of the wall thickness (\1/4\ T) at the limiting 
location (refer to WCAP-16143-P, Figure 4-2), would not exceed 33.2 
ksi[radic] in (even taking into account the NRC staff's consideration 
of ASME Code structural factors), until between 1 and 2 hours into the 
100 [deg]F/hr heatup transient. The temperature at the tip of 
postulated flaws up to \1/4\ T size would be adequate at that time to 
ensure that the limiting Byron and Braidwood flange materials would 
exhibit fracture toughness properties in excess of ASME Code ``lower 
shelf'' behavior.
    The NRC staff has determined that the analysis provided in WCAP-
16143-P has demonstrated, for the most limiting transient addressed by 
10 CFR Part 50, Appendix G, that the combination of factors that would 
have to exist for brittle failure to occur (high stresses in the RPV 
flange region along with low temperature at the metal of the flange 
region) cannot exist simultaneously, and based on consideration of 
Byron and Braidwood's beltline materials, the structural integrity of 
the Byron and Braidwood RPV closure flange materials will not be 
challenged by facility operation in accordance with P-T limit curves. 
Therefore, the more conservative minimum temperature requirements 
related to footnote 2 to Table 1 of 10 CFR Part 50, Appendix G are not 
necessary to meet the underlying intent of 10 CFR Part 50, Appendix G, 
to protect the Byron and Braidwood RPVs from brittle failure during 
normal operation under both core critical and core non-critical 
conditions and RPV hydrostatic and leak test conditions.

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 are 
present whenever, according to 10 CFR 50.12(a)(2):

    (i) Application of the regulation in the particular 
circumstances conflicts with other rules or requirements of the 
Commission; or
    (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; or

[[Page 69600]]

    (iii) Compliance would result in 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; or
    (iv) The exemption would result in benefit to the public health 
and safety that compensates for any decrease in safety that may 
result from the grant of the exemption; or
    (v) The exemption would provide only temporary relief from the 
applicable regulation and the licensee or applicant has made good 
faith efforts to comply with the regulation; or
    (vi) There is present any other material circumstance not 
considered when the regulation was adopted for which it would be in 
the public interest to grant an exemption. If such condition is 
relied on exclusively for satisfying paragraph (a)(2) of this 
section, the exemption may not be granted until the Executive 
Director for Operations has consulted with the Commission.

    The NRC staff finds that special circumstances exist pursuant to 10 
CFR 50.12(a)(2)(ii) in that the application of the regulation is not 
necessary to achieve the underlying purpose of the rule. As stated in 
Section 2.0 above, the more conservative minimum temperature 
requirements related to footnote 2 to Table 1 of 10 CFR Part 50, 
Appendix G are not necessary to meet the underlying intent of 10 CFR 
Part 50, Appendix G, to protect the Byron and Braidwood RPVs from 
brittle failure during normal operation under both core critical and 
core non-critical conditions and RPV hydrostatic and leak test 
conditions.

Authorized by Law

    This exemption would allow the use of an alternative methodology in 
calculating the RPV P-T limits for Byron and Braidwood in lieu of 10 
CFR Part 50, Appendix G, paragraph IV.A.2.c. As stated above, 10 CFR 
50.12 allows the NRC to grant exemptions from the requirement of 10 CFR 
Part 50. Furthermore, Section 50.60(b) to 10 CFR Part 50 allows the use 
of alternatives to 10 CFR Part 50, Appendices G and H, when an 
exemption is granted by the NRC under 10 CFR 50.12. Therefore, this 
exemption is authorized by law.

No Undue Risk to Public Health and Safety

    The underlying purpose of 10 CFR Part 50, Appendix G, paragraph 
IV.A.2.c, is to maintain the appropriate fracture margin in the RPV 
closure head region.
    The proposed methodology for the Byron and Braidwood P-T limits 
relies, in part, on ASME Code, Section XI, Appendix G, which allows the 
use of the KIC fracture toughness curve rather than the 
KIA curve. P-T limits developed using the KIC 
fracture toughness curve permit a much higher allowable pressure 
through the entire range of temperatures.
    The benefit is negated at temperatures below RTNDT +120 
[deg]F because of the additional flange requirement of 10 CFR Part 50, 
Appendix G. Using the KIC fracture toughness curve, the 
analyses presented in WCAP-16143-P show that there is significant 
margin between the applied stress intensity factor at boltup and the 
material fracture toughness at cracks postulated to exist in the 
highest stress region of the closure head/flange region. The analyses 
also show that the boltup temperature requirement for Byron and 
Braidwood could be satisfied at 60 [deg]F or higher, easily justifying 
boltup at ambient temperature.
    Based on its review, the NRC staff finds that the results presented 
in WCAP-16143-P demonstrate that the 10 CFR Part 50, Appendix G RPV 
closure head flange requirement can be eliminated and appropriate 
fracture margins would still be maintained.
    Based on the above, no new accident precursors are created by using 
an alternative methodology in calculating the RPV P-T limits; thus, the 
probability of postulated accidents is not increased. Also, based on 
the above, the consequences of postulated accidents are not increased. 
Therefore, there is no undue risk to public health and safety.

Consistent with Common Defense and Security

    The proposed exemption would allow the use of an alternative 
methodology in calculating the RPV P-T limits, in lieu of 10 CFR Part 
50, Appendix G, paragraph IV.A.2.c. This change has no relation to 
security issues. Therefore, the common defense and security is not 
impacted by this exemption.

Special Circumstances

    Special circumstances, in accordance with 10 CFR 50.12(a)(2), are 
present whenever application of the regulation in the particular 
circumstances would not be necessary to achieve the underlying purpose 
of the rule. The underlying purpose of 10 CFR Part 50, Appendix G, 
paragraph IV.A.2.c is to maintain the appropriate fracture margin in 
the RPV closure head region.
    The NRC staff examined the licensee's rationale to support the 
exemption request and, based on its independent review of the 
information provided in WCAP-16143-P and in EGC's October 3, 2005, 
application, the NRC staff agrees that an acceptable technical basis 
has been established to exempt Byron and Braidwood from requirements 
related to the application of footnote 2 to Table 1 of 10 CFR Part 50, 
Appendix G. The NRC staff finds that the technical basis provided by 
EGC demonstrates that an adequate margin of safety against brittle 
failure would continue to be maintained for Byron and Braidwood RPVs 
without the application of those requirements related to the 
application of footnote 2 to Table 1 of 10 CFR Part 50, Appendix G, for 
normal operation under both core critical and core non-critical 
conditions and RPV hydrostatic and leak test conditions. The NRC staff 
concludes, pursuant to 10 CFR 50.12(a)(2)(ii), that the underlying 
purpose of 10 CFR Part 50, Appendix G will be achieved for Byron and 
Braidwood without the application of those requirements related to the 
application of footnote 2 to Table 1 of 10 CFR Part 50, Appendix G.
    Therefore, since the underlying purpose of 10 CFR Part 50, Appendix 
G is achieved, the special circumstances required by 10 CFR 50.12(a)(2) 
for the granting of an exemption from those requirements related to the 
application of footnote 2 to Table 1 of 10 CFR Part 50, Appendix G, 
exist.

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 EGC an exemption from those 
requirements related to the application of footnote 2 to Table 1 of 10 
CFR Part 50, Appendix G, for Byron and Braidwood.
    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 (71 FR 57577).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 22nd day of November 2006.

    For the Nuclear Regulatory Commission.

Catherine Haney,
Director, Division of Operating Reactor Licensing, Office of Nuclear 
Reactor Regulation.
 [FR Doc. E6-20319 Filed 11-30-06; 8:45 am]
BILLING CODE 7590-01-P