[Federal Register Volume 71, Number 231 (Friday, December 1, 2006)]
[Notices]
[Pages 69598-69600]
From the Federal Register Online via the Government Publishing 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