[Federal Register Volume 65, Number 215 (Monday, November 6, 2000)]
[Notices]
[Pages 66570-66572]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 00-28358]


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NUCLEAR REGULATORY COMMISSION

[Docket No. 50-461]


In the Matter of Amergen Energy Company, LLC (Clinton Power 
Station); Exemption

I

    AmerGen Energy Company, LLC (AmerGen, the licensee) is the holder 
of Facility Operating License No. NPF-62 which authorizes operation of 
the Clinton Power Station (CPS). The license provides, among other 
things, that the facility is subject to all rules, regulations, and 
orders of the U.S. Nuclear Regulatory Commission (the Commission) now 
or hereafter in effect.
    The facility consists of a boiling water reactor located on the 
licensee's CPS site in DeWitt County, Illinois.

II

    The U.S. Nuclear Regulatory Commission (NRC) has established 
requirements in Appendix G of Part 50 to Title 10, Code of Federal 
Regulations (10 CFR Part 50, Appendix G), to protect the integrity of 
the reactor coolant pressure boundary in nuclear power plants. This 
Appendix to Part 50 requires the pressure-temperature (P-T) limits for 
an operating plant to be at least as conservative as those that would 
be generated if the methods of Appendix G to Section XI of the American 
Society of Mechanical Engineers Boiler and Pressure Vessel Code 
(Appendix G to the Code) were applied. The methodology of Appendix G to 
the Code postulates the existence of a sharp surface flaw in the 
reactor pressure vessel (RPV) that is normal to the direction of the 
maximum applied stress. For materials in the beltline and upper and 
lower head regions of the RPV, the maximum flaw size is postulated to 
have a depth that is equal to one-fourth of the thickness and a length 
equal to 1.5 times the thickness. For the case of evaluating RPV 
nozzles, the surface flaw is postulated to propagate parallel to the 
axis of the nozzle's corner radius. The basic parameter in Appendix G 
to the Code for calculating P-T limit curves is the stress intensity 
factor, K<INF>l</INF>, which is a function of the stress state and flaw 
configuration. The methodology requires that licensees determine the 
reference stress intensity (K<INF>la</INF>) factors, which vary as a 
function of temperature, from the reactor coolant system (RCS) 
operating temperatures, and from the adjusted reference temperatures 
(ARTs) for the limiting materials in the RPV. Thus, the critical 
locations in the RPV beltline and head regions are the \1/4\-thickness 
(\1/4\T) and \3/4\-thickness (\3/4\T) locations, which correspond to 
the points of the crack tips if the flaws are initiated and grown from 
the inside and outside surfaces of the vessel, respectively. Regulatory 
Guide (RG) 1.99, Revision 2, provides an acceptable method of 
calculating ARTs for ferritic RPV materials; the methods of RG 1.99, 
Revision 2, include methods for adjusting the ARTs of materials in the 
beltline region of the RPV, where the effects of neutron irradiation 
may induce an increased level of embrittlement in the materials.
    The methodology of Appendix G requires that P-T curves must satisfy 
a safety factor of 2.0 on primary membrane and bending stresses during 
normal plant operations (including heatups, cooldowns, and transient 
operating conditions), and a safety factor of 1.5 on primary membrane 
and bending stresses when leak rate or hydrostatic pressure tests are 
performed on the RCS. Table 1 to 10 CFR Part 50, Appendix G, provides 
the staff's criteria for meeting the P-T limit requirements of Appendix 
G to the Code and 10 CFR Part 50, Appendix G.
    By letter dated August 25, 2000, as supplemented September 21, 
October 14, and October 25, 2000, AmerGen submitted a license amendment 
request to update the P-T limit curves for CPS. In the submittals, 
AmerGen also requested NRC approval for exemptions to use Code Cases N-
588 and N-640 as methods that would allow AmerGen to deviate from 
complying with the requirements in 10 CFR Part 50, Appendix G, for 
generating the P-T limit curves.

Code Case N-588

    AmerGen has requested, pursuant to 10 CFR 50.60(b), an exemption to 
use Code Case N-588 as the basis for evaluating the axial and 
circumferential welds in the CPS RPV. The current methods of appendix G 
to the Code mandate consideration of an axial flaw in full penetration 
RPV welds, and thus, for circumferential welds, dictate that the flaw 
be oriented transverse to the axis of the weld. Postulation of an axial 
flaw in a circumferential weld is unrealistic because the length of the 
flaw would extend well beyond the girth of the circumferential weld and 
into the adjoining base metal material. Industry experience with the 
repair of weld indications found during preservice inspection, and data 
taken from destructive examination of actual vessel welds, confirms 
that any remaining 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 RPV 
welds, the methods of the Code Case therefore postulate the presence of 
a flaw that is oriented in a direction parallel to the axis of the weld 
(i.e., in a circumferential orientation).
    An analysis provided to the American Society of Mechanical 
Engineers (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 
correction factors for

[[Page 66571]]

membrane stress (M<INF>m</INF>) 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.\1\ Appendix G to the Code 
only requires that a safety factor of 2 be placed 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 correction factor, the margin of 2 is maintained for the 
stress integrity calculation for the circumferential weld. 
Consequently, the staff determined that the postulation of an axially-
oriented flaw on a circumferential RPV weld adds a level of 
conservatism in the P-T limits that goes beyond the margins of safety 
required by 10 CFR Part 50, Appendix G, and by Appendix of the Code. 
For this reason, the methods of the Code Case reduce the applied stress 
intensities for primary membrane and bending stresses in 
circumferential flaws by a factor of approximately 2 (<nearly-eq>0.926/
0.443).\2\ This is realistic since the postulated circumferential flaw 
in the vessel will propagate if a stress is applied in a direction 
normal to the axis of the flaw (i.e., by application of an axially 
oriented stress that results in Mode I crack propagation of the 
circumferential flaw). Such tensile stresses in the RPVs are typically 
about half the magnitudes of the corresponding membrane stresses.
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    \1\ The margin of safety of 4.18 is arrived at by dividing 0.926 
by 0.443 and then multiplying by the required safety factor of 2.
    \2\ The Code Case accomplishes this by reducing the 
M<INF>m</INF> factors for circumferential welds that are used for 
calculations of the stress intensities attributed to primary 
membrane stresses (K<INF>lm</INF>) and primary bending stresses 
(K<INF>lb</INF>). As stated previously, for RPVs with wall 
thicknesses in the range of 4.0-12.0 inches, the M<INF>m</INF> 
factor for circumferential welds is 0.443. This is the normal wall 
thickness range for GE designed boiling water reactors.
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    Application of Code Case N-588 will only matter if the Code Case is 
applied for the case where a circumferential weld is the most limiting 
material in the beltline region of the boiling water reactor (BWR) 
designed RPV. Since application of the Code Case methods allow 
licensees to reduce the stress intensities attributed to the 
circumferential weld, the net effect of the Code Case would allow 
AmerGen to use the next most limiting base metal or axial weld material 
in the RPV as the basis for evaluating the vessel and generating the P-
T limit curves, if the circumferential weld (girth weld) is the most 
limiting material in the beltline region of the vessel. In this case, 
the Code Case is relevant to the evaluation of the CPS RPV, because the 
CPS RPV is limited by Circumferential Weld AE (Material Heat 76492).\3\
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    \3\ The most limiting \1/4\T material for the generation of the 
CPS P-T limits is Circumferential Weld AE (Material Heat 76492). 
According to the AmerGen submittal of August 25, 2000, this weld has 
a \1/4\T RT<INF>NDT</INF> value at 32 EFPY of 55 deg.F. Application 
of Code Case N-588 will change the basis for evaluating the vessel 
to the next most limiting plate or vertical weld material, which 
according to AmerGen is material heat 3P4955 (used to fabricate 
vertical welds BE, BF, and BG, which according to AmerGen have a \1/
4\T RT<INF>NDT</INF> value at 32 EFPY of 51 deg.F).
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    WGOPC has concluded that application of Code Case N-588 to plant P-
T limits are still sufficient to ensure the structural integrity of 
RPVs during plant operations. The staff has concurred with WGOPC's 
determination and has previously granted exemptions to use Code Case N-
588 for the Quad Cities Nuclear Power Station (NRC letter to 
Commonwealth Edison dated February 4, 2000). In the staff's letter of 
February 4, 2000, the staff concluded that the procedure in Appendix G 
to the Code was developed for axially oriented flaws and that such a 
procedure was physically unrealistic and overly conservative for 
postulating flaws of this orientation in a circumferential weld. The 
staff also concluded that relaxation of the requirements of Appendix G 
to the Code by application of 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 for the Quad Cities RPVs and reactor coolant pressure. AmerGen's 
proposal to use Code N-588 for generation of the CPS P-T limit curves 
is predicated on the same technical basis as was used for generation of 
the Quad Cities P-T limits. The staff therefore concludes that Code 
Case N-588 is acceptable for application to the CPS P-T limits. Hence, 
the staff concurs that relaxation of the ASME Section XI, Appendix G, 
requirements by application of ASME Code Case N-588 is acceptable for 
CPS 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.

Code Case N-640

    AmerGen has requested, pursuant to 10 CFR 50.60(b), an exemption to 
use ASME Code Case N-640 (previously designated as Code Case N-626) as 
the basis for establishing the P-T limit curves. Code Case N-640 
permits application of the lower bound static initiation fracture 
toughness value equation (K<INF>lc</INF> equation) as the basis for 
establishing the curves in lieu of using the lower bound crack arrest 
fracture toughness value equation (i.e., the K<INF>la</INF> equation, 
which is based on conditions needed to arrest a dynamically propagating 
crack, and which is the method invoked by Appendix G to Section XI of 
the ASME Code). Use of the K<INF>lc</INF> equation in determining the 
lower bound fracture toughness in the development of the P-T operating 
limits curve is more technically correct than the use of the 
K<INF>la</INF> equation 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 K<INF>lc</INF> equation appropriately 
implements the use of the 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 
K<INF>la</INF> equation since 1974 when the equation 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. Therefore, the lower bound static fracture toughness 
K<INF>lc</INF> equation provides an acceptable method for calculating 
P-T limits. In addition, P-T curves based on the K<INF>lc</INF> 
equation will enhance overall plant safety by opening the P-T operating 
window with the greatest safety benefit in the region of low 
temperature operations.
    Generating the RCS P-T limit curves developed in accordance with 
Appendix G to the Code, without the relief provided by ASME Code Case 
N-640, would unnecessarily require the RPV to be maintained at a 
temperature exceeding 212  deg.F during the pressure test. 
Consequently, steam vapor hazards would continue to be one of the 
safety concerns for personnel conducting inspections in primary 
containment. Implementation of the proposed curves, as allowed by ASME 
Code Case N-640, provides an adequate margin of safety and would 
eliminate steam vapor hazards by allowing inspections in primary 
containment to be conducted at a lower coolant temperature. Thus, 
pursuant to 10 CFR 50.12(a)(2)(ii), the underlying purpose of the 
regulation will continue to be served.
    WGOPC has concluded that application of Code Case N-640 to plant P-
T limits are still sufficient to ensure the structural integrity of 
RPVs during plant operations. The staff has concurred with ASME's 
determination and has previously granted exemptions to use Code Case N-
640 for the Quad

[[Page 66572]]

Cities Nuclear Power Station (NRC letter to Commonwealth Edison dated 
February 4, 2000). In the letter of February 4, 2000, the staff 
concluded that application of Code Case N-640 would not significantly 
reduce the safety margins required by 10 CFR part 50, Appendix G, and 
would eliminate steam vapor hazards by allowing inspections in the 
primary containment to be conducted at a lower coolant temperature. The 
staff also concluded that relaxation of the requirements of Appendix G 
to the Code by application of Code Case N-640 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 for the Quad Cities RPVs and reactor coolant pressure boundary. 
AmerGen's proposal to use Code N-640 for generation of the CPS P-T 
limit curves is predicated on the same technical basis as was used for 
generation of the Quad Cities P-T limits. The staff therefore concludes 
that Code Case N-640 is acceptable for application to the CPS P-T 
limits. Hence, the staff concurs that relaxation of the ASME Section 
XI, Appendix G, requirements by application of ASME Code Case N-640 is 
acceptable for CPS 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.

III

    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. The staff accepts the 
licensee's determination that the 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 requests 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; Appendix G of the Code; and Regulatory Guide 1.99, Revision 
2, the staff concludes that application of the code cases 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 
exemption under the special circumstances of 10 CFR 50.12(a)(2)(ii) is 
appropriate and that the methodology of Code Cases N-588 and N-640 may 
be used to revise the P-T limits for Clinton Power Station.

IV

    Accordingly, the Commission has determined that, pursuant to 10 CFR 
50.12(a), the exemption is authorized by law, will not endanger life or 
property or common defense and security, and is, otherwise, in the 
public interest. Therefore, the Commission hereby grants AmerGen Energy 
Company, LLC, exemption from the requirements of 10 CFR Part 50, 
Section 50.60(a) and 10 CFR Part 50, Appendix G, for Clinton Power 
Station.
    Pursuant to 10 CFR 51.32, an environmental assessment and finding 
of no significant impact has been prepared and published in the Federal 
Register (65 FR 61204). Accordingly, based upon the environmental 
assessment, the Commission has determined that the granting of this 
exemption will not result in any significant effect on the quality of 
the human environment.
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 30th day of October 2000.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing, Project Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 00-28358 Filed 11-3-00; 8:45 am]
BILLING CODE 7590-01-P