[Federal Register Volume 65, Number 179 (Thursday, September 14, 2000)]
[Notices]
[Pages 55646-55650]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 00-23610]


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NUCLEAR REGULATORY COMMISSION

[Docket No. 50-352]


PECO Energy Company (Limerick Generating Station, Unit 1); 
Exemption

I

    The PECO Energy Company (PECO, the licensee) is the holder of 
Facility Operating License No. NPF-39 which authorizes operation of the 
Limerick Generating Station, Unit 1 (Limerick Unit 1). The license 
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 boiling water reactor located in 
Montgomery and Chester Counties in Pennsylvania.

II

    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) for normal operating 
and hydrostatic or leak rate testing conditions. Specifically, 10 CFR 
Part 50, Appendix G states, ``The appropriate requirements on both the 
pressure-temperature limits and the minimum permissible temperature 
must be met for all conditions.'' Appendix G of 10 CFR Part 50 
specifies that the P-T limits identified as ``ASME Appendix G limits'' 
in Table 1 require that the limits must be at least as conservative as 
the limits obtained by following the methods of analysis and the 
margins of safety of Appendix G of Section XI of the ASME Code.
    To address provisions of a proposed license amendment to the 
technical specification P-T limits for the Limerick facility, the 
licensee requested in its submittal of May 15, 2000, as supplemented by 
May 19 and August 10, 2000, that the staff exempt Limerick Unit 1 from 
application of specific requirements of 10 CFR Part 50, Section 
50.60(a) and Appendix G, and substitute use of ASME Code Cases N-588 
and N-640. 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. 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, using Code Case N-588 for establishing the P-T limits 
would be less conservative than the methodology currently endorsed by 
10 CFR Part 50, Appendix G, and, therefore, an exemption to apply the 
Code Case would be required by 10 CFR 50.60. Code Case N-640 permits 
the use of an 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 KIc fracture toughness curve shown in ASME Section 
XI, Appendix A, Figure A-2200-1 (the KIc fracture toughness 
curve, KIc curve) provides greater allowable fracture 
toughness than the corresponding KIa fracture toughness 
curve of ASME Section XI, Appendix G, Figure G-2210-1 (the 
KIa fracture toughness curve, KIa curve), using 
Code Case N-640 for establishing the P-T limits would be less 
conservative than the methodology currently endorsed by 10 CFR Part 50, 
Appendix G, and, therefore, an exemption to apply the Code Case would 
also be required by 10 CFR 50.60.

Code Case N-588

    The licensee has proposed an exemption to allow the use of ASME 
Code Case N-588 in conjunction with ASME Section XI, 10 CFR 50.60(a) 
and 10 CFR Part 50, Appendix G, to determine the P-T limits.
    The proposed license amendment to revise the P-T limits for 
Limerick Unit 1 relies, in part, on the requested exemption. These 
proposed P-T limits have been developed 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 1989 Edition of ASME Section XI, Appendix G.
    Postulating the Appendix G [axially-oriented flaw] reference flaw 
in a circumferential weld is physically unrealistic and overly 
conservative, 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 confirm that any remaining flaws are 
small, laminar in nature, and do not transverse the weld bead 
orientation.

[[Page 55647]]

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. A defect with a circumferential orientation is therefore 
postulated by the ASME Code for circumferential welds.
    An analysis provided to the ASME Code's Working Group on Operating 
Plant Criteria (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 membrane stress (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 NRC 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 reactor coolant system (RCS) pressure boundary from 
failure during hydrostatic testing, heatup, and cooldown.
    The NRC staff noted that ASME Code Case N-588 also includes changes 
to the methodology for determining the thermal stress intensity, 
KIT. The staff already accepted the use of Code Case N-588, 
including the modifications made to the KIT methodology, for 
exemption requests from other licensees. Hence, the licensee may use 
the methodology in the edition of ASME Section XI of record, or later 
approved Editions of Section XI through the 1995 Edition, inclusive of 
the 1996 Addenda, or the methodology contained in Code Case N-588 for 
determining KIT.
    In summary, the ASME Section XI, Appendix G, procedure was 
developed for axially-oriented flaws. It is physically unrealistic and 
overly conservative to postulate flaws of this orientation to exist in 
circumferential welds. Hence, the NRC staff concurs that relaxation of 
the ASME Section XI, Appendix G, requirements by application of ASME 
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.

Code Case N-640 (formerly Code Case N-626)

    The licensee has proposed an exemption to allow use of ASME Code 
Case N-640 in conjunction with ASME Section XI, 10 CFR 50.60(a) and 10 
CFR Part 50, Appendix G, to determine P-T limits.
    The proposed license amendment to revise the P-T limits for 
Limerick Unit 1 relies in part on the requested exemption. These 
revised P-T limits have been developed 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 use of 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 NRC 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.
    Since the RCS P-T operating window is defined by the P-T operating 
and test limit curves developed in accordance with ASME Section XI, 
Appendix G, continued operation of Limerick Unit 1 with these P-T 
curves without the relief provided by ASME Code Case N-640 would 
unnecessarily require that the RPV maintain a temperature exceeding 212 
 deg.F in a limited operating window during pressure tests. 
Consequently, steam vapor hazards would continue to be one of the 
safety concerns for personnel conducting inspections in primary 
containment. Implementation of the proposed P-T curves, as allowed by 
ASME Code Case N-640, does not significantly reduce the 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.
    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 
application of ASME 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.

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. These circumstances include 
the special circumstances that ``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; 
* * *'' 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 purpose of 10 CFR Part 50, Appendix G, and 
therefore, application of the assumed flaw types and the KIa 
equation in Appendix G to Section XI of the ASME Code, as invoked by 
the rule, is not necessary to meet the underlying purpose of the 
regulation and thus meets the special circumstance criterion of 10 CFR 
50.12(a)(2)(ii) for granting the exemption requests. 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 ASME 
Code; and Regulatory Guide 1.99, Revision 2; the staff concludes that

[[Page 55648]]

application of the code cases as described would provide an adequate 
margin of safety against brittle failure of the RPV. (See the attached 
safety evaluation supporting these findings.) This is also consistent 
with the determination that the staff has reached for other licensees 
under similar conditions based on the same considerations including 
Quad Cities Nuclear Power Station, Units 1 and 2, exemption dated 
February 4, 2000. Therefore, the staff concludes that granting an 
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 Limerick Unit 1.

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. Also, special circumstances are present. Therefore, 
the Commission hereby grants PECO an exemption from the requirements of 
10 CFR Part 50, Section 50.60(a) and 10 CFR Part 50, Appendix G, for 
Limerick Unit 1.
    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 (65 FR 54081).
    This exemption is effective upon issuance.

    Dated at Rockville, Maryland, this 7th day of September 2000.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear 
Reactor Regulation.

Safety Evaluation by the Office of Nuclear Reactor Regulation 
Exemption Request by the Peco Energy Company to Update the 
Pressure-Temperature Limits for the Limerick Generating Station, 
Unit 1, Facility Operating License No. NPF-39

[Docket No. 50-352]

1.0  Introduction

1.1  Requirements for Generating Pressure-Temperature (P-T) Limits for 
Nuclear Power Generation Facilities
    The U.S. Nuclear Regulatory Commission (NRC) has established 
requirements in Appendix G of Part 50 to Title 10 of the Code of 
Federal Regulations (10 CFR Part 50, Appendix G), to protect the 
integrity of the reactor coolant pressure boundary in nuclear power 
plants. The 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 ASME Code) were applied. The methodology of 
Appendix G to the ASME 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 ASME Code for calculating P-T limit curves is the 
stress intensity factor, KI, which is a function of the 
stress state and flaw configuration. The methodology requires that 
licensees determine the reference stress intensity (KIa) 
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 ASME Code and 10 CFR Part 50, Appendix G.
1.2  PECO Energy Company's Submittal of May 15, 2000
    On May 15, 2000, PECO Energy Company (PECO) submitted a license 
amendment request to update the P-T limit curves for the Limerick 
Generating Station (LGS), Unit 1 (Reference 1). In the license 
amendment request, PECO also requested NRC approval for exemptions to 
use two Code Cases, N-588 and N-640, as methods that would allow PECO 
to deviate from complying with the requirements in 10 CFR Part 50, 
Appendix G, for generating the P-T limit curves. Requests for such 
exemptions are allowed pursuant to 10 CFR 50.60(b), which allows 
licensees to use alternatives to the requirements of 10 CFR Part 50, 
Appendices G and H, if an exemption to use the alternatives is granted 
by the Commission pursuant to 10 CFR 50.12. According to 10 CFR 50.12, 
the Commission may, upon request, grant exemptions to the requirements 
of 10 CFR Part 50, if the exemptions are authorized by law, will not 
present an undue risk to the public health and safety, and are 
consistent with the common defense and security. In considering the 
exemptions, the Commission will not consider granting exemptions unless 
special circumstances are present. These special circumstances include, 
but are not limited to, the following special cases:
     Pursuant to 10 CFR 50.12(a)(2)(ii), the circumstance that 
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,
     Pursuant to 10 CFR 50.12(a)(2)(iii), the circumstance that 
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, and
     Pursuant to 10 CFR 50.12(a)(2)(vi), the circumstance that 
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
    The staff's assessment of the exemption request is given in Section 
2.0 of this safety evaluation (SE).

[[Page 55649]]

2.0  Requests for Exemptions to use Code Cases N-588 and N-640 as Part 
of the Methods Used for Generation of the Updated P-T Curves

2.1  Exemption Request To Use Code Case N-588
    PECO 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 LGS Unit 1 RPV. The current methods of 
Appendix G to the ASME 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 confirm 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 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 membrane stress 
(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.\1\ Appendix G to the ASME 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 G of Section XI of the ASME Code.
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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.
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    The Code Case method for evaluating axially-oriented flaws 
postulated in axial welds or base metal materials does not deviate from 
the methods for evaluating them in the 1995 Edition of Appendix G to 
the ASME Code. Thus, 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 designed RPV. Since application of the Code Case methods allows 
licensees to reduce the stress intensities attributed to the 
circumferential weld, the net effect of the Code Case would allow PECO 
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 really not relevant to the evaluation of the LGS Unit 1 RPV, 
because the LGS Unit 1 RPV is limited in the beltline region by Lower-
Intermediate Plate 17-2 (Heat No. C7677-1).\2\ However, the use of this 
Code Case by the licensee results in a more accurate calculation of the 
applied stress intensity factor for axial welds than would be obtained 
using Appendix G of Section XI of the ASME Code.
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    \2\ The most limiting \1/4\T material for the generation of the 
LGS Unit 1 P-T limits is Lower-Intermediate Shell Plate 17-2 
(Material Heat C7677-1). This plate has \1/4\T nil ductility 
reference temperature (RTNDT) values at 22 effective full 
power years (EFPY) and 32 EFPY of 78  deg.F and 89  deg.F, 
respectively. In contrast, the \1/4\T RTNDT values for 
the most limiting circumferential weld material (i.e, Weld Heat 
640892/J424B27AE) at 22 EFPY and 32 EFPY are considerably less 
conservative, at 37  deg.F and 54  deg.F, respectively.
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    WGOPC has concluded that application of Code Case N-588 to plant P-
T limits is 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 (Quad Cities) (i.e., NRC 
letter to Commonwealth Edison dated February 4, 2000, Reference 2). In 
the staff's letter of February 4, 2000, the staff concluded that the 
procedure in Appendix G to the ASME 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. PECO's proposal to use Code 
Case N-588 for generation of the LGS Unit 1 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 LGS Unit 1 P-T limits; 
however, since the LGS Unit 1 RPV is a plate-limited vessel, 
application of Code Case N-588 in this case will not provide PECO with 
any reduction in burden for the proposed LGS Unit 1 P-T limits.
2.2  Exemption Request To Use Code Case N-640
    PECO 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 (KIc equation) as the basis for 
establishing the curves in lieu of using the lower bound crack arrest 
fracture toughness value equation (i.e., the KIa equation, 
which is based on conditions needed to arrest a dynamically propagating 
crack, and which is the method invoked by Appendix G of Section XI of 
the ASME Code). Use of the KIc 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 
KIa 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 KIc 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 
KIa 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, which

[[Page 55650]]

demonstrates that the lower bound on fracture toughness provided by the 
KIc equation 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 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 ASME 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, does not significantly reduce the 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. However, since use of the 
KIc equation results in the calculations of less 
conservative P-T limits than does use of the KIa equation, 
licensees need staff approval to apply the Code Case methods to the P-T 
limit calculations.
    WGOPC has concluded that application of Code Case N-640 to plant P-
T limits is 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 Quad Cities (i.e., in the NRC letter to Commonwealth Edison 
dated February 4, 2000, Reference 2). In the staff's 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. PECO's proposal to 
use Code Case N-640 for generation of the LGS Unit 1 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 LGS Unit 1 P-T limits.

3.0  Conclusion

    The staff has determined that PECO has provided sufficient 
technical bases for using the methods of Code Cases N-588 and N-640 in 
the calculation of the P-T limits for LGS Unit 1. The staff has also 
determined that application of Code Case N-588 and Code Case N-640 to 
the P-T limit calculations will continue to serve the purpose in 10 CFR 
Part 50, Appendix G, for protecting the structural integrity of the LGS 
Unit 1 RPV and reactor coolant pressure boundary. In this case, since 
strict compliance with requirements of 10 CFR 50.60(a) and 10 CFR Part 
50, Appendix G, is not necessary to achieve the overall intent of the 
regulations, the staff concludes that application of the Code Cases N-
588 and N-640 to the P-T limit calculations meets the special 
circumstance provisions in 10 CFR 50.12(a)(2)(ii), for granting 
exemptions to the regulations, and that, pursuant to 10 CFR 
50.12(a)(1), the granting of these exemptions is authorized by law, 
will not present undue risk to the public health and safety, and is 
consistent with the common defense and security. The staff therefore 
grants exemptions to 10 CFR 50.60(a) and 10 CFR Part 50, Appendix G, to 
allow PECO to use Code Cases N-588 and N-640 as the part of the bases 
for generating the P-T limit curves for LGS Unit 1; however, since the 
LGS Unit 1 RPV is a plate-limited vessel, application of Code Case N-
588 in this case will not provide PECO with any relaxation in the 
burden for the generating the unit's P-T limits.

4.0  References

    1. Letter from J. A. Hutton, Director--Licensing, Limerick 
Generating Station, Unit 1, to the U.S. Nuclear Regulatory Commission, 
Document Control Desk, ``Limerick Generating Station, Unit 1, Technical 
Specifications Change Request No. 00-02-1, Changes to Reactor Pressure 
Vessel Pressure-Temperature Limits,'' dated May 15, 2000.
    2. Letter from S. N. Bailey, U.S. Nuclear Regulatory Commission, to 
O. D. Kingsley, Commonwealth Edison Company, ``Quad Cities--Exemption 
from the Requirements of 10 CFR Part 50, Section 50.60(a) and Appendix 
G,'' dated February 4, 2000.
    Principal Contributors: J. Medoff, B Buckley.
    Date: September 7, 2000.

[FR Doc. 00-23610 Filed 9-13-00; 8:45 am]
BILLING CODE 7590-01-U