[Federal Register Volume 65, Number 171 (Friday, September 1, 2000)]
[Notices]
[Pages 53229-53231]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 00-22498]


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NUCLEAR REGULATORY COMMISSION

[Docket Nos. 50-237 and 50-249]


Commonwealth Edison Company (Dresden Nuclear Power Station, Units 
2 and 3); Exemption

I

    The Commonwealth Edison Company (ComEd, the licensee) is the holder 
of Facility Operating Licenses Nos. DPR-19 and DPR-25 which authorize 
operation of the Dresden Nuclear Power Station, Units 2 and 3 
(Dresden). The licenses provide, 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 boiling water reactors (Units 2 and 3) 
located on the licensee's Dresden site in Grundy County, Illinois. This 
exemption refers to both units.

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) during 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 PT limits must meet the safety margin requirements 
specified in the American Society of Mechanical Engineers (ASME) Boiler 
and Pressure Vessel Code (Code) Section XI, Appendix G.
    To address provisions of the proposed amendments to the technical 
specification (TS) P-T limits, in its submittal of February 23, 2000, 
the licensee requested that the staff exempt Dresden 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 two, 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(a). Code Case N-640 permits the use of an 
alternate reference fracture toughness (K1c fracture 
toughness curve instead of K1a fracture toughness curve) for 
reactor vessel materials in determining the P-T limits. Since the 
K1c fracture toughness curve shown in ASME Code, Section XI, 
Appendix A, Figure A-2200-1 provides greater allowable fracture 
toughness than the corresponding K1a fracture toughness 
curve of ASME Code, Section XI, Appendix G, Figure G-2210-1 (the 
K1a fracture toughness 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(a).

Code Case N-588

    The licensee has proposed an exemption to allow the use of ASME 
Code Case N-588 in conjunction with ASME Code, Section XI, 10 CFR 
50.60(a) and 10 CFR Part 50, Appendix G, to determine the P-T limits.
    The proposed amendments to revise the P-T limits for Dresden rely, 
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 a RPV circumferential weld in 
lieu of an axially-oriented flaw required by the 1989 Edition of ASME 
Code, 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 extent 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. A defect with a circumferential orientation is, 
therefore, postulated for circumferential welds.
    An analysis provided to the ASME Code's Working Group on Operating 
Plant Criteria (WGOPC) (in which Code Case N-588 was developed) 
indicated

[[Page 53230]]

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 two 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 two (1.5 for 
pressure testing condition) is maintained for the contribution of the 
pressure load to the integrity calculation of the circumferential weld. 
Consequently, the staff determined that the posulation 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 pressure 
testing and normal operations, including heatup, cooldown, and 
anticipated operational transients.
    In summary, the ASME Code, Section XI, Appendix G, pocedure was 
developed for axially-oriented flaws, which is physically unrealistic 
and overly conservative for postulation flaws of this orientation to 
exist in circumferential welds. Hence, the NRC staff concurs that 
relaxation of the ASME Code, 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 the use of ASME 
Code Case N-640 in conjunction with ASME Code, Section XI; 10 CFR 
50.60(a); and 10 CFR Part 50, Appendix G, to determine P-T limits.
    The proposed amendment to revise the P-T limits for Dresden rely in 
part on the requested exemption. These revised P-T limits have been 
developed using the K1c fracture toughness curve, in lieu of 
the K1a fracture toughness curve, as the lower bound for 
fracture toughness.
    Use of the K1c curve in determining the lower bound 
fracture toughness in the development of P-T operating limits curve is 
more technically correct that use of the K1a 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 
K1c 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 or the K1a 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 K1a 
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 K1c curve would enhance overall plant 
safety by opening the P-T operating window with the greatest safety 
benefit in the region of low temperature opertions.
    Since the RCS P-T operating window is defined by the P-T operating 
and test limit curves developed in accordance with the ASME Code, 
Section XI, Appendix G, continued operation of Dresden 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 
degrees Fahrenheit 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 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 Code, Section XI, Appendix G, procedure was 
conservatively developed based on the level of knowledge existing tin 
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 Code, 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. 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 ASME 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 and that application of the specific requirements of 
10 CFR 50.60(a) and Appendix G in these circumstances is not necessary 
to achieve the underlying purpose of the rules. This is also consistent 
with the determination that the staff has reached for other licensees 
under similar conditions based on the same considerations (Quad Cities 
Nuclear Power Station, Units 1 and 2, exemption dated February 4, 
2000). Therefore, the staff concludes that requesting 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 Dresden Nuclear Power Station, 
Units 2 and 3.

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, and that special circumstances are present. Therefore, 
the Commission hereby grants Commonwealth Edison Company an exemption 
from the requirements of 10 CFR Part 50, Section 50.60(a) and 10 CFR 
Part 50, Appendix G, for Dresden Nuclear Power Station, Units 2 and 3.
    Pursuant to 10 CFR 51.32, an environmental assessment and finding 
of no significant impact has been prepared and published in the Federal

[[Page 53231]]

Register (65 FR 51344). 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 25th day of August 2000.

    For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 00-22498 Filed 8-31-00; 8:45 am]
BILLING CODE 7590-01-U