[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