[Federal Register Volume 64, Number 206 (Tuesday, October 26, 1999)]
[Notices]
[Pages 57666-57668]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-27950]
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NUCLEAR REGULATORY COMMISSION
[Docket No. 50-395]
South Carolina Electric & Gas Company, V.C. Summer Nuclear
Station, Unit 1; Exemption
I
South Carolina Electric & Gas Company (the licensee) is the holder
of Facility Operating License No. NPF-12, that authorizes operation of
the V.C. Summer Nuclear Station. 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.
[[Page 57667]]
The facility consists of a pressurized water reactor located on the
V.C. Summer Nuclear Station site in Jenkinsville, South Carolina.
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 that ``[t]he appropriate requirements on * *
* the pressure-temperature limits and minimum permissible temperature
must be met for all conditions.'' Appendix G of 10 CFR Part 50
specifies that the requirements for these limits are the American
Society of Mechanical Engineers (ASME) Code, Section XI, Appendix G
limits.
Pressurized water reactor licensees have installed cold
overpressure mitigation systems/low temperature overpressure protection
(LTOP) systems in order to protect the reactor coolant pressure
boundary (RCPB) from being operated outside of the boundaries
established by the P-T limit curves and to provide pressure relief of
the RCPB during low temperature overpressurization events. The licensee
is required by the V.C. Summer Nuclear Station Technical Specifications
(TS) to update and submit the changes to its LTOP setpoints whenever
the licensee is requesting approval for amendments to the P-T limit
curves in the TS.
Therefore, in order to address the provisions of amendments to the
TS P-T limits and LTOP curves, the licensee requested in its submittal
dated August 19, 1999, that the staff exempt V.C. Summer Nuclear
Station from application of specific requirements of 10 CFR Part 50,
Section 50.60(a) and 10 CFR Part 50, Appendix G, and substitute use of
ASME Code Case N-640 as an alternate reference fracture toughness for
reactor vessel materials for use in determining the P-T limits.
The proposed action is in accordance with the licensee's
application for exemption contained in a submittal dated August 19,
1999, and is needed to support the TS amendment that is contained in
the same submittal and is being processed separately. The proposed
amendment would impact the P-T limits of TS 3/4.4 for V.C. Summer
Nuclear Station related to the heatup, cooldown, and inservice test
limitations for the Reactor Coolant System to a maximum of 32 Effective
Full Power Years (EFPY). It will result in a revision to TS 3/4.4.9,
Pressure/Temperature Limits, to reflect the revised P-T limits of the
reactor vessel.
Code Case N-640
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 that the P-T limits meet the
underlying intent of the NRC regulations.
The proposed amendment to revise the P-T limits for V.C. Summer
Nuclear Station relies in part on the requested exemption. The ASME
Code Case N-640 approach for calculating the allowable limit curves for
various heatup and cooldown rates specifies that the total stress
intensity factor, KI, for the combined thermal and pressure
stresses at any time during heatup or cooldown cannot be greater than
the reference stress intensity factor, KIC, for the metal
temperature at that time. KIC is obtained from the reference
fracture toughness curve, defined in Appendix G to Section Xl of the
1996 ASME Code. The KIC curve is based on the lower bound of
static critical KI values measured as a function of
temperature on specimens of SA-533 Grade B Class 1, SA-508-2, and SA-
508-3 steels.
Use of the KIc curve in determining the lower bound
fracture toughness in the development of a P-T operating limits curve
is more technically correct than the KIa curve. The
KIc curve appropriately implements the use of static
initiation fracture toughness behavior to evaluate the controlled heat-
up and cooldown process of a reactor vessel. The licensee has
determined that the use of the initial conservatism of the
KIa curve when the curve was codified in 1974 was justified.
This initial conservatism was necessary due to the limited knowledge of
reactor pressure vessel materials. Since 1974, additional knowledge has
been gained about reactor pressure vessel materials, which demonstrates
that the lower bound on fracture toughness provided by the
KIIa curve is well beyond the margin of safety required to
protect the public health and safety from potential reactor pressure
vessel 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. The two primary
safety benefits in opening the low temperature operating window are a
reduction in the challenges to RCS power-operated relief valves and
elimination of RCP impeller cavitation wear.
Since the RCS P-T operating window is defined by the P-T operating
and test limit curves developed in accordance with the ASME Section XI,
Appendix G procedure, continued operation of Summer with these P-T
curves without the relief provided by ASME Code Case N-640 would
unnecessarily restrict the P-T operating window. This restriction
requires, under certain low temperature conditions, that only one
reactor coolant pump in a reactor coolant loop be operated. The
licensee has found from experience that the effect of this restriction
is undesirable degradation of reactor coolant pump impellers that
results from cavitation sustained when either one pump or one pump in
each loop is operating. Implementation of the proposed P-T curves as
allowed by ASME Code Case N-640 does not significantly reduce the
margin of safety. 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 reactor pressure vessel 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. The staff accepts the
licensee's determination that an exemption would be required to approve
the use of Code Case N-640. The staff examined the licensee's rationale
to support the exemption request and concurred that the use of the Code
case would also 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 RG 1.99, Revision 2, the staff concluded
that application of
[[Page 57668]]
the Code case as described would provide an adequate margin of safety
against brittle failure of the RPVs. 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 the exemption under the special
circumstances of 10 CFR 50.12(a)(2)(ii) is appropriate and that the
methodology of Code Case N-640 may be used to revise the LTOP setpoints
and P-T limits for the Summer reactor coolant system.
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 South Carolina
Electric & Gas Company an exemption from the requirements of 10 CFR
Part 50, Section 50.60(a) and 10 CFR Part 50, Appendix G, for the V.C.
Summer Nuclear Station.
Pursuant to 10 CFR 51.32, the Commission has determined that the
granting of this exemption will not result in any significant effect on
the quality of the human environment (64 FR 56359).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 20th day of October 1999.
For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear
Reactor Regulation.
[FR Doc. 99-27950 Filed 10-25-99; 8:45 am]
BILLING CODE 7590-01-P
