[Federal Register Volume 64, Number 91 (Wednesday, May 12, 1999)]
[Notices]
[Pages 25519-25520]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-11997]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-498 and 50-499]
STP Nuclear Operating Company (South Texas Project Electric
Generating Station, Units 1 and 2); Exemption
I
STP Nuclear Operating Company is the holder of Facility Operating
License No. NPF-76 and Facility Operating License No. NPF-80, which
authorizes operation of the South Texas Project (STP), Units 1 and 2.
The licenses provide, among other things, that the licensee is subject
to all rules, regulations, and orders of the Commission now or
hereafter in effect.
These facilities consist of two pressurized-water reactors at the
licensee's site located in Matagorda County, Texas.
II
Section 50.60(a) to Title 10 of the Code of Federal Regulations (10
CFR) Part 50 requires, in part, that except as provided in Section
50.60(b), all light-water nuclear power reactors, other than reactor
facilities for which the certifications required under Section
50.82(a)(1) have been submitted, must meet the fracture toughness
requirements for the reactor coolant pressure boundary set forth in
Appendix G of 10 CFR Part 50. Section 50.60(b) of 10 CFR Part 50 states
that proposed alternatives to the described requirements of Appendix G
of Part 50 or portions thereof may be used when an exemption is granted
by the Commission under 10 CFR 50.12.
III
By letter dated March 18, 1999, STP Nuclear Operating Company
requested that the NRC exempt STP, Units 1 and 2, from the application
of specific requirements of 10 CFR 50.60 and Appendix G to 10 CFR 50.
Specifically, STP Nuclear Operating Company proposes to use American
Society of Mechanical Engineers (ASME) Code Case N-514 to permit
setting the pressure setpoint of STP's cold overpressure mitigation
system (COMS) such that the pressure-temperature (P-T) limits required
by Appendix G of 10 CFR Part 50 could be exceeded by 10 percent during
a low temperature pressure transient.
The Commission has established requirements in 10 CFR Part 50 to
protect the integrity of the reactor coolant system pressure boundary.
As a part of these, Appendix G of 10 CFR
[[Page 25520]]
Part 50 requires that P-T limits be established for reactor pressure
vessels during normal operation and vessel hydrostatic testing. As
stated in Appendix G, ``The appropriate requirements on . . . the
pressure-temperature limits . . . must be met for all conditions.'' In
order to avoid approaching these P-T limit curves and provide pressure
relief during low temperature overpressurization (LTOP) events,
pressurized water reactor licensees have installed protection systems
(COMS/LTOP) as part of the reactor coolant system (RCS) pressure
boundary. STP Nuclear Operating Company is required, as part of the STP
Technical Specifications, to develop, update, and submit reactor vessel
P-T limits and COMS setpoints for NRC review and approval.
STP Nuclear Operating Company determined that the exemption request
from the provisions of 10 CFR 50.60 and Appendix G was necessary since
these regulations require, as previously noted, that reactor vessel
conditions not exceed the P-T limits established by Appendix G. In
referring to 10 CFR 50.12 on specific exemptions, STP Nuclear Operating
Company cited special circumstances regarding achievement of the
underlying purpose of the regulation as its basis for requesting this
exemption [10 CFR 50.12(a)(2)(ii)].
STP Nuclear Operating Company noted in support of the 10 CFR
50.12(a)(2)(ii) criteria that the underlying purpose of the subject
regulation is to establish limits to protect the reactor vessel from
brittle failure during low temperature operation and that the COMS
provides a physical means of assuring that operation remains within
these limits. STP Nuclear Operating Company proposed that establishing
the COMS pressure setpoint in accordance with the N-514 provisions,
such that the vessel pressure would not exceed 110 percent of the P-T
limit allowables, would still provide an acceptable level of safety and
mitigate the potential for an inadvertent actuation of the COMS. The
use of N-514 was based on the conservatisms that have been explicitly
incorporated into the procedure for developing the P-T limit curves.
This procedure, referenced from Appendix G to Section XI of the ASME
Code, includes the following conservatisms: (1) a safety factor of 2 on
the pressure stresses; (2) a margin factor applied to RTNDT
using Regulatory Guide 1.99, Revision 2, ``Radiation Embrittlement of
Reactor Vessel Materials''; (3) an assumed \1/4\ thickness flaw with a
6:1 aspect ratio; and (4) a limiting material toughness based on
dynamic and crack arrest data.
In addition, STP Nuclear Operating Company stated that a COMS
pressure setpoint must be sufficiently high to prevent the inadvertent
actuation of the COMS as a result of normal operating pressure surges.
STP Nuclear Operating Company requests use of Code Case N-514 to
incorporate pressure instrumentation uncertainty in P-T limit
calculations, while providing an operating band that permits system
makeup and pressure control. Such an inadvertent actuation could lead
to the unnecessary release of reactor coolant inside containment and
could introduce undesirable thermal transients in the RCS.
The Commission has determined that the application of 10 CFR 50.60
in these particular circumstances is not necessary to achieve the
underlying purpose of that rule and that the use of Code Case N-514
would meet the underlying intent of the regulation. Based upon a
consideration of the conservatisms, which are explicitly defined in the
Appendix G methodology, it was concluded that permitting the COMS
setpoint to be established such that the vessel pressure would not
exceed 110 percent of the limit defined by the P-T limit curves would
provide an adequate margin of safety against brittle failure of the
reactor vessel. This is also consistent with the determination that has
been reached for other licensees under similar conditions based on the
same conditions. Therefore, the exemption requested under the special
circumstances of 10 CFR 50.12(a)(2)(ii) was found to be acceptable. The
staff also agrees that limiting the potential for inadvertent COMS
actuation may improve plant safety.
IV
The Commission has determined that, pursuant to 10 CFR 50.12, this
exemption is authorized by law, will not present an undue risk to the
public health and safety, is consistent with the common defense and
security, and is otherwise in the public interest. Therefore, the
Commission hereby grants STP Nuclear Operating Company an exemption
from the requirements of 10 CFR 50.60 in order to apply ASME Code Case
N-514 for determining STP's cold overpressurization mitigation system
pressure setpoint.
Pursuant to 10 CFR 51.32, the Commission has determined that the
granting of this exemption will have no significant effect on the
quality of the human environment (64 FR 23689).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 4th day of May 1999.
For the Nuclear Regulatory Commission.
John A. Zwolinski,
Director, Division of Licensing Project Management, Office of Nuclear
Reactor Regulation.
[FR Doc. 99-11997 Filed 5-11-99; 8:45 am]
BILLING CODE 7590-01-P
