[Federal Register Volume 64, Number 91 (Wednesday, May 12, 1999)]
[Notices]
[Pages 25520-25522]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-11996]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-445 and 50-446]
Texas Utilities Electric Company (Comanche Peak Steam Electric
Station, Units 1 and 2); Exemption
I.
Texas Utilities Electric Company (the licensee/TU Electric) is the
holder of Facility Operating Licenses No. NPF-87 and No. NPF-89, which
authorize operation of the Comanche Peak Steam Electric Station
(CPSES), 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 Somervell County, Texas.
TU Electric seeks this exemption to the 2 percent above licensed
power level assumption to allow for uncertainties specified by Title 10
of the Code of Federal Regulations (10 CFR), Part 50, Appendix K,
``ECCS [Emergency Core Cooling System] Evaluation Models,'' Section
I.A., to support license amendments for modest increases of up to 1
percent in the licensed power levels for both units. This will result
in an exemption from the requirements of 10 CFR Part 50, Appendix K to
allow ECCS evaluation model assumptions to be conducted at no less than
1.01 times licensed power level. The licensee seeks this exemption
based on its proposed use of a new feedwater flow measurement system to
allow more accurate measurement of thermal power (known as the Leading
Edge Flowmeter (LEFM) System), manufactured by Caldon, Inc. The LEFM is
described in Caldon, Inc., Topical Report ER-80P, ``Improving Thermal
Power Accuracy and Plant Safety While Increasing Operating Power Level
Using the LEFM System.'' The subject topical report was approved
subject to the limitations stated in a letter and Safety Evaluation
(SE) dated March 8, 1999.
[[Page 25521]]
II.
Part 50, Appendix K, Section I. A. states, in part, that ``it shall
be assumed that the reactor has been operating continuously at a power
level at least 1.02 times the licensed power level (to allow for such
uncertainties as instrument error).'' The Appendix K rule was written
to ensure that adequate margin for ECCS performance would be available
if a design-basis loss-of-coolant accident (LOCA) ever occurred (39 FR
1002, January 4, 1974). The margin was provided by incorporating
several conservative features into the ECCS performance criteria as
well as maintaining conservative requirements and recommendations for
evaluation models.
The basis for the requirement is discussed in background
documentation, such as the Statement of Consideration for Appendix K
(39 FR 1002, January 4, 1974). The 102 percent assumption is one of
several items listed as conservative factors used to model the energy
available from reactor operation. The Statement of Consideration also
associates the preaccident power level assumption with the modeling of
the rate of heat generation after the LOCA occurs. A comparison is made
between the estimated uncertainty associated with the decay heat
assumption (i.e., 20 percent above the American Nuclear Society (ANS)
standard) and the estimated effect on heat generation resulting from
the 102 percent power assumption. This is a natural connection since
the preaccident power level directly affects the decay heat generation
rate after reactor shutdown.
When it was considering changes to Appendix K to accept the use of
best-estimate evaluations, the staff understood that the rule
incorporated substantial conservatisms (see SECY 83-472, ``Emergency
Core Cooling System Analysis Methods,'' November 17, 1983). These
conservatisms were necessary when the rule was written because of
limited experimental evidence. The major analysis inputs and
assumptions that contribute to the conservatism in Appendix K are
grouped together under Sections A through D of the rule: (A) Sources of
Heat During the LOCA (the 102 percent power provision is one factor);
(B) Swelling and Rupture of the Cladding and Fuel Rod Thermal
Parameters; (C) Blowdown Phenomena; and (D) Post-blowdown Phenomena:
Heat Removal by ECCS. In each of these areas, several assumptions are
typically used to assure conservatism in the analysis results. For
instance, under sources of heat during the LOCA, in addition to the 102
percent requirement, decay heat is modeled on the basis of an ANS
standard with an added 20 percent penalty, and the power distribution
shape and peaking factors expected during the operating cycle are
chosen to yield the most conservative results. As discussed in SECY-83-
472, experimental programs provided ample data, which shed light on the
considerable margin provided by Appendix K, giving the staff confidence
to consider alternative ECCS evaluation models.
III
Section 50.12(a), states that . . .
The Commission may, upon application by any interested person or
upon its own initiative, grant exemptions from the requirements of
the regulations of this part, which are--
(1) Authorized by law, will not present an undue risk to the
public health and safety, and are consistent with the common defense
and security.
(2) The Commission will not consider granting an exemption
unless special circumstances are present. . . .
Section 50.12(a)(2), states that special circumstances are
present whenever . . .
(ii) 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; or
(iv) The exemption would result in benefit to the public health
and safety that compensates for any decrease in safety that may
result from the grant of the exemption; or
(vi) 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. . . .
IV
The staff has reviewed the applicable regulations and the
regulatory history for Appendix K as well as for Section 50.46, and
finds that those regulatory documents do not prohibit the licensee's
proposal to use Caldon Inc.'s, Leading Edge Flowmeter System (Caldon
LEFM System) instrument. Accordingly, the exemption is authorized by
law, as required by 10 CFR 50.12(a)(1).
The staff used Regulatory Guide 1.174 and Standard Review Plan
Chapter 19 to review the application for the exemption. Specifically,
the staff reviewed the application considering the defense-in-depth
philosophy, the maintenance of sufficient safety margin, and the fact
that the increase in risk was small and consistent with the Commission
safety goals. A slightly higher power level will result in a small
increase in decay heat load that could affect required response time of
the ECCS and the available operator response time following transients
and accidents. Results of core and containment consequence analyses
from higher power levels could also be affected. However, NUREG-1230,
``Compendium of ECCS Research for Realistic LOCA Analysis,'' considered
the risk impact of changes associated with the revised ECCS rules,
including power increase, and considered a power increase of 5 percent
or less to have little risk significance. The staff concludes that this
increase of 1 percent is bounded by the NUREG-1230 considerations.
In the safety evaluation for the Caldon topical report ER-80P dated
March 8, 1999, the staff accepted statistical treatment of
uncertainties attributed to the LEFM and venturi-based flow measurement
instruments and the uncertainty values associated with these two types
of flow measurement instruments at CPSES. The use of the Caldon LEFM
System and quantification of power measurement uncertainty do not raise
inconsistencies with the Commission's safety goals. Further, the
Commission has determined that, pursuant to 10 CFR 50.12, the requested
exemption is authorized by law, will not result in an undue risk to the
public health and safety, and is consistent with the common defense and
security and is otherwise in the public interest.
The Commission also finds that special circumstances exist. By
seeking to apply a smaller margin for power measurement uncertainty,
the exemption does not violate the underlying purpose of Appendix K.
The application of 1.02 times the licensed thermal power is not
necessary to achieve the underlying purpose of Appendix K. Indeed, by
quantifying a contributor to the uncertainty where the uncertainty was
not specifically known, the exemption may better serve the underlying
purpose of the requirement. The use of the Caldon LEFM System and the
quantification of power measurement uncertainty appear to offer safety
benefits.
By requesting this exemption, the licensee has undertaken to
quantify a contributor to the uncertainty in power measurement.
Although there is a small safety impact expected from the associated
power increase, it is not considered significant. The use of the LEFM
system and the quantification of power measurement uncertainty appear
to offer safety benefits.
The Caldon LEFM System and the quantification of power measurement
uncertainty associated with use of the Caldon LEFM System constitute
[[Page 25522]]
material circumstances that did not exist when the rule was written.
The current Appendix K rule presumes that the 2 percent margin accounts
for uncertainties associated with measurement of thermal power.
Contributors to the uncertainty were not identified at the time the
rule was written and the magnitude of the uncertainty was not
demonstrated by experiment or analysis. The rule does not require
quantification of actual uncertainties, nor does the regulatory history
reflect any detailed technical basis for the choice of a 2 percent
margin. Therefore, the Commission has determined that special
circumstances as defined in 10 CFR 50.12(a)(2)(ii), (iv), and (vi) are
present.
The Commission hereby grants the licensee an exemption from the
requirements of 10 CFR Part 50, Appendix K to allow ECCS evaluation
model assumptions to be conducted at no less than 1.01 times licensed
power level when the quantification of power measurement uncertainty
can be justified by the use of the Caldon LEFM System instrumentation.
The granting of this exemption does not, however, provide authority to
increase the licensed power of CPSES, Units 1 and 2. A separate license
amendment to increase licensed power level, for each licensed unit,
will be required to be submitted and approved before such authority may
be provided for that unit.
Pursuant to 10 CFR 51.32, the Commission has determined that
granting of this exemption will have no significant effect on the
quality of the human environment (64 FR This exemption is effective
upon issuance.
Dated at Rockville, Maryland, this 6th 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. 99-11996 Filed 5-11-99; 8:45 am]
BILLING CODE 7590-01-P
