ML20206H188
| ML20206H188 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 05/06/1999 |
| From: | Zwolinski J NRC (Affiliation Not Assigned) |
| To: | TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC) |
| Shared Package | |
| ML20206H174 | List: |
| References | |
| NUDOCS 9905110086 | |
| Download: ML20206H188 (7) | |
Text
m 7590-01-P UNITED STATES OF AMERICA NUCLEAR REGULATORY COMMISSION in the Matter of
)
)
TEXAS UTILITIES ELECTRIC COMPANY
) Docket Nos. 50-445 and 50-446
)
)
(Comanche Peak Steam Electric Station,
)
Units 1 and 2)
)
EXEMPTION 1.
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 Reaulations (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 porcent in the licensed power levels for both units. This will result in an exemption from the requirements of l
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
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9905110086 990506 PDR ADOCK 05000445 P
~ proposed use of a new feedwater flow measurement systern 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.
II.
Part 50, Appendix K, Section 1. 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 sue,1 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 frorn the 102 percent power assumption.
This is a natural connection since the preaccident power ievel directly affects the decay heat l
generation rate after reactor shutdown.
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3-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 i
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);
i i
(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 1
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.
111.
4 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....
4 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
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f'. 5 percent or less to have little risk significance. The staff concludes that this increase of f
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1 percent is bounded by the NUREG-1230 considerations.
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In the safety evaluation for the Caldon topical report ER-80P dated March 8,1999, the
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staff accepted statistical treatment of uncertainties attributed to the LEFM and venturi-based j
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 i
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.
6-The Caldon LEFM System and the quantification of power measurement uncertainty associated with use of the Caldon LEFM System constitute 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 i
present.
The Commission hereby grants the licensee an exemption from the requirements of 10 CFR Par 150, Appendix K to allow EC'CS evaluation model assumptions to be conducted at no less than 1.01 times licensed power level when the quantification of power measurement l
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 t
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.
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. 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 23880).
This exemption is effective upon issuance.
Dated at Rockville, Maryland, this 6th day of May 1999.
FOR THE NUCLEAR REGULATORY COMMISSION original signed by:
John A. Zwolinski, Director Division of Licensing Project Management Office of Nuclear Reactor Regulation
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