ML17310A804
| ML17310A804 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 11/19/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17310A803 | List: |
| References | |
| NUDOCS 9311300055 | |
| Download: ML17310A804 (10) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAF TY EV LUATION BY THE OFFICE OF NUCLEAR REACTO REGULA IO S
STE 80 NLET FLOW DIST IBUT 0 SUPP M
1-TO ENCLOSURE 1P TO LD-'82-054 R
ATED TO AM ND ENT 0;
72 TO FACILITY OPERATING LICENSE NO 0
UB C
S RVIC CO AN PALO VERDE NUCLEAR GENERATING STATION UNIT NO.
1 DOCKET NO.
STN 50-528
- 1. 0 INTRODUCTION By letter September 8,
1993, Arizona Public Service Company (APS or the licensee) submitted a request for a change to the Palo Verde Nuclear Generating Station, Unit No.
1 Technical Specifications (TS).
The Arizona Public Service Company submitted this request on behalf of itself and the Salt River Project Agricultural Improvement and Power District, Southern California Edison
- Company, El Paso Electric Company, Public Service Company of New
- Mexico, Los Angeles Department of Water and Power, and Southern California Public Power Authority.
The proposed change would add a methodology supplement entitled, "System 80'nlet Flow Distribution," to the list of methods used to determine the core operating limits.
2.0
~BACKG OUN Enclosure 1P to LD-82-054, "Statistical Combination of Uncertainties - of System Parameter Uncertainties in Thermal Margin Analyses for System 80," is used to calculate the departure from nucleate boiling ratio (DNBR) for the Palo Verde Nuclear Generating Station (Palo Verde) units.
This method has been previously reviewed and approved by the NRC.
In a letter of March 30, 1993 (Ref,. 1), Arizona Public Service Company (APS) requested U.S. Nuclear Regulatory Commission (NRC) review of Supplement 1-P to Enclosure 1-P to LD-82-054, "System 80'Inlet Flow Distribution."
This revised supplement to the methodology for assessing core thermal margin treats core inlet flow distribution data in a statistical manner.
3.0 EVALUATION Supplement 1-P to Enclosure 1P to LD-82-054 describes a revised System 80 core inlet flow distribution for use with ABB-CE Statistical Combination of Uncertainties (SCU) methodology for assessing core thermal margin.
The revised core inlet flow distribution and the associated uncertainties are the 9311300055 931119'DR ADOCK 05000M8
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result of re-evaluating the System 80 reactor flow model data.
The re-evaluation treats the core inlet flow distribution data in a statistical
- manner, as opposed to the deterministic method currently used.
The objective of the revised methodology is to reduce conservatism in the current deterministic approach to gain additional calculated core thermal margin.
The report summarizes the current deterministic method of treating the core inlet flow data as an introduction to the revised methodology,.
The geometric features of the core lower support structure are described in relation to their possible impacts on core inlet flow distribution.
Based on these
- features, the core inlet plane is regionalized to account for potential differences in inlet flow rates.
The System 80 flow model data is then used to determine the core inlet flow'factors and their uncertainties for the core inlet regions.
A statistical test is applied to the resulting core inlet flow factors to support the hypothesis that the selected regions have separate, distinct core inlet flow factors.
APS currently uses a deterministic method to account for the inlet flow distribution and associated uncertainties in thermal margin analysis.
The revised approach applied to determine the System 80 core inlet flow distribution is to identify regional flow factors based on the assumption that the inlet flow for groups of assemblies is determined by some common relationship to the upstream flow geometry.
It consists of the following steps:
Define geometric features in the reactor vessel which influence the core inlet flow distribution.
Examine the core inlet flow distribution data. to identify regions which have similar inlet flow factors, and categorize those regions.
Determine the mean inlet flow factor and standard deviation of the fl'ow factors for each region.
Test the null hypothesis that the mean inlet flow factor values for two regions are from the same population.
If that hypothesis can be rejected at a significance level of 5X for an equal-tails test, assume that the mean flow factors for the two regions are distinct values.
Repeat the test pairwise for each of the regions that have been identified.
The use of this approach resulted in six different regions, each with its own mean value and standard'eviation for inlet flow factors (as identified in Table 3-2 of Reference I).
The licensee's report concludes that these revised flow factors, along with the sample standard deviations of the flow factors, are a valid set to be used for future System 80 thermal margin licensing analyses.
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0 For the System 80 plants, the current SCU methodology treats the core inlet flow distribution in a deterministic manner.
As a result of the reassessment of the System 80 core inlet flow distribution, the core inlet flow distribution and its associated uncertainties will be treated in a statistical manner.
The system parameter SCU methodology consists of developing a minimum departure from. nucleate boiling ratio-(HDNBR) response surface which provides the functional relationship between the dependent vari'able, HDNBR, and the independent system parameter variables.
The response surface is used to combine the probability density functions (PDFs) or the uncertainties associated with each of the independent variables into a resultant DNB PDF.
The DNB PDF is then used to determine the HDNBR value at the 95X probability and 95/o confidence levels.
The 95/95 SCU HDNBR limit is used in conjunction with a best estimate thermal margin model to assess margin to the DNBR limit.
Mith respect to the System 80 SCU methodology described in Reference 2, the revised core inlet flow distribution approach will involve the following changes:
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The sensitivity of DNBR with respect to inlet flow factor will. be determined for the limiting assembly and adjacent assemblies.
These sensitivities will then be used with the appropriate inlet flow factor uncertainties to calculate an overall root-sum-square system parameter uncertainty, using the method presented in Section 5.3 of Reference 3.
This approach will yield an increased HDNBR limit which will include allowances for uncertainties in 'hot assembly and adjacent assembly inlet flow.
The increase in HDNBR limit can be accommodated directly by increasing the limit, or by applying a thermal margin penalty.
Reference 1 has. proposed a set of factors to construct a best estimate core thermal margin model.
Uncertainties in inlet flow to the hot assembly and adjacent assemblies can be accounted for statistically by either increasing DNBR or applying a thermal margin penalty using approved SCU methods.
Uncertainties in inlet flow have been treated statistically using these methods (Ref. 3),,
and have been approved previously by the staff for other Combustion Engineering plants.
The licensee has proposed to implement the uncertainties associated with using a best estimate inlet flow by applying a thermal margin penalty to the addressable constants for Palo Verde Unit 1, cycle 5, as described in References 4 and 5.
This has been reviewed by the staff and is acceptable for use in Palo Verde Unit 1 during cycle. 5.
Based on the above, the staff has concluded that the proposed change to add a
methodology supplement entitled, "System 80'Inlet Flow Distribution," to the list of methods used to determine the core operating limits (as described in the TS change request dated September 8, 1993,) is acceptable.
Additionally, in a meeting with the staff on October 6, 1993, the licensee has committed, to submit a generic implementation of flow uncertainties -utilizing an
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increased HDNBR for Units 2 and 3, and latter cycles of Unit 1.
The NRC staff has allowed the use of applying a thermal margin penalty to the addressable constants for Unit 1, cycle 5.
However, the implementation of this methodology is served best by increasing the-HDNBR setpoint.
The licensee plans to submit a generic application addressing a revised HDNBR setpoint for all three units.
The NRC must review this generic implementation before it is used.
4.0 STA CONSU A
0 In accordance with the Commission s regulations, the Arizona State official was notified of the proposed issuance of the amendment.
The State official had no comments.
5.0 ENVI ONNENTAL CONSIDERATIO The amendment changes a requirement with respect to the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20.
The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure.
The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public comment on such finding (58 FR 53583).
The amendment also changes recordkeeping or reporting requirements.
Accordingly, the amendment meets the el.igibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9) and (10).
Pursuant to 10 CFR 51.22(b). no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
6.0 CONCLUSION
The Commission has concluded, based on the considerations discussed
- above, that (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed
- manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
L. Tran B. Holian G. Schwenk Date:
November,19, 1993
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REFERENCES Letter from W. F.
Conway (APS) to Document Control Desk (NRC),
"Supplement 1-P to Enclosure 1-P to LD-82-054," March 30, 1993.
2.
3.
4.
5.
NUREG-0852, Supplement 2,
CESSAR System 80 SSER 2, dated 'September
.1983.
"Statistical Combination of Uncer tainties, Combination of,System Parameter Uncertainties in Thermal. Margin Analyses for Arkansas Nuclear One Unit,2," CEN-139(P),, November.1980.
"Impact of'mproved Inlet Flow Distribution on Typical CPC Overall Uncertainty 'Factor," CEN-421(v)-P, July 1993'.
"Presentation to NRC on Impact of Improved Inlet Flow Distribution on the CPCS,"
CEN-424(v)-P,, October 1993.
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