ML20248D422
| ML20248D422 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 07/31/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20248D420 | List: |
| References | |
| NUDOCS 8908110023 | |
| Download: ML20248D422 (3) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION n
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,i WASWNGTON D. C. 20555 5
, *,g SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS.120 AND 104 TO FACILITY CPERATING LICENSE NOS. NPF-4 AND NPF-7 VIRGINI A ELECTRIC AND POWER COMPANY OLD DOMINION ELECTRIC COOPERATIVE NORTH ANNA POWER STATION, UNITS NO. 1 AND NO. 2 DOCKET NOS. 50-338 AND 50-339
1.0 INTRODUCTION
By letter dated May 23, 1989, the Virginia Electric and Power Company (the licensee) proposed a change to the Technical Specifications (TS) for the North Anna Power Station, Unit Nos. I and 2 (NA-1&2). The proposed change would reduce the TS 3/4.2.5 limit on the minimum measured flow rate in the reactor coolant system (RCS) for 'ooth NA-1&2. The change allows a lower total RCS flow by taking credit for a previously unused design siaargin without requiring a reanalysis of the Updated Final Safety Analysis Report (UFSAR) Chapter 15 accident analyses.
The reduced RCS flow rate will be offset by the conservatism inherent in the existing calculated Departure from Nucleate Boiling Ratio (DNBR).
This reduction'in DNBR design margin is considered acceptable since it does not adversely affect the UFSAR Chapter 15 accident analysis.
As required by TS 3.2.5 and 4.2.5.2, the licensee performs RCS flow measure-ments at NA-1&2 once per fuel cycle.
Because the RCS flow is sensitive to the steam generator tube plugging (SGTP) which has been performed during refueling outages, the flow rates have been trended as a function of SGTP.
The RCS flow rates have decreased predictably with increased SGTP. As additiocal tubes are plugged, both units are expected to approach the TS flow limit As discussed below, the NA-1&2 TS minimum RCS flow rate may be reduced by absorbing a penalty in the available DNBR design margin. The flow rate wovld be reduced for both NA-1&2 to preserve consistency between the two sets of TS.
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2.0 DISCUSSION l
Two different flow rates have been used in the currently decketed analyses I
for NA-1&2 which were approved as a part of the 1986 core uprating (Amend-ment Nos. 84 and 71 dated August 25,1986).
These are the Westinghouse Improved Thermal Design Procedure (ITDP) flow rate, which is used for statistical DNBR analysis, and a lower, non-ITDP flow rate, which is used for non-statistical analysis of both DNB and non-DNB events such as the Loss of Coolant Accident (LOCA).
8908110023 690731 PDR ADOCK 05000338 l
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. The NA-1&2 TS require that the RCS flow rate be measured every 18 months in order to verify the assumed ITDP flow rate of 289,200 gpm.
No uncer-tainty is applied because the measurement uncertainty has been statisti-cally absorbed by the ITDP methodology.
No comparison with the non-ITDP flow rate is necessary since, even with the application of the required measurement uncertainty, the non-ITDP 284,000 gpm limit is substantially below the ITDP value.
Sufficient analysis margin exists to lower the minimum measured flow rate j
from the ITDP limit uf 289,200 gpm to the non-ITDP limit of 284,000 gpm.
As a part of the Reload Safety Evaluation (RSE) process for each cycle, a i
table of retained DNBR margin (derived from the TS Bases 3/4.2.3) is prepared for the reload cores.
Approximately 10% retained CNBR margin i
remains available after all pertinent penalties have been absorbed. This available margin can be used to offset the desired flow limit reduction.
The maximum DNBR sensitivity to flow rate for NA-182 is 1.6% per percent j
flow. This sensitivity was determined over a range of statepoints which j
bound all operating and DNB-related accident conditions as a part of j
licensee's IIRC-approved Statistical DNBR Methodology. Further, it is slightly more conservative than the ITDP sensitivity which was used to develop the DNBR limit for the NA-1&2 core uprating. Based upon this sensitivity, each percent reduction in the 289,200 gpm limit must be offset by a 1.6% penalty on the retained DNBR margin. Such a penalty may be applied without requiring reanalysis of the ITDP events for NA-1&2.
j Enough DNBR design margin exists (approximately 10%) to lower the NA-1&2
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TS minimum measured RCS flow rate to the non-ITCP flow limit of 284,000
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gpm. This proposed. limit is 1.8% below 289,200 gpm, so that the retained margin DNBR penalty would be (1.8% flow) * (1.6% DNBR/% flow) = 2.9% DNBR.
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The RCS flow limit of TS Table 3.2-1 may therefore be lowered to 284,000 l
gpm. This reduction will be offset by a retained DNBR margin penalty of
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2.9%, which can absorbed by the available retained DNBR margin.
The total
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remaining DNBR margin will still be approximately 7%.
It is noted that measured flow rates below the 284,000 gpm level would invalidate the 1
assumptions of the non-ITDP accident analyses of UFSAR Chapter 15 and are therefore unacceptable at this time.
There are no other areas which are impacted by the limit change.
Flow-rated I
items such as loop transport times or RTD response times either include substantial margin.in the safety analyses when compared to a change of less than 2% or are insensitive to the actual value of the flow (i.e.,
they are sensitive only to relative changes such as a fractional deviation from the measured full power delta T). Therefore it is only necessary to absorb the penalty on retained CNBR margin in order to support the flow limit reduction.
3.0 TS, CHANGES The TS must be updated in two places to implement the RCS minimum flow limit change. These changes are:
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TS Table 3.2-1 The minimum allowable RCS total flow rate will be changed from 289,200.
gpm to 284,000'.gpm. This limit change is offset by a penalty on retained DNBR margin.as discussed above.
TS Bases 3/4.2.5 A flow limit of 289,200 gpm was assumed in the currently docketed UFSAR accident analyses which was performed with the Westinghouse ITDP. The new limit no longer reflects this assumption and the TS Bases is revised accordingly.
4.0 EVALUATION Based on the above, the minimum measured RCS flow rate at. NA-1&2 may be reduced from 289,?00 gpm to 284,000.gpm without reanalysis of the UFSAR Chapter 15 accidents. A 2.9%. penalty will be extracted from available retained DNBR margin (approximately 10%) in order to compennte for this flow reduction.
Further reductions would require. reanalysis or re-eval-uation of all of the UFSAR Chapter 15 events. Potential accident 'conse-quences remain within the bounds of the UFSAR accident analysis. Therefore, we find the proposed change to be acceptable.
5.0. ENVIRONMENTAL CONSIDERATION These amendments involve a change in the installation or use of a facility component located within the-restricted area as defined in 10 CFR Part 20.
The staff has. determined that the amendments involve 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 published a proposed finding'that the amendments involve no significant hazards consideration and there has been no public consnent on such finding. Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 551.22(c)(9).
i Pursuant to 10 CFR 651.22(b), no environmental impact statement or environ-mental assessment need be prepared in connection with the issuance of the amendments.
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60 CONCLUSION We have concluded, based on the considerations discussed above, that (4) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Commission's regulations, and the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Date:
July 31, 1989 Principal Contributor:
Leon Engle
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