ML20059E304
| ML20059E304 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood, Zion File:ZionSolutions icon.png |
| Issue date: | 10/28/1993 |
| From: | Simpkins T COMMONWEALTH EDISON CO. |
| To: | Murley T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9311030163 | |
| Download: ML20059E304 (3) | |
Text
..
Ccmmonwealth Edisan Z
1400 Opus Pface U
Downers Grove. tilinois 60515 o
October 28,1993 Dr. T.E. Murley, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Attention:
Document Control Desk
Subject:
Zion, Byron, and Braidwood Units 1 and 2 Small Break Loss of Coolant Accident Analysis Evaluation Model (Westinghouse NOTRUMP): Resolution of Issues (1) Safety Injection in the Broken Loop and (2) Drift Flux Flow Regime Error
References:
1.
CECO letter, J.A. Bauer to T.E. Murley transmitting the 1993 Annual 50.46 Report for Byron, Braidwood, and Zion Stations, dated July 6,1993.
2.
Westinghouse Letter (ET-NRC-93-3971), N.J. Niparulo to Mr.
1 R.C. Jones, Jr. (NRC), " Notification of a Significant Change to the Westinghouse Small Break LOCA ECCS Evaluation Model, Pursuant to 10CFR50.46 (a)(3)(ii): Safety Injection (SI) in the Broken Loop," dated September 21,1993, Westinghouse Letter Number.
Dear Dr. Murley:
This letter fulfills
.a 30 day reporting requirement of 10CFR50.46(a)(3)(ii) for Units 1 and 2 of Zion, Byron, e-d Braidwood Nuclear Power Stations. Reference 1 transmitted
+
to the NRC the 1993 Annual 10CFR50.46 report for Zion, Byron, and Braidwood Units 1 and 2. Reference 2 transmitted to the NRC the resolution of a modeling discrepancy in.
the Westinghouse NOTRUMP Small Break LOCA ECCS Evaluation Model related to safety injection flow into the broken loop. This model was used for Byron, Braidwood, I
and Zion. The result of the vendor's (Westinghouse) evaluation was that the current model results (Reference 1) provide conservative and more limiting results than a revised model which would incorporate the proposed modeling changes reported in Reference 2.
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Dr. Murley October 28,1993 In a separate transmittal, Westinghouse notified Commonwealth Edison of an additional modeling discrepancy in NOTRUMP. This discrepancy, the " Drift flux flow regime error" was evaluated by the vendor, and resulted in, at a minimum, a 13 F benefit to the predicted peak clad temperature (PCT) for the limiting break.
Commonwealth Edison has reviewed the referenced Westinghouse notifications and concurs with Westinghouse's conclusion that Commonwealth Edison has no reporting obligations under 10CFR21. For both discrepancies, Westinghouse has determined that the issues do not constitute substantial safety hazards, and that all Commonwealth Edison plants continue to comply with the requirements of 10CFR50.46 and 10CFR 50 Appendix K acceptance criteria. After consideration of the effects of these issues on the plants licensing bases, Commonwealth Edison has elected not to incorporate these modeling changes into the evaluation models for Zion, Byron, and Braidwood. The basis for this decision is described below:
1.
The vendor's investigation related to the drift flux flow regime errors determined that the net effect of the evaluation results in a benefit to the predicted PCT for the limiting break case. The issues relate to the NOTRUMP drift flux flow regime map errors. In the evaluation model applications, this model is only used during counter-current flow conditions in vertical flow links.
This error allowed a discontinuity to exist in the flow regime map under certain circumstances. In addition, a closely related logic error was also discovered which led to discontinuities under certain other circumstances. Both of these errors ~ care corrected and resulted in a net benefit (reduction) to the PCT of between 13 F and 55 F. The evaluation of the PCT benefit is based on a.
conservative generic studies for Westinghouse designed NSSSs, but does not constitute a change to the application of the evaluat!
ens in the current licensing bases for Byron, Braidwood, or Zion. If Commonwealth Edison obtains a re-analysis or an evaluation which include these effects these effects will be reported at that time.
2.
The vendor's investigation related to safety injection flow into the broken loop determined that the net effect of the evaluation results in a benefit to the predicted PCT for the limiting break case. The issue relates to the positive impact of modeling steam condensation in the RCS due to the cold safety.
injection flows into the intact loops and relates to the negative impact of the cold safety injection flows into the intact loops and relates to the negative impact of the cold safety injection flows into the broken loop interfacing with steam flow out the postulated break. The vendor's investigation determined, from testing, that the rate of condensation in the intact cold legs was under-predicted by the current model. The current licensing basis analysis assumes a more conservative condensation model and assumes that no injection flow enters the ruptured cold leg, resulting in higher RCS pressures and lower k:nla: zion:nrc:10
j Dr. Murley October 23,1993 injection flows. The PCT decrease due to steam condensation modeling more than offsets the PCT increase due to the safety injection flows interfacing with the steam flow. Therefore, the current licensing basis analysis for Byron, Braidwood, and Zion is conservative. Since the current analysis is conservative, it is acceptable. The evaluations of the PCT benefit (150 F) and penalty (150 F) are based on conservative generic studies for Westinghouse designed NSSSs, but does not constitute a change to the application of the evaluation models in the current licensing bases for Byron, Braidwood, or Zion. If Commonwealth Edison obtains a re-analysis or an evaluation which includes these effects, these effects will be reported at that time.
The Westinghouse Owners Group is currently reviewing these issues and is considering the development of a generic program for their resolution. Because compliance with 10CFR50.46 is maintained with existing PCT penalties, no near term reanalysis is planned for CECO's PWR units. Since these modeling changes are not being made to the licensing basis fbr Zion, Byron, or Braidwood at this time, no credits or penalties have been assigned to the PCT results for their respective evaluation models and no rackup sheets are being provided.
Please direct any questions to this office.
Respectfully, eT,
T.W. Simpkin Nuclear Licensing Administrator cc: J.B. Martin - Region III R.R. Assa, Project Manager - NRR C.Y. Shiraki, Project Manager - NRR S.G. DuPont, Senior Resident Inspector - Braidwood H. Peterson, Senior Resident Inspector - Byron J.L). Smith, Senior Resident Inspector - Zion k:nin:rion:nrc:11