ML19282B575
| ML19282B575 | |
| Person / Time | |
|---|---|
| Issue date: | 02/14/1979 |
| From: | Tedesco R Office of Nuclear Reactor Regulation |
| To: | Vassallo D Office of Nuclear Reactor Regulation |
| References | |
| TAC-46174, NUDOCS 7903150405 | |
| Download: ML19282B575 (3) | |
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UNITED STATES
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NUCLsAR REGULATORY 00MMisSICN
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ru s;.m MEMORANDUM FOR:
D. B. Vassallo, Assistant Director for Light Water Reactors, DPM FROM:
R. L. Tedesco, Assistant Director for Reactor Safety, DSS
SUBJECT:
BOARD NOTIFICATION:
COMBUSTION ENGINEERING RUPTURE STRAIN MODEL IN LOCA CALCULATIONS
References:
1.
Memorandum to Z. Rosztoczy from K. Kniel, "CE Rupture Strain Model," dtd Jan. 26, 1979.
2.
Memorandum to P. S. Check from D. A. Powers, "CE Flow Blockage Review," dtd Sept.1,1977.
3.
Letter to A. E. Scherer from D. F. Ross, Jr.
dtd March 22, 1978.
4.
Letter to D. F. Ross, Jr. from A. E. Senerer dtd Sept. 18, 1978.
The NRC staff has determined (Ref.1 and 2) that a part of the approved flow blockage submodel of the Combustion Engineering ECCS Evaluation Model might, in general, be non-conservative and therefore, might not be in compliance with Appendix K of 10 CFR Part 50. Section IB of Appendix K states that each evaluation model should include provisions for predicting cladding swell and rupture during the LOCA transient based upon applicable data in such a way that the degree of swelling not be under predicted. The staff met with CE on two occasions (March 8,1977 and January 16,1978) to discuss the model in cuestion.
CE stated that other conservatisms in the flow bhckage submodel more than compensate for the current cladding rupture-strain curve in question.
Early in 1978 we sent a letter to CE (Reference 3) that requested CE to submit technical justification for continued acceptance of their ECCS Evaluation Model, and provided staff guidance on implementation of an acceptable rupture strain curve. CE responded to our request in September,1978 (Reference 4).
CE's response included an alternate flow blockage model with a revised cladding rupture-strain curve and a revised cross-flow and heat-transfer model. Calculations showed that use of the revised (less conservative) cross-flow and heat-transfer model will provide compensating margins to offset those lost from the use of the revised (more conservative) rupture-strain curve. Accordingly, CE states that the use of botn revisions would 7903150 % T
D.B. Vassallo result in a decrease in peak cladding temperature and local zirconium-water reaction. Therefore, CE concludes that their overall ECCS Evaluation Model remains sufficiently conservative.
After reviewing tNse and other steam cooling calculations, the staff believes that sufficient margin does exist in the approved CE ficw blockage model to offset the possible non-conservatism of the rupture strain curve.
The alternate model is presently under review. We are confident that the alternate model or soue variation of the alternate model will show compliance with Appendix X should the present model be unacceptable. We anticipate completion of our formal review by June 1979.
t In view of the above, it is concluded that the appropriate licensing boards should be notified.
In accordance with the requirements of NRR Office Letter No.19, the following information is provided:
1.
The Item For Notification When compared to the available data, the CE LOCA induct.d cladding rupture-strain curve underpredicts strains that occur due to bursting in the low temperature alpha phase of Zircaloy. The rupture-strain curve is part of the approved flow blockage submodel in the CE ECCS Evaluation Model.
Since the rupture-strain curve underestimates the data in some areas, the flow blockage model might not be conservative and might not be in compliance with Appendix K of 10 CFR 50 which prohibits under estimation of cladding swelling.
2.
Relevance and Materiality This item is relevant to all LOCA calculations performed in accordance with the CE LOCA Evaluation Model.
3.
Sianificance Test data including recent US and German research results indicate that the CE LOCA induced cladding rupture strain curve is relatively non-conservative in certain regions. However, the staff believes that other features of the approved CE flow blockage model contain sufficient margin to offset the possible non-conservatism of the rupture strain curve.
The staff believes that sufficient margin exists in the approved CE model to assure no undue risk to the health and safety of the general public, It is the staff's judgement that the calculated peak cladding temperature and zirconium-water reaction will not increase as a result of possible model changes, Thus, the proposed ECCS design for Combustion Engineering plants is sui'ficiently conservative.
Also, it is the judgement of CE that the ECCS Evaluation Model with the ficw blockage requirements are in.conformance with 10 CFR 50, App" dix K.
D.B. Vassallo It is our judgement that calculations made with either the current EM or using the revised cladding rupture-strain curve and revised cross-flow and heat transfer model, would yield comparable results.
Therefore, we are able to conclude that the continued use of the CE/EM is appropriate and is in acco.rdance with the general require-ments of Appendix K to 10 CFR 50. We further believe, however, that when our review is completed changes to a portion of the CE/EM may be necessary to more accurately account for clad swelling and rupture during the LOCA transient.
4.
Relation to Projects This relates to all LOCA calculations performed with the CE Evaluation Model.
I recommend that appropriate boards be notified of this concern and our plans to complete the evaluation of the flow blockage model by June, 1979.
[-
dTwc R.L. Tedesco, Assis tant Director for Reactor Safety Division of Systems Safety cc:
R. Mattson D. Powers V. Stello J. Scinto R. Boyd M. Lauben R. Vollmer D. Ross T. Engelhardt H. Denton B. Grimes P. Check P. Norian Z. Rosztoczy T. Novak K. Kniel F. Coffman L. Phillips S. Weiss W. Hodges NEC__ED C R. Meyer
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