ML20153A964

Safety Evaluation Accepting Util Proposed Reflood Steam Cooling Model

ML20153A964
Person / Time
Site:	Maine Yankee
Issue date:	06/28/1988
From:	Office of Nuclear Reactor Regulation
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ML20153A958	List: ML20153A954 ML20153A964
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NUDOCS 8807120670
Download: ML20153A964 (3)

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ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO REFLOOD STEAM COOLING MODEL MAINE YANKEE ATOMIC POWER COMPANY DOCKET NO. 50-309

1.0 INTRODUCTION

In a letter from G. Whittier to V. Nerses, (USNRC), da.ed May 21, 1987, Maine Yankee Atomic Power Company (MYAPCo) provided analyses 'or a proposed revised steam cooling model. The steam cooling model is to be ed to calculate heat transfer during the reflood phase of a loss-of-Coolant-Ai cident (LOCA) for Maine Yankee Nuclear Plant. By a January 5, 1988 letter from G. Papanic to M.

Fairtile (USNRC), Yankee Atomic Electric Company (YAECo) also submitted analyses for a revised steam cooling model for Yankee Rowe Nuclear Plant.

Because the proposals by both licensees are identical, and because the current Maine Yankee and Yankee Rowe steam cooling models for LOCA are virtually identical, this evaluation applies to both referenced proposals.

According to Appendix K of 10CFR50, Section I.D.5, heat transfer must be assumed to be accomplished via cooling by steam only when reflood rates in the core fall below one inch per second. Also, flow blockage must be considered in the steam cooling model in terms of both heat transfer and steam flow. Currently, both MYAPCo and YAEco steam cooling models exemplify and u:e these two characteristic requirements conservatively. With the pr:psed modifications, additional heat transfer capability would be credited in the LOCA analyses, thereby lessening the conservativeness of the current stam cooling model.

As it stands now, the steam cooling model is basically a correlation based on FLECHT test data. A penalty is incorporated in the model, in accordance with Appendix K, to account for the effect of flow chanr.31 blockage. As proposed, the steam cooling model modification would take c' dit for some enhancement in heat transfer as reflected in the more recent tes data. The proposed model would, however, continue to include the specific .onservatisms required by Appendix K. Thus, although the proposal would eliminate excessive conservatism, sufficient conservatism in the model would remain.

2.0 EVALUATION Analyses by MYAPCo and YAEco were done with a wide range of test data.

Included were FLECHT-SEASET data results which showed heat transfer enhancement. The staff notes that the tests were conducted for both blocked and unblocked test assemblies, for densely rodded assembly and sparsely rodded assembly configurations, and with and without bypass flow. Heat transfer was consistently found +,o be enhanced during low reflood rates. Sensitivity studies performed by the licensees were also consistent with these findings.

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Two effects are believed by the licensees to contribute to the overall heat transfer enhancement demonstrated by the test results and by the COBRA-TF computer program results. They are droplet breakup and single phase turbulence. The assumed presence of droplets, however, would violate the Aopendix K requirement that cooling may only be assumed by steam. Single phase (gaseous) turbulence downstream of the blockage is therefore considered by the licensees to be the main contributor for the heat transfer enhancement seen in the more recent test data.

Evaluations were performed by the licensees using various experiments to deduce relationships representing the effects of turbulent steam flow behavior. The relationships represented the. effects in terms of Nusselt numbers and showed an exponential decrease in heat transfer enhancement as distance from the blockage increased. The resultant modification to the model was benchmarked and compared to experimental and test results, and were found to be in good agreement.

We are not convinced that the proposed model truly represents the flow regime downstream of the blockage; the physical basis given for the heat transfer enhancement is questionable. However, the staff recognizes that the Appendix K steam cooling model requirements were administered due to a lack of experimental data at low flooding rate. Data generated since the development of Appendix K has demonstrated that reflood heat transfer characteristics at low reflooding rates (less than one inch per second) are similar to heat transfer characteristics at reflooding rates greater than one inch per second. Thus, staff practice is to assure that steam cooling models developed by licensees meet the specific requirements of Appendix K, i.e., effect on flow diversion, while conservatively predicting heat transfer observed in the experimental data. Regardless of the physical interpretation applied to the model, the proposed model was shown to conservatively predict the recent experimental data in accordance with 10 CFR Part b], Appendix K. No change in the flow divergence model would result from the incorporation of the proposed nodel . Therefore, we find the proposed modification to be acceptable.

3.0 CONCLUSION

S Analyses of test data by the licensees show an overall heat transfer

• enhancement for reflood rotes less than one inch per second. The licensees have incorporated a modi #ication to their steam cooling model which better predicts this increased heat transfer as demonstrated in the more recently accumulated reflood heat transfer data. Although the physical interpretation of the additional enhancement is unclear, the model as proposed was demonstrated to be conservative with respect to FLECHT results. Since the model complies with the requirements of Section I.D.S of Appendix K of 10CFR50, we find the proposed steam cooling model revision acceptable.

4.0 REFERENCES

1. Letter frcm G. Papanic, Jr. (Yankee Atomic Electric Co.) to M. Fairtile (NRC), dated January 5, 1988.

2. Letter from G. Whittier (Maine Yankee Atomic Power Co.) to V. Nerses (NRC), dated May 21, 1987.

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3. Letter from G. Whittier (MYAPCo) to A. Thadani (NRC), dated Navember 10, 1986.

4. Letter from D. Crutchfield (NRC) to G. Ward (Exxon Nuclear Co.), dated July 8, 1986.  !

5. Letter from R. Groce (MYAPCo) to NRR (NRC), dated January 15, 1979.

6. "Application of Yankee-WREM-BASED Generic PWR ECCS Evaluation Model to Maine Yankee," YAEC-116, dated July 1979.

7. L. E. Hochreiter, "FLECHT-SEASET Program Final Report,"

NRC/EPRI/ Westinghouse-16, dated November 1985.

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