SIMMER-II Analysis of simulated core expansion experiments at purdue university

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In the analysis of a core expansion (or postdisassembly expansion) phase by the SIMMER-II code, it was shown that there exist various thermo-hydraulic phenomena available for mitigating effectively the mechanical energy released in a hypothetical core disruptive accident. To utilize SIMMER-II as a standard tool in future safety assessment, the experimental validation of the code is crucial especially on the energetics-mitigating effects. In this study, a series of simulated core expansion experiments performed at Purdue University was analyzed by SIMMER-II as the first effort of the code validation program in Japan. In the experiments, either the nitrogen gas at room temperature or the flashing water at high temperature was injected and expanded into the water pool simulating the outlet plenum of the reactor vessel (a 1/7-scaled model of the Clinch River Breeder Reactor vessel). In the analysis of the nitrogen expansion experiments, SIMMER-II could reproduce the experimentally measured slug impact time without adjusting input parameters. This means that the overall fluid-dynamics model of SIMMER-II is valid. In the flashing water expansion experiments, on the other hand, SIMMER did not reproduce the experimental data very well due to the presence of complex rate-limited processes including heat transfer and phase transiton. This discrepancy is ascribed to lack of modeling the entrainment phenomenon occurring at the interface of a vapor bubble. The effect of the entrainment is very important since the entrained cold liquid efficiently enhances the vapor condensation and hence reduces the slug kinetic energy. It was shown that this effect can be approximated by increasing the heat transfer coefficient between liquid components. Obviously, this result cannot be directly extrapolated to the reacor condition, but implys that the nominal SIMMER parameters are conservative from the energetics point of view because of underestimation of the vapor condensation. ...