FLUENT Analysis of a ROSA Two-Phase Stratification Test

One of the OECD ROSA project tests, investigating temperature stratification in the cold legs and the downcomer during ECCS water injection under two-phase natural circulation conditions was analysed with the FLUENT code. The guidance given in the “Best Practice Guidelines for the Use of CFD in Nuclear Reactor Safety Applications” of the OECD GAMA group was followed. The standard k-ε turbulence model of FLUENT was applied along with the VOF (Volume of Fluid) description of the steam/water phases. In order to model the interface of the two phases more closely, transient calculations were performed. Based on a comparison of calculated and measured results, it is found that the standard k-ε turbulence along with the VOF model gave acceptable description of the temperature distribution within the water layer. However, the model under-estimates mixing at the injection point, while it is over-estimated further downstream in the stratified water flow. The condensation model applied in FLUENT strongly under-estimated the subcooling of the steam phase. Insufficient condensation might explain why the downcomer level drops below the cold leg, which was not the case in the test.