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A filter system to be used in order to reduce the radioactive pollution when reducing the pressure in the containment in case of a severe accident is developed. The radioactive pollution will be captured in the vessel that acts as a bubble column. To be able to maximize the water content in order to ensure that the radioactive pollution is captured it is of importance to predict the swelled water level in the vessel. In this study is three different methods to predict the gas holdup is presented and compared to experimental results; empirical correlations, thermal-hydraulic codes and Computational Fluid Dynamics (CFD). The advantages and disadvantages with the different methods are discussed. Both Computational Fluid Dynamics and the thermal hydraulic code (Gothic) show good predictions of the water level. Using empirical models it is crucial to have knowledge of the two-phase flow region in the bubble column.

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The goal of this work is to design a nozzle system around the Reactor Pressure Vessel (RPV) of a VVER-1000 nuclear reactor which would ensure successful melt retention during a severe accident. Simulations were performed with a multiphase model in ANSYS Fluent 19.1 to determine this. The results suggest that an efficient cooling can be achieved by inducing a flow rising parallel to the RPV walls in the flooded reactor cavity. In order to do this, it is proposed to use one central nozzle below the RPV and a ring of 32 nozzles 0.5 m above the RPV bottom. Assuming a 2 bar pressure, injection of 750 m 3 /h water at 80 o C through the nozzles led to little steam production and a heat flux below the critical value.