Numerical investigation of top-flooding debris bed quenching FLOAT experiments

Safety relevant questions during severe accidents in a nuclear power plant may concern the ex-vessel debris bed coolability in combination with molten corium concrete interaction. The behaviour within the debris bed is expected to be multidimensional, as the bed may be irregular in geometry, composed of heterogeneous particles, exhibit spatially varying porosity, be flooded from different directions, partially re-melt due to local or global dry out, and be affected by non-condensable gases. Fundamental investigation of such conditions requires consideration of multidimensional behaviour. It can be supported by top-flooding experiments and simulations with validated computer codes. This paper evaluates the suitability of an axisymmetric description for analysing multidimensional behaviour during debris bed quenching under top-flooding conditions. Analyses of recent FLOAT experiments (IKE, Germany) show that the multidimensionality of the quenching process is governed by the location of the penetrating water. A detailed numerical investigation performed with the MC3D code (ASNR, France) provides additional valuable insights supporting the interpretation of the experimental results. The axisymmetric modelling shows good qualitative agreement with the measurements. An improved quantitative description is possible with non-axisymmetric modelling. Sensitivity to air flow, initial temperature, porosity, and friction modelling is observed.