In order to investigate the cooling of core melt in a lower plenum during severe accident conditions, a numerical model of natural convection due to volumetric heat generation was prepared. Four different geometries were taken into consideration. As our primary concern was the model verification, the geometries were selected from the experimental work of Asfia and collaborators, which is documented in the open literature [1].

The calculations were performed with the ANSYS CFX fluid dynamics software. The results of transient calculations reveal a stable and thermally stratified lower part of the flow, and a much more dynamic upper layer that is dominated by time dependent structures. The heat flux is the lowest at the stagnation point and increases along the spherical segment. The ratio of maximum and minimum heat transfer coefficient can be as high as 50.
Based on the simulation results, the distributions of local heat transfer coefficients along the wall were obtained and compared with the measurements of Asfia et al. [1]. The comparison shows good agreement with the experimental data. We also made a comparison of the calculated global Nusselt numbers with the correlations of Mayinger et al. [2] and Reineke et al. [3]. The agreement between the results that is well inside the experimental scatter verifies the selected modelling approach.