Experimental study and analytical analysis on the global heat transfer of stratified corium in LWR lower head

Highlights: • Pioneer work with large-scale naturally separated simulant material at high temperature. • The turbulent flow pattern in the upper layer is influenced by upper boundary cooling. • The interlayer crust behavior reacts strongly on upper boundary condition. • Heat flux focusing in upper layer only exists under week upper cooling boundary. • The influence of Pr number in the upper layer heat transfer is discussed. • The influence of the 2D geometry on the heat flux focusing effect is discussed. In-Vessel Melt Retention (IVR) is credited as a promising Severe Accident Management strategy to stabilize the molten corium during severe accident scenarios. This strategy is not only adopted in VVER 440 or AP600 designs, but is also included in higher power reactors around 1000 MWe, such as AP1000 and Chinese CPR 1000. One of the current research priorities bound with large uncertainty is the heat flux at the cooled wall in the metallic layer atop of a ceramic pool, as heat flux focusing effect could appear at this location. The generation of experimental data with naturally separated melt layers is essential to improve the understanding of the heat transfer between the melt layers and on their boundaries. Under the framework of EU H2020 IVMR project, two large-scale test series with simulant materials were conducted in LIVE2D facility at KIT. Rigid top cooling and hot atmosphere on the upper layer were realized in a two-layer melt pool with different upper layer thicknesses. Important global heat transfer phenomena have been obtained, including the effect of the top cooling boundary, the behaviour of the interlayer crust and the impact of upper layer thickness. Finally, the influence of Pr number and geometry are discussed.