In postulated severe accident scenari in which the vessel fails, the molten core (or corium) would interact with concrete. This concrete is either the third barrier, as in current plants, or it is a sacrificial material as in the EPR temporary retention pit. Considering recent experimental findings and the high risk significance to determine the basemat failure, it is recognized that the MCCI and its radial/axial corium erosion is not modelled adequately and large uncertainties subsist, especially for the late phase of MCCI. There is also a lack of experimental database on the role of stratification between immiscible oxidic and metallic phases. Further on, the knowledge on the cooling mechanisms by top flooding of the ex-vessel corium pool needs to be increased because it influences significantly the calculated results of stop or not of the basemat erosion, which affects the containment integrity. A large experimental program is underway within SARNET to tackle these issues. It is complemented by modelling, code benchmarking and reactor calculations.