In the framework of European research activities on nuclear safety, experimental work on ex-vessel core melt behavior under prototypical conditions is being performed. Spreading on various material surfaces and verification of relevant computer codes are the main tasks of the COMAS project, leading to an improvement of the design basis for corium retention systems. In particular, the large-scale two-dimensional spreading test [1:6 with respect to the European pressurized water reactor (EPR) spreading area] shows a quick and uniform melt distribution even for thin melt layers supporting the EPR corium stabilization concept.