The article presents the results of the experimental study carried on molten corium and its crystallization products resulted from contact with sacrificial concretes. Experiments were carried out at COMETA installation using induction melting in cold crucible technology in air. The results of the physical-chemical analysis of the examined composite system U4+-U6+-Zr4+-Fe3+-Fe2+-Si4+-Al3+-Ca2+-O2− presented in this paper are based on computational diagram of fusibility of quasi ternary oxide systems. This work was carried out within the framework program KONTAKT-II project, LH12224 “Modeling and investigation of hypothetical non-standard situations at nuclear reactors of new type”, experimental work has been partly supported by the SUSEN Project CZ.1.05/2.1.00/03.0108 (European Regional Development Fund (ERDF)). Recommendations for further investigation are presented in this paper.