[en] Study of structure and content of melting products obtained in set of experiments simulating conditions of severe accident on light water reactor at the stage of interaction of corium melt with water is carried out. It is established, that at contact of 0.7-0.8 kg melt of both the fuel and the constructional materials heated up to 3,700 K with 1.5 kg of cold or hot water the strong fragmentation of melting have being occurred. This process the high-velocity intensive generation of vapour and hydrogen is accompanying. It is shown, that fragmentation rate depends on corium content and height of it discharge into the water