in a nuclear meltdown, the most severe outcome of a steam explosion is early containment building failure. two possibilities are the ejection at high pressure of molten fuel into the containment, causing rapid heating; or an in-vessel steam explosion causing ejection of a missile (such as the upper head) into, and through, the containment. less dramatic but still significant is that the molten mass of fuel and reactor core melts through the floor of the reactor building and reaches ground water; a steam explosion might occur, but the debris would probably be contained, and would in fact, being dispersed, probably be more easily cooled. see wash-1400 for details.