"Steam explosion" refers to a sudden and powerful blast caused by the rapid release of steam, typically when water comes into contact with something extremely hot like molten lava or heated metal. Full definition
«If that happened it would have triggered a second steam explosion that would have done unimaginable damage and destroyed the entire power station, including the three other reactors,» author Andrew Leatherbarrow wrote in an email to Tech Insider. (businessinsider.com)
Brian McDermott, then - director of the NRC's Division of Preparedness and Response, completed the thought on a conference call, according to the NRC transcripts: «Then you've got potential steam explosions with this melting into the water in the torus. (scientificamerican.com)
UNIDENTIFIED: «And landing in the water under the vessel, it would have caused a little steam explosion.» (scientificamerican.com)