Similarly as in Chernobyl, the chain reaction produced excess heat, and this likely caused water to dissociate into oxygen/hdydrogenium which caused the big explosion. In that sense, it was not a nuclear explosion, but one caused by an out-of-control chain reaction.

If I remember correctly, there were also reports of some observations of short-lived fission products. But we still do not know exactly what happened.

Roughly the same could have happened in Three Miles Island, because the reactor was melting down, but the process was stopped, possibly very little time before the reactor would have been destroyed.