Prompt critical is a necessary, but not sufficient condition for a nuclear weapon to go boom. In a weapon the neutron multiplication time is a million times fas

Prompt critical is a necessary, but not sufficient condition for a nuclear weapon to go boom. In a weapon the neutron multiplication time is a million times faster than in a prompt criticality transient in a thermal reactor. Otherwise it would disintegrate before it could develop any significant yield. But yes, certainly the melted fuel becoming critical and/or causing more steam explosions could have made the Chernobyl accident worse than it already was.