A euphemism coined by Metropolitian Edison vice president Jack Herbein in the wake of the Three Mile Island nuclear incident in 1979.
A mechanical engineer by trade, Herbein spoke more technically (and, ergo, less media-friendly) about the events going on inside the plant as well as nuclear power procedures in general. This coupled with the company's general policy of stonewalling the media as to what was occurring inside made the media very hostile towards MetEd officials and Herbein in particular very quickly.
During one of the question and answer sessions, Herbein was asked what would happen if any of the hydrogen byproduct of the nuclear reactor were to be hit by a spark. Herbein's immortal words emerged; the followup question ("What's the difference between a spontaneous energetic disassembly and an explosion?") was ignored, and ultimately Herbein left the room, refusing to answer any more questions.
Herbein's disarming hostility (telling the same reporters, "I don't see why we have to tell you everything we're doing in there") and general cluelessness (at his first press conference, he announced a radioactive leak 4 times lower than official NRC reports) proved to be more damaging to nuclear power than the TMI incident itself.
A kind of "spontaneous energetic disassembly", if you will.