Go on, my understanding is that the "explosion" was basically a steam pressure explosion which blew the top off of the reactor, if that is incorrect do you have a reasonably easy to understand source?
"However, the sheer force of the second explosion, and the ratio of xenon radioisotopes released during the event, indicate that the second explosion could have been a nuclear power transient; the result of the melting core material, in the absence of its cladding, water coolant and moderator, undergoing runaway prompt criticality similar to the explosion of a fizzled nuclear weapon."
http://en.wikipedia.org/wiki/Chernobyl_disaster#Experiment_a...
NB I'm in the process of reading *"Atomic Accidents - A History of Nuclear Meltdowns and Disasters: from the Ozark Mountains to Fukushima" - but I haven't got to Chernobyl yet....
http://www.amazon.com/Atomic-Accidents-Meltdowns-Disasters-M...
"It is not a bomb. A bomb is a prompt fast criticality." http://www.fairewinds.org/fukushima-daiichi-the-truth-and-th...
In the 50s the US did an open air experiment called Borax - Safety Experiment on a Boiling Water Reactor, where they blew up a reactor. http://nukeprofessional.blogspot.com/p/videos-moderated-prom...