Arnie Gunderson from Fairewinds makes the case that the explosion in fuel pool of Unit 3 at Fukushima was a detonation (faster than the speed of sound) and not a deflagration (speed of sound). A plausible reason was a initial hydrogen explosion in the pool distorted the fuel and initiated a moderated prompt criticality. http://www.fairewinds.org/gundersen-postulates-unit-3-explos...

"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...