During the 50's and 60's a number of experiments where conducted in the US to examine criticality accidents in light water reactors. One basically inserted large amounts of criticality (by ejecting control rods from the core) to see what happens. It is quite fascinating to watch the videos of the experiments. The first two videos are from the Borax experiments and the third one from SPERT, enjoy watching them!
I will try to find some time to write more about criticality, prompt criticality and feedbacks that keep reactors stable and how one calculates transients, but until then if someone technically minded wants to read more about the experiment I recommend searching on DOE's information bride (http://www.osti.gov/bridge/basicsearch.jsp)