It might appear as kind of an abstract concept for someone unfamiliar with the process.
Rather than a poor attempt by me to describe it, I'm going to direct you to this little 60 second video I found on Youtube, after viewing several.
Fission Reaction (HD Rework) - Bellevue University
Additional reading at Wiki: https://en.wikipedia.org/wiki/Nuclear_fission
The above should give you an idea of what the term "going critical" means. Primarily, when the fission process not only becomes self-sustaining, but rapidly ramps up, creating an explosion with release of intense radiation, light, heat, and etc.
Mr. Gunderson, a nuclear engineer, believes the explosion in reactor #3 was a result of a small amount of the fuel pool inside becoming critical.
The reactor vessels are designed to prevent the fuel rods from doing this, but it seems no one thought about the fuel pools or the unusual circumstances encountered by having this much nuclear material in such a close proximity.
This guys seems to think as do I that a leak does not explain the situation. I would not go as far as to say that fission is occurring but there has to be another reason for this surge.
Today’s readings were as high as 1700 milisieverts.
http://rt.com/op-edge/fukushima-radiation-threat-level-288/
Looks like no one thought about a lot of things.
Despite their obvious cerebral superiority, this really puts a dent in how endowed the Japanese are with common sense. Of course, there were probably many lawyers and labor unions involved.
And thank you for that link. Scary stuff.