Basically the faster you go from non-critical to critical, the more energy you get out of a given amount of material. If you do it slowly it just blows itself

Basically the faster you go from non-critical to critical, the more energy you get out of a given amount of material. If you do it slowly it just blows itself apart before it can do much real damage. If you have a small amount of material but enough to be critical and say, generate enough heat to melt itself into a puddle in a minute, it doesn't explode or anything, but before it melts and likely starts itself on fire, everybody nearby is going to get a lethal dose every few seconds. In other words, there's a lot of room between "self-sustaining nuclear reaction" and "bomb". Even storage of materials in warehouses has to be done carefully because too much too close can cause dangerous amounts of reactions.