Parent was alluding to a nuclear explosion, not a steam explosion or other type of explosion. Other kinds of explosions have nothing to do with the fissile material ("rocks" in their parlance) being "too close together". Steam explosions in particular are caused by boiling water, due to increased reactor power, or inadequate circulation of coolant. In a nuclear reactor, the fuel cells are held in a fixed matrix, and are not moving an inch closer to each other, whether the reactor is operating normally, or a steam explosion is imminent.
In general, nobody was disputing the possibility of steam explosions, or other type of failures at nuclear power plants, thus your comment is besides the point, and irrelevant to this subthread.
Impossible. Worst thing that can happen without carefully designed explosive lens is a nuclear fizzle.