Even if you pull all the control rods, it causes a meltdown, which is a permeation of the upper crust. It won't go boom cause critical mass requires the nuclear material to reach a super high density, which is achieved in warheads by a spherical arrangement of explosives, timed down to the millisecond to coordinate the pressure. A nuclear reactor lacks both the right fuel type and the pressure causing variables. Even if pressure builds up, the superstructure fails. Before then, it's likely the whole damn thing will just catch fire.
So no, nuclear reactors can't go boom. But it is pretty.
