[CORed]

As far as I know, the only isotopes that undergo neutron-initiated fission, and are therefore capable of a fission chain reaction, are Uranium 235, Uranium 233 and Plutonium. In a bomb, where the goal is to have as rapid a reaction as possible, you quickly run out of fuel. Another factor limiting the yield of fission bombs is that above a certain size, the explosion will blow the fuel apart before all of it reacts. The biggest bombs use a fission bomb to intitiate a fusion reaction.

As another poster mentioned, in a reactor, there are "control rods" which contain a material that absorbs neutrons, which can be moved in and out of the core to regulate the speed of the reaction. Also, reactors generally use fuel that is not sufficiently concentrated to cause a nuclear explosion, although if things get screwed up badly enough, the reactor can overheat and trash itself (Three Mile Island) or release large amounts of radioactive material (Chernobyl).