How do you control a nuclear reaction?

Chris - They use what are called control rods - these are dense materials which, when dropped down into the reactor, soak up some of the neutrons that are produced by the nuclear chain reaction. The consequence of that is that there are fewer neutrons left to bust open other uranium or fissionable nuclei, and as a result, the chain reaction is slowed down. By putting the fuel rods in, or drawing them out, you can speed up or slow down the chain reaction, and therefore, you can affect how much energy actually comes out of the reactor.