In a nuclear reactor, a fission chain reaction occurs under controlled conditions. The most common type of reactor uses low enriched uranium, these mixtures cannot maintain fission as 238U absorbs too many neutrons. However 238U is a good moderator acting like a catalyst by removing the fast neutrons leaving the more useful slow neutrons. These are more efficient at maintaining a chain reaction as they are less likely to be absorbed by 238U and are more effective at triggering the fission of 235U.
The size, number and location of the uranium rods and the amount and shape of the moderator must be designed so the reactor is critical, i.e. enough neutrons to keep the chain reaction going. The number of neutrons can be finely adjusted by control rods of boron or cadmium, which readily absorb neutrons. By moving the control rods in or out of the reactor the number of neutrons present for fission is controlled. However as with Chernobyl this can go disastrously wrong.