In nuclear fission an atom is broken up into two or more new particles. Sometimes this happens all by itself, but when we want it to happen more rapidly we use a critical assembly of fissile fuel and a moderator, which multiplies the neutron flux. The moderator slows down the neutrons produced in each fission which can then cause further fissions, resulting in a chain reaction. Each fission also releases energy as heat. The chain reaction is controlled at a steady level by control rods which absorb neutrons.