What is necessary for the production of nuclear energy?

The number of neutrons existing in the critical core of the reactor must be steady or increasing, if it is decreasing the reaction will reduce and eventually stop. When a nucleus fissions (ie one of U-235) roughly 2.5 neutrons are produced, on average. Some of these are absorbed by the moderator and other incore materials, and some are lost at the core boundaries, but if for every nucleus that is fissioned, one neutron is then absorbed into another U-235 nucleus causing another fission, then the process will continue at a steady rate and we can call that a chain reaction. If there is a surplus of neutrons the population will increase and this could go on exponentially increasing, but this is contolled by the neutron absorbing control rods so that the neutron flux, or reactor power, is maintained at a steady level, for normal steady operation. In order for a nuclear chain reaction to take place, each nucleus must produce an average of one neutron that causes fission of another nucleus.