The controlled release of nuclear energy in a reactor is accomplished by

Nuclear power refers to the application of nuclear reactions, which discharge nuclear energy to produce heat and is most generally utilized in steam turbines to generate electricity in a nuclear power plant. Nuclear energy can be obtained from nuclear decay, nuclear fission, and nuclear fusion reactions.

The controlled release of nuclear energy in a reactor is attained by controlling the fission without controlling a chain reaction. If a chain reaction occurs, then the amount of energy would increase exponentially, so in order to monitor the release of energy, fission must be controlled without controlling a chain reaction.