There is no clarity in the question. However, the below explanation may help you.
Principal and working of a nuclear reactor:
The device giving large amount of nuclear energy through fission process at a controlled rate is called a nuclear reactor or atomic pile. In the nuclear reactor, the first fission reaction results in the production of fast neutrons. If fast moving neutrons are allowed to pass through moderator they become thermal neutrons. Subsequently, these thermal neutrons are utilized for further fission reactions to produce a large amount of energy.