question_answer

Why is it necessary to slow down the neutrons, produced through the fission of \[_{92}^{235}\text{U}\] nuclei (by neutrons), to sustain a chain reaction? What type of nuclei are  (preferably) needed for slowing down neutrons?                                  

Answer:

                Fast neutrons may escape the fissionable material without causing further fission. Hence they are slowed down to thermal velocities by using moderators like heavy water, graphite, beryllium, etc., which are rich in hydrogen nuclei or protons.