question_answer

Neutrons produced in fission can be slowed down even by using ordinary water. Then, why is heavy water used for this purpose?   

Answer:

                Neutrons produced during fission get slowed if they collide with a nucleus of the same mass. As ordinary water contains hydrogen atoms (of mass nearly that of neutrons), so it can be used as a moderator. But it absorbs neutrons at a fast rate via the reaction: \[n+p\to d+\gamma \] Here d is deuteron. To overcome this difficulty, heavy water is used as a moderator which has negligible cross-section for neutron absorption.