Passage: Neutron emission occurs in very rare situations in which a nucleus contains extra neutrons. Unstable nuclei of this sort will often decay rapidly, with half-lives of a fraction of a second.Fast neutrons are usually undesirable in a steady-state nuclear reactor because most fissile fuel has a higher reaction rate with thermal neutrons. Fast neutrons can be rapidly changed into thermal neutrons via a process called moderation. This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons. These collisions will generally speed up the other particle and slow down the neutron and scatter it. Ideally, a room temperature neutron moderator is used for this process. In reactors heavy water, light water, or graphite are typically used to moderate neutrons.

Question: How is the retardation of neutrons by a moderator?
Fast neutrons can be rapidly changed into thermal neutrons via a process called moderation.