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Why are neutrons slowed by a moderator in most nuclear reactors?

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nuclear engineering Installation of the dome of a containment structure at the Taishan nuclear power plant, Guangdong province, southern China, 2012.

In nuclear engineering, neutrons released during nuclear fission are emitted at very high energies, typically around two million electron volts. These "fast" neutrons are much less likely to induce further fission in fissile material like uranium-235 than slower, "thermal" neutrons.12

A moderator is used to slow these neutrons down through a process called down-scattering. By colliding with light atoms—such as hydrogen, deuterium, beryllium, or carbon—the neutrons lose kinetic energy until they reach thermal equilibrium with the reactor.13

This moderation is essential because it increases the probability that the neutrons will successfully interact with fissile nuclei to trigger subsequent fission events, thereby initiating and sustaining a controlled chain reaction. Without this slowing process, the efficiency of the fission chain reaction in a thermal reactor would be significantly reduced.431