What is the primary function of moderators in a nuclear reactor?

The primary function of moderators in a nuclear reactor is to slow down neutrons to increase fission probability. In nuclear fission reactions, particularly with fuels like uranium-235 or plutonium-239, fast-moving neutrons produced during fission are less likely to cause further fission events. By using a moderator, such as water, heavy water, or graphite, the speed of these neutrons is reduced, resulting in thermal neutrons that are more effective in sustaining a chain reaction.

Thermal neutrons have a higher probability of interacting with fissile material, leading to a greater likelihood of additional fission events. This moderation process is crucial for maintaining the reactor’s criticality and ensuring efficient energy production.

Other functions listed, such as increasing reactor core temperature, generating steam, or containing radioactive materials, do not pertain to the role of moderators. While operators in a nuclear plant will manage heat generation and steam production in other ways, and containment is addressed through reactor design and safety measures, the specific role of moderators is focused solely on neutron speed reduction and enhancing fission likelihood.