Neutron moderation

Neutron moderation is the slowing of fast neutrons through collisions with atomic nuclei. In many reactors, it increases the probability that neutrons will cause fission in fissile fuel.

Elastic scattering: A collision in which total kinetic energy and momentum are conserved, though energy can move between the colliding particles. Neutrons lose kinetic energy by transferring part of it to nuclei in these collisions.
Neutron: An electrically neutral subatomic particle found in atomic nuclei and released in some nuclear reactions. Its mass and lack of electric charge govern how it collides with moderator nuclei.
Light water reactor: A nuclear reactor that uses ordinary water as both coolant and neutron moderator. Water slows neutrons through hydrogen collisions while also carrying heat from the core.
Fast neutron reactor: A nuclear reactor designed to sustain its chain reaction primarily with high-energy neutrons. Unlike thermal reactors, it operates without relying on substantial neutron moderation.
Neutron energy spectrum: The distribution of neutron energies in a population or radiation field. Moderation changes the spectrum from predominantly fast neutrons toward lower energies.
Neutron scattering: The deflection or energy change of a neutron when it interacts with matter. Moderation is the repeated-scattering process that reduces neutron energy.
Heavy water reactor: A nuclear reactor that uses deuterium oxide as a neutron moderator, commonly also as coolant. Deuterium slows neutrons while absorbing fewer of them than ordinary hydrogen.
Neutron reflection: The scattering of neutrons back toward a region from which they might otherwise escape. Reflection returns neutrons toward the core; moderation reduces their energy.
Neutron cross section: A measure of the probability that a neutron will interact with a target nucleus through a specified process. The energy-dependent interaction probabilities determine how scattering and absorption compete during moderation.
Nuclear binding energy: The energy required to separate an atomic nucleus into its constituent protons and neutrons. Nuclear structure influences which neutron interactions scatter or capture a neutron.

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