Written by the Fiveable Content Team • Last updated August 2025
Written by the Fiveable Content Team • Last updated August 2025
Definition
A nuclear moderator is a material used in a nuclear reactor to slow down fast neutrons, thereby increasing the likelihood of further nuclear reactions. Common materials used as moderators include water, heavy water, and graphite.
Moderators slow down neutrons to thermal energies, enhancing the probability of neutron capture by fissile nuclei.
The efficiency of a moderator is determined by its neutron scattering cross-section and its ability to reduce neutron energy.
Common moderators include light water (H2O), heavy water (D2O), and graphite.
Moderators must be chemically stable and capable of withstanding high radiation environments.
The choice of moderator affects the design and safety features of a nuclear reactor.
Related terms
Neutron Cross-Section: A measure of the probability that a neutron will interact with a nucleus, affecting processes such as scattering or absorption.
Thermal Neutrons: Neutrons that are in thermal equilibrium with their environment, typically having lower energies suitable for sustaining chain reactions in reactors.
Fissile Material: $^{235}U$ or $^{239}Pu$, which can sustain a chain reaction when bombarded with neutrons, particularly thermal neutrons.