Moderator
Definition and meaning of Moderator in chemistry.
A moderator is a substance used in a nuclear reactor to purposefully slow down fast neutrons produced by fission reactions. By reducing the kinetic energy of the neutrons, the moderator increases the overall probability that they will successfully cause further fission events, thereby sustaining the nuclear chain reaction.
In more detail
In nuclear fission, the newly emitted neutrons travel at extremely high speeds. Since fissile isotopes, like uranium-235, are much more likely to absorb slow-moving thermal neutrons, a moderator is absolutely necessary to thermalize them. Good moderators are materials with low atomic mass that do not readily absorb neutrons, allowing for efficient elastic collisions. Common moderator materials include light water, heavy water, and nuclear grade graphite. The choice of moderator often directly dictates the overall design and fuel enrichment requirements of the nuclear reactor.
Key facts
| Field | Physical Chemistry |
|---|---|
| Primary Function | Slows down fast neutrons to thermal speeds |
| Common Materials | Light water, heavy water, graphite |
Heavy water (D2O) is used as a highly efficient moderator in CANDU nuclear reactors to slow down fast neutrons without absorbing them significantly.
Frequently asked questions
Why is regular water considered a good nuclear moderator?
Water contains hydrogen atoms, which have a mass very similar to a single neutron, allowing for highly efficient kinetic energy transfer during atomic collisions.