In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.
Commonly used moderators include regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.
| Moderator | Reactors | Design | Country |
|---|---|---|---|
| none (fast) | 1 | BN-600 | Russia (1) |
| graphite | 29 | AGR, Magnox, RBMK | United Kingdom (18), Russia (11) |
| heavy water | 29 | CANDU | Canada (17), South Korea (4), Romania (2), China (2), India (2), Argentina, Pakistan |
| light water | 359 | PWR, BWR | 27 countries |
Read more about Neutron Moderator: Moderation, Reactor Moderators, Form and Location, Moderator Impurities, Non-graphite Moderators, Nuclear Weapon Design, Materials Used