In nuclear engineering, a<unk>  is a medium which reduces the velocity of fast<unk> s, thereby turning them into thermal<unk> s capable of sustaining a nuclear chain reaction involving uranium-235.
Commonly used<unk> s include regular (light) water (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.
Neutron moderator
Metabolic intermediate
Heterocyclic compound
Bifunctional catalyst
Designer drug
Antioxidant
Generic drug
Host-guest chemistry