Question

the purpose of the moderator in a nuclear reactor is to ?

Answer 1

Without wishing to sound flippant, it is to moderate the nuclear reaction. In other words it controls the rate of the reaction so that it doesn't go too fast or too slow

Answer 2

Neutrons are the usual reactant that triggers nuclear reactions, moderators absorb them so the rate of nuclear reactions is adjusted ( which is complicated to establish to begin with) Beryllium is a good neutron absorbent, also graphite has been used.