Thermalizing neutrons is also what the moderator (typically water but can also be eg. graphite, like in the infamous RBMK reactor design) surrounding the fuel r

Thermalizing neutrons is also what the moderator (typically water but can also be eg. graphite, like in the infamous RBMK reactor design) surrounding the fuel rods in a nuclear reactor is for. Neutrons fresh from a fission reaction are too fast to reliably induce additional fissions before escaping and being absorbed by the reactor walls.