PINKY: Your migration area is tiny, Brain! The neutron will never be able to slow down from fission to thermal in here! BRAIN: Please, Pinky, let me finish! PINKY: I mean, it’s got to be at least one-sixth of the mean square distance between—
Pinky’s science checks out! The migration area of a medium is one-sixth the mean square distance that a neutron travels from its birth in fission until its absorption. The medium involved here is apparently a neutron moderator, which reduces the speed of fast neutrons produced by nuclear fission, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.