Water is used as a moderator to slow down neutrons in a light water nuclear reactor. By slowing down the neutrons they have more of a window to strike fission ready nuclei thus promoting the self perpetuating chain reaction when more neutrons are released from the fissions and slowed neutrons.
All fine and good so far. I got it. Now, what I don't get is why fast neutrons are needed for the breeder nuclear reactor. According to my research, the main reason breeder reactors have not met expectations is the problems encountered with the liquid sodium cooling system. Why liquid sodium instead of water? Because water is a moderator that slows neutrons, while liquid sodium does not slow down neutrons.
Why do we need fast neutrons to breed Pu 239 from U 238. Wouldn't the same slow neutrons used in fission chain reactions also increase the probability of the neutron encountering and hitting the U238 nucleus?