Sometimes the unlikeliest of sources provides clarity. Silverstein’s book is entertaining but rather off topic for me, except take a look at this:
Manhattan Project scientists discovered that some neutrons move at about seventeen million miles per hour, one fortieth of the speed of light. If they are “moderated” to about five thousand miles per hour, they have a better chance of being absorbed by another atom and provoking fission.
Reverting to my units, I’m staggered to discover that the difference between “normal” reactors and “breeders”/”fast” reactors is 8,000 kms/hour versus 27,000,000 kms/hour. When we’re told that “moderation” slows down neutrons, we can lose sight of how much slowing down takes place. A fast neutron is over 3,000 times faster than a slow one.
Silverstein, Ken. 2004. The Radioactive Boy Scout: The True Story of a Boy and His Backyard Nuclear Reactor. Random House, New York, p. 117.