Fast vs. slow neutrons
•Natural uranium that is mined from the ground is 0.7% U-235 and 99.3% U-238.
•The result of fission is fission products that are radioactive, radiation, fast (or energetic) neutrons and heat.
•The fast neutrons have a low probability of inducing further fissions, and hence have a low probability of generating more neutrons and thus sustaining a chain reaction.