Suppose 3 neutrons are released by an atom of U-235 in the first stage of a fission chain reaction. Each of these neutrons has enough energy to cause another fission reaction in another nucleus of U-235, releasing more neutrons. And if all of those 2nd stage neutrons cause other nuclei to fission in a 3rd stage, how many total U-235 atoms will have undergone fission after these 3 stages of the chain reaction?

Question

anonymous · Answer

the first stage is 1 U-235 giving off 3 neutrons.  Those 3 neutrons collide with 3 U-235 in the 2nd stage producing various products and in particular 7 more neutrons.  Those 7 collide with 7 more U-235 atoms in the 3rd stage.  That gives you a total of 11 U-235 atoms in the chain reaction.