Hi all, I am trying to study fission, and my crude understanding has some gaps. a) U235 fissions when it absorbs a thermal neutron because U236 is energetically favorable, and the energy difference is larger than the activation barrier to splitting. The activation barrier in the simple model arises from the larger surface-tension energy of two small nuclei compared to the one larger one. So it's U236 that splits, right? b) Can a fast neutron also induce the reaction? I'm thinking yes since bombs fission and must have mostly fast neutrons, not thermal... c) Would that imply that the purpose of moderators in reactors is to absorb excess neutrons to keep the fuel from going critical? c) If U238 absorbs a thermal neutron it becomes Pu239. Where do the two extra protons come from? Thanks in advance.