Hello, I am reading about this and I have a question , so let me explain how I understood this and please correct where I am wrong. I will ask about nuclear reactors because obviously in bombs only prompt neutrons matter since there is no need for any control only an exponential increase in reaction rate. So I'll start from ground up , one assembles a fresh new nuclear reactor core , the fuel is loaded and the reactor is kickstarted by an external neutron source like a radium rod or whatever.In the first few seconds I assume there are only prompt neutrons , the ones that start the chain reaction and split the U235 nucleus creating the first isotopes of U235(or should I say nuclides? since they haven't yet decayed to isotopes?) , seconds to minutes later (depending on the radionuclide) secondary fission starts correct? the main U235 nucleus has been split into many isotopes which are now decaying , some with beta decay and some with direct neutron release if they are energetic enough to brake the bond? I assume that once this beta decay is over for a given element it then releases a neutron which is the delayed neutron that can then go on and if not absorbed create a new split of U235? This is the part that I don't understand , a nuclear reactor has neutron moderators usually of two kind , one is water or heavy water which also acts as coolant , the others are boron etc used in control rods which are inserted or taken out to compensate reactivity and control the reactor but then how do the neutron absorbers decide to catch the Prompt or fast neutrons and leave out or let the delayed ones do the work , how come the delayed ones aren't absorbed too and the chain reaction killed altogether? Is it because the prompt neutrons have higher energies (fast neutrons) than the delayed ones, so the neutron absorber catches more high energy neutrons than lower energy delayed ones allowing them to do the main deal of splitting new nucleus? I don't quite get this because the water acts as a moderator for the fast neutrons making them thermal neutrons , so if the delayed neutrons are weaker than the fast prompt ones how come the water doesn't slow them down so much so that they can't initiate a fission anymore? Also in that case do fast reactors like a fast molten salt breeder for example have delayed neutrons at all and if yes then do they play any role? do the fast ones catch into the breeder substance like thorium more and the delayed ones fission the primary fissile core due to the difference in neutron absorbtion between nuclear fuels like U235 and Th232? In other words the fast ones make the throium232 into fissile U233, while the delayed ones split the U235 in the core and also the added U233? Thanks for your answers.