Why is energy released during fission reactions?

Question

If the energy released during a fission reaction is the binding energy then what energy is left to bind the nucleons together? I'm very confused if there's a better explanation on fission I'd appreciate that.

Answer 1

This is the curve of binding energy per nucleon for the list of nuclei, versus the number of nucleons in a nucleus.

Up to iron Fe, which has 56 nucleons the more nucleons the higher average binding energy per nucleon This means if one can fuse two nuclei below 56 energy may be released energy.

after that, the more nucleons, the less binding energy per nucleon. This means that a break up of a high nucleon content nucleus may release energy, because the fragments will be more bound than the original .

Not all heavy nuclei will break conveniently, releasing energy, because this is a quantum mechanical scattering process and quantum numbers have to be conserved as well as energy. For example uraneum-235 can be broken ( is fissile) into pieces by a neutron scattering, that will release energy.

Answer 2

The total rest masses of the fission products is less then the original fuel nucleus. The excess mass is the invariant mass of the energy that is released as gamma rays, and kinetic energy of the fission fragments.

You are asking whether if all the binding energy is released then what binds the fragments (after fission) together. Now not all binding energy is released. Typically, when a nucleus fissions into two daughter nuclei fragments, only 0,1 % of the original mass of the nucleus is released fission energy. Most of this fission energy comes in the kinetic energy of the daughter nuclei which fly apart at 3% of the speed of light.

If you look at a nucleus, 99% of its rest mass comes from binding energy and only 1% is the rest mass of its constituents. Now if only 0.1% of the mass is released as fission energy, that will mean that the daughter nuclei will have less rest mass, by 0.1% then the original nucleus. But 99% of that rest mass (of the original nucleus) was binding energy, and 1% was rest mass of the constituents. Only 0.1% of that 99% binding energy is released as fission energy (and only 0.1% of the 1% rest mass of the constituents), so most of the binding energy is still going to be there that will hold the daughter nuclei in one piece.

What you are missing is that the binding energy is not only between neutrons and protons. If you look at a neutron, 99% of its rest mass comes from binding energy between (color force) the quarks that make up the neutron, and only 1% of its rest mass comes from the rest mass of the quarks themselves.

So when there is nuclear fission, and a nucleus (made of protons and neutrons) fissions into daughter nuclei, that themselves have protons and neutrons, there is still binding energy that holds the daughter nuclei in one piece, binding energy between the dughter nucleus's neutrons and protons, and between the quarks that make up the neutrons and protons. Only 0.1% of that binding energy is released as fission energy.