<P> In fission there is a preference to yield fragments with even proton numbers, which is called the odd - even effect on the fragments' charge distribution . However, no odd - even effect is observed on fragment mass number distribution . This result is attributed to nucleon pair breaking . </P> <P> In nuclear fission events the nuclei may break into any combination of lighter nuclei, but the most common event is not fission to equal mass nuclei of about mass 120; the most common event (depending on isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 u and the other the remaining 130 to 140 u . Unequal fissions are energetically more favorable because this allows one product to be closer to the energetic minimum near mass 60 u (only a quarter of the average fissionable mass), while the other nucleus with mass 135 u is still not far out of the range of the most tightly bound nuclei (another statement of this, is that the atomic binding energy curve is slightly steeper to the left of mass 120 u than to the right of it). </P> <P> Nuclear fission of heavy elements produces exploitable energy because the specific binding energy (binding energy per mass) of intermediate - mass nuclei with atomic numbers and atomic masses close to Ni and Fe is greater than the nucleon - specific binding energy of very heavy nuclei, so that energy is released when heavy nuclei are broken apart . The total rest masses of the fission products (Mp) from a single reaction is less than the mass of the original fuel nucleus (M). The excess mass Δm = M--Mp is the invariant mass of the energy that is released as photons (gamma rays) and kinetic energy of the fission fragments, according to the mass - energy equivalence formula E = mc . </P> <P> The variation in specific binding energy with atomic number is due to the interplay of the two fundamental forces acting on the component nucleons (protons and neutrons) that make up the nucleus . Nuclei are bound by an attractive nuclear force between nucleons, which overcomes the electrostatic repulsion between protons . However, the nuclear force acts only over relatively short ranges (a few nucleon diameters), since it follows an exponentially decaying Yukawa potential which makes it insignificant at longer distances . The electrostatic repulsion is of longer range, since it decays by an inverse - square rule, so that nuclei larger than about 12 nucleons in diameter reach a point that the total electrostatic repulsion overcomes the nuclear force and causes them to be spontaneously unstable . For the same reason, larger nuclei (more than about eight nucleons in diameter) are less tightly bound per unit mass than are smaller nuclei; breaking a large nucleus into two or more intermediate - sized nuclei releases energy . The origin of this energy is the nuclear force, which intermediate - sized nuclei allows to act more efficiently, because each nucleon has more neighbors which are within the short range attraction of this force . Thus less energy is needed in the smaller nuclei and the difference to the state before is set free . </P>

Where does the energy from nuclear fission come from
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