• $summary_{type}$: concept
• $word_{count}$: 160

In nuclear fission, a heavy nucleus (typically U-235 or Pu-239) absorbs a slow (thermal) neutron and splits into two medium-mass fragments plus 2-3 neutrons, releasing ~200 MeV per fission (~0.9 MeV per nucleon). The classic reaction: U-235 + n -> Ba-141 + Kr-92 + 3n + 200 MeV. The released neutrons can trigger further fissions, creating a chain reaction. Critical mass is the minimum amount of fissile material needed for a self-sustaining chain reaction. In nuclear reactors: moderators (heavy water, graphite) slow neutrons to thermal energies for efficient capture, control rods (cadmium, boron) absorb excess neutrons to regulate the reaction rate. Energy per fission is calculated from mass defect: Q = (mass before - mass after) x 931.5 MeV. Fission products are neutron-rich and undergo beta-minus decay chains. The ~200 MeV splits into KE of fragments (~167 MeV), neutron KE, gamma rays, and neutrinos.