The fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize. On the other hand most of the energy released by one fission (~170MeV of total ~200MeV) appears as kinetic energy of these fission fragments. Therefore part of the released energy is radiated away from the reactor (See also: Reactor antineutrinos). Most of the fission fragments are highly unstable (radioactive) nuclei and undergo further radioactive decays to stabilize itself. It is much more probable to break up into unequal fragments, and the most probable fragment masses are around mass 95 (Krypton) and 137 (Barium). The average of the fragment atomic mass is about 118, but very few fragments near that average are found. Typically, when uranium 235 nucleus undergoes fission, the nucleus splits into two smaller nuclei (triple fission can also rarely occur), along with a few neutrons (the average is 2.43 neutrons per fission by thermal neutron) and release of energy in the form of heat and gamma rays. About 85% of all absorption reactions result in fission. Therefore about 15% of all absorption reactions result in radiative capture of neutrons. The cross-section for radiative capture for thermal neutrons is about 99 barns (for 0.0253 eV neutron). Most absorption reactions result in fission reaction, but a minority results in radiative capture forming 236U. For fast neutrons, its fission cross-section is on the order of barns. Uranium 235 is a fissile isotope, and its fission cross-section for thermal neutrons is about 585 barns (for 0.0253 eV neutron).
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