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. The average of the fragment atomic mass is about 118, but very few fragments near that average are found. 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).
Most of the fission fragments are highly unstable (radioactive) nuclei and undergo further radioactive decays to stabilize itself. Therefore part of the released energy is radiated away from the reactor (See also: Reactor antineutrinos). On the other hand most of the energy released by one fission (~170MeV of total ~200MeV) appears as kinetic energy of these fission fragments. The fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize. Creation of ion pairs requires energy, which is lost from the kinetic energy of the charged fission fragment causing it to decelerate. The positive ions and free electrons created by the passage of the charged fission fragment will then reunite, releasing energy in the form of heat (e.g., vibrational energy or rotational energy of atoms). This is the principle how fission fragments heat up fuel in the reactor core.
Source of nuclear data: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1
Fission fragments after a nucleus fission. Fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize. Uranium 235 decays via alpha decay into 231Pa. Uranium 235. Comparison of total fission cross-section and cross-section for radiative capture.See above:
Uranium 235