Conversion of nuclear fission energy to electrical energy
The fission of a 235U92 (or plutonium or other transuranic atom) nucleus with atomic mass A uranium = 235 within a fuel element converts the mass of that nucleus to the lesser masses of the (usually two) "fission product" nuclei and the mass of two to three neutrons plus about 200 MeV of kinetic energy (energy of motion) of these particles, plus the energy of one or more "gamma rays" (similar to X-rays). Most of this energy is in the form of kinetic energy of the two recoiling intermediate-mass "fission product" nuclei, plus the considerable kinetic energy of the two to three neutrons, gamma rays, and neutrinos.