Fission, i.e. splitting of nuclei of heavy isotopes like 235U (or 239Pu) generates energy in a nuclear rector. A neutron collision with a heavy nucleus results in fission into two nuclei called fission fragments and into several neutrons.

Fission products and fission neutrons have a very high velocity and quickly slowdown heating the closest environment.

Energy released in fission comes from a phenomenon discovered by A. Einstein - equivalence of mass and energy. A nuclei before fission is heavier than a sum of fission fragments and fission neutrons. This difference in mass is converted into kinetic energy of fission product and fission neutrons, as well as some other particles and gamma radiation.