Self-perpetuating reaction. In a fissionSee 'nuclear fission'. chain reaction, a fissile nucleusPositively charged nucleus of an atom. Its diameter amounts ... absorbs a neutronUncharged elementary particle with a mass of 1.67492716 ·10..., splits and simultaneously releases several neutrons (in the case of U-235 an average of 2.46). These neutrons can be absorbed in turn by other fissile nuclei, initiate fissions and release other neutrons.