Nuclear fission is the splitting of a large and unstable nucleus (eg uranium or plutonium). • Spontaneous fission is rare. Usually, for fission to occur the unstable nucleus must first absorb a neutron. • The nucleus undergoing fission splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons plus gamma rays. Energy is released by the fission reaction. • All of the fission products have kinetic energy. • The neutrons may go on to start a chain reaction. • The chain reaction is controlled in a nuclear reactor to control the energy released. The explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction. • Students should be able to draw/interpret diagrams representing nuclear fission and how a chain reaction may occur.