Nuclear fission and fusion 

[[Nuclear fission[[

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.

\ Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation.