Nuclear fusion
Nuclear fusion is the process of forcing two nuclei close enough together so they form a single larger nucleus. It can be brought about by making two light nuclei collide at a very high speed. Energy is released when two light nuclei are fused together. Nuclear fusion in the sun’s core released energy. A fusion reactor needs to be at a very high temperature before nuclear fusion can take place. The nuclei to be fused are difficult to contain.
Nuclear fission
Nuclear fission is the splitting of a nucleus into two, approximately equal fragments and the release of two or three neutrons. It occurs when a neutron hits a uranium-235 nucleus or a plutonium-239 nucleus and the nucleus splits. When a nucleus undergoes fission, it releases:
two or three neutrons at high speeds ;
energy in the form of radiation, plus kinetic energy of the fission neutrons and the fragment nuclei.
The fission neutrons may cause further fission, resulting in a chain reaction.