22-4-nuclear-fission-and-fusion_summary

This section is about nuclear fission and fusion, processes that release energy from the atom. Nuclear fission is the splitting of a nucleus, which releases a large amount of energy but requires a catalyst, such as a free neutron, for it to occur. In the process, some neutrons are also released, which can cause further fission in other nuclei if they are directed back towards them by a dense shield or neutron reflector. The minimum amount of fissionable material needed to support a self-sustaining chain reaction is called its critical mass. The chain reaction can become self-sustaining in a nuclear reactor, which uses heat produced in the fission reaction to create steam for generating electrical energy. In nuclear fusion, two nuclei combine, releasing energy, but this requires overcoming the Coulomb repulsion force between the nuclei, which can be achieved by heating the fusion fuel to high temperatures. Fusion gives off energy when low-mass nuclei combine to form medium-mass nuclei. The principal sequence of fusion reactions in the Sun forms what is called the proton-proton cycle and results in the production of helium from hydrogen. Nuclear weapons use uncontrolled nuclear fission reactions to release large amounts of energy very quickly. The first fission weapon was detonated in 1945 and led to the end of World War II. The first hydrogen bomb, a thermonuclear weapon, was detonated in 1952. Fusion reactors are still in development as a potentially safer, cleaner, and more abundant source of energy.