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In natural nuclear radiation, the by-products are very small compared to the nuclei from which they originate. Nuclear fission is splitting a nucleus into roughly equal parts and releasing energy and neutrons into the process. If another unstable nucleus captures these neutrons, they can fission and lead to a chain reaction. The average number of neutrons released per nucleus that continues to fission another nucleus is called k. Values ​​of k more significant than 1 mean that the fission reaction releases more neutrons than it absorbs and is called a self-sustaining chain reaction. A mass of fissile material large enough (and in an appropriate configuration) to induce a self-sustaining chain reaction is called critical mass.

When an appropriate nucleus captures a neutron, fission can occur immediately, or the nucleus can persist in an unstable state for a short time. Suppose there is enough immediate decay to continue the chain reaction. In that case, the mass is rapidly critical, and the release of energy will increase rapidly and uncontrollably, usually resulting in an explosion.

When discovered on the eve of World War II, this idea led several countries to launch programs on the possibility of building an atomic bomb. This weapon uses fission reactions to generate much more energy than which could be created with chemical explosives. The Manhattan Project, managed by the United States with the help of the United Kingdom and Canada, developed multiple fission weapons used against Japan in 1945 in Hiroshima and Nagasaki. During the project, the first fission reactors were also developed, although they are mainly intended to manufacture weapons and do not produce electricity.

However, if the mass is only critical when the delayed neutrons are included, the reaction can be controlled by introducing or eliminating neutron absorbers. This is what makes it possible to build nuclear reactors. Fast neutrons are not easily capture fast neutrons; they must be slowed down (slow neutrons), usually by collision with a neutron moderator’s nuclei, before they can be easily captured. Today, this type of fission is commonly used to generate electricity.

Nuclear fusion was first pursued only at theoretical stages during the Second World War when scientists from the Manhattan Project (led by Edward Teller) studied it to build a bomb. The project abandoned the merger after concluding that it would take a fission reaction to explode. It was not until 1952 that the first complete hydrogen bomb to be detonated, so-called because it used reactions between deuterium and tritium.

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