kernespaltning

Kernespaltning, or nuclear fission, is the process in which a heavy atomic nucleus splits into two lighter nuclei, usually after absorbing a neutron. The splitting releases energy and additional neutrons. The energy mainly arises from the binding energy released due to the mass defect, and it is typically on the order of 200 MeV per fission, distributed between the kinetic energy of the fragments, prompt gamma rays, and neutrons.

In fissile materials such as uranium-235 or plutonium-239, a thermal (slow) neutron can initiate fission, creating two smaller nuclei and several neutrons. The emitted neutrons can trigger further fission events, producing a chain reaction. The rate is governed by neutron economy, the presence of a moderator to slow neutrons, and materials such as control rods to absorb excess neutrons. A chain reaction can be controlled, self-sustaining, or explosive depending on conditions.

Historically, the phenomenon was discovered in 1938 by Otto Hahn and Fritz Strassmann, with the theoretical explanation provided by Lise Meitner and Otto Frisch. The discovery revealed a new source of energy and led to rapid developments in science and technology, including civilian power generation and nuclear weapons.

Applications and safety: In nuclear power plants, fission is maintained in a controlled manner using moderators and control rods to achieve a steady, sustained release of energy. In weapons, the aim is an uncontrolled, rapid chain reaction. Fission products from uranium or plutonium determine the radiological hazards associated with spent fuel and accidents.