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Please help to improve this article by introducing more precise citations. Nuclear reactor physics is the branch of science that deals with the study and application of chain reaction to induce a controlled rate of fission in a nuclear reactor for the production of energy. Most nuclear reactors use a chain reaction to induce a controlled rate of nuclear fission in fissile material, releasing both energy and free neutrons.
The physics of nuclear fission has several quirks that affect the design and behavior of nuclear reactors. This article presents a general overview of the physics of nuclear reactors and their behavior. When the reactor’s neutron production exceeds losses, characterized by increasing power level, it is considered “supercritical”, and when losses dominate, it is considered “subcritical” and exhibits decreasing power. This equation’s factors are roughly in order of potential occurrence for a fission born neutron during critical operation.
Reactivity is an expression of the departure from criticality. When reactivity ρ is expressed in units of delayed neutron fraction β, the unit is called the dollar. 2 and 3 for both 235U and 239Pu. The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all.
Most nuclear reactors include a “starter” neutron source that ensures there are always a few free neutrons in the reactor core, so that a chain reaction will begin immediately when the core is made critical. The primary sources described above have to be used with fresh reactor cores. Antimony becomes activated in the reactor and produces high-energy gamma photons, which produce photoneutrons from beryllium.
Uranium-235 undergoes a small rate of natural spontaneous fission, so there are always some neutrons being produced even in a fully shutdown reactor. When the control rods are withdrawn and criticality is approached the number increases because the absorption of neutrons is being progressively reduced, until at criticality the chain reaction becomes self-sustaining. Note that while a neutron source is provided in the reactor, this is not essential to start the chain reaction, its main purpose is to give a shutdown neutron population which is detectable by instruments and so make the approach to critical more observable.
The reactor will go critical at the same control rod position whether a source is loaded or not. Although the chain reaction is not self-sustaining, it acts as a multiplier that increases the equilibrium number of neutrons in the core. This subcritical multiplication effect can be used in two ways: as a probe of how close a core is to criticality, and as a way to generate fission power without the risks associated with a critical mass.