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The BN-350 fast-neutron reactor at Aktau, Kazakhstan.It operated between 1973 and 1994. A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV, on average), as opposed to slow thermal neutrons used in thermal-neutron reactors.
The ratio of neutrons released per neutron absorbed (η) in 233 U is greater than two over a wide range of energies, including the thermal spectrum. A breeding reactor in the uranium–plutonium cycle needs to use fast neutrons, because in the thermal spectrum one neutron absorbed by 239 Pu on average leads to less than two neutrons.
A fast neutron is a free neutron with a kinetic energy level close to 1 M eV (100 T J/kg), hence a speed of 14,000 km/s or higher. They are named fast neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes:
A deuteron beam impinges on a target; the target nuclei absorb either the neutron or proton from the deuteron. The deuteron is so loosely bound that this is almost the same as proton or neutron capture. A compound nucleus may be formed, leading to additional neutrons being emitted more slowly. (d,n) reactions are used to generate energetic ...
Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. [1] Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.
The extremely energetic nature of the fast neutrons emitted during the fusion events (up to 0.17 the speed of light) can allow normally non-fissioning 238 U to undergo fission directly (without conversion first to 239 Pu), enabling refined natural Uranium to be used with very low enrichment, while still maintaining a deeply subcritical regime.
Reactivity (denoted ρ or ΔK/K) is related to the effective neutron multiplication factor (k eff), the average number of all neutrons from one fission that cause another fission. [2] ρ = k eff - 1 / k eff But in nuclear physics, it useful to talk about the reactivity contributed by just the prompt neutrons. This is the reactivity in ...
In the case of protons, very fast neutrons will spall off the target, while in the case of the electrons, very high energy photons will be generated. These high-energy neutrons and photons will then be able to cause the fission of the heavy actinides. Such reactors compare very well to other neutron sources in terms of neutron energy: