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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.
A fast neutron reactor uses fast neutrons, so it does not use a moderator. Moderators may absorb a lot of neutrons in a thermal reactor , and fast fission produces a higher average number of neutrons per fission, so fast reactors have better neutron economy making a plutonium breeder reactor possible.
238 U is a good neutron reflector, so the position of the blanket controlled the number of neutrons available to the reaction. When the blanket was raised, more neutrons were reflected back into the core, causing a greater number of fissions and, consequently, greater power output. [6]
Some reactions are only possible with fast neutrons: (n,2n) reactions produce small amounts of protactinium-231 and uranium-232 in the thorium cycle which is otherwise relatively free of highly radioactive actinide products. 9 Be + n → 2α + 2n can contribute some additional neutrons in the beryllium neutron reflector of a nuclear weapon.
An example of a nucleogenic nuclide is neon-21 produced from neon-20 that absorbs a thermal neutron (though some neon-21 is also primordial). [1] Other nucleogenic reactions that produce heavy neon isotopes are (fast neutron capture, alpha emission) reactions, starting with magnesium-24 and magnesium-25, respectively. [2]
A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in a system may be typified by either slow neutrons (i.e., a thermal system) or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.
The multiplication factor, k, is defined as (see nuclear chain reaction): k = number of neutrons in one generation / number of neutrons in preceding generation . If k is greater than 1, the chain reaction is supercritical, and the neutron population will grow exponentially.
Fission product yields by mass for thermal neutron fission of U-235 and Pu-239 (the two typical of current nuclear power reactors) and U-233 (used in the thorium cycle). This page discusses each of the main elements in the mixture of fission products produced by nuclear fission of the common nuclear fuels uranium and plutonium.