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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:
Crucially, when a reactor runs on fast neutrons, the 239 Pu isotope is likely to fission 74% of the time instead of the 62% of fissions when it captures a thermal neutron. In addition the probability of a 240 Pu atom fissioning upon absorbing a fast neutron is 70% while for a thermal neutron it is less than 20%. Fast neutrons have a smaller ...
The bounds of this factor are 0 and 1, with a value of 1 describing a system for which all fast neutrons that do not leak out and do not cause fast fissions eventually reach thermal energies. P T N L {\displaystyle P_{\mathrm {TNL} }} describes the probability that a thermal neutron will not escape the system without interacting.
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.
Nonradioactive 133 Cs capturing a neutron and becoming 134 Cs, which is radioactive with a half-life of 2 years; Many of the fission products with mass 147 or greater such as 147 Pm, 149 Sm, 151 Sm, and 155 Eu have significant cross sections for neutron capture, so that one heavy fission product atom can undergo multiple successive neutron ...
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.
In nuclear physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. The neutron cross section σ can be defined as the area in cm 2 for which the number of neutron-nuclei reactions taking place is equal to the product of the number of incident neutrons that would pass through the area and the number of ...
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.