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The newly released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down or "moderated", typically to speeds of a few kilometres per second, if they are to be likely to cause further fission in neighbouring 235 U nuclei and hence continue the chain reaction. This speed occurs at temperatures in the few hundred Celsius ...
Fast neutrons can be rapidly changed into thermal neutrons via a process called moderation. This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons. These collisions will generally speed up the other particle and slow down the neutron and scatter it.
In comparison to conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei so hydrogen-rich material is more effective at shielding than iron nuclei. The light atoms serve to slow down the neutrons by elastic scattering so they can then be absorbed by nuclear ...
The trick to achieving criticality using only natural or low enriched uranium, for which there is no "bare" critical mass, is to slow down the emitted neutrons (without absorbing them) to the point where enough of them may cause further nuclear fission in the small amount of 235 U which is available.
Heavy water is used in certain types of nuclear reactors, where it acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile uranium-235 than with uranium-238, which captures neutrons without fissioning. The CANDU reactor uses this design.
Neutron moderators are thus materials that slow down neutrons. Neutrons are most effectively slowed by colliding with the nucleus of a light atom, hydrogen being the lightest of all. To be effective, moderator materials must thus contain light elements with atomic nuclei that tend to scatter neutrons on impact rather than absorb them.
It also characterizes the loss of slowing neutrons in the resonance region. As the 238 U concentration increases, the absorption of resonant neutrons increases and hence fewer neutrons are slowed down to thermal energies. The resonance absorption is influenced by the slowing down of neutrons.
A thermal-neutron reactor is a nuclear reactor that uses slow or thermal neutrons.. ("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and structure, which is much lower energy than the fast neutrons initially produced by fission.)