Neutron temperature

[1] The precise boundaries of neutron energy ranges are not well defined, and differ between sources,[2] but some common names and limits are given in the following table.

This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons.

Moderation substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-239.

[15] An increase in fuel temperature also raises uranium-238's thermal neutron absorption by Doppler broadening, providing negative feedback to help control the reactor.

When the coolant is a liquid that also contributes to moderation and absorption (light water or heavy water), boiling of the coolant will reduce the moderator density, which can provide positive or negative feedback (a positive or negative void coefficient), depending on whether the reactor is under- or over-moderated.

Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator.

See caption for explanation. Lighter noble gases (helium and neon depicted) have a much higher probability density peak at low speeds than heavier noble gases, but have a probability density of 0 at most higher speeds. Heavier noble gases (argon and xenon depicted) have lower probability density peaks, but have non-zero densities over much larger ranges of speeds.
A chart displaying the speed probability density functions of the speeds of a few noble gases at a temperature of 298.15 K (25 C). An explanation of the vertical axis label appears on the image page. Similar speed distributions are obtained for neutrons upon moderation .