Doppler broadening is a broadening of spectral lines due to thermal agitation. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ...

In a nuclear reactor, this effect reduces the power generated as the reactor temperature increases. Nuclear reactor - Wikipedia, the free encyclopedia /**/ @import /skins-1. ... The concept of power occurs in multiple areas. ...

When a reactor gets hotter, the more rapid motion of the atoms in the fuel decreases the probability of neutron capture by U-238 atoms. Uranium nuclei are best split by neutrons moving at specific speeds. When the Uranium is heated, its nuclei move more rapidly in random directions, and therefore see and generate a wider range of relative neutron speeds. This reduces the number of neutrons available to cause U-235 fission, reducing the power output by the reactor.

In some reactors, such as pebble bed reactors, this natural negative feedback places an inherent upper limit on the temperature of the fuel without any operator intervention. Such reactors are said to be "inherently safe" because a reactor failure cannot generate a criticality excursion. The Pebble Bed Reactor is an advanced nuclear reactor design. ... Negative feedback is a type of feedback, during which a system responds so as to reverse the direction of change. ... A criticality accident (also sometimes referred to as an excursion or power excursion) occurs when a nuclear chain reaction is accidentally allowed to occur in fissile material, such as enriched uranium or plutonium. ...