Fuel temperature coefficient of reactivity is the change in reactivity of the nuclear fuel per degree change in the fuel temperature. It is a measure of the stability of the reactor operations (that is, its value is normally negative). This coefficient is also known as the Doppler coefficient due to the contribution of Doppler broadening, which is the dominant contribution to the effect.
Contributing effects
Doppler broadening The thermal motion of atoms within the fuel causes a broadening of the resonance peaks of the neutron capture cross-section of the non-fissile portions of the fuel (notably, uranium-238). Since the resonances are self-shielding, their broadening results in an overall reduction of neutron flux as the temperature increases.
Thermal expansion Thermal expansion of the fuel at higher temperatures results in a lower density which reduces the likelihood of a neutron interacting with the fuel.
See also Nuclear fission Nuclear reactor physics Void coefficient Temperature coefficient
References
External links Fuel temperature coefficient of reactivity – USNRC Glossary