The Doppler broadening
of resonances is very important phanomenon, which improves reactor stability
. The prompt temperature coefficient of most thermal reactors is negative
, owing to the nuclear Doppler effect (fuel temperature coefficient – FTC). The broadening of uranium 238 resonances constitute fast acting negative feedback effect that is essential to ensure safe operation of all reactors.
In power reactors this effect acts as follows:
The reactor power increase (heating of a fuel) causes broadening of 238U resonances. This results in a higher probability of neutron absorbtion, thus causes negative reactivity insertion (reduction of reactor power).
In the thorium fuel cycle 232Th is analogous to 238U in the U-Pu fuel cycle, because it is the fertile component. Some of the radiative captures in 232Th are resonance events, so that the Doppler broadening role of 238U is similar.