Doppler (fuel temperature) coefficient: typical sign in thermal reactors?

• A. Reactivity change due to Doppler broadening of resonances with increasing fuel temperature; typically positive.
• B. Reactivity change due to Doppler broadening of resonances with increasing fuel temperature; sign is unpredictable.
• C. Reactivity change due to Doppler broadening of resonances with increasing fuel temperature; typically negative.
• D. Reactivity change due to Doppler broadening of resonances with increasing fuel temperature; sign depends on moderator.

Answer: (C)

Raising fuel temperature causes Doppler broadening of the resonances in the fuel nuclei. With broader resonances, neutrons at energies near those resonances are more likely to be captured by the fuel without causing fission. That extra absorption removes neutrons from the fission process, lowering the neutron multiplication and reducing reactivity. So, in thermal reactors, the Doppler coefficient is typically negative: as temperature increases, reactivity decreases.

This sign is a robust, stabilizing feedback in most light-water–moderated, thermal-fission systems. The idea that the sign could be positive is not consistent with the physical mechanism here, and while the exact magnitude depends on fuel composition and neutron spectrum, the direction of the effect is generally negative. The moderator influences the spectrum and thus the magnitude, but not the fundamental sign of the Doppler effect in the fuel.