What is Doppler feedback and how does it affect reactor reactivity with temperature changes?

What is Doppler feedback and how does it affect reactor reactivity with temperature changes?

• A. Temperature increase has no effect
• B. Temperature decrease causes resonance broadening
• C. Temperature increase causes resonance broadening, reducing cross-section and reactivity
• D. Temperature increase causes resonance narrowing, increasing cross-section

Answer: (C)

Doppler feedback is a negative temperature feedback in which the fuel’s thermal motion broadens neutron absorption resonances. When the fuel heats up, nuclei move more, causing the resonance peaks in cross-sections to broaden (Doppler broadening). This broader shape increases the chance that neutrons—especially those near resonance energies—are captured rather than causing fission, so more neutrons are removed without sustaining the chain reaction. The net effect is a decrease in reactor reactivity as temperature rises. The peak cross-section can drop while the broader resonance increases absorption over the neutron spectrum, leading to less reactivity overall.

Doppler feedback is a negative temperature feedback in which the fuel’s thermal motion broadens neutron absorption resonances. When the fuel heats up, nuclei move more, causing the resonance peaks in cross-sections to broaden (Doppler broadening). This broader shape increases the chance that neutrons—especially those near resonance energies—are captured rather than causing fission, so more neutrons are removed without sustaining the chain reaction. The net effect is a decrease in reactor reactivity as temperature rises. The peak cross-section can drop while the broader resonance increases absorption over the neutron spectrum, leading to less reactivity overall.