Doppler Broadening is a critical phenomenon in the operation of Gen 4 pebble bed gas-cooled reactors, particularly in China's new reactor design. This effect influences neutron absorption by altering the thermal energy spectrum of moderated neutrons. As the reactor temperature increases, the energy of the neutrons also rises, leading to a decrease in the fission reaction rate due to the lower fission cross-section of high-energy neutrons. This mechanism serves as a vital safety feature, ensuring that increased reactor activity results in a self-regulating reduction of the reaction rate.
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Thank you very much.mfb said:Moderated neutrons get a thermal energy spectrum. A hotter reactor has hotter neutrons. Typically low-energetic neutrons have a higher fission cross-section, so heating the reactor reduces the reaction rate. That is an important safety feature: an increased reactor activity raises the temperature of the fuel, which reduces the reactor activity.