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Fuel temperature coefficient of reactivity is the change in reactivity of the nuclear fuel per degree change in the fuel temperature. The coefficient quantifies the amount of neutrons that the nuclear fuel (such as uranium-238 ) absorbs from the fission process as the fuel temperature increases.
Doppler broadening, the physical phenomenon driving the fuel temperature coefficient of reactivity also been used as a design consideration in high-temperature nuclear reactors. In principle, as the reactor fuel heats up, the neutron absorption spectrum will broaden due to the relative thermal motion of the fuel nuclei with respect to the neutrons.
LFTR designs use a strong negative temperature coefficient of reactivity to achieve passive inherent safety against excursions of reactivity. The temperature dependence comes from 3 sources. The first is that thorium absorbs more neutrons if it overheats, the so-called Doppler effect. [42]
The Doppler coefficient, which would have automatically reduced the neutron multiplication as the temperature increased, was relied upon to limit the power in the event of criticality. [1] If, contrary to all expectations, the plant had gotten out of control, the cadmium piece, which acted as a neutron absorber , would have been thrown into the ...
The change in reactivity caused by a change of voids inside the reactor is directly proportional to the void coefficient. A positive void coefficient means that the reactivity increases as the void content inside the reactor increases due to increased boiling or loss of coolant; for example, if the coolant acts predominantly as neutron absorber.
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The issue of the Doppler coefficient remained untested. [14] The EBR meltdown brought to a head another lingering concern with the breeder design. As the core was highly enriched, even slight movements of the fuel could cause significant changes in reactivity, as occurred on EBR.