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. Given the shape of the neutron absorption spectrum, this has the result of reducing neutron absorption cross section, reducing the likelihood of absorption and fission. The end result is that reactors designed to take advantage of Doppler broadening will decrease their reactivity as temperature increases, creating a passive safety measure. This tends to be more relevant to gas-cooled reactors, as other mechanisms are dominant in water cooled reactors.