Change in measured frequency of a wave structure due to the movement of the receiver or the wave source. The moved receiver cuts more or fewer waves per time unit, depending on whether the receiver moves towards the wave source or away from it. Analogously, the oscillations of the uranium atoms in a fuel element within a reactor lead to a Doppler effect due to the increasing operating temperature, since the fission cross-section depends on the relative velocity of the neutrons and uranium atoms. This Doppler effect may modify the reactivity of the reactor.