The Doppler coefficient represents that reactivity is decreased by an increase in neutron resonance absorption (mainly by 238U and 240Pu) resulting from power and then fuel temperature rises. A reactivity variation to a 1 % power rise is referred to as the Doppler power coefficient, and a reactivity variation to a 1 ° C fuel temperature rise is referred to as the Doppler temperature coefficient. As units of reactivity change, pcm and %Ak/k are used where 1 pcm = 10—5Ak/k and 1 % Ak/k = 10—2Ak/k. The former unit is mainly used for a relatively small reactivity change such as with reactivity coefficients and the latter is used for a relatively large reactivity change. Production of 240Pu as burnup proceeds makes the Dopp­ler temperature coefficient more negative, while the fuel temperature incre­ment by the power rise becomes smaller. Therefore, the Doppler power coefficient becomes slightly less negative as burnup increases. Typical Doppler power coefficient as a function of power is shown in Fig. 3.37.

The Doppler temperature coefficient is about —3 to —5 pcm/°C. LWRs have negative Doppler coefficient and reactivity is decreased by the feed­back when power increases.