## Doppler coefficient

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.