Doppler reactivity in heavy water lattice; Effect due to nonuniform temperature within fuel rod

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Doppler reactivity has been calculated with uniform temperature within fuel rod for reactor design. Therefore, actual temperature distribution is close to parabolic, depending on the central and surface temperature, because the dependence of PuO-UO conductivity with the temperature and nonuniformity of heat generation within the fuel rod affect the temperature distribution in opposite sense. Doppler broadening effect is considered mainly due to resonance absorption in U on the surface of fuel rod. In the present study, doppler reactivity effect due to nonuniform temperature within fuel rod was investigated by the RABBLE/WIMS-ATR code. We considered several temperature distributions in the cases of normal operation, transient and LOCA for a 36-rod cluster cell. The following results are summarized. (1)It is necessary to divide the fuel rod into about 10 concentric-annular sub-regions at equal intervals for calculations with nonuniform temperatures. (2)Doppler reactivities calculated with nonuniform temperatures in the cases of normal operation and transient are reduced by about 5% in comparison with ones calculated with uniform temperatures. In the case of LOCA, doppler reactivities calculated with nonuniform temperatures are about 6% larger than ones calculated with uniform temparatures.