Study on the Doppler reactivity worth considering thermal agitation in epithermal range for various types of nuclear fuels

The Doppler reactivity worth of various types of nuclear fuels is evaluated using verified modeling of thermal agitation in epithermal range in order to investigate the effect of thermal agitation on the Doppler reactivity worth of practical nuclear fuels including thorium-based fuel. The Doppler reactivity worth of UO2 fuel, MOX fuel and thorium-based fuel is evaluated using sensitivity coefficient so as to reveal the breakdown of the impact. The result shows that the thermal agitation effect on the Doppler reactivity worth is mainly observed in the energy range from 10 eV to around 100 eV because of following two facts. One is that the probability of the scattering reaction becomes small at lower energy range and second is that the impact of thermal agitation on the scattering kernel becomes small at higher energy range. Regarding MOX fuel, it is found that thermal agitation effect on the Doppler reactivity worth is smaller than UO2 fuel since the amount of Pu-240 is smaller than that of U-238 and Pu-239 has larger absorption from 10 eV to around 100 eV compared to U-238. As for thorium-based fuel, thermal agitation effect on the Doppler reactivity worth is smaller than that of UO2 fuel since Th-232 has small number of resonances with large ratio σs/σt from 10 eV to around 100 eV.