Monte Carlo sensitivity analyses of isothermal temperature coefficient in solid-moderated and solid-reflected cores at Kyoto University critical assembly

•The characteristics of the ITC in the KUCA A-core are analysed by McCARD.•The sensitivity of the ITC is calculated by the MC perturbation techniques.•The thermal scattering library variation according to the temperature change in the reflector region can make the ITC positive. The Monte Carlo sensitivity analyses are carried out for the isothermal temperature coefficient (ITC) of two solid-moderated and solid-reflected cores characterized by medium-fast and thermal spectra at the Kyoto University Critical Assembly. From the isotope-wise and region-wise sensitivity analyses, the ITCs of two cores are found to be mainly determined by variations of the thermal scattering cross section of hydrogen in the moderator and reflector regions due to the spectral shift effect. The ITC contribution of the moderator is negative due to the degradation of neutron moderation while that of the reflector is positive due to a decrease in the neutron absorption in both cores. As the 1H/235U ratio is smaller, the ITC contribution of the moderator is less negative due to the smaller degradation of neutron moderation, while that of the reflector is more positive due to the larger thermal peak of neutron flux distribution in the reflector region.