Monte Carlo Perturbation Analysis of Isothermal Temperature Reactivity Coefficient in Light-Water Moderated Critical Assembly

The isothermal temperature reactivity coefficient (ITRC) of the light water-moderated core with or without the D₂O tank in the Kyoto University Critical Assembly are analyzed by the Monte Carlo (MC) neutron transport analysis code, McCARD. Through the temperature changes of H₂O and D₂O, the effects of the coolant density changes in the moderator and reflector regions, and the microscopic cross section variations on ITRC are investigated by the sensitivity analyses by the MC adjoint-weighted perturbation method. Also, to obtain ITRC contributions of hydrogen and deuterium cross sections in thermal neutron energy region, a new perturbation technique for the thermal scattering library is presented. From results of MC perturbation analyses with help of the new technique, the ITRCs of the two core configurations are accurately estimated by competitions between a negative contribution of number density change of hydrogen in the moderator region and a positive contribution of the thermal scattering cross section change of hydrogen in the reflector region