Monte Carlo Perturbation Analysis of Isothermal Temperature Reactivity Coefficient in Kyoto University Critical Assembly

Experiments on the isothermal temperature reactivity coefficient (ITRC) have been carried out at the light water–moderated core with or without a D₂O tank in the Kyoto University Critical Assembly. The ITRC experiments are analyzed by a continuous-energy Monte Carlo (MC) neutron transport analysis code, McCARD. Through the temperature changes of H₂O and D₂O, effects of the coolant density changes in moderator and reflector regions and the microscopic cross-section variations on the ITRC are investigated by sensitivity analyses with the use of the MC adjoint-weighted perturbation method. An adjoint-weighted correlated sampling method for the stochastic mixing technique of cross-section libraries is devised to estimate the reactivity change from a perturbation of the thermal scattering cross sections due to the temperature change. From results of the MC perturbation analyses, it is clearly seen that the ITRCs of the two core configurations are dominated by a negative contribution of the 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.