Study of the Effects of Moderators on ADS System Performance Based on UN-ThO₂ Fuel

The neutron spectrum is an essential factor in making possible the increase of ²³³U isotope breeding from thorium fuel in an accelerator-driven subcritical (ADS) system; therefore, studying the effects of various moderators and coolants on ²³³U breeding is an important step in ADS performance. This study aims to evaluate the effect of using different moderators and coolants on the ADS system characteristics. Sodium, which was the most common coolant used in ADS reactors, was replaced by light water (LW) and graphite + CO₂, separately. In this study, we used uranium nitride as the seed fuel associated with ThO₂ as the blanket fuel for all cases. The Monte Carlo transport code MCNPX 2.7.0 was used to calculate neutronic parameters such as effective multiplication factor (K_eff), power peaking factor (P_max/P_av) in the radial direction of the ADS reactor core, actinide isotope evolution during fuel burnup, and power fraction from each fuel type for all cases. The results show that the utilization of graphite as the moderator with CO₂ as the coolant allows more ²³³U production in thorium fuel compared with sodium and LW. On the other hand, LW showed great ability for plutonium and minor actinide transmutation and for energy generation.