Abstract: A Monte-Carlo parametric study was carried out to investigate the nuclear properties of Th-Pu-U fueled model of the LR-0 reactor when moderated by mixtures of heavy/light water at molecular ratios ranging from 0% up to 100% D2O at increments of 10% in D2O. The mass of control rods needed to make the reactor critical and the potential reactivity in heavy water were tallied at the 11 heavy water percentage moderators being studied. It was found that the changes in these tallied parameters with heavy water percentage in moderator were not monotonic. Very large negative reactivity was found at 90% heavy water moderator.