Thorium can be introduced into the energy vector in combination with high-grade plutonium (HG-Pu). Excellent neutron economy of heavy-water moderator allows use of mixed ThO2/HG-PuO2 fuel in heavy-water reactors with high efficiency, leading to the exploitation of large world thorium reserves and extending the availability of the nuclear energy by two orders of magnitude. In the present work, the criticality calculations have been performed with the code MCNPX 3-D geometrical modeling of a typical heavy-water reactor, where the structure of all fuel rods and bundles is represented individually. In the course of time calculations, nuclear transformation and radioactive decay of all actinide elements as well as fission products are considered. Five different fuel compositions have been selected for investigations: (1) 97% thoria (ThO2) + 3% PuO2; (2) 96% ThO2 + 4% PuO2; (3) 95% ThO2 + 5% PuO2; (4) 94% ThO2 + 6% PuO2; and (5) 92% ThO2 + 8% PuO2. The behavior of the criticality k(infinity) and the burn-up values of the reactor have been pursued by full power operation at 640 MWel (2180 MWth) for approximately 7 years. Time calculations have been conducted with MCNPX and CINDER codes under consideration of all nuclear transformation and radioactive decay processes on the actinide isotopes and fission fragments. As the reactor allows fuel recharging at on-power operation mode, the reactor criticality has been followed down to k(eff,end) = similar to 1.05. The corresponding burn-up values and operation periods for the investigated modes are (1) 18 GWd/MT and 780 days; (2) 27 GWd/MT and 1200 days; (3) 35 GWd/MT and 1560 days; (4) 44 GWd/MT and 1940 days; and (5) 60 GWd/MT and 2640 days. Among the investigated four modes, 94% ThO2 + 6% PuO2 seems a reasonable choice under consideration of the high price of the HG-Pu as driver fuel. The mixed fuel has the potential of an extensive exploitation of thorium resources. Reactor will run with the same fuel charge for approximately 5 years and allow a fuel burn-up approximately 44 GWd/MT, comparable with conventional light-water reactors (LWRs). Plutonium component of the mixed fuel will become nonprolific after few months of plant operation through the accumulation of even isotopes. Addition of few percent natural uranium to the initial mixed fuel charge will keep the U-233 component below 22% and hence at nonprolific level over the entire plant operation period. Replacement of 4% ThO2 with 4% nat-UO2 will practically not change main technical parameters of the reactor.