Decrease of the economically accessible uranium resources and the inherent proliferation resistance of thorium fuel motivate its application in nuclear power systems. Estimation of the nuclear reactor's neutronic parameters during different operational situations is of key importance for the safe operation of nuclear reactors. In the present research, thorium oxide fuel burn-up calculations for a demonstrative model of a heavy water-cooled reactor have been performed using MCNPX 2.6 code. Neutronic parameters for three different thorium fuel matrices loaded separately in the modelled thermal core have been investigated. U-233, U-235 and (PU)-P-239 isotopes have been used as fissile element in the thorium oxide fuel, separately. Burn-up of three different fuels has been calculated at 1 MW constant power. X-135 and Sm-149 concentration variations have been studied in the modelled core during 165 days burn-up. Burn-up of thorium oxide enriched with U-233 resulted in the least Sm-149 and Xe-135 productions and net fissile production of U-233 after 165 days. The negative fuel, coolant and void reactivity of the used fuel assures safe operation of the modelled thermal core containing (U-233-Th) O-2 matrix. Furthermore, utilisation of thorium breeder fuel demonstrates several advantages, such as good neutronic economy, U-233 production and less production of long-lived a emitter high radiotoxic wastes in biological internal exposure point of view.