U-233 isotope is used as a booster fissile fuel material in the form of mixed ThC2/(UC2)-U-233 fuel in a Canada Deuterium Uranium (CANDU) fuel bundle in order to assure the initial criticality at startup. Three different fuel compositions have been used: (1) 97% ThC2+3% (UC2)-U-233, (2) 98% ThC2+2% (UC2)-U-233 and (3) 99% ThC2+1% (UC2)-U-233. The temporal variation of the criticality k(infinity) and the burn-up values of the reactor have been calculated by full-power operation for a period of 20 years. The criticality starts by k(infinity) = 1.541, 1.355 and 0.995 for modes of (1), (2) and (3) fuel compositions, respectively. A sharp decrease in the criticality has been observed in the first 2 years as a consequence of rapid U-233 burnout fuelling with (1) and (2) modes. The criticality becomes quasi-constant after the second year and remains above k(infinity) similar to 1.06 for 20 years. After the second year, the CANDU reactor begins to operate practically as a thorium burner. Very high burnup could be achieved with the same fuel materials (up to 500 000 MW day t (1)), provided that the fuel rod claddings would be replaced periodically (after every 500 00 or 100 000 MW day t(-1)). The reactor criticality will be sufficient for fuelling with (1) and (2) modes until a great fraction of the thorium fuel is burnt up. This would reduce fuel fabrication costs and nuclear waste mass for final disposal per unit energy drastically. Copyright (C) 2010 John Wiley & Sons, Ltd.