Minor actinide burning in a CANDU thorium reactor

Nuclear waste actinides can be used as a booster fissile fuel material in form of mixed fuel with thorium in a CANDU reactor in order to assure the initial criticality at startup. Two different fuel compositions have been found useful to provide sufficient reactor criticality over a long operation period: (1) 95% thoria (ThO2) + 5% minor actinides MAO2 and (2) 90% ThO2 + 5% MAO2 + 5% UO2. The latter allows a higher degree of nuclear safeguarding through denaturing the new 233U fuel with 288U. The temporal variation of the criticality k∞ and the burn-up values of the reactor have been calculated by full power operation for a period of 10 years. The criticality starts by k∞ > 1.3 for both fuel compositions. A sharp decrease of the criticality has been observed in the first year as a consequence of rapid plutonium burnout in the actinide fuel. The criticality becomes quasi constant after the 2nd year and remains close to k∞ = ∝ 1.06 for ∝ 10 years. After the 2nd year, the CANDU reactor begins to operate practically as a thorium burner. Very high burn up could be achieved with the same fuel material (up to 200 000 MW.D/MT), provided that the fuel rod claddings would be replaced periodically (after every 50 000 or 100 000 MW.D/MT). The reactor criticality can be maintained 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. (orig.)