Comparison of (Th-233U) O2 and (Th-235U) O2 fuel burn up into a thermal research reactor using MCNPX 2.6 code

Background: Decrease of economically accessible uranium resources motivates consideration of breeding of fertile elements such as thorium. Material and Method: Thorium oxide fuel burn up calculation of a simulated research reactor cooled heavy water has been proposed in the present work using MCNPX 2.6 code. Two 233U and 235U isotopes have been used as fissile element of thorium oxide fuel. 135Xe and 149Sm reactivity variations has been studied in the core loaded (Th- 233U)O2 or (Th- 235U)O2 fuel matrixes during 3 months burn up process. Results: Thorium oxide having 4% 233U burned 1 MW power results in less 149Sm reactivity than thorium oxide having 4% 235U burned in 0.5 MW power. 135Xe reactivity has an overestimated shift by 15 days in the core operated in 0.5 MW than the other, after 15 days both the cores behave similarly. 480 g of 235U burns into the core using 0.5 MW power and 364 g of 233U invents after 3 months. Burn up calculation of the modeled core of (Th- 233U)O2 fuel shows a fissile mass reduction by 60 days while the consumed fissile mass reaches to its initial value after 120 days. The core flux is constant during 3 months for both modeled cores. A considerable negative reactivity occurs up to 15 days in both cores which can be refer to xenon inventory during this time and then neutron multiplication factor is steadier up 3 months. Conclusion: Breeder thorium fuel enriched 233U make several advantages of good neutronic economy, 233U inventory and less inventory of long-lived alpha emitter wastes.