Neutronic design of homogeneous thorium/uranium fuel for 24 month fuel cycles in the European pressurized reactor using MCNP6

Abstract

Abstract A new approach to designing thorium based fuels for a European pressurized reactor (EPR) core is introduced by considering a three dimensional full core MCNP6 model. Existing information on designing thorium/uranium fuel for Generation II pressurized water reactors (PWRs) have been analyzed and the new fuel was proposed for the EPR (Gen III+). The newly designed fuel assembly (Th-B1) can reach 24-month fuel cycles without altering the geometry or disregarding any design limits. The Th-B1 was developed by reducing and ultimately removing Gd2O3 and slightly reducing the overall initial fissile content compared to the uranium only EPR. The same overall initial fissile content of the U-EPR was maintained, with the exception for fuel pin sections where pure ThO2 replaced the UO2 and Gd2O3 containing sections of the original U-EPR design. Results showed that there is no need to increase the initial fissile content or soluble boron enrichment as predicted by previous studies. Both xenon and samarium fission product yields for the Th-B1 are lower than for the U-B1, which contributes to a higher value for k ∞ and better breeding. Results also showed that the total fissile content for the Th-B1 shows a smaller decrease with time, which implies a smaller decrease for k ∞ with time and subsequently an extension of the fuel cycle. The innovative optimized fuel design of the EPR (five axial zones), enabled fuel enrichment savings and longer fuel cycles for Th-EPR. Not only does one save on fuel costs by keeping the overall initial fissile content similar, but this also results in reactor properties that are similar to those of the U-EPR and no serious changes need to be made. The only concerns are a lower delayed neutron fraction and boron worth for the Th-B1 at EOL. However, not all assemblies in a full core will be at the end of their life and if a three batch reloading scheme is assumed, the boron worth and delayed neutron fraction will most likely be within acceptable limits. Other differences between the Th-EPR and U-EPR such as the axial power profile and difference in boron letdown curve can be accounted for by amending the operating procedures during the design phase.