a b s t r a c tIn this study, we have sought to determine the advantages, disadvantages, and viability of open cycle thorium-uranium-fuelled (Th-U-fuelled) nuclear energy systems. This has been done by assessing three such systems, each of which requires uranium enriched to $20% 235 U, in comparison to a reference uranium-fuelled (U-fuelled) system over various performance indicators, spanning material flows, waste composition, economics, and proliferation resistance. The values of these indicators were determined using the UK National Nuclear Laboratory's fuel cycle modelling code ORION. This code required the results of lattice-physics calculations to model the neutronics of each nuclear energy system, and these were obtained using various nuclear reactor physics codes and burn-up routines. In summary, all three Th-U-fuelled nuclear energy systems required more separative work capacity than the equivalent benchmark U-fuelled system, with larger levelised fuel cycle costs and larger levelised cost of electricity. Although a reduction of $6% in the required uranium ore per kWh was seen for one of the Th-U-fuelled systems compared to the reference U-fuelled system, the other two Th-U-fuelled systems required more uranium ore per kWh than the reference. Negligible advantages and disadvantages were observed for the amount and the properties of the spent nuclear fuel (SNF) generated by the systems considered. Two of the Th-U-fuelled systems showed some benefit in terms of proliferation resistance of the SNF generated.Overall, it appears that there is little merit in incorporating thorium into nuclear energy systems operating with open nuclear fuel cycles.

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