Thorium based fuel cycle options for PWRs

Thorium has been considered as an option to uranium-based fuel since the earliest days of the nuclear industry, initially based on considerations of resource utilization (thorium is approximately three times more plentiful than uranium), and more recently as a result of concerns about proliferation and waste management (e.g., reduced production of plutonium and higher actinides, improved physical and nuclear properties for reactor and potential waste management applications). Thorium can be used in both once-through and recycle options, and in both thermal and fast spectrum systems. Since there are no naturally-occurring thorium isotopes that can fission under reactor conditions, thorium is only useful as a resource for breeding new fissile materials, in this case U-233. Consequently, an isotope, such as U-233, U-235, or Pu-239, must be present in sufficient quantities for the reactor to operate. Most of the recent work on the utilization of thorium in a pressurized water reactor (PWR) has been limited to once-through cycle applications consistent with current U.S. practice. The present study is an initial examination of options for the utilization of thorium-based fuels if reprocessing of used nuclear fuel (UNF) is considered to make U-233 and/or LWR TRU available to serve as the source of initial fissile material required in place of LEU. Analyses have been performed to assess the potential performance of these options in a reference current PWR (1000 MWe Westinghouse design with J 7x17 fuel assemblies) under the constraint that minimal or no modifications would be required to the assembly and reactor designs to facilitate implementation. These calculations address several "recycle" scenarios and complement earlier studies which were restricted to once-through options. The results confirm the expected low production of plutonium and other actinides, and the ability to burn plutonium more efficiently than with conventional U-Pu MOX. Results of initial calculations of reactivity coefficients and control worths are similar to those for conventional MOX fuel, and remain negative over the parameter ranges of interest. However, the presence of plutonium and/or U-233, and the absence of U-235 will result in lower delayed neutron fractions and shorter prompt neutron lifetimes which will likely make transient and accident response and control more challenging. The analyses were limited to infinite lattice assembly calculations, and assuming standard UOXfuel assembly parameters with homogeneously loaded fuel. While this approach enhances the ability to retrofit thorium-based fuels into existing commercial PWRs with minimal or no changes required to reactor hardware, it does not necessarily represent the optimum performance achievable. The potential role of thorium-based fuels within the nuclear enterprise will also depend on the implementation and deployment scenarios.