Innovative use of Thorium in LWR fuel assemblies

In this paper, the use of thorium in pressurized water reactor fuel assemblies is investigated. The novelty of the reported work is that the fuel design in this study is primarily intended to control the excess reactivity at beginning of life, and flatten the intra-assembly power distribution rather than converting fertile Th-232 into fissile U-233.The fuel assembly corresponds to the layout of a classical 17x17 pressurized water reactor assembly. Most of the fuel pins contain a mixture of uranium and thorium oxides, while a few additional fuel pins contain a mixture between uranium and gadolinium oxides. Two-dimensional transport calculations were performed with the Studsvik Scandpower CASMO-4E code in order to determine the main neutronic properties of the new fuel design, with a traditional uranium-based fuel assembly containing gadolinium used as a reference. The calculations demonstrated that most of the neutronic properties of the thorium-based fuel assembly were comparable to the properties of classical uranium-based fuel assemblies. The isothermal temperature coefficient of reactivity and the moderator temperature coefficient of reactivity were found to be appreciably more negative in the thorium-based design, while still remaining within acceptable limits. The main advantage of the thorium-based design is a significant reduction of the pin peak power at beginning of life. This special feature is of particular importance from an operational and safety viewpoint, since the margin to departure from nucleate boiling becomes larger. Consequently, this new type of fuel assembly could also be used in power-uprated cores