Thorium as an alternative fuel for SCWRs

This paper presents the research activity performed on a SCWR, which uses thorium fuel with 233U as fissile material. One- and two-dimensional calculations have been carried out to find the optimal assembly configuration including fuel pin diameters, pitches, number of fuel rings, etc. The goal has been to come up with an assembly configuration which has a conversion ratio higher than 1.0 for 233U, thus only the diminution of the 232Th has to be compensated. A two-pass configuration is proposed in order to combine the advantages of the regions with different coolant densities (without additional moderator). It is found that an assembly with pins of larger diameter in the high density coolant region and smaller ones in the low density coolant region is favorable since it minimizes the moderator-to-fuel volume ratio. Furthermore, an appropriate fitting of the regions is another measure to increase the conversion ratio. The two-dimensional calculations were followed by three-dimensional coupled neutronics – thermal hydraulics calculations, which prove that the assembly design is feasible from a neutronics point of view: the break-even point of FIR > 1.0 is achieved up to a burnup of 40.0 MWd/kgHM. Additionally, the fuel temperature and power reactivity coefficients have been determined. Both are negative proving the inherent safety of the reactor. A first sketch of the reactor pressure vessel including the flow path of the coolant is also presented.

► We examine thorium fuel in Supercritical Water Cooled Reactors.
► We propose a two-pass doubly heterogeneous assembly configuration.
► We have carried out one-, two-dimensional analyses to optimize the geometry.
► We have carried out three-dimensional coupled neutronics – thermal hydraulics calculations.
► We have achieved the break-even point of FIR > 1.0 up to a burnup of 40.0 MWd/kgHM.