Introduction of Thorium-Based Fuels in High Conversion Pressurized Water Reactors

Abstract

Pressurized water reactors (PWRs) are likely to produce the major portion of nuclear electricity during the 21st century. Nevertheless, even with the recycling of plutonium within MOX fuel, the utilization rate of uranium is very low and can be improved. Indeed, it grows significantly with the conversion ratio (CR) above the value of 0.8. The CR measures the competition between the production and the consumption rate of fissile isotopes as a function of the burnup. Thus, a CR higher than unity corresponds to a breeder reactor. The CR is the key factor that must be improved to allow a better use of natural uranium resources. A way to improve the CR would be to use thorium instead of uranium as a fertile material through the excellent qualities of its daughter, ²³³U.

Consequently, the aim of this paper is to investigate the use of thorium in high conversion pressurized water reactors (HCPWR) with a reduced moderator-to-fuel volume ratio using a high plutonium content in a hexagonal lattice. This study focuses on two heterogeneous concepts that fulfill the following criteria: a large production of ²³³U, the respect of safety aspects, and a cycle length higher or equal to 300 equivalent full-power days. The first core, named M-ThPu, has 21% of fertile fuel assemblies composed of depleted uranium and 79% of MOX fuel assemblies containing ThPuO₂ fuel, whereas the second core, named FA-Th, has ThO₂ fertile assemblies and U_depletedPuO₂ fuel assemblies, including axial layers of depleted uranium only. For each concept, the recycling of ²³³U with thorium in order to decrease the plutonium content in core has also been discussed. The conclusion for both concepts is that [approximately]25% of the PWR (with UOX fuel) could be replaced by HCPWR if ²³³U is reintroduced directly in each core concept. Therefore, this transition study shows no penalty in terms of natural uranium economy in moving toward a thorium fuel cycle in combination with the existing uranium cycle.