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 UdepletedPuO2 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.