THE UTILIZATION OF THORIUM-232 IN ADVANCED PWR – FROM SMALL TO BIG REACTORS

Since the beginning of Nuclear Energy Development, thorium was considered as a potential fuel, mainly due to the potential to produce fissile 233U. Several Th/U fuel cycles, using thermal and fast reactors were proposed and are still under investigation. However, the technical feasibility to use thorium was made in PWR; the USA PWR Indian Point Reactor was the first to utilize a core load with (Th0-0.9./U1-0.1)O2, with highly enriched U (93 w/0), achieving a maximum burn up of 32 MWD/kg HM. Also the last core of the Shipping port PWR (shutdown in 1982) was ThO2 and (Th/U)O2, operating as a Light Water Breeder Reactor (Seed- Blanket Concept) during 1200 effective full power days of operation (60 MWD/kg HM). More recently, many researchers turned their attention to Th fuel cycles in PWRs aiming at reducing the generation of minor actinide waste, at improving the nuclear power sustainability and at better fuel utilization. These studies were interested in assessing the feasibility of using 233U-Th fuels in PWR without worrying about how to obtain the initial 233U fuel load or the transition from an uranium to a thorium core in the current nuclear power plants. In this paper a review of the recent initiatives to utilize mixed oxide of U-Th in PWR is going to be provided, with an emphasis in two types of Advanced Reactors, the first a Small Modular Reactor (SMR); and the second a Generation III Advanced PWR (APWR). For the SMR, we use as criteria the fact that the core is designed to stand for a complete cycle, without the need to be refueled, but they need to be strongly poisoned at the beginning of life. So, since thorium can be used as a poison and a fertile fuel it could be a good option to be used as mixed oxide with uranium, and so we could reduce the burnable poison and have an extended burnup cycle. For the APWR, we use as criterion that the transition from the current UO2 APWR core to one with mixed U/Th fuels should be such that minimum changes occur on its current core design and operational parameters. Thus, one could consider the following requirements in this study: produce important amounts of 233U (maximization) for future 233U/Th cores; keep the current fuel assembly geometry, i.e., fuel rod diameter and pitch and meet the current thermal-hydraulic limits such as maximum center line fuel rod temperature and maximum linear power density; keep the current fuel cycle length of 18 months. As case studies, for the SMR, we used the Korean SMART reactor (the first integrated PWR to receive design certification), and for the APWR we used the Westinghouse AP 1000, due to its commercial success, with units being constructed in the USA and China. For both reactors we used a parametric study using homogeneous and heterogeneous fuel assemblies keeping the same geometry as the original UO2 core, and just changing the pellet material for (U-Th) O2. All the calculations were made by Monte Carlo codes. The results for both reactors show the feasibility to utilize thorium and satisfying the criterium imposed, even with advantages such as an extended discharge burn up, reducing the burn up poison, and a lower linear power density. As conclusion, we notice that the utilization of thorium in small or big PWR could be done successfully, without needing any changes in the current Nuclear Power Plants.