An overview of thorium utilization in nuclear reactors and fuel cycles

The Nuclear Power Plants (NPP) constructed in the XX century, also called generation II reactors, are still in operation, most of them Light Water Reactors, but are being decommissioned. These reactors have a low burn up (30 MWD/kgU) and utilize UO2 as nuclear fuel and are operating in an Once Through Cycle (OTC); they use a very low energy content of the natural resources (0,5%). To overcome economic and political and partly safety issues, since the end of last century, and beginning of this century, the nuclear industry launched a new generation of evolutionary reactors, called Generation III, such as the Westinghouse AP 1000, and AREVA EPR. These reactors still use uranium as primary source but have an increased burn up (60 MWD/KgU), which although increasing the utilization of the natural resources (up to 1%), still are not significant to be considered sustainable: if only uranium is used in an OTC, uranium will be exhausted in this century. To increase the utilization of natural resources, recycling of uranium and plutonium is already in use in many countries and used as Mixed Oxide of U-Pu fuel (MOX) in the same thermal reactors. Moreover, another issue which a controversy question is still is the solution for the High-Level Waste (HLW), mainly related with the political and public acceptance of the nuclear energy. To turn nuclear energy sustainable, a long-term deployment of innovative reactors is underway. These reactors and their associated fuel cycle are old concepts with technological improvements and generically denominated as Generation IV, are in development and, in some cases, they are breeders, HLW burn, and efficient concepts, such as the High Temperature Reactors (Thermal and Fast), Sodium or Lead Fast Breeder Reactors, Molten Salt Reactors, and the water super critical reactor. Another concept that although not new is constitute by the Small Modular Reactors (SMR), with power less than 300 MWe, which nowadays are deserving a lot of attention by the nuclear industry. Another option is to utilize thorium as a primary source of energy. Although not fissile at thermal energy, it produces 233U, which is one of best fissile nuclide (number of neutrons produced per neutron absorbed). Also, it is three times more abundant than uranium in the earth crust, and has thermal physics properties when used as (U-Th)O2 better than UO2. 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 (93w/0), achieving a maximum burn up of 32 MWD/kg HM. Also the last core of the Shippingport 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 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). Besides the thorium utilization in PWR, this paper will discuss the thorium utilization in PHWR, HTR, etc, and in Generation IV reactors, mainly the LFTR - Liquid Fluoride Thorium Reactor, which is a self-sustainable Molten Salt Reactor, promising to turn nuclear energy by fission in a sustainable source, with a utilization of the natural resources of 100%. (author).