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/kg U) and utilize UO2 as nuclear fuel and are
operating in a 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/Kg
U), 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. 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 burners, and efficient concepts. 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. Although, the first reactors to utilize thorium were
PWR, using (U-Th)O2, such as the Indian Point, and Shipping Port, thorium has been proposed as
fuel for the molten salt reactor, the advanced heavy water reactor, High Temperature Reactors,
Pebble Bed reactor, fast breeder reactors, and more recently, for the innovative accelerator driven
system in a double strata fuel cycle and for the Generation IV, such as 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%. This paper,
besides an introduction of the present time uranium fuel cycles, will give an over view of the
thorium utilization in nuclear reactors and fuel cycles, with an emphasis in Advanced PWR