An overview of Thorium Utilization in Nuclear Reactors and Fuel Cycle

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

Simulation of rod ejection accident in a WWER-1000 Nuclear Reactor by using PARCS code

The rod ejection accident is defined as the postulated rupture of a control rod drive mechanism housing that results in the complete ejection of a rod cluster control assembly from the reactor core. The consequences of the mechanical failure are a rapid positive reactivity insertion and an increase in the local power peaking with high local energy deposition in the fuel assembly, accompanied by an initial pressure increase in the reactor cooling system. In this study, the REA has been simulated in a WWER-1000 reactor by using WIMSD-5B and PARCS v2.7 codes. First, macroscopic cross-sections have been calculated for various types of fuel assemblies using WIMSD-5B. Results have been fed as input to PARCS v2.7 code. Steady-state, transient and specially thermal–hydraulic feedback blocks of PARCS code have been handled in this simulation. Finally, results have been compared with Final Safety Analysis Report of WWER-1000 reactor. The results show a great similarity and confirm the ability of PARCS code in simulation of transient accidents

The utilization of thorium in Small Modular Reactors – Part I: Neutronic assessment

This work presents a neutronic assessment to convert a Small Modular Reactor (SMR) with uranium core to the thorium mixed oxide core with minimum possible changes in the geometry and main parameters of SMR core. This option is due to most of SMR are designed to be strongly poisoned in the beginning of cycle and to have a long cycle. Thorium can be used as an absorber in the beginning of the cycle and also be used as a fertile material during the cycle, it seems to be a good option to use (Th/U)O2 as SMR’s fuel. The main neutronic objectives of this study is achieving longer cycle length for SMR by using the minimum possible amount of burnable poison and soluble boron in comparison with reference core. The Korean SMART reactor as a certified design SMR has been chosen as the reference core. The calculations have been performed by MCNP code for homogeneous and heterogeneous seed and blanket concept fuel assemblies. The results obtained show that the heterogeneous fuel assembly is the one which gives longer cycle length and used lower amount of burnable poison and soluble boron, and also consumes almost the same amount of 235U