Thorium Fuel Performance in a Tight Pitch LWR Lattice

The aim of this work is to study the utilization of thorium-based fuels in the intermediate neutron spectrum of a tight pitch LWR lattices. The analysis was performed using MCNP-5 and the WIMSD5 codes. For the sake of showing the potential benefits of using thorium in LWR fuel, a thorium fueled benchmark was analyzed. Comparison was made between the state-of-the-art codes, MCNP-5 and the WIMSD5. Eigenvalue and isotope concentrations were compared for a PWR pin -cell model up to high burnup (60GWd⁄ton). The first step of the investigation of the thorium fuel cycle is the validation of the results obtained from the codes for this particular type of fuel. To complete this first task we performed calculation of the benchmark announced by IAEA. The benchmark was based on a simplified PWR model of the assembly with reduced fuel composition. This calculation was focused on the comparison of the methods and basic nuclear data. The calculated and compared values are criticality and fuel composition as a function of burn up. The results showed that thorium-based fuels in the intermediate spectrum of tight pitch LWR have considerable advantages in terms of conversion ratio, reactivity control, non-proliferation characteristics and a reduced production of long-lived radio toxic wastes. Due to the high conversion ratio of thorium-based fuels in intermediate spectrum reactors, the total fissile inventory required to achieve a given fuel burnup is only 11% to 17% higher than that of 238U fertile fuels. However, unlike 238U fertile fuels, the void reactivity coefficient with thorium based fuels is negative in an intermediate spectrum reactor. This provides motivation for replacing 238U with 232Th in advanced high conversion intermediate spectrum LWRs (AHCLWR), such as the RMWR or SCR reactors.