Comparative study of the neutronic performance of thorium-based metallic fuel and MOX fuel in an ASTRID-like fast reactor core

Abstract

As the continuation of a previous research work, in this paper the utilization of thorium-based metallic fuel is investigated in an ASTRID-like fast reactor core. The main goal of this paper is to compare the neutronic behavior and the isotopes evolution between the previously studied reference core, based on oxide fuel, and a core loaded with metallic thorium-based fuel for the same ASTRID-like reactor. The reference core, in which the fuel is a material composed of a mixture of U-Pu MOX, was compared with an equivalent core with a U-Pu-Zr metallic alloy fuel (Met-UP) and two alternative thorium-fueled strategies. For the first strategy, a combination of 232Th/233U was introduced in the fertile zone instead of the uranium isotopes, keeping the fissile zone composition unchanged (Met-UPT). For the second strategy, the fertile zone was kept with the same composition as in the first strategy, while a mix of 232Th/233U replaced the fissile fuel zone composition (Met-UT). The calculations were made with the Monte Carlo MCNP6 code and the ENDF/B-VII.0 cross section library. The parameters analyzed were: the neutron multiplication factor for a burnup of one year, the neutron energy spectrum, the Doppler effect, the coolant density reactivity effect, the delayed neutrons fraction and the control rods reactivity worth (shutdown margin). The radial power distribution and the concentration evolution of the main isotopes for each fueling strategy in the core were also analyzed. The main findings of this study were: the effective neutron multiplication factor of the reference core and that of the Met-UPT case is very close, meanwhile, the configuration Met-UT has an important loss of reactivity along the burnup. Regarding the neutron energy spectrum, it becomes harder with a higher thorium fraction in the core. Related to the fissile isotopes, in the reference core 239Pu is produced, meanwhile, 233U is bred in the fertile zone of the configuration with 232Th/233U. The Am and Cm actinides production are lower for the 232Th/233U in the fertile zone compared to the reference configuration and null in the case of the full core with 232Th/233U. Doppler constants have negative values and show similar behavior in all the core configurations. Regarding the coolant density reactivity effect, the metallic fuel configuration with 232Th/233U in the full core shows the better behavior. The power distribution is similar in all the cases, showing a region with the highest power in the outer fuel zone. The metallic fuel configurations have higher power peaks. The full 232Th/233U fuel configuration has the flattest power distribution and the lower effective delayed neutrons fraction. All core configurations show a very good shutdown margin, widely greater than 1000 pcm at the beginning of the cycle. As a result of this study, it can be concluded that the Met-UP configuration can be considered as the best option followed by the Met-UPT configuration.