Neutronic study of an ASTRID-like fast reactor core with thorium-based fuel

Abstract

Abstract In this work the utilization of thorium was investigated in a sodium-cooled ASTRID-like fast reactor core. Two thorium fueled strategies were analyzed and compared with the reference core in which the fuel vector is composed of a mixture of U-Pu MOX fuel. For the first strategy, all uranium isotopes in the fertile zone (depleted uranium) were replaced by a mixture of 232Th/233U, leaving the fissile zone with the U-Pu MOX composition. For the second strategy, the whole core, that is, the fertile and fissile zones were loaded with a mixture of 232Th/233U. The Monte Carlo MCNP6 code with the ENDF/B-VII.0 cross section library was used to perform the calculations. Six neutronic parameters were analyzed for each fuel strategy: the neutron energy spectrum, the effective neutron multiplication factor (keff) for an operating cycle of 365 days, the Doppler constant, the reactivity effect of the coolant density, the effective delayed neutrons fraction (βeff) and the shutdown margin. The main isotopes evolution and the radial power distribution were also analyzed. Results show that the energy spectrum is harder with high thorium fraction, i.e. in the case with 232Th/233U in the whole core. The behavior of keff along the operating cycle of the reference core and that of the case with 232Th/233U in the fertile zone is very similar, getting the second one a slightly higher keff at the end of cycle. The keff with 232Th/233U in the whole core has a significant loss in reactivity and need further research. As for the isotopes, in the reference core 239Pu is bred, the same happens with 233U in the core with 232Th/233U in the fertile zone. This does not happen with 232Th/233U in the whole core. The Am and Cm actinides concentrations are lower with 232Th/233U in the fertile zone than in the reference core and are completely absent with 232Th/233U in the whole core. For all the configurations the Doppler constant and the reactivity effect of coolant density behave well, as the keff drops with temperature raise or with void raise. In the case of sodium voiding, a better performance is observed with 232Th/233U in the whole core. The power distribution is alike in all core configurations, having a ring with the higher power in the outer fuel region. The full 232Th/233U fuel configuration have the more even power distribution. All core configurations have similar βeff and show a satisfactory shutdown margin, as it is higher than 1% dk/k at the beginning of cycle.