Core design and fuel behaviour of a small modular pressurised water reactor using (Th,U)O[formula omitted] fuel for commercial marine propulsion

Abstract

There is an increasing interest in employing technologies to decarbonise the shipping sector; one option is the use of nuclear energy to power civilian ships. Previous work has investigated the use of UO2 fuel and its behaviour in a novel, soluble-boron-free, low power density PWR that offered a 15 year core life to power a container ship with a demand of 110 MWe. Here, building on that previously-presented UO2 core design, we investigate the analogous design of a (Th,U)O2 core to examine the neutronic and fuel performance benefits of using (Th,U)O2 fuel. The motivations for studying (Th,U)O2 fuel include understanding how (Th,U)O2 would behave in a novel low power density system and its behaviour as a fertile poison in a long-life core. Core design was developed using CASMO-SIMULATE and fuel performance was analysed using ENIGMA. In this new design, the introduction of (Th,U)O2 fuel exhibited similar neutronic characteristics (shutdown margins, xenon transient behaviour and reactivity coefficients) relative to the UO2 core. However, the fuel behaviour predicted by ENIGMA for (Th,U)O2 in this low power density core was found to be significantly impaired compared with using UO2 fuel. In particular, the introduction of uranium to ThO2 was found to degrade thermal conductivity and thorium-based fuels exhibited worse creep behaviour than UO2 fuels; both conclusions are supported by published experimental evidence. A factor behind the degraded fuel performance characteristics predicted by ENIGMA is the assumption that athermal phonon scattering by fission products has the same effect on thermal conductivity as for UO2. This assumption is necessary to address the limited burnup data for (Th,U)O2 fuels.