Consideration of fuel behaviour in AGR-like FHR reactor design

Abstract

The Fluoride Salt-cooled High-temperature Reactor (FHR) is a subclass of Molten Salt Reactor (MSR) that uses solid fuel separated from the molten coolant salt. Previous work showed that British Advance Gas-cooled Reactor (AGR) technology can be used to potentially reduce the time required to develop FHRs. Using such proven technology ensures that much of the experience acquired can be retained while providing solutions to the technical challenges faced by other FHR concepts. The AGR-like FHR is a reactor that combines AGR geometry and its pin-type fuel with the molten salt FHR concept. Since the AGR-like FHR is a low-pressure system, the integrity of the cladding is a major concern but previous studies have not examined the fuel performance aspects of the pin-type fuel assemblies so their findings have limited value and significance. For this reason, this thesis addresses the unique challenges arising from shifting to using molten salt coolant. The TRANSURANUS (TU) fuel performance code was expanded to make it suitable for simulating the AGR-like FHR by introducing the necessary materials and by updating the burnup model integrated into the code. These extensions have also been made available to other users of the code. The stainless-steel cladding of conventional AGRs creeps-out until it ruptures and this was avoided by switching the cladding material to Hastelloy-N, with a lower thermal expansion and better corrosion resistance to molten salt. Although the Hastelloy-N cladding does not fail, the deformation is still large, exacerbating fuel temperature, fission gas release (FGR) and pressure buildup, highlighting the need for mitigation measures such as increasing the size of fuel grains by introducing Cr-doped fuel, an Advanced Technology Fuel (ATF). Additionally, a different distribution of the linear power among the fuel pins within the fuel element was found beneficial for improving the fuel performance. Finally, a 2D analysis of the fuel element to assess the impact of the power asymmetries revealed notable differences, particularly in regions closer to the moderator. This highlights the importance of considering azimuthally non-uniform power distribution effects in the analysis of AGR-like FHR fuel pins, even more so if the power increases.