Molten Salts and Pyrochemical Processing Progress at the UK's National Nuclear Laboratory

The UK's National Nuclear Laboratory offers infrastructure and capabilities to support research and development programmes for advanced fuel cycles and partitioning and transmutation (P and T). Our world-leading facilities have been widely used as part the UK government funded Advance Fuel Cycle Programme. Part of the programme involved the development of molten salt and pyrochemical processing capabilities including fuel manufacture, salt science, engineering, online monitoring, and fuel cycle integration. This has led to a resurgence of activity in molten salt technologies for nuclear application. Molten salt reactors (MSRs) are receiving increasing attention due to their potential for passive safe, efficient energy production. Combined with pyrochemical processing they offer the potential for closing the nuclear fuel cycle, contributing to energy security and meeting the UK's 2050 Net Zero commitments. Molten salt reactors come in a variety of models from those using salt purely as a coolant to those with fuel dissolved in the molten salt. The choice salt and fuel, their resultant behaviour in the reactor and the energy extraction methods are all fundamental variables. These choices influence the reactor capabilities and more specifically their application. The chemistry of MSRs is complex: the varying composition over time as fuel is converted to fission products impact the physical and chemical properties, and neutronics of the system. This leads to challenges in control of corrosion, salt behaviour and the ability to sustain the nuclear reaction. The fact that the fuel is a fluid is both an advantage and disadvantage in this respect. The ability to add to and remove from the salt allows fuel and redox control agents to be added and poisons removed, but it creates added complexity in modelling, assurance measurement and radiation shielding leading to control and regulatory challenges. The treatment of wastes from MSRs is also far from trivial. While the design and control of the reactor is challenging the promise of a suite of meltdown proof reactors, offering economic benefits (capital and operational cost) over alternatives, with a reduced waste burden means the race to solve this tricky puzzle is on