Enabling Pin-type Graphite-Free Salt-Cooled Reactor Designs (EPIGRAF)

Among the designs of advanced nuclear reactors being investigated worldwide, one concept holds a particular promise to make significant impact and address the global needs for clean energy sources in the near future - the molten slat-cooled high-temperature reactor, also known as Fluoride slat-cooled High-temperature Reactor (FHR). Unlike traditional Molten Salt Reactors (MSRs), the FHR uses solid fuel, which simplifies the concept greatly and can speed up its development and deployment. The advantages of FHR reactors over the current LWR designs and their economic benefits were highlighted in the AGRESR project concluded in 2021. Even though FHR concept presents many advantages in simplicity (when compared to molten slat-fuelled reactors), safety and economics, a significant amount of development is still needed. The concepts pursued elsewhere require substantial investment in the development of new fuel, core design and materials. Therefore, taking advantage of existing technologies and experience can substantially reduce the time needed to develop and deploy a salt-cooled reactor. Previous work explored the possibility of adopting features from the British Advanced Gas-Cooled Reactors (AGRs) to speed up the development and commercialisation of FHRs. AGR development in the UK suffered from notable initial challenges, but the resulting product was an impressive fleet of nuclear reactors, which remains unmatched in its availability and efficiency. Thus, borrowing knowledge and expertise gained in years of AGR operation and their implementation in FHR is an attractive proposition. Recent analyses showed that the best performance of an AGR-like FHR system is achieved in configurations with only a small amount of graphite in the core, and, possibly, no graphite at all. This finding implies substantial deviation from typical AGR fuel geometry and core layout affecting the behaviour of the system. Therefore, several questions remain to be addressed, which will form the scope of this proposed project. It was shown that power density of the core can be substantially increased, allowing to produce more power from the same for or significantly reduce the core. Small modular AGR-like FHR core design has never been attempted. The previous fuel cycle analysis used simplified assembly models. Therefore, the findings would need to be confirmed on a more realistic full core level. It would be required to identify and analyse the most relevant limiting accident scenarios for the new core in order to make a compelling safety case. Unlike AGRs, the new system operates at nearly atmospheric pressure, suggesting that refuelling procedures can be simplified. Currently operating AGRs do not refuel at full power. This is partly due to concerns over vibrations and coolant flow redistribution among the neighbouring channels introduced by the fuel assembly extraction. Demonstrating possibility of refuelling at full power would be a major advantage of the new system. The changes in the core configuration mean that many questions related to salt fluid dynamics need to be addressed through experimental investigation. These, for example, should include evaluation of pressure losses imposed by support grids and those due to deposits and corrosion of surfaces which can change its roughness. These effects need to be quantified for different flow regimes. Finally, alternative fuel and coolant options can be explored. For example, metallic and dispersed particles fuels as well as coolants other than traditional FLiBe salt can offer many performance advantages.