Recent Progress and Challenges in Molten Salt Reactors: Materials, Chemistry, and Corrosion Control toward Carbon-Neutral Energy Systems

Molten salt reactors (MSRs) have regained attention because they offer high-temperature heat delivery, low primary-system pressure, and flexible fuel-cycle options. Their technological feasibility, however, is governed by a comparatively narrow materials and chemistry window. Structural alloys are exposed for long times to fluoride or chloride melts whose redox state, impurity content, dissolved corrosion products, and fission-product inventory evolve during operation. This review examines MSR concepts and salt-chemistry constraints before discussing corrosion of candidate structural alloys, particularly Ni-based alloys. The principal degradation modes are selective Cr dissolution, intergranular attack, redox-driven mass transfer, corrosion-product redistribution, and the additional effects of impurities, irradiation, and fission products. Redox control is therefore considered not only as an electrochemical operation, but also as a materials-compatibility problem involving alloy composition, diffusion kinetics, salt purification, buffering chemistry, reference electrodes, and diagnostic access. Recent in situ/operando measurements and data-driven approaches are also reviewed, with emphasis on their proper use: they are valuable for identifying chemical states, defining response variables, and selecting experiments, but they require uncertainty treatment and validation against service-relevant corrosion data. The review argues that MSR corrosion management should be developed as an integrated materials-chemistry-control problem rather than as a sequence of independent alloy, salt, sensor, and modelling tasks.