The introduction of Small Modular Reactors (SMRs), particularly innovative non-water-cooled designs, marks a new paradigm in the global energy sector. Among these, Molten Salt Reactors (MSRs) are increasingly recognized as a transformative technology within the nuclear power landscape, promising advancements in safety, flexibility, and efficiency for sustainable and clean energy deployment.
As part of the International Atomic Energy Agency’s (IAEA) efforts to support the development and deployment of innovative reactor technologies, MSRs offer unique advantages, including low operating pressure, inherent safety characteristics, and fuel flexibility. These features make MSRs attractive options for future energy systems, including floating nuclear power plants, power supply for ports and remote regions, and maritime nuclear propulsion. With their ability to operate at high temperatures and accommodate a wide range of fuel cycles, MSRs are also particularly suited for hybrid energy applications, such as thermal energy storage, hydrogen production, industrial process heat, and closing the nuclear fuel cycle through the transmutation of transuranic elements from spent fuel of conventional water-cooled reactors.
Typical MSR fuels include enriched uranium, plutonium, TRansUranium (TRU) elements from reprocessed spent nuclear fuels, and uranium-233 produced from thorium. MSR fuels generally consist of fission material halides (fluorides or chlorides) dissolved in a carrier salt.
MSR technology provides significant advantages over conventional solid-fuel reactor designs:
· A high coefficient of thermal expansion which provides a large negative temperature coefficient of reactivity. Because the fuel is liquid, it expands when heated, thus slowing down the rate of nuclear reactions and making the reactor self-regulating.
· The possibility of continuous fission-product removal using physical (helium sparging) and pyrochemical processes. Fuel salt can be processed in an online mode or in batches to retrieve fission products and actinides. Actinides are then reintroduced into the fuel circuit.
· Minimization of excess reactivity, which in solid-fuel systems must be compensated through complex control mechanisms. The continuous circulation and adjustment of the liquid fuel during operation allow for active management of fuel composition and reactivity. Furthermore, MSRs operate at low system pressure due to the high boiling point of fuel salt and the continuous removal of gaseous fission products, thereby enhancing inherent safety and simplifying containment requirements.
· Better resource utilization by achieving higher fuel burn-up than with conventional reactors using uranium solid fuel. TRU elements could in principle remain in the fluid fuel of the core, be destroyed in the neutron flux, either by direct fissioning or transmutation to fissile elements until they eventually all undergo fission.
· The avoidance of the expense of transport and fabrication of new fuel elements.
In MSRs, radionuclides (including activation and fission products) are distributed within the reactor circuit, gas off and processing systems, rather than being confined within the solid fuel cladding. This introduces new challenges for materials compatibility, corrosion control, and containment integrity.
Currently, numerous countries are actively pursuing MSR research and development, like: Belgium, Canada, China, Czech Republic, Denmark, France, Germany, India, Indonesia, Italy, Japan, the Republic of Korea, the Netherlands, the Russian Federation, Sweden, Switzerland, the United-Kingdom and the United States of America.
The IAEA Workshop on Molten Salt Reactor Fuels: Recent Developments and Future Challenges (held on 21–25 July 2025) highlighted several common challenges and priorities across the community. Ensuring high fuel salt purity and establishing international standards for synthesis and quality assurance are essential for obtaining reliable and comparable data. Significant gaps remain in thermophysical and thermochemical properties, and even trace impurities can strongly influence corrosion, performance, and waste management. Radiolytic effects on fuel salt chemistry, are not yet well understood, requiring further experimental and analytical development. There is a clear need for consistent modelling frameworks and a shared international database to support validation and cross-comparison of simulation results. Isotopic enrichment, particularly of ⁷Li and ³⁷Cl, poses a major supply challenge, and isotopic impurities must be managed within fuel cycle strategies. Large-scale facilities for salt production are lacking, and the development of continuous purification, advanced instrumentation, and real-time monitoring is critical for industrial deployment. Finally, accurate system modelling requires integrated coupling of neutronics, thermal-hydraulics, and chemistry codes, supported by collaborative international efforts and open-source development to accelerate technology validation and optimization.
A consistent theme emerging from these findings was that structural materials for reactor systems and related fuel cycles represent one of the critical enabling technologies for MSRs. These container materials must withstand high temperatures, fuel salts containing fission products, radiation damage, and long-term mechanical stresses while maintaining integrity, safety, and manufacturability.
A large body of literature exists on the corrosion of metallic alloys by molten fluoride and chloride salts, both in thermal and forced convection loops. The major impurities that must be removed to prevent severe corrosion of container metals are moisture and oxide contaminants. Considerable effort has been devoted worldwide to salt purification using HF/H2 sparging of molten salts. However, significant work remains to develop analytical and purification methods capable of identifying oxygen-containing species (oxide type, hydroxyl), purifying the molten-salt mixture, and accurately determining the oxygen content in salt melts.
The adoption of new MSR designs, with both fast and thermal spectra, as well as the potential application of MSRs as industrial heat sources, introduces new challenges. These include higher salt operating temperatures, up to 750°C, metallic barriers, graphite moderators, and reflectors capable of functioning under strong neutron fluxes. Ensuring compatibility of salts with structural materials in fuel and coolant circuits, alongside the development of suitable fuel processing materials, is therefore a critical priority.
Next steps for selecting container materials and solid moderators must involve irradiation testing, corrosion assessment, tellurium exposure evaluation, mechanical property characterization, and fabrication trials to finalize compositions for scale-up. This includes the procurement of large commercial heats of reference materials and conducting long-term mechanical property and corrosion tests of at least 10 000 hours. Additionally, development of design methods and rules is required to enable the safe and reliable design of selected materials.
To support design optimization and licensing, an integrated multi-physics modelling framework is needed to capture the complex interactions among neutronics, thermal-hydraulics, fuel salt chemistry, and materials corrosion phenomena. Such a framework should incorporate depletion calculations to track changes in fuel salt composition during reactor operation, including makeup additions and fission products’ removal. It should enable multi-physics simulations integrating neutron transport, heat transfer, and mass transport within the fuel salt, as well as thermodynamic and kinetic models describing fuel salt chemistry, including the behaviour of fission products (e.g., tellurium) and impurities (e.g., O, Fe, Ni, Cr). Finally, system-level transient models and corrosion models are necessary to evaluate structural material surface damage under operational conditions.
Building on the outcomes of the IAEA Technical Meeting on Reactor Physics, Thermal Hydraulics and Plant Design of Molten Salt Reactors (held on 22–25 April 2025) and the IAEA Workshop on Molten Salt Reactor Fuels: Recent Developments and Future Challenges (held on 21–25 July 2025), the IAEA identified the urgent need for coordinated international collaboration for the development of MSR technologies. Members of the IAEA Technical Working Group on Fuel Cycle Facilities (TWG-FCF) and participants of previous IAEA MSR events recommended that the IAEA organize a Workshop on Structural Material Development for Molten Salt Reactors and Related Challenges, to address these cross-cutting issues and foster collaboration among Member States.
