This article outlines key standardisation needs for specific topics related to molten salt reactors (MSR), namely measurements of thermo-physical properties, safety evaluation, qualification of fuels and fuel cycles, and codes & standards for materials and components. It also explores strategies for bridging gaps in international standardisation, harmonisation and collaboration, and raises the importance of building MSR prototypes and investing in testing facilities. This article is based on a survey and direct inputs from a Putting Science into Standards workshop held on 18 and 19 March 2024 and organised by the European Commissions’ Joint Research Centre (JRC) and European Committee for Standardization (CEN) and European Committee for Electrotechnical Standardization (CENELEC). The workshop gathered 100 experts from research, industry and policy making to discuss the development and standardisation of MSR technologies. By working together at European and international levels, standardisation will be a key instrument to enable the development and commercialisation of mature MSR technologies to support the European Union's goal of achieving fully functioning small modular reactors by 2030.

NRG together with JRC Karlsruhe has set out to perform a series of molten fuel salt irradiations in the High Flux Reactor (HFR) Petten, to build up irradiation experience with molten salt samples, the handling of irradiated salt and the treatment of salt waste produced by these irradiations. As a first step, an irradiation of small 78LiF-22ThF₄ fuel samples in graphite crucibles is being conducted under the name SALIENT-01 (SALt Irradiation ExperimeNT). The specific goals for SALIENT-01 are (i) to confirm claims of good fission product retention in the salt by post-irradiation Knudsen-cell effusion by fission product release measurements, in particular with respect to Cs and I which dominate the radioactive release during thermal transient and fuel failure in conventional reactor fuel, as for example occurred in the Fukushima accident, (ii) to obtain size distributions for noble metal particles using Transmission Electron Microscopy and (iii) to assess possible interactions between fuel salt and fine-grained nuclear graphite, as well as possible uptake of fission products by the graphite. The SALIENT-01 irradiation has started in August 2017 and is projected to finish in August 2019. In this contribution the design, irradiation history and status of the experiment are treated, as well as plans for post-irradiation examinations. The limitations of the experiment will also be discussed, as well as the follow-up actions taken to assess its representativeness and to improve the quality of future molten salt irradiations.

A thermodynamic assessment of the KF-ThF₄ binary system using the CALPHAD method is presented, where the liquid solution is described by the modified quasichemical formalism in the quadruplet approximation. The optimization of the phase diagram is based on experimental data reported in the literature and newly measured X-ray diffraction and differential scanning calorimetry data, which have allowed to solve discrepancies between past assessments. The low temperature heat capacity of α-K₂ThF₆ has also been measured using thermal relaxation calorimetry; from these data the heat capacity and standard entropy values have been derived at 298.15 K: C_p,m^o(K₂ThF₆,cr,298.15K)=(193.2±3.9) J·K^-1·mol^-1 and S_m^o(K₂ThF₆,cr,298.15K)=(256.9±4.8) J·K^-1·mol^-1. Taking existing assessments of the relevant binaries, the new optimization is extrapolated to the ternary systems LiF-KF-ThF₄ and NaF-KF-ThF₄ using an asymmetric Kohler/Toop formalism. The standard enthalpy of formation and standard entropy of KNaThF₆ are re-calculated from published e.m.f data, and included in the assessment of the ternary system. A calculated projection of the NaF-KF-ThF₄ system at 300 K and the optimized liquidus projections of both systems are compared to published phase equilibrium data at room temperature and along the LiF-LiThF₅ and NaF-KThF₅ pseudobinaries, with good agreement.

The mixture LiF-ThF₄-UF₄-PuF₃ (77.5–6.6-12.3–3.6 mol%), a fuel option for the Molten Salt Fast Reactor (MSFR), has been measured by differential scanning calorimetry (DSC) for determination of phase transitions, and by Knudsen effusion mass spectrometry (KEMS) for measurement of partial vapour pressures. The boiling point of the mixture was determined by extrapolation of total vapour pressure to 1 bar. Thermodynamic calculations were performed and compared with the experimental results. Novel experimental data on pure plutonium trifluoride are presented: melting point, vaporization enthalpy, vapour pressure and ionization energies by electron impact.