## 2 Experimental  
In the present study, tests for determining the corrosion properties of materials were carried out under static conditions (ampoule corrosion tests). The corrosion tests were conducted in FLiNaK-based melts. In a number of experiments, uranium tetrafluoride was added to the melt to assess the effect of the MSR main fuel component on the corrosion performance of the construction materials. The procedure for synthesis of FLiNaK was described earlier [47; 48]. Uranium (IV) fluoride was prepared from uranyl sulfate (UO2 SO4.3H2O, depleted in U-235). First, uranium was reduced to an oxidation state of +4. High-purity iron powder was added to an acidified solution of uranyl sulfate to enable the reduction:  
\[\mathrm{UO}_{2}{}^{2+}+\mathrm{Fe}+4\mathrm{H}^{+}\rightarrow\mathrm{U}^{4+ }+\mathrm{Fe}^{2+}+2\mathrm{H}_{2}\mathrm{O} \tag{1}\]  
Any excess of iron was dissolved in the sulfuric acid present in the solution. Then, uranium tetrafluoride was precipitated by adding an excess solution of hydrofluoric acid:  
\[\mathrm{U}^{4+}+4\mathrm{F}^{-}+\mathrm{nH}_{2}\mathrm{O}\rightarrow\mathrm{UF }_{4}\mathrm{\cdot nH}_{2}\mathrm{O}. \tag{2}\]  
Precipitated hydrate of uranium fluoride was washed four times with a hot 1% solution of hydrofluoric acid and then twice with distilled water. The solid was dried in air at 150 \({}^{\circ}\)C to remove the moisture and then dehydrated by two-stage heating under vacuum, first at 360 \({}^{\circ}\)C for 2 h and then at 550 \({}^{\circ}\)C for 1 h. The phase composition of uranium tetrafluoride was confirmed by X-ray powder diffraction analysis (PANalytical X'Pert Pro MPD, PANalytical, Almelo, The Netherlands). Fluoride salts (the mixture of lithium, sodium, and potassium fluorides) used as the corrosive media are hygroscopic. All operations involving FLiNaK, including synthesis of the eutectic mixture, grinding, loading, and storage were carried out in a glovebox (Glovebox Systemtechnik GmbH, Malsch, Germany) in an argon atmosphere (<4 ppm O\({}_{2}\) and <0.1 ppm H\({}_{2}\)O). The FLiNaK + UF\({}_{4}\) salt system was prepared by mixing the salts in a FLiNaK-to-UF\({}_{4}\) mass ratio of 93.4:6.6. Salts were mixed in the glovebox and then fused together at 850 \({}^{\circ}\)C in the furnace located inside the glovebox. The salts were kept at said temperature for 2 h to homogenize the melt. The chemical composition of the electrolytes is presented in Table 1, and the impurity content is shown in Table 2. The results from the chemical analysis showed that addition of uranium fluoride to FLiNaK did not lead to a significant increase in the impurity content, thus confirming the purity of the synthesized UF\({}_{4}\). The corrosion experiments were performed at a temperature range of 550-750 \({}^{\circ}\)C using a custom-built setup that included a set of steel cells closed using water-cooled lids; each cell connected to a gas-vacuum system and was heated by an external resistance furnace. The tests were performed in a high-purity argon (99.9998%) atmosphere with controlled temperature and pressure inside the retort. The setup and the methodology for conducting ampoule corrosion tests were described in detail previously [47; 48]. This setup allows long-term corrosion tests to be performed in various working media at elevated temperatures. Three samples of each alloy were used in the corrosion experiments. The weight of the samples was recorded prior to the test. The samples were placed in the crucibles and covered with the required amount of powdered salt. The crucibles were then covered with lids. The loading procedure was performed in an argon-filled glove box. The crucibles were transferred into stainless-steel cells, which could be tightly closed using steel lids. A metallic titanium sponge was positioned above the crucible lid in each cell to act as a getter for additional purification of the atmosphere from possible oxygen contamination. A schematic of the experimental cell used for the corrosion studies is shown in Figure 1.