Molten alkali metal halides, especially fluorides, are an extremely aggressive corrosive medium. The engineering implementation of molten salt reactor essentially depends on how successfully the problem of selection and creation of appropriate structural materials can be solved. Materials that are stable in these environments have not yet been created, and other ways of protecting structural elements of high-temperature technological devices are becoming extremely relevant. Methods of corrosion protection in molten salts are few, both due to external corrosion factors (temperature, radiation) and internal—the possibility of phase transitions, modification and degradation of materials at high temperatures). In the presented work, the corrosion behavior of copper (M1) was studied in a lithium, sodium, and potassium fluoride melt containing additives of neodymium fluorides, cerium as imitators of actinide fluorides and uranium fluoride from 0 to 5 wt % in a high purity argon atmosphere at 550 and 800°C for 100 h. Gravimetry, energy-dispersion analysis of the surface and cross-sectional sections of the samples were used to assess the corrosion resistance of copper. The dependences of the corrosion rate of the material on the temperature and concentration of additives of fluoride imitators of actinides and uranium fluoride are presented. The morphology of the surface and the cross-section section is presented depending on the composition of the melt. Based on the totality of data from gravimetric analysis, atomic absorption and microrentgenospectral methods of investigation, it was found that copper manifests itself as a material stable in lithium, sodium, and potassium fluorides.