In the pyrochemical reduction of uranium dioxide or other actinide metal oxides by reaction with magnesium, magnesium oxide byproduct is produced. The use of a salt flux comprising magnesium chloride and a rare earth element trichloride such as neodymium chloride is disclosed. The neodymium chloride reacts with magnesium oxide to form magnesium chloride and neodymium oxychloride. The resulting magnesium chloride-neodymium oxychloride salt mixture can readily be subjected to electrolysis to regenerate magnesium and neodymium chloride for reuse in the pyrochemical reduction process. Other uses of the magnesium chloride-neodymium chloride salt flux are also proposed.