[en] Uranium dicarbide is metastable at a temperature below 1500⁰C so that it gradually changes into diuranium tricarbide with the liberation of carbon, an undesirable phenomenon from the viewpoint of using uranium dicarbide as a nuclear fuel. The present invention discloses a technique for preparing a stable uranium sulfide (preferably monosulfide) is heated at a temperature above 1000⁰C, particularly in the range of 1500 to 1800⁰C in vacuum or in an inert atmosphere to convert them into a stable solid solution. In one example, a mixture of uranium dicarbide and uranium monosulfide (95 mol : 5 mol) was pulverized, formed into a block, and heated to 1800⁰C for 5 hours to give a solid solution. When heated to 1450⁰C for 5 hours to give a solid solution. When heated to 1450⁰C for 100 hours, the solid solution underwent no change. Uranium carbide itself, when heated to 1450⁰C, formed diuranium tricarbide in one hour or less. (Kaichi, S.)