S1: generating a temperature of 1200 to 1600° C. in a center of a fuel element, and at the same time introducing a cooling medium to an outer surface of the fuel element, to introduce a temperature difference to heat the fuel element; wherein the generating a temperature of 1200 to 1600° C. in a center of a fuel element comprises: inserting a metal probe into the center of the fuel element, so as to generate the temperature of 1200 to 1600° C. in the center of the fuel element; the fuel element is a metallic spent fuel element, and the fuel element contains the spent fuel

S2: soaking cooled spent fuel in an ionic liquid to selectively dissolve fission products and transuranic elements, so that the fission products and the transuranic elements are pre-separated from the metallic spent fuel; wherein the ionic liquid is betaine-bis(trifluoromethylsulfonyl) imide.