The spent fuel dissolver solution is heated for Pu oxidation, and cooled for U-Pu cocrystallization. The U-Pu crystal (the color is orange) and mother liquor are separated by filtration. The crystal is dissolved in nitric acid solution, and Pu in the solution is reduced to Pu(IV). The solution is cooled again, resulting in U crystal (the color is yellow) and U-Pu mixed solution.