PURPOSE: To make PuO2 and (U.Pu)O2 of a hardly soluble component being contained dissolvable by using an aqueous soln. of U(NO3)4 as a dissolving soln. when a fuel composed of mixed U.Pu oxide is retreated by a wet process.

CONSTITUTION: An aqueous soln. of U(NO3)4 is used as a dissolving soln. in case of the retreatment of a fuel composed of mixed U.Pu oxide by a wet process. Said soln. of U(NO3)4 is manufactured more easily by electrolyzing at about 1.5V voltage the aqueous soln. of UO2(NO3)2-added hydrazine (N2H4) to prevent the oxidation of uranous (U4+) due to nitrous acid (HNO2). PuO2 and (U.Pu)O2 as a hardly soluble component being contained in the fuel composed of mixed U.Pu mixed oxide can be dissolved by using said aqueous soln. of U(NO3)4.