Reprocessing method of spent fuel

Spent fuels are melted in molten chlorides, and then at least one of oxygen or a material releasing oxygen is supplied to the molten chlorides. Uranium (U), plutonium (Pu), neptunium (Np) and americium (Am) contained in the spent fuels are converted to insoluble oxides and precipitated. Then, U, Pu, Np and Am are separated from the molten chloride and recovered. In this case, the amount of precipitated insoluble oxides is detected by monitoring the amount of supplied oxygen and the change of the concentration of the dissolve oxygen ions. According to this method, a U/Pu mixture having a predetermined Pu/U ratio can be produced easily by controlling the amount of oxygen to be supplied, and can be recovered as a mixture not separated from each other. Accordingly, a mixture having U/Pu ratio suitable to MOX fuels can be provided by a convenient method at a low cost. (T.M.)