PURPOSE: To provide a method for dissolving plutonium dioxide with OH- radical generated by water radiolysis, which is suitable for processing dissolved micro materials and plutonium group waste.

CONSTITUTION: In a reactor 1, under presence of reagent which can trap redox mediator such as solvent sum electron, OH- radical generated in a similar method, and silver which is appropriately added, a solid product containing PuO2 ejected from a hopper 5 is made to contact to aqueous solution of nitric acid 3 irradiated with radiation or charged particles, and through rediolysis of the salvation, free OH- radical is generated. The irradiation is made by αor β emitter which is present in the solution or by such an external radiation source as an irradiator or an electron accelerator.