Sheared fuel pins are fed intermittently to the first stage of the drum that is divided into eight stages by a helical auger and heated by a surrounded heating device, while nitric acid is supplied into the 8th stage and emerges as the dissolver solution from the 1st stage continuously. The sheared fuel pins are transferred from one stage to the next stage by rotating the drum periodically, and the solid contents such as hulls and wires are finally discharged from the 8th stage to an outer canister.