To reduce radioactive waste and separate and remove sodium with good efficiency by evaporating metal sodium adhered to a reactor core constituting element at a high temperature under vacuum.

An inert gas is filled in a container 21 through an inert gas supply passage, and this is raised in temperature by the residual decay heat of a reactor core constituting element 22, and also heated to a required temperature by an inert circuit 36 provided on the container 21. Since the metal sodium adhered to the constituting material surface of the element 22 is then evaporated, the drive of a circulating gas blower 38 is stopped. The inert gas accompanied by sodium vapor is sucked by the vacuum pump unit 41 of a vacuum exhaust passage 39, and cooled by a cold heat transfer surface 28 within a sodium separator 27 to condense and separate sodium. Thereafter, the residual sodium vapor is removed by a sodium trap 40 and exhausted. The metal sodium separated by the separator 27 is recovered in a sodium recovering vessel 29.