PURPOSE: To achieve a lighter weight and a smaller size of a plant equipment according to the present invention by assembling a cold trap into a cesium trap to control the flow of a coolant with a vertical movement of the cold trap.

CONSTITUTION: A cold trap cylinder 9 is pulled up in a direction of the black arrow and under such a condition, a cylinder top coolant outflow hole 13 of the trap cylinder 9 is closed with an upper stopper 18 of a cesium trap cylinder 8. On the other hand, a lower coolant inflow hole 14 of the trap cylinder 9 and an upper coolant outflow hole 11 are opened. As a result, the coolant flows from a coolant inlet piping 6 as indicated by the arrow and cooled down with a cooling pipe through which a fluid such as cooling gas flows between an outer cylinder 1 and an external side wall of the cesium trap cylinder 8 reaching a lower end, where with the direction of the flow thereof reversed, the coolant enters a radioactive impurities removing material 10 passing through the lower coolant inflow hole 14 of the cesium trap cylinder 8 to remove radioactive substance.