[en] The invention refers to a device for shielding annular gaps against ionizing radiations, in particular of a gap between the test cabin or platform and the reactor pressure vessel wall. The pre-assembled device is movable in such a way that it does not hinder the derive-in or withdrawal of the test cabin. A hollow cylinder made of radiation shielding material is divided alternately into the same number of small and large segments which are radially movable and which form an internal and an external ring before the drive-in of the test cabin