A container for heat evolving radioactive materials has a metallic outer casing formed with outwardly projecting heat dissipating or cooling members, such as pins or fins, while each of its ends is formed with a flat flange which extends radially beyond the outer ends of the cooling members. A perforated wall extends between the flanges to define with same and with the periphery of the outer casing an annular space within which the cooling members are enclosed. This perforated wall is adapted to support a flexible covering sleeve the ends of which are clamped by inflatable seals between the periphery of the flanges and outer rings removably secured to the latter. Spraying means are provided within the aforesaid space to permit of projecting an uncontaminated liquid on the cooling members to cool the container before and/or while the latter is immersed in a loading and unloading pond with the sleeve mounted in position. The lower flange is provided with liquid collecting and evacuating means and compressed air may be injected into the said space to force the collected liquid outwardly.