Abstract: The invention relates to a protective structure for a concrete enclosure able to contain a fluid, which can be raised to a high temperature.Starting from the floor of the enclosure, the structure comprises a porous layer, intersecting shaped sections and a tight metal skin formed from plates, welded edge-to-edge along the axes of the shaped sections, after being fixed to the latter e.g. by weld spots. The cavities formed between the shaped sections receive concrete blocks and flexible joints. The metal skin has a peripheral rising portion along the walls of the enclosure and extending up to the maximum fluid level in the enclosure.Application to enclosures having to receive hydrocarbons or liquid sodium, e.g. on the site of a fast neutron nuclear reactor.