Disclosed is a storage tank for molten salts, preferably of the thermocline type. The tank is provided with an insulation on the inside, which is provided by molten salts captured and retained in an appropriate metal structure. The metal structure has openings allowing molten salts to flow into it, and may consist of metal supports that hold elements which allow retaining molten salts, such as a structured packing, metal boxes, or gutter wall.