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Interim storage plants for spent fuel elements, based on dry storage technology in transport casks, are planned in the Federal Republic of Germany. The casks are arranged in storage buildings. The decay heat is removed from the outer cask surfaces by natural convection of air entering the building through openings in the walls, and leaving through outlets in the roof. As the differential equations describing the complex three-dimensional flow and temperature field can only be solved for simple boundary conditions, experiments were conducted using scaled-down models of the casks and the building. The relevant similarity conditions have been investigated and used for design and operation of the 1:5 scale test setup. The cask models were heated electrically. Cask temperatures, air temperatures, as well as air flow and velocities, were measured. It was found that the cooling conditions at the different cask positions show very small differences and that the cask surface temperatures inside the building are a maximum of 10/sup 0/C higher than on a free-standing cask.