Sodium-cooled reactor vessels must not resist high pressures, and their walls are relatively thin. These vessels are usually suspended by their lateral wall to the cold roof slab. The upper part of this wall is subjected to periodic thermal stresses and to a permanent tension, corresponding to the weight of the sodium and of the components inside the vessel. In order to avoid a progressive deformation, the temperature of the vessel wall is limited to ∼400°C. This necessitates setting up relatively expensive baffles to isolate the wall from the hot sodium flowing out of the core. In order to relieve the wall from weight-related stresses, vessels resting on the installation basemat by means of sliding or articulated supports were proposed in early projects. A current design proposal consists of resting the vessel bottom on welded, concentric skirts, the chosen temperature of the bottom being relatively low; then a stable thermal gradient appears in sodium layers below the core. The corresponding heat flux lost toward the bottom proves remarkably low; an important simplification of the vessel walls and internal structures, as well as interesting safety features, may be obtained.