In a dry storage silo system, the spent nuclear fuel is stored in metallic canisters, inside a concrete cylinder on the floor (silos). Silos are monolithic or modular concrete reinforced structures, which provide the radiation shielding (as the building exterior does in the case of a vault) while the sealed inner metal liner or canister provides containment. The storage position can be vertical or horizontal. Spent fuel is received (either dry or wet) at a vault facility using transfer or transportation casks. Canisters are then loaded into silos. Concrete is the structural material and the radiation shielding (like the building in vault storage), as the canisters provide containment. Heat is removed by air convection through ducts in the concrete cylinders. The metal canisters are also fitted with a double lid closure system, which following welding is tested for leak tightness.