Vault systems have operated for several decades beginning with Wylfa in the UK in the 1970s. Their principal advantages are corrosion mitigation (due to use of inert gas), spent fuel condition monitoring (including via continuous gas monitoring for leakages), and ease of reversibility of storage operations. The modular construction of MVDS-type vaults allows progressive expansion and straightforward maintenance of individual modules. The main limitation is the higher initial construction cost relative to cask or silo systems. Expansion of MVDS-type vaults is also more difficult than in the case of other dry storage options due to the complexity of the operations involved. Vaults, overall, provide a proven, long-term dry-storage option offering both environmental isolation and flexible, reversible storage for a wide range of spent fuel types.