The building and its facilities at the Dounreay nuclear power complex cost about Â£20m to construct.
Each vault will be able to hold the equivalent of between 370 and 450 double decker buses.
The floor of the vault is 36ft (11m) below ground and the construction work involved 260 tonnes of steel.
The Dounreay experimental nuclear power plant was built in the 1950s.
Buildings, from its time when its reactors were operational, are being demolished and the site cleaned up by 2025.
The land where the vaults are located will remain a restricted area for 300 years after that date because of the radioactive material stored inside the facilities.
The first of two massive vaults where low-level radioactive material will be stored on the far north Caithness coast has been completed.