For a cask to be used, current regulations require that its design be approved by the Directorate General of Energy Policy and Mines (DGEPM) with a mandatory report from the CSN.

Any person or organisation can apply to the CSN for a favourable appraisal of the design of a spent fuel cask, whether it is a new design or the approval of a cask design already approved in other countries. The DGEPM may use this favourable appraisal to approve its use in nuclear facilities. The approval of the cask design is granted on the basis of the cask’s Safety Study and its Quality Plan, both of which include the limits and conditions of the authorisation.

In Spain, the DGEPM has issued a resolution to authorise Enresa to use casks called ENSA-DPT for the fuel from the Trillo nuclear power plant, HI-STORM 100Z for that of the José Cabrera nuclear power plant and HI-STORM 100 for that of the Ascó nuclear power plant, and has approved the original design as well as later modifications to it. Recently, the resolution of 20/11/2014 approved the design of the ENUN-52B cask for the fuel from the Santa María de Garoña nuclear power plant and authorised ENSA, as the owner of the design, to manufacture this cask.