If the Public Utilities Commission determines that casks or other containers that allow for transportation as well as storage of spent nuclear fuel exist and are economically feasible for storage and transportation of spent nuclear fuel generated by the Prairie Island nuclear power generating plant, the commission shall order their use to replace use of the casks that are only usable for storage, but not transportation. If the commission orders use of dual-purpose casks under this section, it must authorize use of a number of dual-purpose casks that provides the same total storage capacity that is authorized under sections 116C.77 to 116C.779; provided, that the total cask storage capacity permitted under sections 116C.77 to 116C.779 may not exceed the capacity of the TN-40 casks authorized under section 116C.77.