This is primarily used for spent nuclear fuel transport but is also licensed for transport and storage of spent nuclear fuel and Greater than Class C waste. It is engineered to store spent nuclear fuel in a vertical orientation and to transport it horizontally. These overpacks meld the roles of a nuclear pressure vessel and a robust transport device. From a structural standpoint, HI-STAR casks are made up of three shells: an innermost shell that acts as the containment boundary, a series of thick steel intermediate shells for gamma shielding, and an outer or enclosure shell that houses the neutron shielding material.