In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, SNF is first placed in a canister, which is typically a hermetically sealed canister that creates a confinement boundary about the SNF. The loaded canister is then transported and stored in a large cylindrical container called a cask. Generally, a transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store SNP for a determined period of time.
One type of storage cask is a ventilated vertical overpack (“VVO”). A VVO is a massive structure made principally from steel and concrete and is used to store a canister loaded with spent nuclear fuel. VVOs come in both above-ground and below-grade versions. In using a VVO to store SNF, a canister loaded with SNF is placed in the cavity of the body of the VVO. Because the SNP is still producing a considerable amount of heat when it is placed in the VVO for storage, it is necessary that this heat energy have a means to escape from the VVO cavity. This heat energy is removed from the outside surface of the canister by ventilating the VVO cavity. In ventilating the VVO cavity, cool air enters the VVO chamber through air-inlet ducts, flows upward past the loaded canister as it is warmed from the heat emanating from the canister, and exits the VVO at an elevated temperature through air-outlet ducts. Such VVOs do not require the use of equipment to force the air flow through the VVO. Rather, these VVOs are passive cooling systems as they use a natural convective flow of air induced by the heated air to rise within the VVO (also know as the chimney effect).
While it is necessary that the VVO cavity be vented so that heat can escape from the canister, it is also imperative that the VVO provide adequate radiation shielding and that the SNF not be directly exposed to the external environment. Being that VVOs (and the canisters loaded therein) are intended to be used as long term storage solutions for SNF, it is imperative that both VVOs and the canisters exhibit a long life, in which corrosion, cracking and/or any type of compromise of structural integrity is minimized and/or avoided entirely.