FIELD OF THE INVENTION

The present invention related generally to the field of storing spent nuclear fuel, and specifically to systems and methods for storing spent nuclear fuel in ventilated vertical modules.

BACKGROUND OF THE INVENTION

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 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, spent nuclear fuel is first placed in a canister. The loaded canister is then transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel for a determined period of time.

In a typical nuclear power plant, an open empty canister is first placed in an open transfer cask. The transfer cask and empty canister are then submerged in a pool of water. Spent nuclear fuel is loaded into the canister while the canister and transfer cask remain submerged in the pool of water. Once fully loaded with spent nuclear fuel, a lid is typically placed atop the canister while in the pool. The transfer cask and canister are then removed from the pool of water, the lid of the canister is welded thereon and a lid is installed on the transfer cask. The canister is then properly dewatered and filled with inert gas. The transfer cask (which is holding the loaded canister) is then transported to a location where a storage cask is located. The loaded canister is then transferred from the transfer cask to the storage cask for long term storage. During transfer from the transfer cask to the storage cask, it is imperative that the loaded canister is not exposed to the environment.

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 stand above ground and are typically cylindrical in shape and extremely heavy, weighing over 150 tons and often having a height greater than 16 feet. VVOs typically have a flat bottom, a cylindrical body having a cavity to receive a canister of spent nuclear fuel, and a removable top lid.

In using a VVO to store spent nuclear fuel, a canister loaded with spent nuclear fuel is placed in the cavity of the cylindrical body of the VVO. Because the spent nuclear fuel 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 bottom ventilation ducts, flows upward past the loaded canister, and exits the VVO at an elevated temperature through top ventilation ducts. The bottom and top ventilation ducts of existing VVOs are located circumferentially near the bottom and top of the VVO's cylindrical body respectively, as illustrated inFIG. 1.

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 spent nuclear fuel not be directly exposed to the external environment. The inlet duct located near the bottom of the overpack is a particularly vulnerable source of radiation exposure to security and surveillance personnel who, in order to monitor the loaded overpacks, must place themselves in close vicinity of the ducts for short durations.

Additionally, when a canister loaded with spent nuclear fuel is transferred from a transfer cask to a storage VVO, the transfer cask is stacked atop the storage VVO so that the canister can be lowered into the storage VVO's cavity. Most casks are very large structures and can weigh up to 250,000 lbs. and have a height of 16 ft. or more. Stacking a transfer cask atop a storage VVO/cask requires a lot of space, a large overhead crane, and possibly a restraint system for stabilization. Often, such space is not available inside a nuclear power plant. Finally, above ground storage VVOs stand at least 16 feet above ground, thus, presenting a sizable target of attack to a terrorist.

FIG. 1illustrates a traditional prior art VVO2. Prior art VVO2comprises flat bottom17, cylindrical body12, and lid14. Lid14is secured to cylindrical body12by bolts18. Bolts18serve to restrain separation of lid14from body12if prior art VVO2were to tip over. Cylindrical body12has top ventilation ducts15and bottom ventilation ducts16. Top ventilation ducts15are located at or near the top of cylindrical body12while bottom ventilation ducts16are located at or near the bottom of cylindrical body12. Both bottom ventilation ducts16and top ventilation ducts15are located around the circumference of the cylindrical body12. The entirety of prior art VVO2is positioned above grade.

DISCLOSURE OF THE PRESENT INVENTION

It is an object of the present invention to provide a system and method for storing spent nuclear fuel that reduces the height of the stack assembly when a transfer cask is stacked atop a storage VVO.

It is another object of the present invention to provide a system and method for storing spent nuclear fuel that requires less vertical space.

Yet another object of the present invention is to provide a system and method for storing spent nuclear fuel that utilizes the radiation shielding properties of the subgrade during storage while providing adequate ventilation of the spent nuclear fuel.

A further object of the present invention is to provide a system and method for storing spent nuclear fuel that provides the same or greater level of operational safeguards that are available inside a fully certified nuclear power plant structure.

A still further object of the present invention is to provide a system and method for storing spent nuclear fuel that decreases the dangers presented by earthquakes and other catastrophic events and virtually eliminates the potential damage from a World Trade Center or Pentagon type of attack on the stored canister.

It is also an object of the present invention to provide a system and method for storing spent nuclear fuel that allows an ergonomic transfer of the spent nuclear fuel from a transfer cask to a storage VVO.

Another object of the present invention is to provide a system and method for storing spent nuclear fuel below grade.

Yet another object of the present invention is to provide a system and method of storing spent nuclear fuel that reduces the amount of radiation emitted to the environment.

Still another object of the present invention is to provide a system and method of storing spent nuclear fuel that affords adequate heat removal capabilities from a stored canister during flood conditions, including “smart flood” conditions.

These and other objects are met by the present invention which in one aspect is a method of storing spent nuclear fuel comprising: providing a below grade hole; providing a system comprising a shell forming a cavity for receiving a canister of spent nuclear fuel, at least a portion of the shell positioned below grade, and at least one inlet ventilation duct extending from an inlet to an outlet at or near a bottom of the cavity, the inlet ventilation duct connected to the shell; positioning the apparatus in the hole so that the inlet of the inlet ventilation duct is above grade and the outlet of the inlet ventilation duct into the cavity is below grade; filling the hole with engineered fill; and lowering a spent fuel canister into the cavity.

In another aspect, the invention is a method of storing spent nuclear fuel comprising: providing a system comprising a structure forming a cavity for receiving and storing a spent nuclear fuel canister, the cavity having a top, a bottom, and a bottom surface, at least one inlet ventilation duct forming a passageway from an ambient air inlet to an outlet at or near the bottom of the cavity, and at least one outlet ventilation duct forming a passageway from at or near the top of the cavity to ambient air; lowering a canister loaded with spent nuclear fuel into the cavity until a bottom surface of the canister is lower than a top of the outlet of the at least one inlet ventilation duct; and supporting the canister in the cavity in a position where the bottom surface of the canister is lower than the top of the outlet of the at least one inlet ventilation duct.

In yet another aspect, the invention is a method of storing spent nuclear fuel comprising: providing a system comprising a body having a cavity for receiving and storing a spent fuel canister, a major portion of the body positioned below grade, and the body having at least one inlet ventilation duct extending from an above grade inlet to a below grade outlet in the cavity; lowering the spent fuel canister into the cavity so that a major portion of the canister is below grade; and placing a lid atop the body so as to enclose the cavity, the lid having at least one outlet ventilation duct for allowing heated air to exit the cavity; wherein ventilation of the canister is provided by cold air entering the cavity through the inlet ventilation duct in the body, the cold air being heated within the cavity by the spent nuclear fuel, and warm air exiting the cavity through the outlet ventilation duct in the lid.

In even yet another aspect, the invention is a method of storing spent nuclear fuel having a low level heat load comprising: providing a system comprising a structure forming a cavity for receiving a canister of spent nuclear fuel, at least a portion of the cavity being positioned below grade, and at least one ventilation duct forming a passageway from at or near the top of the cavity to an ambient atmosphere, wherein the cavity is hermetically sealed to ingress of below grade fluids; lowering a canister of low heat spent nuclear fuel into the cavity until at least a major portion of the canister is below grade; and supporting the canister in the cavity.

