Apparatus for storing and/or transporting high level radioactive waste, and method for manufacturing the same

A system for storing and/or transporting high level radioactive waste, and a method of manufacturing the same. In one aspect, the invention is a ventilated vertical overpack (“VVO”) having specially designed inlet ducts that refract radiation back into the storage cavity. A clear line-of-sight does not exist through the inlet ducts and, thus, the canister can be supported on the floor of the VVO. Also disclosed is a method of manufacturing a variable height VVO that falls within a regulatory license previously obtained for a shorter and taller version of the VVO.

FIELD OF THE INVENTION

The present invention relates generally to apparatus, systems and methods for storing and/or transporting high level radioactive waste, and specifically to such apparatus, systems and methods that utilize a ventilated vertical overpack that allows natural convection cooling of the high level radioactive waste, which can be spent nuclear fuel (“SNF”) in certain instances.

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 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 SNF 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. SNF is loaded into the canister while the canister and transfer cask remain submerged in the pool of water. Once the canister is fully loaded with SNF, a lid is placed atop the canister while in the pool. The transfer cask and canister are then removed from the pool of water. Once out of the water, the lid of the canister is welded to the canister body and a cask lid is then installed on the transfer cask. The canister is then dewatered and backfilled lied with an 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 of the canister 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. Traditional VVOs stand above ground and are typically cylindrical in shape and are extremely heavy, often weighing over 150 tons and having a height greater than 16 feet. VVOs typically have a flat bottom, a cylindrical body having a cavity to receive a canister of SNF, and a removable top lid.

In using a VVO to store SNF, a canister loaded with SNF is placed in the cavity of the cylindrical body of the VVO. Because the SNF 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 as it is warmed from the heat emanating from the canister, and exits the VVO at an elevated temperature through top ventilation 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 the natural air flow 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. 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 VVOs, must place themselves in close vicinity of the ducts for short durations. Therefore, when a typical VVO is used to store a canister of SNF in its internal cavity, the canister is supported in the cavity so that the bottom surface of the canister is higher than the top of inlet ventilation ducts. This is often accomplished by providing support blocks on the floor of the cavity. By positioning the bottom surface of the canister above the inlet ventilation ducts, a line of sight does not exist from the canister to the external atmosphere through the inlet ventilation ducts, thus eliminating the danger of radiation shine out of inlet ventilation ducts. However, as discussed below, positioning a canister in the cavity of a VVO so that the bottom surface of the canister is above the top of the inlet ventilation ducts creates two issues: (1) a potential cooling problem during a “smart flood” condition; and (2) an increased height of the VVO.

Subpart K of 10 C.F.R. §72 provides for a “general certification” of casks for on-site storage of SNF. A number of casks have been licensed by the United States Nuclear Regulatory Committee (“U.S.N.R.C.”) and are listed in subpart L of 10 C.F.R. §72. These casks are certified to store a whole class of SNF (including SNF coming from pressurized water reactors (PWRs) or boiling water reactors (BWRs)). Unfortunately, reactors burn fuel in a wide variety of lengths. For example, PWRs in the U.S. presently burn fuel as short as 146″ (e.g., Ft. Calhoun) and as long as 198″ (e.g., South Texas). A general certified cask has been licensed in one or two fixed lengths (models) by the U.S.N.R.C. However, if the SNF is too long to fit in a licensed cask, then the cask simply cannot be used. Moreover, if the SNF is too short, then axial spacers are used to fill the open space in the storage cells to limit the movement of SNF in the axial direction. Thus, most casks and canisters used in the on-site storage of SNF have significant open spaces in their storage cells. This condition is particularly undesirable for VVOs because of the adverse consequence to the occupational dose to the plant personnel and cost (because of physical modifications forced on the plant), as set forth below.

First, the dose received by the workers performing the loading operations is directly influenced by the amount of shielding material per unit length in the body of the cask. The total quantity of shielding that can be installed in a transfer cask is governed by the lifting capacity of the plant's cask crane. A longer than necessary transfer cask means less shielding per unit length installed in the cask which in turn results in increased dose to the workers.

In VVOs, the VVO is often loaded inside the plant's truck bay by stacking the transfer cask over the VVO. Minimizing the height of the VVO's body is essential to allow the VVO to be moved out through the plant's truck bay (typically, a roll-up door) after the canister is installed therein. The loaded VVO is typically moved out across the roll-up door without its lid, and the lid is then installed on it immediately after the VVO body clears the door. Therefore, a key objective in the storage VVO design is to minimize the height of VVO body.

In another variation, the transfer cask itself is taken outside through the plant's truck bay and carried over to a pit where the transfer of the canister to the VVO takes place. In this case, the height of the transfer cask must be short enough to clear the plant's roll-up door to avoid the need to shorten the transfer cask (or alternatively, to increase the height of the roll-up door). Shortening the transfer cask is not always possible.

SUMMARY OF THE INVENTION

The present invention, in one aspect, is a ventilated overpack having specially designed inlet ducts that allow a canister loaded with SNF (or other high level radioactive waste) to be positioned within the overpack so that a bottom end of the canister is below a top of the inlet ducts while still preventing radiation from escaping through the inlet ducts. This aspect of the present invention allows the overpack to be designed with a minimized height because the canister does not have to be supported in a raised position above the inlet ducts within the cavity of the overpack. Thus, it is possible for the height of the cavity of the overpack to be approximately equal to the height of the canister, with the addition of the necessary tolerances for thermal growth effects and to provide for an adequate ventilation space above the canister.

When the canister is supported within the overpack cavity so that the bottom end of the canister is below the top end of the inlet ducts, the canister is protected from over-heating during a “smart flood” condition because a substantial portion of the canister will become submerged in the flood water prior to the incoming air flow from the inlet duct being choked off. Moreover, the design and arrangement of inlet ducts of the inventive overpack result in the cooling air flow within the overpack to not be significantly impacted by high wind conditions exterior to the overpack.

In one embodiment, the invention can be an apparatus for transporting and/or storing high level radioactive waste comprising: an overpack body having an outer surface and an inner surface forming an internal cavity about a longitudinal axis; a base enclosing a bottom end of the cavity; a plurality of inlet ducts in a bottom of the overpack body, each of the inlet ducts extending from an opening in the outer surface of the overpack body to an opening in the inner surface of the overpack body so as to form a passageway from an external atmosphere to a bottom portion of the cavity; a columnar structure located within each of the inlet ducts, the columnar structures dividing each of the passageways of the inlet ducts into first and second channels that converge at the first and second openings, wherein for each inlet duct a line of sight does not exist between the opening in the inner surface of the overpack body and the opening in the outer surface of the overpack body; a lid enclosing a top end of the cavity; and a plurality of outlet ducts, each of the outlet ducts forming a passageway from a top portion of the cavity to the external atmosphere.

In another embodiment, the invention is an apparatus for transporting and/or storing high level radioactive waste comprising: a cylindrical radiation shielding body forming an internal cavity and having a vertical axis; a base enclosing a bottom end of the cavity; a plurality of inlet ducts in a bottom of the radiation shielding body, each of the inlet ducts forming a horizontal passageway from an external atmosphere to a bottom portion of the cavity; a radiation shielding structure located within each of the inlet ducts that divides the horizontal passageway of the inlet duct into at least first and second horizontally adjacent portions and blocks a line of sight from existing from the cavity to the external atmosphere through the inlet duct; a radiation shielding lid enclosing a top end of the cavity; and a plurality of outlet ducts, each of the outlet ducts forming a passageway from a top portion of the cavity to the external atmosphere.

In another aspect, the invention is directed to a method of utilizing a general license obtained for two different ventilated vertical overpacks to manufacture a third ventilated vertical overpack that is covered by the general license without filing an application for certification of the third ventilated vertical overpack.

In one embodiment, the invention can be a method of manufacturing a licensed ventilated vertical overpack without filing an application for certification comprising: designing a first ventilated vertical overpack comprising: a first cavity for receiving a first canister containing high level radioactive waste, the first cavity having a first horizontal cross section and a first height; a first ventilation system for facilitating natural convection cooling of the first canister within the first cavity, the first ventilation system comprising a first plurality of inlet vents for introducing cool air into a bottom of the first cavity and a first plurality of outlet vents for allowing heated air to escape from a top of the first cavity; and wherein the first ventilated vertical overpack is designed to withstand an inertial load resulting from a postulated tip-over event so as to maintain the integrity of the first canister within the cavity; designing a second ventilated vertical overpack comprising: a second cavity for receiving a second canister containing high level radioactive waste, the second cavity having a second horizontal cross section that is the same as the first horizontal cross section and a second height that is less than the first height; a second ventilation system for facilitating natural convective cooling of the second canister within the second cavity, the second ventilation system comprising a second plurality of inlet vents for introducing cool air into a bottom of the second cavity and a second plurality of outlet vents for allowing heated air to escape from a top of the second cavity, wherein the second plurality of inlet vents have the same configuration as the first plurality of inlet vents and the second plurality of outlet vents have the same configuration as the first plurality of outlet vents; and wherein the second ventilated vertical overpack is designed to achieve a heat rejection capacity; obtaining a license from a regulatory agency for the first and second ventilated vertical overpacks; manufacturing a third ventilated vertical overpack comprising: a third cavity for receiving a third canister containing high level radioactive waste, the third cavity having a third horizontal cross section that is the same as the first and second horizontal cross sections and a third height that is less than the first height and greater than the second height; a third ventilation system for facilitating natural convective cooling of the third canister within the third cavity, the third ventilation system comprising a third plurality of inlet vents for introducing cool air into a bottom of the third cavity and a third plurality of outlet vents for allowing heated air to escape from a top of the third cavity, wherein the third plurality of inlet vents have the same configuration as the first and second plurality of inlet vents, and the third plurality of outlet vents have the same configuration as the first and second plurality of outlet vents; and wherein the third ventilated vertical overpack is automatically covered by the license without filing a new application for certification with the regulatory agency.

In another embodiment, the invention can be a method of manufacturing a licensed ventilated vertical overpack without filing an application for certification comprising: designing a first ventilated vertical overpack having a first cavity for receiving a first canister containing high level radioactive waste and having a structural configuration that can withstand an inertial load resulting from a postulated tip-over event so as to maintain the integrity of the first canister within the cavity, the first cavity having a first height that corresponds to a height of the first canister; designing a second ventilated vertical overpack having a second cavity for receiving a second canister containing high level radioactive waste and an inlet and outlet duct configuration for facilitating natural convective cooling of the second canister that achieves a heat rejection capacity, the second cavity having a second height that corresponds to a height of the second canister, the first height being greater than the second height; obtaining a license from a regulatory agency for the first and second ventilated vertical overpacks; manufacturing a third ventilated vertical overpack comprising: a third cavity for receiving a third canister containing high level radioactive waste, the third cavity having a third height that corresponds to a height of the third canister, the third height being greater than the second height and less than the first height; a structural configuration that is the same as the structural configuration of the first ventilated vertical overpack; and an inlet and outlet duct configuration for facilitating natural convective cooling of the third canister that is the same as the inlet and outlet duct configuration of the second ventilated vertical overpack; and wherein the first, second and third cavities have the same horizontal cross-sections and the first, second and third canisters have the same horizontal cross-sections; wherein the third ventilated vertical overpack is automatically covered by the license without filing a new application for certification with the regulatory agency.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring toFIGS. 1-4concurrently, a ventilated vertical overpack (“VVO”)1000according to an embodiment of the present invention is illustrated. The VVO1000is a vertical, ventilated, dry, SNF storage system that is fully compatible with 100 ton and 125 ton transfer casks for spent fuel canister transfer operations. The VVO1000can, of course, be modified and/or designed to be compatible with any size or style of transfer cask. Moreover, while the VVO1000is discussed herein as being used to store SNF, it is to be understood that the invention is not so limited and that, in certain circumstances, the VVO1000can be used to transport SNF from location to location if desired. Moreover, the VVO1000can be used in combination with any other type of high level radioactive waste.

The VVO1000is designed to accept a canister for storage at an Independent Spent Fuel Storage Installation (“ISFSI”). All canister types engineered for the dry storage of SNF can be stored in the VVO1000. Suitable canisters include multi-purpose canisters (“M PCs”) and, in certain instances, can include thermally conductive casks that are hermetically sealed for the dry storage of high level radioactive waste. Typically, such canisters comprise a honeycomb basket250, or other structure, to accommodate a plurality of SNF rods in spaced relation. An example of an MPC that is particularly suited for use in the VVO1000is disclosed in U.S. Pat. No. 5,898,747 to Krishna Singh, issued Apr. 27, 1999, the entirety of which is hereby incorporated by reference.

The VVO1000comprises two major parts: (1) a dual-walled cylindrical overpack body100which comprises a set of inlet ducts150at or near its bottom extremity and an integrally welded baseplate130; and (2) a removable top lid500equipped with radially symmetric outlet vents550. The overpack body100forms an internal cylindrical storage cavity10of sufficient height and diameter for housing an MPC200fully therein. As discussed in greater detail below, the VVO1000is designed so that the internal cavity10has a minimized height that corresponds to a height of the MPC200which is to be stored therein. Moreover, the cavity10preferably has a horizontal (i.e., transverse to the axis A-A) cross-section that is sized to accommodate only a single MPC200.

The overpack body100extends from a bottom end101to a top end102. The base plate130is connected to the bottom end101of the overpack body100so as to enclose the bottom end of the cavity10. An annular plate140is connected to the top end102of the overpack body100. The annular plate140is ring-like structure while the base plate130is thick solid disk-like plate. The base plate130hermetically encloses the bottom end101of the overpack body100(and the storage cavity10) and forms a floor for the storage cavity10. If desired, an array of radial plate-type gussets112may be welled to the inner surface121of an inner shell120and a top surface131of the base plate130. In such an embodiment, when the MPC200is fully loaded into the cavity10, the MPC200will rest atop the gussets112. The gussets112have top edges that are tapered downward toward the vertical central axis A-A. Thus, the gussets112guide the MPC200during loading and help situate the MPC200in a co-axial disposition with the central vertical axis A-A of the VVO1000. In certain embodiments; the MPC200may not rest on the gussets112but rather may rest directly on the top surface131of the base plate130. In such an embodiment, the gussets112may still be provided to not only act as guides for properly aligning the MPC200within the cavity10during loading but also to act as spacers for maintaining the MPC200in the desired alignment within the cavity10during storage.

By virtue of its geometry, the overpack body100is a rugged, heavy-walled cylindrical vessel. The main structural function of the overpack body is provided by its carbon steel components while the main radiation shielding function is provided by an annular plain concrete mass115. The plain concrete mass115of the overpack body100is enclosed by concentrically arranged cylindrical steel shells110,120, the thick steel baseplate130, and the top steel annular plate140. A set of four equispaced steel radial connector plates111are connected to and join the inner and outer shells110,120together, thereby defining a fixed width annular space between the inner and outer shells120,110in which the plain concrete mass115is poured.

The plain concrete mass115between the inner and outer steel shells120,110is specified to provide the necessary shielding properties (dry density) and compressive strength for the VVO1000. The principal function of the concrete mass115is to provide shielding against gamma and neutron radiation. However, the concrete mass115also helps enhance the performance of the VVO1000in other respects as well. For example, the massive bulk of the concrete mass115imparts a large thermal inertia to the VVO1000, allowing it to moderate the rise in temperature of the VVO1000under hypothetical conditions when all ventilation passages150,550are assumed to be blocked. The case of a postulated fire accident at an ISFSI is another example where the high thermal inertia characteristics of the concrete mass115of the VVO1000controls the temperature of the MPC200. Although the annular concrete mass115in the overpack body100is not a structural member, it does act as an elastic/plastic filler of the inter-shell space.

Four threaded steel anchor blocks (not illustrated) are also provided at the top of the overpack body100for lifting. The anchor blocks are integrally welded to the radial plates111, which join the inner and outer shells120,110. The four anchor blocks are located at 90° angular spacings around the circumference of the top of the overpack body100.

While the cylindrical body100has a generally circular horizontal cross-section, the invention is not so limited. As used herein, the term “cylindrical” includes any type of prismatic tubular structure that forms a cavity therein. As such, the overpack body can have a rectangular, circular, triangular, irregular or other polygonal horizontal cross-section. Additionally, the term “concentric” includes arrangements that are non-coaxial and the term “annular” includes varying width.

The overpack body100comprises a plurality of specially designed inlet vents150. The inlet vents150are located at a bottom of the overpack body100and allow cool air to enter the VVO1000. The inlet vents150are positioned about the circumference of overpack body100in a radially symmetric and spaced-apart arrangement. The structure, arrangement and function of the inlet vents150will be described in much greater detail below with respect toFIGS. 4-6and10.

Referring now toFIGS. 1-4and7concurrently, the overpack lid500is a weldment of steel plates510filled with a plain concrete mass515that provides neutron and gamma attenuation to minimize skyshine. The lid500is secured to a top end101of the overpack body100by a plurality of bolts501that extend through bolt holes502formed into a lid flange503. When secured to the overpack body100, surface contact between the lid500and the overpack body100forms a lid-to-body interface. The lid500is preferably non-fixedly secured to the body100and encloses the top end of the storage cavity10formed by the overpack body100.

The top lid500further comprises a radial ring plate505welded to a bottom surface504of the lid500which provides additional shielding against the laterally directed photons emanating from the MPC200and/or the annular space50(best shown inFIG. 9) formed between the outer surface201of the MPC200and the inner surface121of the inner shell120. The ring plate505also assists in locating the top lid500in a coaxial disposition along axis A-A of the VVO1000through its interaction with the annular ring140. When the lid500is secured to the overpack body100, the outer edge of the ring plate505of the lid500abuts the inner edge of the annular plate140of the overpack body100. A third function of the radial ring501is to prevent the lid500from sliding across the top surface of the overpack body100during a postulated tipover event defined as a non-mechanistic event for the VVO1000.

As mentioned above, the lid500comprises a plurality of outlet vents550that allow heated air within the storage cavity10of the VVO1000to escape. The outlet vents550form passageways through the lid500that extend from openings551in the bottom surface504of the lid500to openings552in the peripheral surface506of the lid500. While the outlet ducts550form L-shaped passageways in the exemplified embodiment, any other tortuous or curved path can be used so long as a clear line of sight does not exist from external to the VVO1000into the cavity10through the inlet ducts550. The outlet vents550are positioned about the circumference of the lid500in a radially symmetric and spaced-apart arrangement. The outlet ducts550terminate in openings552that are narrow in height but axi-symmetric in the circumferential extent. The narrow vertical dimensions of the outlet ducts550helps to efficiently block the leakage of radiation. It should be noted, however, that while the outlet vents550are preferably located within the lid500in the exemplified embodiment, the outlet vents550can be located within the overpack body100in alternative embodiments, for example at a top thereof.

Referring briefly toFIG. 10, the purpose of the inlet vents150and the outlet vents550is to facilitate the passive cooling of an MPC200located within the cavity10of the VVO1000through natural convection/ventilation. InFIG. 10, the flow of air is represented by the heavy black arrows3,5,7. The VVO1000is free of forced cooling equipment, such as blowers and closed-loop cooling systems. Instead, the VVO1000utilizes the natural phenomena of rising warmed air, i.e., the chimney effect, to effectuate the necessary circulation of air about the MPC200stored in the storage cavity10. More specifically, the upward flowing air5(which is heated from the MPC200) within the annular space50that is formed between the inner surface121of the overpack body100and the outer surface201of the MPC200draws cool ambient air3into the storage cavity10through inlet ducts150by creating a siphoning effect at the inlet ducts150. The rising warm air5exits the outlet vents550as heated air7. The rate of air flow through the VVO1000is governed by the quantity of heat produced in the MPC200, the greater the heat generation rate, the greater the air upflow rate.

To maximize the cooling effect that the ventilating air stream3,5,7has on the MPC200within the VVO1000, the hydraulic resistance in the air flow path is minimized to the extent possible. Towards that end, the VVO1000comprises eight inlet ducts150(shown inFIG. 6). Of course, more or less inlet ducts150can be used as desired. In one preferred embodiment, at least six inlet ducts150are used. Each inlet duct150is narrow and tall and has an internally refractive contour (shown inFIG. 6) so as to minimize radiation streaming while optimizing the size of the airflow passages. The curved shape of the inlet ducts150also helps minimize hydraulic pressure loss. The structure of the inlet ducts150will be described below in much greater detail with respect toFIGS. 4-6.

Referring back toFIGS. 1-4and7concurrently, in order to decrease the amount of radiation scattered to the environment, an array of duct photon attenuators (DPAs) may be installed in the inlet and/or outlet ducts150,550. An example of a suitable DPA is disclosed in U.S. Pat. No. 6,519,307, the entirety of which is hereby incorporated by reference. The DPAs scatter any radiation streaming through the ducts150,550, thereby significantly decreasing the local dose rates around the ducts150,550. The configuration of the DPAs is such that the increase in the resistance to air flow in the air inlet ducts150and outlet ducts550is minimized.

The inlet ducts150permit the MPC200to be positioned directly atop the top surface131of the base plate130of the VVO1000if desired, thus minimizing the overall height of the cavity10that is necessary to house the MPC200. Naturally, the height of the overpack body100is also minimized. Minimizing the height of the overpack body100is a crucial ALARA-friendly design feature for those sites where the Egress Bays in their Fuel Buildings have low overhead openings in their roll-up doors. To this extent, the height of the storage cavity10in the VVO1000is set equal to the height of the MPC200plus a fixed amount to account for thermal growth effects and to provide for adequate ventilation space above the MPC200, as set forth in Table 1 below.

TABLE 1OPTIMIZED MPC, TRANSFER CASK, AND VVO HEIGHTDATA FOR A SPECIFIC UNIRRADIATED FUEL LENGTH, lMPC Cavity Height, cl + Δ1MPC Height (including top lid), hc + 11.75″VVO Cavity HeightH + 3.5″Overpack Body Body Height (height fromH + 0.5″the bottom end to the top end of theoverpack body)Transfer Cask Cavity Heighth + 1″Transfer Cask Height (loaded over the pad)h + 27″Transfer Cask Total HeightH + 6.5″1Δ shall be selected as 1.5″ < Δ < 2″ so that c is an integral multiple of ½ inch (add 1.5″ to the fuel length and round up to the nearest ½″ or full inch).

As can be seen from Table 1, the first step in the height minimization plan is to minimize the height of the MPCs200. The MPC cavity height, c, is customized for each plant (based on its fuel) so that there is no unnecessary (wasted) space.

The MPC200can be placed directly on the base plate130such that the bottom region of the MPC200is level with the inlet ducts150because radiation emanating from the MPC200is not allowed to escape through the specially shaped inlet ducts150due to: (1) the inlet ducts150having a narrow width and being curved in shape so as to wrap around a columnar structure155made of alloy steel or steel (or a combination of steel and concrete); (2) the configuration of the inlet ducts150is such that that there is no clear line of sight from inside the cavity10to the exterior environment; and (3) there is enough steel and/or concrete in the path of any radiation emanating from the MPC200to de-energize it to acceptable levels. The columnar structure155is configured to be cylindrical so as to be internally refractive, but it can also be of rectangular, elliptical, or other prismatic cross-sections to fulfill the essence of the above design features. With the radiation streaming problem at the inlet ducts150solved, the top102of the overpack body100can be as little as ½″ higher than the top surface202of the MPC200. Table 1 above gives typical exemplary dimensions but, of course, is not limiting of the present invention.

Finally, with reference toFIG. 4, to protect the concrete mass115of the VVO1000from excessive temperature rise due to radiant heat from the MPC200, a thin cylindrical liner160of insulating material, can be positioned concentric with the inner shell120. This insulating liner140is slightly smaller in diameter than the inner shell120. The liner acts as a “heat shield” and can be hung from top impact absorbers165or can be connected directly to the inner shell120or another structure. The insulating layer140can be constructed of, without limitation, blankets of alumina-silica fire clay (Kaowool Blanket), oxides of alimuna and silica (Kaowool S Blanket), alumina-silica-zirconia fiber (Cerablanket), and alumina-silica-chromia (Cerachrome Blanket). The underside of the overpack lid500may also include a liner of insulating material if desired.

The top impact absorbers165are connected to the inner surface121of the inner shell120in a circumferentially spaced apart arrangement at or near the top end of the cavity10. Similarly, bottom impact absorbers166are connected to the inner surface121of the inner shell120in a circumferentially spaced apart arrangement at or near the bottom end of the cavity10. The top and bottom impact absorbers165,166are designed to absorb kinetic energy to protect the MPC200during an impactive collision (such as a non-mechanistic tip-over scenario). In the exemplified embodiment, the top and bottom impact absorbers165,166are hollow tube like structures but can be plate structures if desired. The impact absorbers165,166serve as the designated locations of impact with the MPC lid210and the base plate220of the MPC200in case the VVO1000tips over. The impact absorbers165,166are thin steel members sized to serve as impact attenuators by crushing (or buckling) against the solid MPC lid210and the solid MPC base220during an impactive collision (such as a non-mechanistic tip-over scenario).

Referring now toFIGS. 4-6concurrently, the details of the inlet ducts150will be discussed in detail. Generally, each of the inlet ducts150extend from an opening151in the outer surface112of the overpack body100(which in the exemplified embodiment is also the outer surface of the outer shell110) to an opening152in the inner surface121of the overpack body100(which in the exemplified embodiment is also the inner surface of the inner shell120). Each of the inlet ducts150forms a passageway153from an atmosphere external to the VVO1000to a bottom portion of the cavity10so that cool air can enter the cavity10.

A columnar structure155is located within each of the inlet ducts150. Each of the columnar structures155extend along their own longitudinal axis B-B. In the exemplified embodiment, the longitudinal axes B-B of the columnar structures155are substantially parallel with the central vertical axis A-A of the VVO1000. Thought of another way, the longitudinal axes B-B extend in the load bearing direction of the overpack body100. Of course, the invention will not be so limited in all embodiments and the longitudinal axes B-B of the columnar structures155may be oriented in a different manner if desired.

The columnar structures155are formed by a combination of steel plates156,157and concrete115. The plates157are cylindrical in shape and bound the outer circumferences of the columnar structures155, thereby forming the outer surfaces of the columnar structures155. The plates156are flat plates that are thicker than the plates157and are centrally positioned within the columnar structures155so as to extend along the axes B-B. The plates156provide structural integrity to the columnar structures155(similar to rebar) and also add additional gamma shielding to the columnar structures155. The columnar structures155have a transverse cross-section that is circular in shape. However, the invention is not so limited and the columnar structures155can have a transverse cross-section of any prismatic shape.

The columnar structures155divide each of the passageways153of the inlet ducts150into a first channel153A and a second channel153B. For each inlet duct150, the first and second channels153A,153B converge at both openings151,152, thereby collectively surrounding the entire circumference of the outer surface of the columnar structure155. Thought of another way, for each inlet duct150, the first and second channels153A,153B collectively circumferentially surround the longitudinal axes B-B of the columnar structures155, forming a circular (or other prismatic) passageway contained within the walls of the overpack body100.

Importantly, for each inlet duct150, a line of sight does not exist between the opening152in the inner surface121of the overpack body100and the opening151in the outer surface112of the overpack body100. This is because the columnar structures155block such a line-of-sight and provide the required radiation shielding, thereby preventing radiation shine into the environment via the inlet ducts150. As such, the MPC200can be positioned within the cavity10so as to be horizontally and vertically aligned with the inlet ducts150without radiation escaping into the external environment (seeFIGS. 8-9). Stated conceptually, for each inlet duct150, the opening152in the inner surface121of the overpack body100is aligned with the opening151in the outer surface112of the overpack body100so that: (i) a first reference plane D-D that is perpendicular to the longitudinal axis A-A of the overpack body100intersects both the opening152in the inner surface121of the overpack body100and the opening151in the outer surface112of the overpack body100; and (ii) a second reference plane C-C that is parallel with and includes the longitudinal axis A-A of the overpack body100intersects both the opening152in the inner surface121of the overpack body100and the opening151in the outer surface112of the overpack body100. When an MPC200is positioned in the cavity10as shown inFIGS. 8-9, the MPC200is also intersected by the reference plane C-C and the reference plane D-D.

The inlet vents150(and thus the first and second channels153A, B) are lined with steel. For each inlet duct160, the steel liner includes the cylindrical plate157of the columnar structure155, two arcuate wall plates158, an annular roof plate159, and the base plate130. All connections between these plates can be effectuated by welding. As can best be seen inFIGS. 5 and 6, the width of the first and second channels153A, B is defined by a gap located between the cylindrical plate157of the columnar structure155and the two arcuate plates158. Preferably, the cylindrical plate157of the columnar structure155and the two arcuate plates158are arranged in a concentric and evenly spaced-apart manner so that the first and second channels153A, B have a constant width. Most preferably, the first and second channels153A, B are curved so as to reduce hydraulic pressure loss. Finally, it is also preferred that the inlet ducts150have a height that is at least three times that of its width.

Referring now toFIGS. 8-11concurrently, the benefits achieved by the special design of the inlet ducts150with respect to MPC200storage will be discussed. During use of the VVO1000, an MPC200is positioned within the cavity10. An annular gap50exists between the outer surface201of the MPC200and the inner surface121of the overpack body100The annular gap50creates a passageway along the outer surface201of the MPC200that spatially connects the inlet vents150to the outlet vents550so that cool air3can enter VVO1000via the inlet vents150, be heated within the annular space50so as to become warm air5that rises within the annular space50, and exit the VVO1000via the outlet vents550.

The MPC200is supported within the cavity10so that the bottom surface of the MPC200rests directly atop the top surface131of the base plate130. This is made possible because the inlet ducts150are shaped so as not to allow radiation to shine therethrough because a clear line-of-sight does not exist from the cavity10to the atmosphere outside of the VVO1000through the inlet ducts150. Thus, the cavity10(and as a result the overpack body100) can be made as short as possible and substantially correspond to the height of the MPC200, as discussed above with respect to Table 1.

Additionally, positioning the MPC200in the cavity10so that the bottom surface of the MPC200is below the top of the opening152of the inlet vents150ensures adequate MPC cooling during a “smart flood condition.” A “smart flood” is one that floods the cavity10so that the water level is just high enough to completely block airflow though the inlet ducts150. In other words, the water level is just even with the top of the openings152of the inlet ducts150. Because the bottom surface of the MPC200is situated at a height that is below the top of the openings152of the inlet ducts150, the bottom of the MPC200will be in contact with (i.e. submerged in) the water during a “smart flood” condition. Because the heat removal efficacy of water is over 100 times that of air, a wet bottom is all that is needed to effectively remove heat and keep the MPC200cool. The MPC cooling action effectively changes from ventilation air-cooling to evaporative water cooling. Additionally, as shown inFIG. 11, the MPC200is particularly suited for “smart-flood” cooling because the MPC200is designed to achieve an internal natural thermopshion cyclical flow. Thus, in a smart-flood,” the thermosiphon flow in the MPC200will circulate the internal gas so that the hot gas is circulated to the top of the MPC where its heat can be effectively removed.

As mentioned above, the design discussed above for the VVO1000allows the VVO1000to be constructed so that the height of the cavity10(and thus the VVO1000) is minimized to the extent possible to accommodate an MPC200that, in turn, corresponds in height to the length of the SNF assemblies at issue. It has been further discovered that because the MPC200does not have to be positioned above the inlet ducts150, the same configuration of inlet ducts150can be used for any and all VVOs1000, irrespective of the height of the MPC200to be positioned therein. Additionally, it has been further discovered that if the outer horizontal cross-section of the MPC200and the inner horizontal cross-section of the VVO1000are also kept constant, that it is possible to manufacture VVOs1000of variable heights under a single N.R.C. (or other regulatory agency) license without having to obtain a new license, so long as a taller and shorter version of the VVO1000has already been licensed.

Licensing of the shorter VVO1000is necessary because the shorter a VVO1000is, the less effective the heat rejection capacity of that VVO's natural ventilation system becomes. This is because decreasing the height of the MPC200results in a decreased upward flow of air within the annular space50, thereby reducing the ventilation of the MPC200. Licensing of the taller VVO1000is necessary because the taller a VVO1000is, the more susceptible it becomes to inertial loading resulting from a postulated tip-over event that would destroy the integrity of the MPC200within the cavity10. Stated simply, assuming that the ventilation system of the taller and shorter VVOs are held constant, if the shorter VVO meets the required heat rejection capacity, it can be assumed that all taller VVOs will also meet the required heat rejection capacity. Similarly, assuming that the structural configuration of the taller and shorter VVOs are held constant, if the taller VVO can withstand an inertial load resulting from a postulated tip-over event and maintain the integrity of the MPC within its cavity, it can be assumed that all shorter VVOs will also withstand the inertial load resulting from the postulated tip-over event and maintain the integrity of the MPC within its cavity. As used herein, the structural configuration of two VVOs are held constant if the structural components and arrangements remain the same, with exception of the height of the shells110,120and possibly the diameter of the outer shell110.

Thus, in on embodiment, the invention is directed to a method of designing embodiments of the VVO1000so that its height is variable and greater than the plant's fuel length by a certain fixed amount. Thus, VVOs1000of varying heights can be manufactured under a single U.S.N.R.C. license and be suitable to store SNF in an optimized configuration at all nuclear plants in the world. An embodiment of the present invention will now be described in relation to VVO1000discussed above with the addition to suffixes “A-C” to distinguish between the tall version of the VVO1000A the short version of the VVO1000B, and the intermediate version of the VVO10000respectively.

According to one embodiment of the present invention, a VVO1000A having a first cavity10A for receiving a first MPC200A containing high level radioactive waste is designed. This first VVO1000A comprises a structural configuration that can withstand an inertial load resulting from a postulated tip-over event of the VVO1000A so as to maintain the integrity of the first MPC200A within the cavity. The first cavity10A has a first height H1that corresponds to the height of the first MPC200A as discussed above in relation to Table 1.

A second VVO1000B having a second cavity10B for receiving a second MPC200B containing high level radioactive waste is then be designed. The second VVO1000B comprises a configuration of inlet and outlet ducts150B,550B for facilitating natural convective cooling of the second MPC200B that achieves a required heat rejection capacity. The second cavity10B has a second height H2that corresponds to the height of the second MPC200B as discussed above in relation to Table 1. The first height H1is greater than the second height H2.

The designs of the first and second VVOs1000A,1000B are then submitted to the appropriate regulatory agency, such as the U.S.N.R.C., for licensing. A license is obtained from the regulatory agency for the first and second VVOs1000A,10008.

After the licenses are obtained, a third VVO1000C comprising a third cavity10C for receiving a third MPC200C containing high level radioactive waste is manufactured. The third cavity10C has a third height H3that corresponds to a height of the third MPC200C as discussed above in relation to Table 1. The third height H3is greater than the second height H2and less than the first height H1. The VVO1000C is manufactured to have a structural configuration that is the same as the structural configuration of the first VVO1000A and a configuration of inlet and outlet ducts150C,550C for facilitating natural convective cooling of the third MPC200C that is the same as the configuration of the inlet and outlet ducts150B,550B of the second VVO1000B. The first, second and third cavities10A,10B,10C all have the same horizontal cross-sections and the first, second and third MPCs200A,200B,200C all have the same outer horizontal cross-sections.

Thus, the third VVO1000C will automatically be covered by the license granted for the VVOs1000A and1000B without filing a new application for certification with the regulatory agency.

In the example above, the taller VVO1000A may also be designed to comprise a configuration of inlet and outlet ducts150A,550A for facilitating natural convective cooling of the second MPC200B that achieves a required heat rejection capacity. The configuration of inlet and outlet ducts150A,550A may be the same as the configuration of inlet and outlet ducts150B,550B of the shorter VVO1000B. Similarly, the shorter VVO1000B may also be designed to comprise a structural configuration that can withstand an inertial load resulting from a postulated tip-over event of the VVO1000B so as to maintain the integrity of the first MPC200B within the cavity10B. The structural configuration of the VVO1000B may be the same as the structural configuration of the VVO1000A.