Thermally split insert for spent nuclear fuel casks and method

Abstract

To enable safe storage when one or more air intakes are completely or partially blocked to some extent so that air flow is insufficient, the air flow flowing into the air intake and passing through a cask in order to properly cool hazardous nuclear material.SOLUTION: One or more air inlets near a bottom of an overpack communicate air from outside environment into an annular region. One or more outlet vents near a top of the overpack communicate air from the annular region to the outside environment. A thermal divider insert extends through each outlet vent and into the annular region and is designed to establish two separate and opposite air flows flowing through each outlet vent and the annular region when the air inlets of the overpack have been blocked. When not blocked in normal operation, the two air flows both flow upwardly through the annular region and flow outwardly from the outlet vent.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE Embodiments of the present disclosure relate generally to the storage of hazardous radioactive materials, and more particularly to dry storage, spent nuclear fuel casks for containing spent nuclear fuel or other hazardous radioactive materials. [Background technology]

[0002] Spent nuclear fuel has historically been stored in deep reservoirs called "spent fuel pools" within nuclear power plants. This spent fuel storage technique is often referred to as "wet storage." Nuclear reactors' spent fuel pools are reaching their spent fuel capacity limits, raising concerns about the need to shut down the reactors as there will be no room for more spent fuel. Dry spent nuclear fuel storage techniques (called "dry storage") are being deployed around the world to extend the capacity of nuclear power plants to release and store spent fuel outside of the reactor's spent fuel pool, thereby extending the operating life of the plants.

[0003] There are two basic classes of technologies used for dry spent fuel storage: (a) metal casks with a final closure lid that is bolted closed at the power plant after loading with spent fuel, and (b) concrete storage casks that include a metal canister with a canister final closure lid that is welded closed or sealed by mechanical means at the power plant after loading with spent fuel. This latter dry storage technology is called "canister-based concrete spent fuel storage." The concrete cask serves as an enclosure, or "overpack," that provides mechanical protection, heat removal characteristics, and radiation shielding for the inner metal canister that encases the radioactive material. The use of this technology tends to have significant capital cost and other economic advantages over the use of metal cask technology for storage. Summary of the Invention

[0004] Disclosed are embodiments of a thermal splitting insert for dry storage, spent nuclear fuel casks that allows for the safe storage of hazardous nuclear material when one or more air intakes are fully or partially blocked to the extent that there is insufficient air flow into the air intakes and through the cask to adequately cool the hazardous nuclear material.

[0005] In one embodiment, among others, the cask includes a metal canister having a top, a bottom, and a sidewall. The canister contains hazardous nuclear material. The concrete overpack contains the metal canister with the hazardous nuclear material. The overpack has a top, a bottom, and a sidewall. The overpack has an interior surface spaced apart from an exterior surface of the canister to form an annular region that allows air to flow between the surfaces to cool the canister. One or more air intakes near the bottom of the overpack communicate air from an external environment into the annular environment. One or more exhaust vents near the top of the overpack communicate air from the annular region to the external environment. The thermal splitter inserts extend through the respective exhaust vents into the annular region and are designed to establish two separate and opposing air flows (i.e., an inward air flow and an outward air flow) through the respective exhaust vents and the annular region when the overpack air intakes are blocked. When unblocked in normal operation, the two airways both flow upward through the annular region and outward through the exhaust vent.

[0006] One embodiment of the thermal splitting insert, among others, comprises (a) a planar horizontal radial plate and (b) a curved vertical plate extending from the radial plate to establish two separate and opposing air flows through the exhaust vent. The horizontal radial plate extends through the exhaust vent of the overpack. The radial plate has a curvature along its inner and outer edges that corresponds to the curvature associated with the exhaust vent of the overpack. The radial plate establishes a lower airflow region and an upper airflow region within the exhaust vent of the overpack. When one or more air intakes are blocked, the lower airflow region allows inward airflow from the external environment and the upper airflow region allows outward airflow to the external environment. When one or more air intakes are unblocked, the upper airflow region and the lower airflow region allow outward airflow to the external environment.

[0007] With respect to the curved vertical plate, it extends downward at a right angle from the inner edge of the radial plate and has a curvature corresponding to the curvature associated with the annular region. The curved vertical plate essentially establishes an outer annular region and an inner annular region. When one or more air intakes are blocked, the outer annular region allows inward airflow from the lower airflow region in the exhaust vent, and the inner annular region allows outward airflow to the upper airflow region of the exhaust vent. When one or more air intakes are unblocked, the outer and inner annular regions allow upward airflow into the lower and upper airflow regions, respectively, and then outward airflow from the exhaust vent into the external environment.

[0008] In particular, where one or more air inlets are fully or partially blocked to the extent that airflow into the air inlets and through the cask is insufficient for adequate cooling of the hazardous nuclear material, an embodiment of a method for safely storing hazardous nuclear material includes the steps of allowing airflow into the air inlets, through the annular region, and then out through one or more air vents when the one or more air inlets are unblocked, and allowing an inward air flow through the exhaust vents, then through the annular region, and then out through the exhaust vents when the air inlets are blocked. Yet another embodiment is an apparatus having means for performing each of the foregoing steps.

[0009] Other embodiments, devices, methods, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following figures and detailed description, and it is intended that all such additional embodiments, devices, methods, features, and advantages be included within this disclosure, be within the scope of the present invention, and be protected by the accompanying claims. [Brief description of the drawings]

[0010] Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. In the drawings, like reference numbers indicate corresponding parts throughout the several views.

[0011] [Figure 1] FIG. 1 is a cross-sectional view of a typical prior art dry storage spent nuclear fuel cask having an overpack with a canister containing radioactive material, with typical air movement through the annular region between the overpack and the canister.

[0012] [Diagram 2] FIG. 2 is a cross-sectional view of the cask of FIG. 1 having stagnant air due to substantial blockage of one or more overpack air intakes.

[0013] [Diagram 3] FIG. 3 shows a cask having an overpack with a canister containing radioactive material, with the overpack having an inner annular air flow established by a thermal splitting insert of the present invention, which establishes a separated air flow, thereby cooling the canister and radioactive contents despite substantial blockage of the overpack's air intake.

[0014] [Figure 4] FIG. 4 is an enlarged cross-sectional view of a portion of a cask having the thermally splitting insert of FIG.

[0015] [Diagram 5] FIG. 5 is a perspective view of the thermal split insert of FIGS. 3 and 4 with removed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] 1 shows a cross-sectional view of a typical prior art dry storage spent nuclear fuel cask 10 having an overpack 12 with a canister 14 containing radioactive material 15 stored therein, with typical air 16 flowing into one or more air inlets 17, through an annular region 18 between the overpack 12 and the canister 14, and then out one or more air outlet vents 22. The canister 14 in the preferred embodiment is primarily (or substantially) metal, such as stainless steel, and generally cylindrical in shape with a flat top, a flat bottom, and cylindrical sidewalls. The overpack 12 in the preferred embodiment is primarily (or substantially) concrete and generally cylindrical in shape with a flat top, a flat bottom, and cylindrical sidewalls.

[0017] The overpack heat removal function associated with canister-based spent fuel storage relies on the natural circulation of air through an annular region 18 between the overpack vertical interior boundary and the vertical exterior boundary of the metal canister 14 that contains the radioactive material stored within the overpack 12. Cooler, denser air 16 is introduced into the annular region 18 via one or more air intakes 17 where the air 16 absorbs heat being released from the radioactive material 15 within the canister 14, thereby becoming less dense and more buoyant. This increased buoyancy causes the less dense air 16 to rise upwardly through the annular region 18 until it reaches an upper region where the air 16 exits the overpack 12 via one or more of a plurality of exhaust vents 22. As discussed above, the movement of air 16 through the annular region 18 is a continuous process that results in the removal of heat from the radioactive material 15 stored within the canister 14, thereby ensuring that the temperature of the radioactive material 15 is maintained below a predetermined limit.

[0018] 2, it is believed that in the unlikely event of flooding of the area in which cask 10 is stored, ingressed water 24 may completely or partially cover one or more overpack air inlets 17, thereby blocking the introduction of air 16 (FIG. 1) into annular region 18. In general, FIG. 2 illustrates a cross-sectional view of cask 10 having stagnant air 16' due to substantial blockage of overpack air inlets 17 by ingressed water 24. This blockage of air 16 flow may result in an undesirable and dangerous increase in the temperature of radioactive material 15, potentially above desirable and / or acceptable levels.

[0019] The annular region 18 within the overpack 12 acts as a single column for the air 16 to move upward as the air absorbs heat and becomes less dense. When the normal introduction path for the cooler, less dense air 16 at the bottom of the overpack 12 becomes blocked, this single column of air 16 becomes stagnant, thereby eliminating a means for creating a thermally induced driving force based on the differing air densities.

[0020] As shown in Figures 3-5, the present disclosure provides thermal divider inserts 26 that are specially designed to address this stagnant air condition when the air intake 17 becomes blocked. One or more thermal divider inserts 26 are installed within the overpack 12. Each thermal divider insert 26 extends through a respective air exhaust vent 22 into the annular region 18.

[0021] As shown in FIG. 5, each heat splitting insert 26 is an angle plate configured to have a complex right angle profile that is simultaneously radially shaped to conform to the inner radial dimensions of the overpack 12 along both the vertical and horizontal planes. The heat splitting inserts 26 may be made of any suitable material, but in a preferred embodiment are primarily metal, such as stainless steel. The heat splitting inserts 26 may be of any suitable thickness. The material and thickness should provide sufficient rigidity to the structure. Additionally, the heat splitting inserts 26 are attached via bolts, welding, or other suitable known methods.

[0022] 3, the thermal splitter insert 26 is installed within the overpack 12 such that the horizontal portion 28 of the insert 26 is configured to effectively divide the overpack exhaust vent 22 into two separate regions, a lower region and an upper region. The vertical plate 32 of the thermal splitter insert 26 is aligned within the overpack annular region 18 between the inner boundary wall of the overpack 12 and the outer wall of the canister 14 containing the radioactive material 15 stored within the overpack 12, thereby dividing the annular region 18 into two separate regions, an inner annular region and an outer annular region. The curved vertical plate 32 extends a substantial vertical distance downward through the annular region 18, preferably at least half the vertical span of the annular region 18. In a preferred embodiment, the vertical plate 32 extends approximately 60% of the vertical distance of the annular region.

[0023] The thermal divider insert 26 acts as a thermal material shield during normal system operation (i.e., no water-filled conditions blocking the overpack inlets 17). When one or more of the air inlets is unblocked, the outer and inner annular regions allow airflow upward into the lower and upper airflow regions of the vent 22, respectively, and then outward from the vent 22 to the external environment.

[0024] Upon blockage of the overpack inlet 17 by flooding (or any other hypothetical condition that prevents or otherwise inhibits the introduction of cooler, denser air 16 into the overpack inlet 17), a thermal imbalance will initially be encountered within the annular region 18, resulting in an initial stagnant air condition. As the radioactive material 15 within the canister 14 continues to release heat, the air 16 closest to the canister 14 will continue to absorb heat, thereby creating a density difference compared to the air 16 closest to the interior surface of the overpack 12. As shown by the arrows in FIG. 3, the presence of the thermal splitter insert 26 will establish a separation of air masses of different density such that the air 16 closest to the canister 14 will begin to rise due to buoyancy and exit the overpack 12 through the upper region of the overpack vent 22. Conversely, the cooler, denser air enters overpack 12 through the lower region of overpack exhaust vent 22, travels downwardly within the outer annular region of annular region 18, and then travels inwardly around and upwardly within the inner annular region of annular region 18 established by thermal splitter insert 26, thereby re-establishing airflow through annular region 18 and removing heat released from radioactive material 15 stored within canister 14.

[0025] Finally, it should be emphasized that the above-described embodiments of the present invention, particularly any "preferred" embodiments, are merely non-limiting examples of possible implementations and are set forth merely for a clear understanding of the principles of the present invention. Many variations and modifications of the above-described embodiments of the present invention are possible without substantially departing from the spirit and principles of the present invention. All such modifications and variations are intended to be included herein within the scope of the present disclosure and the present invention.

[0026] As an example, it is contemplated that other embodiments of the thermal splitting insert 26 of FIG. 5 may be designed with different configurations, shapes, sizes, etc. as compared to the preferred embodiment to achieve the desired goal of establishing two separate and opposing air flows (an inward air flow and an outward air flow) through the respective vents and annular regions when the overpack air intake 17 is blocked.

Claims

1. 1. A dry storage cask for safely storing hazardous nuclear material when one or more air intakes are fully or partially blocked to the extent that air flow into the air intakes and through the cask is insufficient to adequately cool the hazardous nuclear material, comprising: a metallic cylindrical canister having a top, a bottom, and a side wall, the canister containing hazardous nuclear material; a concrete cylindrical overpack for containing the canister containing the hazardous nuclear material, the overpack having a top, a bottom and side walls and an inner surface spaced from an outer surface of the canister to define an annular region permitting air flow between surfaces; an overpack air intake adjacent the bottom of the overpack that allows air to communicate from the external environment into the annular region; an overpack exhaust vent adjacent the top of the overpack capable of communicating air from the annular region to the external environment; a thermal splitting insert extending from the exhaust vent through a top of the annular region and dividing the top of the annular region into an outer annular region on the overpack side and an inner annular region on the canister side; Equipped with a lower end of the thermal splitter insert is located within the annular region and vertically above the bottom of the canister; 1. A dry storage cask, comprising: a thermal splitting insert configured to: (a) establish a first air flow when the air intake is unblocked, in which air taken in through the air intake flows upward through each of the outer and inner annular regions and is discharged outwardly through the exhaust vent; and (b) establish a second air flow when the air intake is blocked, in which air taken inwardly through the exhaust vent flows downwardly through the outer annular region and then upwardly through the inner annular region and is discharged outwardly through the exhaust vent.

2. The heat splitting insert is a planar horizontal radial plate extending through the exhaust vent of the overpack and having a curvature along an inner edge and an outer edge corresponding to the curvature associated with the exhaust vent of the overpack, establishing a lower airflow region and an upper airflow region within the exhaust vent of the overpack, the lower airflow region for inward airflow from the external environment and the upper airflow region for outward airflow to the external environment; a curved vertical plate extending perpendicularly downward from the inner edge of the horizontal radial plate, the curved vertical plate having a curvature corresponding to the curvature associated with the annular region, establishing the outer annular region and the inner annular region, the outer annular region permitting inward airflow from the lower airflow region within the exhaust vent and the inner annular region permitting outward airflow to the upper airflow region of the exhaust vent; 2. The dry storage cask of claim 1, comprising:

3. 3. The dry storage cask of claim 2, wherein the curved vertical plate extends a substantial vertical distance through the annular region, the substantial distance being at least half the vertical length of the annular region.

4. A dry storage cask as described in claim 1, wherein the air flow in the annular region is vertically below the air flow at the exhaust vent.

5. The dry storage cask of claim 1 , wherein the thermally splitting insert is substantially metallic.

6. 1. A cask for safely storing hazardous nuclear material when one or more air intakes are fully or partially blocked to the extent that air flow into the air intakes and through the cask is insufficient to adequately cool the hazardous nuclear material, comprising: a metallic cylindrical canister having a top, a bottom, and a side wall, the canister containing hazardous nuclear material; a concrete cylindrical overpack for containing the canister containing the hazardous nuclear material, the overpack having a top, a bottom and side walls and an inner surface spaced from an outer surface of the canister to define an annular region permitting air flow between surfaces; an overpack air intake adjacent the bottom of the overpack that allows air to communicate from the external environment into the annular region; an overpack exhaust vent adjacent the top of the overpack capable of communicating air from the annular region to the external environment; A thermally split insert; Equipped with The heat splitting insert is a planar horizontal radial plate extending through the exhaust vent of the overpack, the horizontal radial plate having a curvature along an inner edge and an outer edge corresponding to the curvature associated with the exhaust vent of the overpack, establishing a lower airflow region and an upper airflow region within the exhaust vent of the overpack, the lower airflow region and the upper airflow region being for outward airflow to the external environment when the air intake is unblocked, and when the air intake is blocked, the lower airflow region being for inward airflow from the external environment and the upper airflow region being for outward airflow to the external environment; a curved vertical plate extending perpendicularly downward from the inner edge of the horizontal radial plate, the curved vertical plate having a curvature corresponding to the curvature associated with the annular regions, establishing outer and inner annular regions which allow upward airflow into the lower and upper airflow regions, respectively, in the exhaust vent when the air intake is unblocked, and when the air intake is blocked, the inner annular region allows upward airflow into the upper airflow region of the exhaust vent and the outer annular region allows inward airflow from the lower airflow region in the exhaust vent; A cask wherein a lower end of the curved vertical plate is located vertically above the bottom of the canister within the annular region.

7. The cask of claim 6 , wherein the thermally splitting insert is substantially metallic.

8. The cask of claim 6 , wherein the curved vertical plate extends a substantial vertical distance through the annular region, the substantial distance being at least half the vertical length of the annular region.

9. A cask for storing hazardous nuclear material, comprising: a canister having a top, a bottom, and a sidewall, the canister containing hazardous nuclear material; a concrete overpack having a top, a bottom, and side walls, the overpack having an inner surface spaced apart from an outer surface of the canister to define an annular region permitting air flow between the surfaces; an overpack air intake adjacent the bottom of the overpack that allows air to communicate from the external environment into the annular region; an overpack exhaust vent adjacent the top of the overpack capable of communicating air from the annular region to the external environment; (a) establishing a first air flow when the air intake is unblocked, whereby air is taken in through the air intake and flows upward through the annular region and discharged outwardly through the exhaust vent; and (b) establishing a second air flow when the air intake is blocked, whereby air is taken inwardly through the exhaust vent and flows downwardly through an overpack side of the annular region and then upwardly through a canister side of the annular region and discharged outwardly through the exhaust vent, A cask, wherein the thermal division means includes a plate arranged within the annular region so as to divide the annular region into an overpack side and a canister side, and a lower end of the plate is positioned vertically above the bottom of the canister.

10. 10. The cask of claim 9, wherein said canister and said thermal division means are substantially made of metal and said overpack is substantially made of concrete.

11. The cask of claim 9 , wherein the sidewalls associated with the canister and the overpack are generally cylindrical in shape.

12. The thermal division means is a planar horizontal radial plate extending through the exhaust vent of the overpack, the horizontal radial plate having a curvature along an inner edge and an outer edge corresponding to the curvature associated with the exhaust vent of the overpack, establishing a lower airflow region and an upper airflow region within the exhaust vent of the overpack, the lower airflow region and the upper airflow region being for outward airflow to the external environment when the air intake is unblocked, and when the airflow is blocked, the lower airflow region being for inward airflow from the external environment and the upper airflow region being for outward airflow to the external environment; a curved vertical plate extending perpendicularly downward from the inner edge of the horizontal radial plate, the curved vertical plate having a curvature corresponding to the curvature associated with the annular regions, establishing outer and inner annular regions which permit upward airflow into the lower and upper airflow regions, respectively, in the exhaust vent when the air intake is unblocked, and in the event that the exhaust vent is blocked, the inner annular region permits upward airflow into the upper airflow region of the exhaust vent and the outer annular region permits inward airflow from the lower airflow region in the exhaust vent; in the structure, 10. The cask of claim 9, wherein the curved vertical plate is the plate disposed within the annular region and divides the annular region into an outer annular region on the overpack side and an inner annular region on the canister side.

13. The cask of claim 9 , wherein the cask includes a plurality of air inlets, a plurality of exhaust vents, and a plurality of heat division means respectively corresponding to the plurality of exhaust vents.

Images