Patent Publication Number: US-11393603-B2

Title: Thermal divider insert and method for spent nuclear fuel cask creating both air inlets and air outlets at the top of the overpack

Description:
FIELD OF THE INVENTION 
     The embodiments of the present disclosure generally relate to storage of hazardous radioactive materials and, more particularly, to dry storage, spent nuclear fuel casks for containing spent nuclear fuel or other hazardous radioactive material(s). 
     BACKGROUND OF THE INVENTION 
     Spent nuclear fuel has historically been stored in deep reservoirs of water, called “spent fuel pools,” within nuclear power plants. This spent fuel storage technology is often termed “wet storage.” Spent fuel pools at reactors are reaching their spent fuel capacity limits, causing concerns about the need to shut down reactors because there is no more room for the spent fuel. Dry nuclear spent fuel storage technology (termed “dry storage”) is deployed throughout the world to expand the capabilities of nuclear power plants to discharge and store nuclear spent fuel external to a reactor&#39;s spent fuel pool, thereby extending the operating lives of the power plants. 
     There are two fundamental classes of technology used in dry spent fuel storage: (a) metal casks with final closure lids that are bolted closed at the power plants after loading with spent fuel, and (b) concrete storage casks containing metal canisters having canister final closure lids that are welded closed or sealed with mechanical methods at the power plants following spent fuel loading. This latter dry storage technology is referred to as “canister-based concrete spent fuel storage.” The concrete cask serves as an enclosure, or “overpack” that provides mechanical protection, heat removal features, and radiation shielding for the inner metal canister that encloses the radioactive materials. 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 
     Embodiments of a thermal divider insert and method for a dry storage, spent nuclear fuel cask are disclosed. The thermal divider insert enables safe storage of the hazardous nuclear material when one or more air inlets have been fully or partially blocked to an extent that insufficient air flows into the air inlets and through the cask for adequate cooling of the hazardous nuclear material. 
     In one embodiment, among others, a cask comprises a metal canister having a top, bottom, and sidewall. The canister contains the hazardous nuclear material. A concrete overpack contains the metal canister with the hazardous nuclear material. The overpack has a top, bottom, and sidewall. The overpack has an inside surface that is spaced from an outer surface of the canister to create an annular region that permits flow of air between the surfaces for cooling the canister. One or more air inlets near the bottom of the overpack communicates air from an outside environment into the annular region. One or more outlet vents near the top of the overpack communicates air from the annular region to the outside environment. The thermal divider insert extends through a respective outlet vent and into the annular region and is designed to establish two separate and opposite air flows (i.e., inward air flow and outward air flow) through the respective vent and the annular region when the overpack air inlets have been blocked. When not blocked in normal operation, the two air flows both flow upwardly through the annular region and outwardly from the vent. 
     An embodiment of the thermal divider insert, among others, comprises (a) a planar horizontal radial plate and (b) a curved vertical plate extending from the radial plate, in order to establish the two separate and opposite air flows through the vent. The horizontal radial plate extends through the overpack outlet vent. The radial plate has a curvature along its inside and outside edges that corresponds to a curvature associated with the overpack outlet vent. The redial plate establishes a lower air flow region and an upper air flow region within the overpack outlet vent. When the one or more air inlets are blocked, then the lower air flow region enables inward air flow from the outside environment, and the upper air flow region enables outward air flow to the outside environment. When the one or more air inlets are not blocked, then the upper and lower air flow regions enable outward air flow to the outside environment. 
     As for the curved vertical plate, it extends downwardly at a right angle from the inside edge of the radial plate and has a curvature that corresponds to a curvature associated with the annular region. The curved vertical plate essentially establishes an outer annular region and an inner annular region. When the one or more inlets are blocked, the outer annular region enables inward air flow from the lower air flow region within the vent, and the inner annual region enables outward air flow to the upper air flow region of the vent. When the one or more air inlets are not blocked, the outer and inner annular regions enable upward air flow into the lower and upper air flow regions, respectfully, and then outwardly from the vent into the outside environment. 
     An embodiment of a method, among others, for safely storing hazardous nuclear material when one or more air inlets have been fully or partially blocked to an extent that insufficient air flows into the air inlets and through the cask for adequate cooling of the hazardous nuclear material, comprises the steps of: when the one or more air inlets are not blocked, enabling air flow into the air inlets, through the annular region, and then through and out of the one or more air vents; and when the air inlets are blocked, enabling air flow inwardly through the vents, then through the annular region, and then through and out of the vents. Furthermore, another embodiment is an apparatus having a means for performing each of the foregoing steps. 
     Other embodiments, apparatus, methods, features, and advantages of the present invention will be apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional embodiments, apparatus, 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 
       Many aspects of the 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. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a cross-sectional view of a typical, prior art dry storage, spent nuclear fuel cask having an overpack with canister containing radioactive material(s) stored therein with the typical air movement through the annular region between the overpack and the canister. 
         FIG. 2  is a cross-sectional view of the cask of  FIG. 1  with stagnant air due to substantial blockage of one or more overpack air inlets. 
         FIG. 3  shows a cask having an overpack with canister containing radioactive material stored therein with an overpack having an inner annular air flow established by the thermal divider insert of the present invention, which establishes a separated air flow, thereby cooling the canister and radioactive contents despite substantial blockage of the overpack inlets. 
         FIG. 4  a partial enlarged cross-sectional view of the cask with thermal divider insert of  FIG. 3 . 
         FIG. 5  is a perspective view of the uninstalled thermal divider insert of  FIGS. 3 and 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a cross-sectional view of a typical, prior art, dry storage, spent nuclear fuel cask  10  having an overpack  12  with canister  14  containing a radioactive material(s)  15  stored therein with the typical air  16  into one or more air inlets  17 , through an annular region  18  between the overpack  12  and the canister  14 , and then out of 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 sidewall. 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 sidewall. 
     The overpack heat removal function associated with canister-based spent fuel storage relies upon natural circulation of air though the annular region  18  between the overpack vertical inner boundary and the vertical outer boundary of the metal canister  14  containing the radioactive material stored within the overpack  12 . The cooler, more dense air  16  is introduced into the annular region  18  via the one of more inlets  17  where the air  16  absorbs heat which is being emitted from the radioactive material  15  in the canister  14 , thereby becoming less dense and more buoyant. This increased buoyancy results in the less dense air  16  rising upward through the annular region  18  until the air  16  reaches the upper area where is exits the overpack  12  via the one or more outlet vents  22 . The movement of air  16  through the annular region  18 , as described, 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. 
     With reference to  FIG. 2 , in the unlikely event that water flooding of the area where the cask  10  is stored, it is conceivable that the flood water  24  could cover the one or more overpack air inlets  17 , in whole or in part, thereby interrupting the introduction of air  16  ( FIG. 1 ) into the annular region  18 . Generally,  FIG. 2  shows a cross-sectional view of the cask  10  with stagnant air  16 ′ due to substantial blockage of the overpack inlets  17  by the flood water  24 . This interruption of the flow of air  16  could result in an undesirable and dangerous increase in temperature of the radioactive material  15 , potentially above desirable and/or allowable levels. 
     The annular region  18  within the overpack  12  serves to act as a single column for air  16  to travel upward through as the air  16  absorbs heat, becoming less dense. With the blockage of the normal introduction path for cooler, less dense air  16  at the bottom of the overpack  12 , this single column for air  16  becomes stagnated, thereby resulting in no means to create a thermally induced driving force based on different air densities. 
     As illustrated in  FIGS. 3 through 5 , the present disclosure provides a thermal divider insert  26  that is devised specifically to address this stagnant air condition when the air inlets  17  are blocked. One of more of the thermal divider inserts  26  are installed in the overpack  12 . Each thermal divider insert  26  extends through a respective air outlet vent  22  and into the annular region  18 . 
     As shown in  FIG. 5 , each thermal divider insert  26  is an angular plate configured in such a manner so as to have a complex right-angle appearance that is concurrently radially shaped to conform to the inner radial dimension of the overpack  12  along both vertical and horizontal surfaces. The thermal divider insert  26  can be made from any suitable materials, but in the preferred embodiment, is primarily metal, such as stainless steel. The thermal divider insert  26  can be any suitable thickness. The material and thickness should give sufficient rigidity to the structure. Furthermore, the thermal divider insert  26  is mounted via bolts, welding, or some other suitable known method. 
     With reference to  FIG. 3 , the thermal divider insert  26  is installed in the overpack  12  and is configured in such a manner that the horizontal portion  28  of the insert  26  effectively divides the overpack outlet vent  22  into two distinct areas: a lower area and an upper area. The vertical plate  32  of the thermal divider insert  26  is aligned in the overpack annular region  18  between the inner boundary wall of the overpack  12  and the outer wall of the canister  14  containing radioactive material  15  stored within the overpack  12 , thereby dividing the annular region  18  into two distinct areas: an inner annular region and an outer annular region. The curved vertical plate  32  extends a substantial vertical distance downwardly through the annular region  18 , preferably at least half the vertical span of the annular region  18 . In the preferred embodiment, the vertical plate  32  extends about sixty percent of the vertical distance of the annular region. 
     The thermal divider insert  26  acts as a thermal material shield during normal system operation (i.e., no flood condition present that blocks the overpack inlets  17 ). When the one or more air inlets are not blocked, then the outer and inner annular regions enable upward air flow into the lower and upper air flow regions, respectfully, of the vents  22 , and then outward air flow from the vents  22  into the outside environment. 
     Upon blockage of the overpack inlets  17  due to flood waters (or any other postulate condition that prevents or otherwise inhibits the introduction of cooler, more dense air  16  into the overpack inlets  17 ), a thermal imbalance is initially encountered within the annular region  18 , resulting initially in a stagnant air condition. Since the radioactive material  15  within the canister  14  will continue to emit heat, the air  16  closest to the canister  14  will continue to absorb heat, thereby creating a difference in density as compared to the air  16  closest to the inner surface of the overpack  12 . As shown by the arrows in  FIG. 3 , due to the presence of the thermal divider insert  26 , a separation of the different density air masses will be established such that the air  16  closest to the canister  14  will begin to rise due to buoyancy and will exit the overpack  12  via the upper region of the overpack outlet vent  22 . Conversely, relatively cooler, more dense air will enter into the overpack  12  via the lower region of the overpack outlet vent  22 , travelling downward into the outer annular region of the annular region  18 , then turning inward and travelling upward within the inner annular region of the annular region  18  that has been established by the thermal divider insert  26 , thereby re-establishing air flow through the annular region  18  and removing heat being emitted from the radioactive material  15  stored within the canister  14 . 
     Finally, it should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible nonlimiting examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention. 
     As an example, it is envisioned that other embodiments of the thermal divider insert  26  of  FIG. 5  can designed with a different configuration, shape, size, etc., as compared to the preferred embodiment to achieve the desired goal of establishing two separate and opposite air flows (inward air flow and outward air flow) through the respective vent and the annular region when the overpack air inlets  17  have been blocked.