Abstract:
Example embodiments provide a Basemat-Internal Melt Arrest and Coolability device (BiMAC) that offers improved spatial and mechanical characteristics for use in damage prevention and risk mitigation in accident scenarios. Example embodiments may include a BiMAC having an inclination of less than 10-degrees from the basemat floor and/or coolant channels of less than 4 inches in diameter, while maintaining minimum safety margins required by the Nuclear Regulatory Commission.

Description:
GOVERNMENT SUPPORT 
     This invention was made with Government support under contract number DE-FC07-07ID14778, awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     1. Field 
     Example embodiments generally relate to risk mitigation components used in nuclear power plants. 
     2. Description of Related Art 
     Nuclear reactors use a variety of damage prevention/mitigation devices and strategies to minimize the risk of, and damage during, unexpected plant events. An important aspect of risk mitigation is prevention of radioactive material escape into the environment. A containment building is conventionally constructed for this purpose to surround the reactor core, and several risk mitigation devices are used to ensure that the containment building is not breached during transient events. 
     A known damage and risk mitigation device is a Basemat-Internal Melt Arrest and Coolability device (BiMAC). A BiMAC is designed to prevent or reduce damage to the containment building in the case of a severe reactor accident involving reactor vessel breach and forceful relocation of molten core components to the containment building floor, or basemat. The ultimate purpose of the BiMAC, combined with several other layers of risk mitigation components and strategies, is to maintain containment integrity at least for 24 hours following the most probable severe nuclear plant accidents and, for known accident scenarios involving core-concrete interaction, reduce the likelihood of containment breach to 0.1% or less. 
       FIG. 1  is an illustration of a conventional containment building  10  cross section. Although containment  10  is shown in  FIG. 1  having components and characteristics of an Economic Simplified Boiling Water Reactor (ESBWR), it is understood that components described therein are usable with other plant configurations. As shown in  FIG. 1 , containment  10  includes a reactor vessel  50  containing a core  55  filled with nuclear fuel bundles. A number of control blades  56  and control blade drives may be positioned below the core  55  and may be extended into core  55  to control the nuclear reaction therein. In an ESBWR, containment  10  may also include a gravity-driven coolant system  20 , which may be a large, water-filled tank used to cool the core  55  in the event of a loss of primary coolant. Further, a suppression pool  25  may be within containment  10  and used to condense steam from vessel  50  and relieve pressure in the event of an accident. 
     A reinforced concrete containment vessel  15  surrounds the vessel  50  and several reactor components, so as to contain any radioactive materials that may escape from vessel  50  or other components during normal operation or during an accident. A lower drywell  60  is formed below the vessel  50  to house control blades  56 , control blade drives, and core instrumentation and to provide space for core debris in the instance of vessel  50  breach or leak. For an ESBWR, the lower drywell  60  is largely circular with a diameter of approximately 11.2 meters. A liner  61  conventionally is placed over the concrete containment wall  15  in order to reduce corrosion and damage to wall  15  in the event of an hazardous material release in containment  10 . 
     Basemat  62  is conventionally the lowest point of containment  10  and fabricated of similar materials as the reinforced concrete containment wall  15 , directly above the ground. A BiMAC  100  may be placed above the basemat  62  to mitigate damage to basemat  62  in the event that molten or other core  55  debris is relocated to lower drywell  60 , such as in the event of vessel  50  breach. 
       FIG. 2  is a detailed view of a conventional BiMAC  100  useable in ESBWRs. BiMAC  100  is described in Nuclear Regulatory Commission document NEDO-33201, Revision 2, “ESBWR DESIGN CERTIFICATION PROBABILISTIC RISK ASSESSMENT” (“NEDO-33201 Document”), which is incorporated herein by reference in its entirety. As shown in  FIG. 2 , BiMAC  100  may be placed immediately above basemat  62  and/or liner  61 . Further, BiMAC  100  may line lower portions of the walls of the lower drywell  60 . 
     A coolant supply line  65  may be connected to and may deliver coolant material to the BiMAC  100 . Coolant may include a liquid having a high heat-absorption capacity, such as water. Coolant supply line  65  may be connected to a coolant source, and coolant may be delivered through coolant supply line  65  by a pump or other driving mechanism. Alternatively, coolant may be driven through coolant supply line  65  to BiMAC  100  by gravity alone, so as to be more fail-safe. For example, supply line  61  may be a lower drywell deluge line that connects to a pool of the gravity-driven coolant system  25  ( FIG. 1 ) or other pool within containment  10 . A fail-safe valve or other control mechanism, such as a squib valve, may initiate coolant flow through coolant supply line  65  to BiMAC  100  in the case of lower vessel  50  ( FIG. 1 ) breach or other event. 
     BiMAC  100  includes a distributor line  120  that may connect to the coolant supply line  65  and/or other coolant source. Distributor line  120  may extend the entire length of the drywell  60  along basemat  62 . Several parallel coolant channels  130  may extend, perpendicularly or otherwise, off of distributor line  120  at a 10-degree upward angle from the basemat. Coolant channels  130  may then extend up a portion of the lower drywell wall, where they terminate with an open end. In this way, coolant may flow into distributor line  120 , feed into each coolant channel  130 , and eventually flood into lower drywell  60 . Distributor line  120  and coolant channels  130  may be fabricated of a material that substantially maintains its physical properties in an operating and transient nuclear reactor environment. For example, distributor line  120  and coolant channels  130  may be fabricated from a zirconium-based alloy, stainless steel, etc. 
     An ablation shield  110  may be placed over and/or may coat coolant channels  130  and distributor line  120 . The ablation shield  110  may protect coolant channels  130  and distributor line  120  from thermal and chemical damage caused by molten core components forcefully relocating to lower drywell  60  in the event of vessel  50  breach. The ablation shield  110  may be fabricated from an inert, heat resistant, and conductive material, such as a ceramic or concrete. Additional shielding material  140  may be placed adjacent to coolant channels  130  to support the weight of core components relocated on top of BiMAC  100  during a vessel breach event. Additional shielding material  140  may be fabricated of a number of strong materials, such as concrete, ceramics, etc. 
       FIG. 3  is a detailed cross-sectional view of the coolant channels  130  of BiMAC  100 . As shown in  FIG. 3 , channels  130  may be parallel and touch, so as to form a continuous wedge-shaped jacket of channels  130  capable of cooling the BiMAC and materials relocated thereon. Each channel  130  is 3.937 inches in inner diameter to provide sufficient coolant flow therethrough, as approved in the NEDO-33201 document. Ablation shield  110  may be formed directly atop and cover each channel  130 , in order to provide heat conduction and cooling therethrough. Ablation shield  110  may be formed to different thicknesses than that shown in  FIG. 3 , depending on the material used to fabricate ablation shield  110  and the characteristics of the material to be cooled on top of ablation shield  110 . 
       FIG. 4  is a top-down perspective view of BiMAC  100  illustrating operation of BiMAC  100  during an accident scenario. As shown in  FIG. 4 , during an initiating event, a valve is opened, permitting coolant flow down through a coolant supply line  65  to distribution line  120 . Coolant flows into either end of distribution line  120  and then into coolant channels  130  up at a 10-degree angle toward walls  15 . As molten or other hot debris is relocated to the lower drywell on top of an ablation shield  110  covering coolant channels  130 , forced coolant flow through coolant channels  130  removes heat from the relocated debris, preventing continued melt and/or damage to basemat  62  and containment walls  15 . Coolant exits the coolant channels  130  at a higher open end point of each channel  130 , eventually flooding the lower drywell  60  and further aiding in cooling debris therein. As such, open end points of each channel  130  are typically located such that relocated debris cannot clog the channels  130 . Further, if coolant flow to BiMAC  100  is provided by gravity, coolant flow and cooling may continue even if other plant mechanical systems fail that would otherwise be required to pump coolant into BiMAC  100 , resulting in continuous, natural-circulation cooling. 
     The structure and function of BiMAC  100  described in  FIGS. 1-4  has been extensively tested and submitted for approval to the Nuclear Regulatory Commission with dimensions of 3.937 inch inner diameter for coolant channels  130  and a 10-degree incline for coolant channels  130  with respect to basemat  62  in an ESBWR having an 11.2 meter diameter lower drywell  60 . 
     SUMMARY 
     Example embodiments provide a Basemat-Internal Melt Arrest and Coolability device (BiMAC) that offers improved spatial and mechanical characteristics for use in damage prevention and risk mitigation in accident scenarios. Example embodiments may include a BiMAC having an inclination of less than 10-degrees from the basemat floor and/or coolant channels of less than 4 inches in diameter, while maintaining minimum safety margins required by the Nuclear Regulatory Commission. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       Example embodiments will become more apparent by describing, in detail, the attached drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus do not limit the example embodiments herein. 
         FIG. 1  is an illustration of a schematic of a conventional containment building for a nuclear reactor. 
         FIG. 2  is an illustration of a conventional lower drywell and Basemat-Internal Melt Arrest and Coolability device. 
         FIG. 3  is a detailed view of a conventional Basemat-Internal Melt Arrest and Coolability device. 
         FIG. 4  is another view of a conventional Basemat-Internal Melt Arrest and Coolability device. 
         FIG. 5  is an illustration of an example embodiment Basemat-Internal Melt Arrest and Coolability device. 
         FIG. 6  is a detail view of an example embodiment Basemat-Internal Melt Arrest and Coolability device. 
         FIG. 7  is a graph of experimental results showing example embodiment Basemat-Internal Melt Arrest and Coolability device compliance with performance criteria. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed illustrative embodiments of example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. For example, although example embodiments may be described with reference to an Economic Simplified Boiling Water Reactor (ESBWR), it is understood that example embodiments may be useable in other types of nuclear plants and in other technological fields. The example embodiments may be embodied in many alternate forms and should not be construed as limited to only example embodiments set forth herein. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “connected,” “coupled,” “mated,” “attached,” or “fixed” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the language explicitly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The inventors have recognized that a BiMAC having a decreased height, and additional volume of lower drywell  60  created by the lower height, may improve accident mitigation in certain cases of core melting and relocation to drywell  60 , including scenarios where the relocation occurs rapidly and under very high pressure. A BiMAC having a lower height will have a smaller surface area and coolant channel length, potentially providing greater protection against damage from crushing and explosive forces experienced in the instance of high-pressure vessel rupture onto the BiMAC. A BiMAC having a lower height will have a more horizontal, and less “wedged” surface, potentially reducing natural convection currents in coolant channels subject to the greatest heat flux loads at edges of conventional BiMAC  100 . Additionally, the inventors have recognized that the lower height permits smaller internal channels, offering improved crushing protection. 
     Further, the inventors have recognized that a larger lower drywell volume may better accommodate and contain larger amounts of caustic, radioactive molten core and plant components and reduce the likelihood of materials being relocated to other, less confined areas of containment  10 . The inventors have further recognized that additional volume in lower drywell  60  may permit greater access to lower drywell  60  for maintenance of control rod drives and instrumentation housed in lower drywell  60 . 
       FIG. 5  illustrates an example embodiment BiMAC  200 , and  FIG. 6  is a detailed cross-sectional view of example embodiment BiMAC  200 . Example embodiment BiMAC  200  may share several features with conventional BiMAC  100  described in  FIGS. 1-4 , with like reference numbers indicating like features, whose redundant description is omitted. As shown in  FIG. 5 , coolant channels  130  are inclined at less than 10 degrees, such as at approximately 5 degrees, in example embodiment BiMAC  200 . As shown in  FIG. 6 , coolant channels  130  may have a reduced inner diameter of less than 4 inches, such as approximately 2 inches. For a lower drywell  60  with an 11.2 meter diameter, example embodiment BiMAC  200  has a maximum coolant channel  130  length of approximately 5.62 meters, compared to a maximum coolant channel  130  length of approximately 5.69 meters in conventional BiMAC  100  having a 10-degree incline. Thus, example embodiment BiMAC  200  has a lower surface area, and will experience less loading strain, than conventional BiMAC  100 . 
     Also, example embodiment BiMAC  200  has a maximum height of about 0.49 meters, compared to conventional BiMAC  100  that is about 0.99 meters high. Thus, example embodiment BiMAC  200  frees approximately half a meter of vertical space in the lower drywell  60  compared to conventional BiMAC  100 , because of the approximate 5-degree coolant channel incline. Further example embodiment BiMAC  200 , having a lower and more level floor, is useable with a wider variety of ablation shield  110  materials, including poured concrete, which may benefit from a level pouring surface. 
     While example embodiment BiMAC  200  possesses several spatial and mechanical advantages over conventional BiMAC  100 , there was no expectation that example embodiment BiMAC  200  would successfully function in the same accident scenarios used to test and certify conventional BiMACs in the related art. Only the conventional 10-degree BiMAC  100  has known functionality in molten core relocation accident scenarios. In fact, the NEDO-33201 Document shows that a BiMAC having greater than 10-degree incline with more than 4-inch diameter coolant channels would be required for such an accident scenario in conventional ESBWR commercial power plants. Thus, the inventors subjected example embodiment BiMAC  200 , having less than 10-degree incline and 4-degree diameters, to lengthy testing to ensure cooling and risk mitigation functionality. 
     A model of example embodiment BiMAC  200 , having an approximately 5-degree incline and 2-inch inner diameter coolant channel  130  as shown in  FIGS. 5 and 6 , was constructed and subject to thermal loads encountered in the same scenarios discussed in the NEDO-33201 Document. Specifically, central coolant channels  130  were subjected to an average thermal load of 100 kW/m 2  with local peaking loads of 125 kW/m 2 ; coolant channels  130  near edges of example embodiment BiMAC  200  were subjected to an average heat load of 100 kW/m 2  with local peaking loads of 300 kW/m 2 ; and portions coolant channel  130  extending vertically upward in example embodiment BiMAC  200  were subject to an average heat load of 320 kW/m 2  with local peaking loads of 450 kW/m 2 . Heating was supplied by electric-powered copper cartridge and band heaters. 
     Criteria measuring sustainability and failure of example embodiment BiMAC  200  included instantaneous flow rates in each coolant channel  130 , temperature of coolant channels  130 , coolant pressure drop through coolant channels  130 , and coolant void fraction at open end of coolant channels  130 , through which coolant exited. Heat loads and coolant flow were applied over the course of 10 hours to ascertain sustainability. 
     Results of the testing indicated that required thermal load absorption was maintained throughout the experiment involving example embodiment BiMAC  200 . No flow instability, changes in pressure distribution, or complete voiding were experienced in the experiment. Further, complete voiding and burnout within coolant channels  130  could not be achieved in the experiment, even when effectively doubling the thermal loads discussed above.  FIG. 7  is a graph illustrating these beneficial and unexpected results of example embodiment BiMAC  200 .  FIG. 7  illustrates inlet sub-cooling temperature (K) at inlet channel  64  as a function of flow rate (GPM) through all coolant channels  130  in example embodiment BiMAC  200 . Four test series A-D were run, with point heat loads ranging from approximately 50 to 60 kW applied to the BiMAC surface. As shown in  FIG. 7 , example embodiment BiMAC  200  handled all heat loads with little variation in performance, including no dry-out, reflecting the improved they and hydro-dynamic properties of the BiMAC  200  discussed above. 
     Thus, the inventors have determined that example embodiment BiMACs having inclination of less than 10 degrees and more than about 5 degrees and coolant channel  130  having inner diameters of less than 4 inches and more than approximately 2 inches can adequately provide cooling and melt arrest in the same accident scenarios addressed by conventional BiMAC  100 , while increasing free space in lower drywell  60  and reducing material stress and fatigue over conventional BiMAC  100 . 
     Example embodiments thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied through routine experimentation and without further inventive activity. Variations are not to be regarded as departure from the spirit and scope of the exemplary embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.