Abstract:
A passive containment air cooling system for a nuclear power plant that enhances air flow over a metal containment that houses the reactor system to improve heat transfer out of the containment. The heat transfer is improved by employing swirl vanes to mix the air as it rises over the walls of the containment due to natural circulation and a vortex engine proximate an exit along the cooling air path to increase the quantity of air drawn along the containment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to co-pending patent application Ser. No. 13/444,932, filed concurrently herewith. 
     BACKGROUND 
     1. Field 
     This present invention relates to a passive containment cooling system for a nuclear reactor power plant and more specifically to a passive containment air cooling system that relies on natural circulation of air over the surface of a metal containment. 
     2. Related Art 
     Nuclear power has played an important part in the generation of electricity since the 1950s and has advantages over thermal electric and hydraulic power plants. The generation of electricity by nuclear power is accomplished by the nuclear fission of radioactive materials. Due to the volatility of the nuclear reactions, nuclear power plants are required by practice to be designed in such a manner that the health and safety of the public is assured even for the most adverse accident that can be postulated. For plants utilizing water as a coolant, the most adverse accident is considered to be a double ended break of the largest pipe in the reactor cooling system and is termed a loss of coolant accident (LOCA). 
     For accident protection, these plants utilize containment systems that are designed to physically contain water, steam and any entrained fission products that may escape from the reactor cooling system. The containment system is normally considered to encompass all structures, systems and devices that provide ultimate reliability and complete protection for any accident that may occur. Engineered safeguard systems are specifically designed to mitigate the consequences of an accident. Basically, the design goal of a containment system is that no radioactive material escapes from the nuclear power plant in the event of an accident so that the lives of the surrounding populous are not endangered. 
     Recently, reactor manufacturers have offered passive plant designs, i.e., plants that will shut down in the event of an accident without the operator intervention or off-site power. Westinghouse Electric Company LLC offers the AP1000 passive plant design that employs a passive containment cooling system that uses a large steel shell. The containment cooling system suppresses the rise in pressure that will likely occur within the containment in the unlikely event of a loss of coolant accident. The passive containment cooling system is an engineered safety feature system. Its objective is to reduce the containment temperature and pressure, following a loss of coolant accident or steam line break accident inside the containment, by removing thermal energy from the containment atmosphere. The passive containment cooling system also serves as a means of transferring heat for other events resulting in a significant increase in containment pressure and temperature. The passive containment cooling system also limits release of radioactivity (post accident) by reducing the pressure differential between the containment atmosphere and the external environment, thereby diminishing the driving force for leakage of fission products from the containment to the atmosphere. The passive containment cooling system also provides a source of make-up water to the spent fuel pool cooling water. To achieve the foregoing objectives, the containment building is made of steel to provide efficient heat transfer from within to outside of the containment. During normal operation, heat is removed from the containment vessel by continuous natural circulation of air. During an accident, however, more heat removal is required and air cooling is supplemented by evaporation of water, provided by a passive containment cooling system water storage tank and gravity feed. 
     An AP1000 containment system  10  is schematically illustrated in  FIG. 1 , surrounding an AP1000 reactor system including a reactor vessel  12 , steam generator  14 , pressurizer  16  and main coolant circulation pump  18 ; all connected by the piping  20 . The containment system  10  in part comprises a steel dome containment vessel enclosure  22  surrounded by a concrete shield building  24  which provides structural protection for the steel dome containment vessel  22 . 
     The major components of the passive containment cooling system are a passive containment cooling water storage tank  26 , an air baffle  28 , air inlet  30 , air exhaust  32  and water distribution system  34 . The passive containment cooling water storage tank  26  is incorporated into the shield building structure  24 , above the steel dome containment vessel  22 . An air baffle  28  located between the steel dome containment vessel  22  and the concrete shield building  24  defines the cooling air flow path which enters through an opening in the shield building  24  at an elevation approximately at the top of the steel dome containment vessel  22 . After entering the shield building  24 , the air path travels down one side of the air baffle  28  and reverses direction around the air baffle at an elevation adjacent the lower portion of the steel dome containment vessel. The air path then flows up between the baffle and the steel dome containment vessel  22  and exits at the exhaust opening  32  in the top of the shield building  24 . The exhaust opening  32  is surrounded by the passive containment cooling water storage tank  26 . 
     In the unlikely event of an accident, the passive containment cooling system provides water that drains by gravity from the passive containment cooling water storage tank  26  and forms a film over the steel dome containment vessel  22 . The water film evaporates thus removing heat from the containment building  22 . 
     The passive containment cooling system is capable of removing sufficient thermal energy, including subsequent decay heat, from the containment atmosphere following a Design Basis event resulting in containment pressurization such that the containment pressure remains below the design value with no operator action required for at least 72 hours. 
     The air flow path that is formed between the shield building  24 , which surrounds the steel dome containment vessel  22 , and the air baffle  28  results in the natural circulation of air upward along the containment vessel&#39;s outside steel surface. This natural circulation of air is driven by buoyant forces when the flowing air is heated by the containment steel surface and when the air is heated by and evaporates water that is applied to the containment surface. The flowing air also enhances the evaporation that occurs from the water surface. In the event of an accident, the convective heat transfer to the air by the heated containment steel surface only accounts for a small portion of the total heat transfer that is required, such total heat transfer being primarily accomplished by the evaporation of water from the wetted areas of the containment steel surface, which cools the water on the surface which then cools the containment steel, which then cools the inside containment atmosphere and condenses steam within the containment. 
     Water is continuously applied via gravity from the passive containment cooling water storage tank  26  to the containment vessel steel surface  22  for the first seventy two hours following a Design Basis event. The application of water to the containment vessel steel surface  22  enhances heat transfer through the vessel and aids in condensing the steam within the containment, therefore also limiting the pressure increase within the containment. After the first seventy two hours, active onsite pumping methods will provide makeup water to the passive containment cooling water storage tank  26  for at least an additional four days. Additional onsite and offsite water sources and pumping methods continue to provide makeup water to the passive containment cooling water storage tank  26  after seven days. 
     It is an object of this invention to enable air cooling alone of the containment vessel to provide sufficient decay heat removal to maintain acceptably low containment pressure after the initial three days. 
     Furthermore, it is an object of this invention to enable air cooling of the containment vessel to provide such sufficient decay heat removal with no reliance on active components, operator actions, or non-safety onsite or offsite water supplies. 
     Additionally, it is an object of this invention to provide sufficient air cooling of the containment vessel that will enable a reduction in the size of the passive containment cooling water storage tank that is required. 
     SUMMARY 
     These and other objects are achieved in accordance with the embodiments set forth hereafter which include the application of swirl generators, guide vanes, and a vortex engine. More particularly, the embodiments set forth include a solid metal nuclear containment shell having sides and a cover, sized to surround at least a portion of the primary coolant loop of a nuclear reactor system. An outer housing having sides and a roof substantially surrounds and is spaced from an exterior surface of the solid metal shell forming an annular cooling fluid passage around the exterior surface of the sides of the solid metal shell that communicates with a passage between the cover and the roof. A fluid intake communicates between the outside of the outer housing and a lower portion of the annular passage and a fluid exit extends through a portion of the roof. A swirl vane assembly is supported in the annular passage between an interior of the sides of the housing and the exterior surface of the sides of the metal shell to enhance the turbulence and mixing of the air rising in the annular passage along the solid metal shell as the air is heated by the heat transmitted through the solid metal shell. 
     Preferably, a swirl vane assembly, comprising at least two swirl vanes which are positioned at approximately the same elevation and proximate each other with at least two swirl vanes arranged in a counter rotating pair to enhance the mixing of the air as it rises along the solid metal shell. In a preferred embodiment, the swirl vane assembly comprises a plurality of the counter rotating pairs of swirl vanes, which are circumferentially spaced around the exterior surface of the sides of the solid metal shell at approximately the same elevation. Desirably, the same elevation is in a lower portion of the annular passage. In one embodiment, the swirl vane assembly is supported from the wall opposite the exterior surface of the solid metal shell and, preferably, a baffle is interposed between the sides of the housing and the exterior surface of the solid metal shell so that the baffle extends approximately from a lower surface of the roof of the housing to an elevation juxtaposed to a lower portion of the sides of the solid metal shell below the swirl vane assembly so that the cooling air intake through the housing communicates cooling air between the baffle and the housing and into the annular passage below the swirl vane assembly. Desirably, the swirl vane assembly is supported on an interior of the baffle. As a further improvement, the swirl vane assemblies comprise at least two swirl vane assemblies supported at two spaced elevations, one above the other. 
     In still another embodiment, the nuclear reactor containment includes a vortex engine  42  supported proximate to or within the air exhaust  32 . 
     In still another embodiment, directional guide vanes  44  are supported in the passage between the cover and the roof, that are oriented to direct the cooling fluid from the annular passage to an intake on the vortex engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a simplified schematic of an AP1000 nuclear power plant; 
         FIG. 2  is a perspective view of a swirl vane which can be employed in the containment shown in  FIG. 1 ; 
         FIG. 3  is a plan view of a vortex engine which can be employed proximate to or within the chimney in the shield building shown in  FIG. 1 ; 
         FIG. 4  is a side view of the vortex engine shown in  FIG. 3 ; 
         FIG. 5  is a plan view of the guide vanes that can be employed in the space between the roof of the shield building and the cover of the steel dome containment vessel to guide the rising heated air into the vortex engine; and 
         FIG. 6  is a simplified schematic view of the containment system shown in  FIG. 1  incorporating the swirl vanes, guide vanes and vortex engine of one embodiment described herein. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As previously mentioned, in an AP1000® passive cooling containment system, the convective heat transfer to the air by the heated containment steel surface only accounts for a small portion of the total heat transfer; such total heat transfer being primarily accomplished by the evaporation of water from the wetted areas of the containment steel surface, which cools the water on the surface, which then cools the containment steel, which then cools the inside containment atmosphere and condenses steam. It is an object of the embodiment described herein to enable air cooling alone to provide sufficient heat removal to maintain acceptably low containment pressure with no reliance on active components, operator actions, or auxiliary water supplies, after the initial three days when the initial water volume in the passive containment cooling water storage tank  26  has been exhausted. 
     The foregoing objective is achieved in related co-pending application Ser. No. 13/444,932, filing date Apr. 12, 2012, by creating a tortuous air path and in effect creating an increased surface area over the steel containment vessel over which cooling air flows. The embodiments described herein achieve the same objective by promoting better mixing of the air within the annular passage  34  between the baffle  28  and the steel dome shell  22  and by drawing more air per unit time through that passage. Either of these concepts can be used alone or they can be used together to promote more efficient cooling of the reactor containment. Though, the dome containment shell  22  is identified as being constructed out of steel it should be appreciated that the containment vessel can be constructed out of other materials that have relatively good thermal conductivities and the necessary integrity and strength. Also, it should be appreciated that the water film during the discharge of the passive containment cooling water storage tank  26  will follow a flow path over the steel dome containment vessel that is opposite to the direction of flow of the air path. 
     The design of the AP1000 nuclear power plant passive containment cooling system utilizes the steel containment vessel  22 , the shield building  24  and the air baffle  28  to form an air flow path driven by natural circulation. The cooler outside air is drawn into the sides of the shield building  24  through inlet vents  30  and directed downward around the baffle  28 . The cool air then turns back upwards and travels countercurrent to water flowing down the containment vessel shell  22 . Heat is transferred from the vessel steel to the water and finally to the air flowing up and out the chimney  32 . The buoyancy of the warm air leaving the chimney helps to drive the air flow through the annulus  34 . 
     The AP1000 passive containment cooling system utilizes water from the passive containment cooling system storage tank  26  on the top of the shield building  24 , a safety related source, for the first 72 hours. Traditionally, from 72 hours to seven days, water is supplied by an ancillary tank on the plant site, although this action requires operator intervention as well as AC power. It is the object of this embodiment to enhance the air flow through the annulus  34  such that continued water cooling after  72  hours can be replaced with passive air-only cooling that will maintain the pressure within the containment within design limits. To accomplish the foregoing objective, the preferred embodiment employs a swirl generator such as the swirl vane assembly  36  shown in  FIG. 2  to improve decay heat removal from the containment vessel  22  by passively rotating the air in the annulus so that the cooler air adjacent the baffle  28  is rotated towards the steel dome enclosure  22  as the air moves up the annulus to disrupt the thermal boundary layer and enhance the heat transfer across the steel dome enclosure. The term “passive” is employed to indicate that there are no moving parts and the action is accomplished without any need for an outside power source. The swirl generator illustrated in  FIG. 2  is a swirl vane assembly  36  that has a tubular housing that supports a number of curve vanes whose arc transforms an axial intake of air into a swirling pattern. 
     The embodiment employed herein may also employ a vortex engine such as the one illustrated in  FIGS. 3 and 4  to generate a virtual chimney in combination with the air exhaust exit  32  on top of the shield building  24  and use this virtual chimney to improve decay heat removal. The combination of the swirl generators  36  with a vortex engine  42  can be further enhanced with the use of guide vanes  44  in the space between the underside of the roof of the shield building  24  and the cover of the steel dome enclosure  22 . None of the swirl generators, vortex engine or guide vanes require moving parts or are maintenance intensive. Nevertheless, this combination of elements enhances the natural draft through the annulus of the shield building without the use of fans or AC power, or physically increasing the height of the shield building chimney. Preferably, the swirl generators  36  are arranged in counter-rotating pairs in the annulus  34 . 
     Thus, in accordance with this embodiment, swirl generators  36  are attached to the inside surface of the baffle  28  adjacent to the steel dome enclosure  22  in a lower portion of the annular passage  34 . The purpose of the swirl generators  36  is to make the air flow rising up the annulus region  34 , counter current to the water flow streaming down the surface of the steel dome enclosure  22 , turbulent by means of rotating the air. This mixing of the warmer air near the vessel  22  with cooler air adjacent the baffle  28  using a swirling motion will thin the thermal boundary layer, thereby reducing resistance to heat transfer. Preliminary testing suggests the vortices produced by the swirl generators  36  will travel for an extensive distance before dissipating. An additional bank of swirl generators  36 , as illustrated in  FIG. 6 , higher up in the annulus  34 , may be employed to regenerate the vortices as necessary. Each swirl vane assembly  36  comprises two swirl vanes oriented to establish counter rotating air paths, i.e., air paths that rotate in opposite directions. At each elevation where the swirl vane assemblies are supported, the assemblies are equidistantly spaced circumferentially around the outer surface of the steel dome enclosure vessel  22 . 
     By use of the swirl generators  36 , heat transfer from the containment steel enclosure  22  to the water and then to the air is improved. However, adding any device in the air flow path through the annulus  34  will tend to impose a penalty on pressure drop and air velocity. This embodiment further contemplates that these losses can be recouped by increasing the thermal buoyancy of the system. This can be accomplished by the development of a virtual chimney, figuratively illustrated by reference character  46  in  FIG. 6 . The virtual chimney  46  extends from the top of the shield building chimney  48 . The virtual chimney  46  is a high velocity cylinder of air rotating in tornado-like fashion that will help to pull air through the annulus region  34  and increase air velocity such that losses incurred by the swirl generators  36  are compensated for and heat transfer is further increased. 
     A vortex generator is an aerodynamic surface consisting of a small vane or vanes that create a vortex. A vortex engine works on the principal that mechanical energy is produced when water descends or when warm air rises. The atmospheric vortex engine captures the energy produced when warm air rises by creating a river of rising air using an air vortex which acts as a vertical conduit. The vortex is produced by admitting warm or humid air tangentially into a circular arena. Tangential entries cause the warm moist air to spin as it rises forming an “anchored vortex.” Centrifugal forces in the vortex prevents the rising air from becoming diluted by cooler ambient air and thereby losing its buoyancy. Once the turbulent air flow produced by the swirl generators  36  in the annulus  34  has progressed to the dome region between the steel containment vessel  22  and the shield building  24 , guide vanes  44  will aid in “pre-swirling” the air flow towards the shield building chimney  48  in a favorable swirling motion. The guide vanes  44  assist in developing the larger vortex required to be generated by the vortex engine  42 . The vortex engine  42  is positioned within, on or near the chimney  48  of the shield building  24  and may, in principal, be a larger scale version of a swirl generator. The air entering the inlet  50  of the vortex engine  42  will do so in a tangential fashion and will produce a high velocity cylinder of air that will extend some distance above the top of the shield building chimney  48  into the outside air with the portion of the extension above the shield building chimney making up the virtual chimney  46 . This virtual chimney will extend the effective height of the shield building chimney  48 , thereby increasing buoyancy and increasing the driving force for air flow. 
     A plan view of a vortex engine that can be employed for this purpose is illustrated in  FIG. 3  with  FIG. 4  showing a side view exposing the inlet to the vanes  52  that are capped by a base plate  54  having a central opening  56  and a cover  58 . The air enters the vanes  52  and accelerates as it moves around to the arena and up the chimney drawing more air into the intake  50  as it extends upward and out the virtual extension of the chimney. Accordingly, more air is drawn through the system to overcome the pressure drop imparted by the swirl vane banks  36 . 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.