Abstract:
A method and apparatus for providing an alternative cooling system for the suppression pool of a Boiling Water Reactor (BWR) nuclear reactor. The cooling system is operated to cool the suppression pool in the event of a plant accident when normal plant electricity is not available for the conventional residual heat removal system and pumps. The cooling system may also be used to supplement the cooling of the suppression pool via the residual heat removal system. The cooling system is operated and controlled from a remote location, which is ideal during a plant emergency.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Example embodiments relate generally to nuclear reactors, and more particularly to a method and apparatus for an alternative cooling system for the suppression pool of a Boiling Water Reactor (BWR) nuclear reactor. The cooling system may be particularly beneficial in the event a plant emergency that causes plant electrical power to be disrupted, or normal cooling of the suppression pool to otherwise become impaired. The cooling system may also be used by the suppression pool to supplement the conventional residual heat removal system. 
         [0003]    2. Related Art 
         [0004]      FIG. 1  is a cut-away view of a conventional boiling water nuclear reactor (BWR) reactor building  5 . The suppression pool  2  is a torus shaped pool that is part of the reactor building primary containment. Specifically, the suppression pool  2  is an extension of the steel primary containment vessel  3 , which is located within the shell  4  of the reactor building  5 . The suppression pool  2  is positioned below the reactor  1  and spent fuel pool  10 , and is used to limit containment pressure increases during certain accidents. In particular, the suppression pool  2  is used to cool and condense steam released during plant accidents. For instance, many plant safety/relief valves are designed to discharge steam into the suppression pool  2 , to condense the steam and mitigate undesired pressure increases. Conventionally, a BWR suppression pool  2  is approximately 140 feet in total diameter (i.e., plot plan diameter), with a 30 foot diameter torus shaped shell. During normal operation, the suppression pool  2  usually has suppression pool water in the pool at a depth of about 15 feet (with approximately 1,000,000 gallons of suppression pool water in the suppression pool  2 , during normal operation). 
         [0005]    The pool  2  is conventionally cleaned and cooled by the residual heat removal (RHR) system of the BWR plant. During normal (non-accident) plant conditions, the RHR system can remove water from the suppression pool  2  (using conventional RHR pumps) and send the water through a demineralizer (not shown) to remove impurities and some radioactive isotopes that may be contained in the water. During a plant accident, the RHR system is also designed to remove some of the suppression pool water from the suppression pool  2  and send the water to a heat exchanger (within the RHR system) for cooling. 
         [0006]    During a serious plant accident, normal plant electrical power may be disrupted. In particular, the plant may be without normal electrical power to run the conventional RHR system and pumps. If electrical power is disrupted for a lengthy period of time, water in the suppression pool may eventually boil and impair the ability of the suppression pool to condense plant steam and reduce containment pressure. 
         [0007]    In a plant emergency, use of the RHR system may cause highly radioactive water (above acceptable design limits) to be transferred between the suppression pool and RHR systems (located outside of primary containment). The transfer of the highly radioactive water between the suppression pool and RHR system may, in and of itself, cause a potential escalation in leakage of harmful radioactive isotopes that may escape the suppression pool. Additionally, radiation dosage rates in areas of the RHR system could be excessively high during an accident, making it difficult for plant personnel to access and control the system. 
       SUMMARY OF INVENTION 
       [0008]    Example embodiments provide a method and an apparatus for providing an alternative cooling system for the suppression pool. The cooling system may be a single or multiple-stage, once-through heat exchanger that does not pose a hazard to the environment. The cooling system could be operated to cool the suppression pool even in the event of a plant accident when normal plant electricity is not available to run the conventional RHR systems and pumps. The cooling system may also be used simply to supplement the conventional RHR system, in the event that the RHR system remains functional during a serious plant accident. The cooling system could be operated and controlled from a remote location, which is ideal during a plant emergency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and other features and advantages of example embodiments will become more apparent by describing in detail, example embodiments with reference to the attached drawings. The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the intended scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
           [0010]      FIG. 1  is a cut-away view of a conventional boiling water nuclear reactor (BWR) reactor building; 
           [0011]      FIG. 2  is an overhead view of a suppression pool, in accordance with an example embodiment; and 
           [0012]      FIG. 3  is a flowchart of a method of cooling the suppression pool, in accordance with an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
         [0014]    Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments. Like numbers refer to like elements throughout the description of the figures. 
         [0015]    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. 
         [0016]    It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may 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.). 
         [0017]    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 context clearly 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. 
         [0018]    It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
         [0019]      FIG. 2  is an overhead view of a suppression pool  2 , in accordance with an example embodiment. The cooling system  20  may provide an in situ heat exchanger (within the suppression pool  2 ), without the need for removing water from the suppression pool  2 , itself. The cooling system  20  may include a cooling pipe  26  that provides a flow of cooling water through the suppression pool  2 . The cooling pipe  26  may include a single cold water inlet  22  and a single warm water outlet  24 , to provide a single-stage, once-through heat exchanger within the suppression pool  2 . Benefits of a single-stage, once-through cooling system  20  include an increased efficiency, as the greatest amount of heat may be exchanged per gallon of water flowing through the cooling pipe  26 . Alternatively to a single-stage, once-through cooling system  20  (as shown in  FIG. 2 ), a multi-stage cooling system  20  (not shown) may be used. The multi-stage cooling system  20  may include multiple single-stage passes of cooling pipe  26  that may each be the same as the single-stage cooling system  20  of  FIG. 2 . 
         [0020]    To help mitigate the chance of radiation leakage from the suppression pool  2  into the cooling pipe  26 , the pressure of cooling water flowing through the cooling pipe  26  may be maintained above the pressure of the water in the suppression pool  2 . The suppression pool has a normal operating pressure of about 1 atmosphere. However, during plant accidents the suppression pool  2  is designed to reach pressures of about 58 psig. Therefore, to be conservative, the pressure of the fluid flowing through the cooling pipe  26  may be maintained at a pressure that is about 20 psig above the design pressure for the suppression pool  2 . By maintaining pressure in the cooling pipe  26  above the pressure of the suppression pool  2 , any leaks in the cooling pipe  26  will cause cooling water in pipe  26  to leak into the suppression pool  2  (as opposed to causing water in the suppression pool  2  to leak into the cooling pipe  26 ), which may reduce the possibility of highly radioactive water escaping the suppression pool  2  through the warm water outlet  24 . 
         [0021]    In addition to maintaining the pressure of the cooling pipe  26  above the pressure of the water in the suppression pool  2  (to mitigate the chance of radiation leakage), a radiation monitor  28  may also be located on the warm water outlet  24  piping. The radiation monitor  28  may measure radiation levels of cooling water flowing out of the suppression pool  2 , to ensure that radiation leakage out of the pool  2  does not occur. 
         [0022]    To pump cooling water through the cooling pipe  26 , a dedicated cooling system pump  30  may be used. The pump  30  may run on a back-up diesel generator  58  or be directly driven by a diesel engine  58 , to ensure that the pump  30  is not reliant on normal plant electrical power that may be unavailable in the event of a serious plant emergency. The size of the pump  30  may vary, depending on the size of the suppression pool  2 . The size of the pump  30  may also vary based on design calculations for worst-case accidents that may cause suppression pool  2  temperatures to rise in the event of RHR system failure. In order to mitigate a plant accident for most BWR designs, the pump  30  may provide a cooling water flow-rate of about 300 gallons/minute. It should be understood that a greater cooling water flow-rate will cause increased heat to be exchanged, at the expense of a reduced efficiency of the cooling system  20 . 
         [0023]    It should be noted that conventional emergency portable pumps (not shown), which are generally available in a BWR nuclear plant, may be used as the cooling system pump  30 . If a single-stage, once-through cooling pipe  26  is used, a single pump  30  may be adequate. If a multi-stage cooling pipe  26  is used, a single pump  30  for each stage of the cooling pipe  26  may be used (i.e., the multi-stage configuration may include multiple cooling systems  20 , similar to the one shown in  FIG. 2 ). 
         [0024]    Alternative to using a cooling system pump  30 , gravity draining of cooling water through the cooling pipe  26  may be implemented. Gravity draining of cooling water through the cooling pipe  26  offers an additional level of security for the cooling system  20 , as no pumping power would be required to use the system. However, such a configuration would require a cooling water source  50  to be located at an elevation above the plant elevation of the suppression pool  2 . A cooling water source  50  may be an ocean, a river, a large outdoor body of water, or a man-made structure containing a source of water. The warm water outlet  24  would then need to be discharged to a water discharge  52  location with an elevation that is below the plant elevation of the suppression pool  2 . The water discharge  52  may also be an outdoor body of water, or a man-made structure used to collect the discharged water. 
         [0025]    Whether gravity draining or a cooling system pump  30  is used for the cooling system  20 , all controls (see controller  58 ) associated with the system  20  may be positioned in a remote location  60  that is remote to the suppression pool  2 , for the safety of plant personnel. That is to say, locations of the pumps  30 , inlet/outlet valves  32   a / 32   b  (if the valves are not manually operated), and radiation monitor  28 , may be located a distance from the suppression pool  2 . Similarly, inlet valves  32   a  (on the cold water inlet  22 ) and/or outlet valves  32   b  (on the warm water outlet  24 ), used to control the flow of water through the cooling pipe  26 , may be positioned in locations remote from the suppression pool  2  (especially in the event that valves  32   a / 32   b  are manually operated). This is to ensure that plant personnel may safely operate the system  20  without being exposed to potentially high levels of radiation that may be present near the suppression pool  2  during an accident condition. 
         [0026]    The configuration of the cooling pipe  26  may include a single loop through the suppression pool  2 , as shown in  FIG. 2 . Alternatively, the cooling pipe  26  may entail other configurations, which may include additional loops or a “snake”-shaped configuration (not shown) through the pool. The cooling pipe  26  may be finned, or otherwise configured to maximize the surface area of the pipe  26  to increase the heat exchange capacity between the pipe  26  and the water in the suppression pool  2 . Additionally, the cooling system pipe  26  may include branching of the cooling water pipe, which may also increase the heat that is exchanged between the cooling pipes  26  and the water in the suppression pool  2 . Branched cooling system pipe  26  may still have a single cold water inlet  22  and a single warm water outlet  24 , to reduce the amount of cooling piping  26  being exposed to areas of the plant other than the suppression pool  2 . The single cold water inlet  22  and single warm water outlet  24  configuration may further reduce the possibility of radiation leakage to other areas of the plant by reducing the number of penetrations that are required in the secondary containment of the reactor building. 
         [0027]    Elevation of the cooling pipe  26  within the suppression pool  2  should be low enough that the cooling pipe is fully submersed in suppression pool water, during both accident and non-accident conditions. Otherwise, the cooling system  20  may be ineffective in exchanging heat with the suppression pool  2 . For more effective heat exchange, the cooling pipe  26  should also not be positioned at the lowest elevations of the suppression pool  2 . By locating the pipe  26  a distance from the floor of the suppression pool  2 , a natural convection current may be formed. Specifically, the cooling pipe  26  may produce cool water within the suppression pool  2  that may settle to the bottom of the pool  2 , as warmer suppression pool  2  water (located under the pipe  26 ) may rise and be displaced by the settling cool water. This natural convection current may increase the efficiency of the cooling system  20 . To ensure a natural convection current while still locating the cooling pipe  26  below the liquid level of the suppression pool, the cooling pipe may be located at an elevation of about 4 feet above the bottom floor of the suppression pool  2 . 
         [0028]    The cooling pipes  26  may be anchored to the walls of the pool  2  using anchors  54 , for extra support. The cooling pipes  26  may be installed prior to BWR plant operation, to ensure that the cooling system  20  is in place prior to a potential plant accident. Alternatively, the cooling system  20  may be installed as a retro-fitted system. 
         [0029]    It should be understood that cooling system  20  may be used during periods of time other than plant accident conditions. For instance, the cooling system  20  may be used simply to supplement the normal cooling of the suppression pool via the RHR system, to provide the suppression pool system with extra temperature design margins. It should also be understood that the temperature of the cooling water supply for the cooling system  20  will impact system performance. That is to say, the cooling system  20  will be more effective and efficient if colder cooling water supply is used. 
         [0030]      FIG. 3  is a flowchart of a method of cooling the suppression pool, in accordance with an example embodiment. As shown in method step S 40 , a cooling pipe  26  may be inserted into the suppression pool  2 . As shown in step S 42 , cooling water from a cooling water source may be run through the cooling pipe  26 . As shown in step S 44 , the cooling water in the cooling pipe  26  may be maintained at a pressure that is above the pressure of the water in the suppression pool  2 . The cooling water in the cooling pipe may also be maintained at a temperature that is below the temperature of the water in the suppression pool  2 . 
         [0031]    Example embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the intended spirit and scope of example 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.