Abstract:
A method and apparatus for providing an alternative remote spent fuel pool cooling system for the spent fuel pool. The cooling system is operated to cool the spent fuel pool in the event of a plant accident when normal plant electricity is not available for the conventional fuel pool cooling and cleanup system, or when the integrity of the spent fuel has been jeopardized. 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 remote spent fuel pool cooling system for a Light Water Reactor (LWR) 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 spent fuel pools to otherwise become impaired. The cooling system may also be used to supplement fuel pool cooling via the conventional fuel pool cooling and cleanup system. 
         [0003]    2. Related Art 
         [0004]      FIG. 1  is a cut-away view of a conventional boiling water nuclear reactor (BWR) reactor building  5 , which is one example of a light water reactor (LWR). It should be understood that this is merely an example, as other reactor design layouts may be used for other LWRs. The spent fuel pools  10  are storage pools used to store spent fuel  12  that remain following the use of the fuel to power the BWR reactor  1 . The spent fuel pools  10  are generally positioned in locations adjacent to, and toward the top of, the reactor  1  (as shown in  FIG. 1 , the spent fuel pool  10  is located in secondary containment, outside of the steel containment vessel  3  and concrete shell  4  protecting reactor  1 ). The spent fuel pool may be located at a plant elevation that is above a location of the suppression pool  2 . It should be noted that in other reactor designs, the spent fuel pool may be located at a same plant elevation as the reactor, or at an elevation that is below the reactor. The spent fuel  12  is generally stored in the spent fuel pools  10  for a period of at least 5 years before being sent to reprocessing or cask storage. The spent fuel pools  10  are typically 40 feet or greater in depth, with the bottom 14 feet being equipped with storage racks that hold the fuel assemblies that are removed from the reactor. About 8 feet of water (above the top of the spent fuel, itself) is generally needed to keep radiation levels in the spent fuel pools  10  within acceptable limits (see spent fuel pool water level  10   b , which is above the spent fuel  12 ). 
         [0005]    A flow of cooling water, provided by conventional fuel pool cooling and cleanup system (not shown), provides shielding from radiation and maintains the spent fuel pools  10  at cool temperatures that ensure the cooling water does not boil (thereby exposing the spent fuel to open air). The conventional spent fuel cooling pumps provide cooling of the spent fuel pools. Specifically, the conventional fuel pool cooling pumps transfer the water from the spent fuel pool to the fuel pool cooling and cleanup system. The conventional fuel pool cooling and cleanup system cools and cleans the water, using a heat exchanger and demineralizers (removing some radioisotopes, and other impurities). The fuel pool cooling pumps then send the cool, clean water back to the spent fuel pool  10 . 
         [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 fuel pool cooling pumps, or operate the fuel pool cooling and cleanup system. If electrical power is disrupted for a lengthy period of time, disruption in the use of the fuel pool cooling and cleanup system may cause water in the spent fuel pool to warm and eventually boil. When enough boiling occurs, water levels in the pool may drop to levels that no longer provide enough cooling water to effectively shield radiation that may be caused by the spent fuel. In very serious emergencies, water in the spent fuel pool may boil and evaporate to the point that the spent fuel may become exposed to open air. Such an emergency may pose grave dangers for plant personnel and the environment. 
         [0007]    In a plant emergency, even if the spent fuel in the spent fuel pool is not exposed to open air (in the event of a worst-case accident scenario), there are still concerns with radiation leakage leaving the spent fuel pool and escaping to the environment. In particular, the fuel pool cooling and cleanup system may become over-loaded in handling the cooling and radiation reduction needs of the spent fuel pool. This may particularly be the case, in the event that fuel damage occurs in the spent fuel pool. If the integrity of the fuel rods within the spent fuel pool becomes jeopardized, use of the fuel pool cooling and cleanup system may pose risks to plant personnel and the environment, as highly radioactive water (above acceptable design limits) may be transferred to the fuel pool cooling and cleanup system. In such a scenario, the fuel pool cooling and cleanup system may be unable to assist in effectively reducing radiation levels of the spent fuel pool water. Therefore, the transfer of the highly radioactive water to the fuel pool cooling and cleanup system may, in and of itself, cause a potential escalation in the abilities to contain harmful radioactive isotopes within secondary containment. 
       SUMMARY OF INVENTION 
       [0008]    Example embodiments provide a method and an apparatus for providing an alternative remote spent fuel pool cooling system for the spent fuel pool. The cooling system may be a single-stage, once-through heat exchanger that does not pose a hazard to the environment. The cooling system could be operated to cool the spent fuel pool even in the event of a plant accident where normal plant electricity is not available to run the conventional spent fuel pool cooling and cleanup system or the fuel pool cooling pumps. Additionally, the cooling system may be particularly beneficial in instances when fuel damage has occurred and the conventional spent fuel pool cooling and cleanup system become ineffective in containing radiation leakage to other areas of the plant. 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 one example design of a conventional light water nuclear reactor (LWR) reactor building; 
           [0011]      FIG. 2  is an overhead view of a spent fuel pool, in accordance with an example embodiment; 
           [0012]      FIG. 3  is a side-view of a spent fuel pool, in accordance with an example embodiment; and 
           [0013]      FIG. 4  is a flowchart of a method of cooling the spent fuel pool, in accordance with an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    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. 
         [0015]    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. 
         [0016]    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. 
         [0017]    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.). 
         [0018]    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. 
         [0019]    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. 
         [0020]      FIG. 2  is an overhead view of a spent fuel pool  10 , in accordance with an example embodiment. The cooling system  20  may provide an in situ heat exchanger (within the spent fuel pool  10 ), to cool the pool  10  without the need for removing water from the spent fuel pool  10 . The cooling system  20  may include a cooling pipe  26  that provides a flow of cooling water through the pipe  26 . The cooling pipe  26  may be positioned within the spent fuel pool  10 , and around the area of the spent fuel  12  within the pool  10 . 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 spent fuel pool  10 . 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  shown in  FIG. 2 . 
         [0021]    To help mitigate the chance of radiation leakage from the spent fuel pool  10  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 spent fuel pool  10 . Because the spent fuel pool is exposed to open air within the plant, the atmospheric pressure above the spent fuel pool  10  is approximately 1 atmosphere of pressure. Therefore, to mitigate radiation leakage, the pressure of the fluid flowing through the cooling pipe  26  may be maintained at a pressure of 1 atmosphere or greater, plus the static pressure of the water at the lowest depth upon which the cooling pipe  26  extends. To be conservative, the pressure of the cooling pipe  26  may be maintained at a pressure of 1 atmosphere, plus the static pressure of the water at the deepest depth of the spent fuel pool  10 . 
         [0022]    In addition to maintaining the pressure of the cooling pipe  26  above the pressure of the water in the spent fuel pool  10  (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 spent fuel pool  10 , to ensure that radiation leakage out of the pool  10  does not occur. 
         [0023]    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  56  or directly driven by a diesel engine  56 , 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 spent fuel pool  10 . The size of the pump  30  may also vary based on design calculations for worst-case heat output of the spent fuel pool  10  during an accident scenario. In order to mitigate a plant accident for most LWR 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 exchange, at the expense of a reduced efficiency of the cooling system  20 . 
         [0024]    It should be noted that conventional emergency portable pumps (not shown), which are generally available in a LWR 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 ). 
         [0025]    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 safety 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 liquid level  10   b  (see  FIGS. 1 and 3 ) of the spent fuel pool  10 . 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 lowest elevation of the cooling water pipe  26  that runs through the spent fuel pool  10 . The water discharge  52  may also be an outdoor body of water, or a man-made structure used to collect the discharged water. 
         [0026]    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 spent fuel pool  10 , for the safety of plant personnel. That is to say, locations of the pumps  30 , or locations of controller  58  used to operate the pump  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 pool  10 . 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 pool  10  (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 in the spent fuel pool  10  during an accident condition. 
         [0027]    The configuration of the cooling pipe  26  may include a single loop around the spent fuel pool  10 , 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 spent fuel pool  10 . Additionally, the cooling system pipe  26  may include branching  26   a / 26   b / 26   c  (see  FIG. 3 ) of the cooling water pipe, which may also increase the heat that is exchanged between the cooling pipes  26  and the water in the spent fuel pool  10 . 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 spent fuel pool  10 . 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. 
         [0028]      FIG. 3  is a side-view of a spent fuel pool  10 , in accordance with an example embodiment. Conventionally, the spent fuel  12  is located at a depth of about ⅓ the overall depth of the spent fuel pool  10 . Therefore, the cooling pipe  26  (including braches  26   a / 26   b / 26   c ) may be located in a position that is generally above the spent fuel  12  and below the water level  10   b  of the pool  10 . By locating the cooling pipe  26  above the spent fuel  12 , the cooling pipe  26  will create a natural convection current to form. Specifically, the cooling pipe  26  will produce cool water above the locations of the spent fuel  12 , and this cooler water will naturally settle to the bottom of the pool  10 . Likewise, the spent fuel  12  will produce warmer water near the bottom of the spent fuel pool  10 , and this warmer water will naturally rise within the pool  10 . Therefore, by locating the cooling pipe  26  above the locations of the spent fuel  12 , the heat exchanging process of the cooling system will be more efficient. 
         [0029]    The cooling pipes  26  may be anchored to the pool walls  10   a  of the spent fuel pool  10  using anchors  54  (see  FIGS. 2 and 3 ), for extra support. The cooling pipes  26  may be installed prior to LWR 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. The cooling pipes  26  may be permanently installed in the spent fuel pool  10 , in which case the cooling pipes  26  may be located in positions within the pool  10  that do not interfere with the installment and removal of spent fuel  12  within the pool. Alternatively, the cooling pipes  26  may be temporarily held in place within the pool  10  via brackets, in which case the cooling pipes  26  may be located directly above locations of the spent fuel  12 . 
         [0030]    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 spent fuel pool via the conventional fuel pool cooling and cleanup system. 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. 
         [0031]      FIG. 4  is a flowchart of a method of cooling the spent fuel pool, in accordance with an example embodiment. As shown in method step S 40 , a cooling pipe  26  may be inserted into the spent fuel pool  10 . 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 spent fuel pool  10 . The cooling water in the cooling pipe may also be maintained at a temperature that is below the temperature of the water in the spent fuel pool  10 . 
         [0032]    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.