Abstract:
An environmentally sequestered nuclear spent fuel pool in one embodiment includes sidewalls and a base slab that confine a water impoundment. The pool includes fuel racks containing spent fuel assemblies which heat the water via radioactive decay. A dual liner system enclosing the pool forms an impervious barrier providing redundant provisions for preventing leakage of contaminated pool water into the environment. An interstitial space is formed between the liners which may be maintained at sub-atmospheric pressures by a vacuum pump system that evacuates the space. By maintaining the pressure in the space at a negative pressure with corresponding boiling point less than the temperature of the pool water, any leakage through the inner-most liner into the interstitial space will vaporize and be extracted via the pump for signaling a potential leak in the liner system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of priority to U.S. Provisional Application No. 62/061,089 filed Oct. 7, 2014, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention generally relates to storage of nuclear fuel assemblies, and more particularly to an improved spent fuel pool for wet storage of such fuel assemblies. 
         [0003]    A spent fuel pool (sometimes, two or more) is an integral part of every nuclear power plant. At certain sites, standalone wet storage facilities have also been built to provide additional storage capacity for the excess fuel discharged by the reactors. An autonomous wet storage facility that serves one or more reactor units is sometimes referred to by the acronym AFR meaning “Away-from-Reactor.” While most countries have added to their in-plant used fuel storage capacity by building dry storage facilities, the French nuclear program has been the most notable user of AFR storage. 
         [0004]    As its name implies, the spent fuel pool (SFP) stores the fuel irradiated in the plant&#39;s reactor in a deep pool of water. The pool is typically 40 feet deep with upright Fuel Racks positioned on its bottom slab. Under normal storage conditions, there is at least 25 feet of water cover on top of the fuel to ensure that the dose at the pool deck level is acceptably low for the plant workers. Fuel pools at most (but not all) nuclear plants are at grade level, which is desirable from the standpoint of structural capacity of the reinforced concrete structure that forms the deep pond of water. To ensure that the pool&#39;s water does not seep out through the voids and discontinuities in the pool slab or walls, fuel pools in nuclear plants built since the 1970s have always been lined with a thin single-layer stainless steel liner (typically in the range of 3/16 inch to 5/16 inch thick). The liner is made up of sheets of stainless steel (typically ASTM 240-304 or 304L) seam welded along their contiguous edges to form an impervious barrier between the pool&#39;s water and the undergirding concrete. In most cases, the welded liner seams are monitored for their integrity by locating a leak chase channel underneath them (see, e.g.  FIG. 1 ). The leak chase channels&#39; detection ability, however, is limited to welded regions only; the base metal area of the liner beyond the seams remains un-surveilled. 
         [0005]    The liners have generally served reliably at most nuclear plants, but isolated cases of water seepage of pool water have been reported. Because the pool&#39;s water bears radioactive contaminants (most of it carried by the crud deposited on the fuel during its “burn” in the reactor), leaching out of the pool water to the plant&#39;s substrate, and possibly to the underground water, is evidently inimical to public health and safety. To reduce the probability of pool water reaching the ground water, the local environment and hence some AFR pools have adopted the pool-in-pool design wherein the fuel pool is enclosed by a secondary outer pool filled with clean water. In the dual-pool design, any leakage of water from the contaminated pool will occur into the outer pool, which serves as the barrier against ground water contamination. The dual pool design, however, has several unattractive aspects, viz.: (1) the structural capacity of the storage system is adversely affected by two reinforced concrete containers separated from each other except for springs and dampers that secure their spacing; (2) there is a possibility that the outer pool may leak along with the inner pool, defeating both barriers and allowing for contaminated water to reach the external environment; and (3) the dual-pool design significantly increases the cost of the storage system. 
         [0006]    Prompted by the deficiencies in the present designs, a novel design of a spent nuclear fuel pool that would guarantee complete confinement of pool&#39;s water and monitoring of the entire liner structure including seams and base metal areas is desirable. 
       SUMMARY 
       [0007]    The present invention provides an environmentally sequestered spent fuel pool system having a dual impervious liner system and leak detection/evacuation system configured to collect and identify leakage in the interstitial space formed between the liners. The internal cavity of the pool has not one but two liners layered on top of each other, each providing an independent barrier to the out-migration (emigration) of pool water. Each liner encompasses the entire extent of the water occupied space and further extends above the pool&#39;s “high water level.” The top of the pool may be equipped with a thick embedment plate (preferably 2 inches thick minimum in one non-limiting embodiment) that circumscribes the perimeter of the pool cavity at its top extremity along the operating deck of the pool. Each liner may be independently welded to the top embedment plate. The top embedment plate features at least one telltale hole, which provides direct communication with the interstitial space between the two liner layers. In one implementation, a vapor extraction system comprising a vacuum pump downstream of a one-way valve is used to draw down the pressure in the inter-liner space through the telltale hole to a relatively high state of vacuum. The absolute pressure in the inter-liner space (“set pressure”) preferably should be such that the pool&#39;s bulk water temperature is above the boiling temperature of water at the set pressure as further described herein. 
         [0008]    In one embodiment, an environmentally sequestered nuclear spent fuel pool system includes: a base slab; a plurality of vertical sidewalls extending upwards from and adjoining the base slab, the sidewalls forming a perimeter; a cavity collectively defined by the sidewalls and base slab that holds pool water; a pool liner system comprising an outer liner adjacent the sidewalls, an inner liner adjacent the outer liner and wetted by the pool water, and an interstitial space formed between the liners; a top embedment plate circumscribing the perimeter of the pool at a top surface of the sidewalls adjoining the cavity; and the inner and outer sidewalls having top terminal ends sealably attached to the embedment plate. 
         [0009]    In another embodiment, an environmentally sequestered nuclear spent fuel pool with leakage detection system includes: a base slab; a plurality of vertical sidewalls extending upwards from and adjoining the base slab, the sidewalls forming a perimeter; a cavity collectively defined by the sidewalls and base slab that holds pool water; at least one fuel storage rack disposed in the cavity that holds a nuclear spent fuel assembly containing nuclear fuel rods that heat the pool water; a pool liner system comprising an outer liner adjacent the sidewalls and base slab, an inner liner adjacent the outer liner and wetted by the pool water, and an interstitial space formed between the liners; a top embedment plate circumscribing the perimeter of the pool, the embedment plate embedded in the sidewalls adjoining the cavity; the inner and outer liners attached to the top embedment plate; a flow plenum formed along the sidewalls that is in fluid communication with the interstitial space; and a vacuum pump fluidly coupled to the flow plenum, the vacuum pump operable to evacuate the interstitial space to a negative set pressure below atmospheric pressure. 
         [0010]    A method for detecting leakage from a nuclear spent fuel pool is provided. The method includes: providing a spent fuel pool comprising a plurality of sidewalls, a base slab, a cavity containing cooling water, and a liner system disposed in the cavity including an outer liner, an inner liner, and an interstitial space between the liner; placing a fuel storage rack in the pool; inserting at least one nuclear fuel assembly into the storage rack, the fuel assembly including a plurality of spent nuclear fuel rods; heating the cooling water in the pool to a first temperature from decay heat generated by the spent nuclear fuel rods; drawing a vacuum in the interstitial space with a vacuum pump to a negative pressure having a corresponding boiling point temperature less than the first temperature; collecting cooling water leaking from the pool through the liner system in the interstitial space; converting the leaking cooling water into vapor via boiling; and extracting the vapor from the interstitial space using the vacuum pump; wherein the presence of vapor in the interstitial space allows detection of a liner breach. The method may further include discharging the vapor extracted by the vacuum pump through a charcoal filter to remove contaminants. The method may further include: monitoring a pressure in the interstitial space; detecting a first pressure in the interstitial space prior to collecting cooling water leaking from the pool through the liner system in the interstitial space; and detecting a second pressure higher than the first pressure after collecting cooling water leaking from the pool through the liner system in the interstitial space; wherein the second pressure is associated with a cooling water leakage condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The features of the exemplary embodiments will be described with reference to the following drawings where like elements are labeled similarly, and in which: 
           [0012]      FIG. 1  is a cross sectional diagram of a known approach used to monitor the integrity of weld seams for leakage in a single spent fuel pool liner system; 
           [0013]      FIG. 2  is a side cross-sectional view of an environmentally sequestered nuclear spent fuel pool having a dual liner and leakage collection and monitoring system according to the present disclosure; 
           [0014]      FIG. 3  is a top plan view of the fuel pool with liner and leakage collection/monitoring system of  FIG. 2 ; 
           [0015]      FIG. 4  is a detail taken from  FIG. 2  showing a bottom joint of the liner system at the intersection of liners from the sidewalls and base slab of the fuel pool; 
           [0016]      FIG. 5  is a detail taken from  FIG. 2  showing a top joint of the liner system at the terminal top ends of the sidewall liners; 
           [0017]      FIG. 6  is a perspective view of an example nuclear fuel assembly containing spent nuclear fuel rods; and 
           [0018]      FIG. 7  is a schematic diagram of a vacuum leakage collection and monitoring system according to the present disclosure. 
       
    
    
       [0019]    All drawings are schematic and not necessarily to scale. Parts shown and/or given a reference numerical designation in one figure may be considered to be the same parts where they appear in other figures without a numerical designation for brevity unless specifically labeled with a different part number and described herein. References herein to a figure number (e.g.  FIG. 1 ) shall be construed to be a reference to all subpart figures in the group (e.g.  FIGS. 1A, 1B , etc.) unless otherwise indicated. 
       DETAILED DESCRIPTION 
       [0020]    The features and benefits of the invention are illustrated and described herein by reference to exemplary embodiments. This description of exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. Accordingly, the disclosure expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features. 
         [0021]    In the description of embodiments disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. 
         [0022]    Referring to  FIGS. 2-6 , an environmentally sequestered spent fuel pool system includes a spent fuel pool  40  comprising a plurality of vertical sidewalls  41  rising upwards from an adjoining substantially horizontal base wall or slab  42  (recognizing that some slope may intentionally be provided in the upper surface of the bottom wall for drainage toward a low point if the pool is to be emptied and rinsed/decontaminated at some time and due to installation tolerances). The base slab  42  and sidewalls  41  may be formed of reinforced concrete in one non-limiting embodiment. The fuel pool base slab  42  may be formed in and rest on the soil sub-grade  26  the top surface of which defines grade G. In this embodiment illustrated in the present application, the sidewalls are elevated above grade. In other possible embodiments contemplated, the base slab  42  and sidewalls  41  may alternatively be buried in sub-grade  26  which surrounds the outer surfaces of the sidewalls. Either arrangement may be used and does not limit of the invention. 
         [0023]    In one embodiment, the spent fuel pool  40  may have a rectilinear shape in top plan view. Four sidewalls  41  may be provided in which the pool has an elongated rectangular shape (in top plan view) with two longer opposing sidewalls and two shorter opposing sidewalls (e.g. end walls). Other configurations of the fuel pool  40  are possible such as square shapes, other polygonal shapes, and non-polygonal shapes. 
         [0024]    The sidewalls  41  and base slab  42  of the spent fuel pool  40  define a cavity  43  configured to hold cooling pool water W and a plurality of submerged nuclear spent fuel assembly storage racks  27  holding fuel bundles or assemblies  28  each containing multiple individual nuclear spent fuel rods. The storage racks  27  are disposed on the base slab  42  in typical fashion. With continuing reference to  FIGS. 1-6 , the spent fuel pool  40  extends from an operating deck  22  surrounding the spent fuel pool  40  downwards to a sufficient depth D 1  to submerge the fuel assemblies  28  (see, e.g.  FIG. 6 ) beneath the surface level S of the pool water W for proper radiation shielding purposes. In one implementation, the fuel pool may have a depth such that at least 10 feet of water is present above the top of the fuel assembly. 
         [0025]    A nuclear fuel assembly storage rack  27  is shown in  FIGS. 2 and 3 , and further described in commonly assigned U.S. patent application Ser. No. 14/367,705 filed Jun. 20, 1014, which is incorporated herein by reference in its entirety. The storage rack  27  contains a plurality of vertically elongated individual cells as shown each configured for holding a fuel assembly  28  comprising a plurality of individual nuclear fuel rods. An elongated fuel assembly  28  is shown in  FIG. 6  holding multiple fuel rods  28   a  and further described in commonly assigned U.S. patent application Ser. No. 14/413,807 filed Jul. 9, 2013, which is incorporated herein by reference in its entirety. Typical fuel assemblies  28  for a pressurized water reactor (PWR) may each hold over 150 fuel rods in 10×10 to 17×17 fuel rod grid arrays per assembly. The assemblies may typically be on the order of approximately 14 feet high weighing about 1400-1500 pounds each. 
         [0026]    The substantially horizontal operating deck  22  that circumscribes the sidewalls  41  and pool  40  on all sides in one embodiment may be formed of steel and/or reinforced concrete. The surface level of pool water W (i.e. liquid coolant) in the pool  40  may be spaced below the operating deck  22  by a sufficient amount to prevent spillage onto the deck during fuel assembly loading or unloading operations and to account to seismic event. In one non-limiting embodiment, for example, the surface of the operating deck  22  may be at least 5 feet above the maximum 100 year flood level for the site in one embodiment. The spent fuel pool  40  extending below the operating deck level may be approximately 40 feet or more deep (e.g. 42 feet in one embodiment). The fuel pool is long enough to accommodate as many spent fuel assemblies as required. In one embodiment, the fuel pool  40  may be about 60 feet wide. There is sufficient operating deck space around the pool to provide space for the work crew and for staging necessary tools and equipment for the facility&#39;s maintenance. There may be no penetrations in the spent fuel pool  40  within the bottom 30 feet of depth to prevent accidental draining of water and uncovering of the spent fuel. 
         [0027]    According to one aspect of the invention, a spent fuel pool liner system comprising a double liner is provided to minimize the risk of pool water leakage to the environment. The liner system is further designed to accommodate cooling water leakage collection and detection/monitoring to indicate a leakage condition caused by a breach in the integrity of the liner system. 
         [0028]    The liner system comprises a first outer liner  60  separated from a second inner liner  61  by an interstitial space  62  formed between the liners. An outside surface of liner  60  is disposed against or at least proximate to the inner surface  63  of the fuel pool sidewalls  41  and opposing inside surface is disposed proximate to the interstitial space  62  and outside surface of liner  61 . The inside surface of liner  61  is contacted and wetted by the fuel pool water W. It bears noting that placement of liner  60  against liner  61  without spacers therebetween provides a natural interstitial space of sufficient width to allow the space and any pool leakage there-into to be evacuated by a vacuum system, as further described herein. The natural surface roughness of the materials used to construct the liners and slight variations in flatness provides the needed space or gap between the liners. In other embodiments contemplated, however, metallic or non-metallic spacers may be provided which are distributed in the interstitial space  62  between the liners if desired. 
         [0029]    The liners  60 ,  61  may be made of any suitable metal which is preferably resistant to corrosion, including without limitation stainless steel, aluminum, or other. In some embodiments, each liner may be comprised of multiple substantially flat metal plates which are seal welded together along their peripheral edges to form a continuous liner system encapsulating the sidewalls  41  and base slab  42  of the spent fuel pool  40 . 
         [0030]    The inner and outer liners  61 ,  60  may have the same or different thicknesses (measured horizontally or vertically between major opposing surfaces of the liners depending on the position of the liners). In one embodiment, the thicknesses may be the same. In some instances, however, it may be preferable that the inner liner  61  be thicker than the outer liner  60  for potential impact resistant when initially loading empty fuel storage racks  27  into the spent fuel pool  40 . 
         [0031]    The outer and inner liners  60 ,  61  (with interstitial space therebetween) extend along the vertical sidewalls  41  of the spent fuel pool  40  and completely across the horizontal base slab  42  in one embodiment to completely cover the wetted surface area of the pool. This forms horizontal sections  60   b ,  61   b  and vertical sections  60   a ,  61   a  of the liners  60 ,  61  to provide an impervious barrier to out-leakage of pool water W from spent fuel pool  40 . The horizontal sections of liners  60   b ,  61   b  on the base slab  42  may be joined to the vertical sections  60   a ,  61   a  along the sidewalls  41  of the pool  40  by welding. The detail in  FIG. 4  shows one or many possible constructions of the bottom liner joint  64  comprising the use of seal welds  65  (e.g. illustrated fillet welds or other) to seal sections  60   a  to  60   b  along their respective terminal edges and sections  61   a  to  61   b  along their respective terminal edges as shown. Preferably, the joint  64  is configured and arranged to fluidly connect the horizontal interstitial space  64  between horizontal liner sections  60   b ,  61   b  to the vertical interstitial space  64  between vertical liner sections  60   a ,  61   a  for reasons explained elsewhere herein. 
         [0032]    The top liner joint  65  in one non-limiting embodiment between the top terminal edges  60   c ,  61   c  of the vertical liner sections  60   a ,  61   a  is shown in the detail of  FIG. 5 . The top of the spent fuel pool  40  is equipped with a substantially thick metal embedment plate  70  which circumscribes the entire perimeter of the fuel pool. The embedment plate  70  may be continuous in one embodiment and extends horizontally along the entire inner surface  63  of the sidewalls  41  at the top portion of the sidewalls. The embedment plate  70  has an exposed portion of the inner vertical side facing the pool which extends above the top terminal ends  60   c ,  61   c  of the inner and outer liners  60 ,  61 . The opposing outer vertical side of the plate  70  is embedded entirely into the sidewalls  41 . A top surface  71  of the embedment plate  70  that faces upwards may be substantially flush with the top surface  44  of the sidewalls  41  to form a smooth transition therebetween. In other possible implementations, the top surface  71  may extend above the top surface  44  of the sidewalls. The embedment plate  70  extends horizontal outward from the fuel pool  40  for a distance into and less than the lateral width of the sidewalls  41  as shown. 
         [0033]    The embedment plate  70  has a horizontal thickness greater than the horizontal thickness of the inner liner  61 , outer liner  60 , and in some embodiments both the inner and outer liners combined. 
         [0034]    The top embedment plate  70  is embedded into the top surface  44  of the concrete sidewalls  41  has a sufficient vertical depth or height to allow the top terminal edges  60   c ,  61   c  of liners  60 ,  61  (i.e. sections  60   a  and  61   a  respectively) to be permanently joined to the plate. The top terminal edges of liners  60 ,  61  terminate at distances D 2  and D 1  respectively below a top surface  71  of the embedment plate  70  (which in one embodiment may be flush with the top surface of the pool sidewalls  41  as shown). Distance D 1  is less than D 2  such that the outer liner  60  is vertical shorter in height than the inner liner  61 . In one embodiment, the embedment plate  70  has a bottom end which terminates below the top terminal edges  60 i cl ,  61 i c l of the liners  60 ,  61  to facilitate for welding the liners to the plate. 
         [0035]    In various embodiments, the embedment plate  70  may be formed of a suitable corrosion resistant metal such as stainless steel, aluminum, or another metal which preferably is compatible for welding to the metal used to construct the outer and inner pool liners  60 ,  61  without requiring dissimilar metal welding. 
         [0036]    As best shown in  FIG. 5 , the top terminal edges  60   c ,  61   c  of inner and outer liners  60 ,  61  may have a vertically staggered arranged and be separately seal welded to the top embedment plate  70  independently of each other. A seal weld  66  couples the top terminal edge  61   c  of liner  61  to the exposed portion of the inner vertical side of the embedment plate  70 . A second seal weld  67  couples the top terminal edge  60   c  of liner  60  also to the exposed portion of the inner vertical side of the embedment plate  70  at a location below and spaced vertical apart from seal weld  66 . This defines a completely and hermetically sealed enclosed flow plenum  68  that horizontal circumscribes the entire perimeter of the spent fuel pool  40  in one embodiment. The flow plenum  68  is in fluid communication with the interstitial space  62  as shown. One vertical side of the flow plenum is bounded by a portion of inner liner  61  and the opposing vertical side of the plenum is bounded by the inner vertical side of the top embedment plate  70 . 
         [0037]    The top flow plenum  68  may be continuous or discontinuous in some embodiments. Where discontinuous, it is preferable that a flow passageway  105  in the top embedment plate  70  be provided for each section of the separate passageways. 
         [0038]    Seal welds  66  and  67  may be any type of suitable weld needed to seal the liners  60 ,  61  to the top embedment plate  70 . Backer plates, bars, or other similar welding accessories may be used to make the welds as needed depending on the configuration and dimensions of the welds used. The invention is not limited by the type of weld. 
         [0039]    In one embodiment, the outer and inner liners  60 ,  61  are sealably attached to the spent fuel pool  40  only at top embedment plate  70 . The remaining portions of the liners below the embedment plate may be in abutting contact with the sidewalls  41  and base slab  42  without means for fixing the liners to these portions. 
         [0040]    It bears noting that at least the inner liner  61  has a height which preferably is higher than the anticipated highest water level (surface S) of the pool water W in one embodiment. If the water level happens to exceed that for some reason, the top embedment plate  70  will be wetted directly by the pool water and contain the fluid to prevent overflowing the pool onto the operating deck  22 . 
         [0041]    According to another aspect of the invention, a vapor extraction or vacuum system  100  is provided that is used to draw down the air pressure in the interstitial space between the outer and inner liners  60 ,  61  to a relatively high state of vacuum for leakage control and/or detection.  FIG. 7  is a schematic diagram of one embodiment of a vacuum system  100 . 
         [0042]    Referring to  FIGS. 5 and 7 , vacuum system  100  generally includes a vacuum pump  101  and a charcoal filter  102 . Vacuum pump  101  may be any suitable commercially-available electric-driven vacuum pump capable of creating a vacuum or negative pressure within the interstitial space  62  between the pool liners  60  and  61 . The vacuum pump  101  is fluidly connected to the interstitial space  68  via a suitable flow conduit  103  which is fluidly coupled to a telltale or flow passageway  105  extending from the top surface  71  of the top embedment plate  70  to the top flow plenum  68  formed between the pool liners  60  and  61 . Flow conduit  103  may be formed of any suitable metallic or non-metallic tubing or piping capable of withstanding a vacuum. A suitably-configured fluid coupling  104  may be provided and sealed to the outlet end of the flow passageway  105  for connecting the flow conduit  103 . The inlet end of the flow passageway penetrates the inner vertical side of top embedment plate  70  within the flow plenum  68 . The flow passageway  105  and external flow conduit  103  provides a contiguous flow conduit that fluidly couples the flow plenum  68  to the vacuum pump  101 . A one-way check valve is disposed between the flow plenum  105  and the suction inlet of the vacuum pump  101  to permit air and/or vapor to flow in a single direction from the liner system to the pump. 
         [0043]    The absolute pressure maintained by the vacuum system  100  in the interstitial space  62  between the liners  60 ,  61  (i.e. “set pressure”) preferably should be such that the bulk water temperature of the spent fuel pool  40  which is heated by waste decay heat generated from the fuel rods/assemblies is above the boiling temperature of water at the set pressure. The table below provides the boiling temperature of water at the level of vacuum in inches of mercury (Hg) which represent some examples of set pressures that may be used. . 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Pressure in inch, HgA 
                 Boiling Temp, deg F. 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1 
                 79 
               
               
                   
                 2 
                 101 
               
               
                   
                 3 
                 115 
               
               
                   
                 4 
                 125 
               
               
                   
                 5 
                 133 
               
               
                   
                   
               
             
          
         
       
     
         [0044]    Any significant rise in pressure would indicate potential leakage of water in the interstitial space  62  between the liners  60 ,  61 . Because of sub-atmospheric conditions maintained by the vacuum pump  101  in the interstitial space, any water that may leak from the pool into this space through the inner liner  61  would evaporate, causing the pressure to rise which may be monitored and detected by a pressure sensor  104 . The vacuum pump  101  preferably should be set to run and drive down the pressure in the interstitial space  62  to the “set pressure.” 
         [0045]    In operation as one non-limiting example, if the vacuum pump  101  is operated to create a negative pressure (vacuum) in the interstitial space  62  of 2 inches of Hg, the corresponding boiling point of water at that negative pressure is 101 degrees Fahrenheit (degrees F.) from the above Table. If the bulk water temperature of pool water W in the spent fuel pool  40  were at any temperature above 101 degrees F. and leakage occurred through the inner pool liner  61  into the interstitial space  62 , the liquid leakage would immediately evaporate therein creating steam or vapor. The vacuum pump  101  withdraws the vapor through the flow plenum  68 , flow passageway  105  in the top embedment plate  70 , and flow conduit  103  (see, e.g. directional flow arrows of the water vapor in  FIGS. 5 and 7 ). Pressure sensor  104  disposed on the suction side of the pump  101  would detect a corresponding rise in pressure indicative of a potential leak in the liner system. In some embodiments, the pressure sensor  104  may be operably linked to a control panel of a properly configured computer processor based plant monitoring system  107  which monitors and detects the pressure measured in the interstitial space  62  between the liners on a continuous or intermittent basis to alert operators of a potential pool leakage condition. Such plant monitoring systems are well known in the art without further elaboration. 
         [0046]    The extracted vapor in the exhaust or discharge from the vacuum pump  101  is routed through a suitable filtration device  102  such as a charcoal filter or other type of filter media before discharge to the atmosphere, thereby preventing release of any particulate contaminants to the environment. 
         [0047]    Advantageously, it bears noting that if leakage is detected from the spent fuel pool  40  via the vacuum system  100 , the second outer liner  60  encapsulating the fuel pool provides a secondary barrier and line of defense to prevent direct leaking of pool water W into the environment. 
         [0048]    It bears noting that there is no limit to the number of vapor extraction systems including a telltale passageway, vacuum pump, and filter combination with leakage monitoring/detection capabilities that may be provided. In some instances, four independent systems may provide adequate redundancy. In addition, it is also recognized that a third or even fourth layer of liner may be added to increase the number of barriers against leakage of pool water to the environment. A third layer in some instances may be used as a palliative measure if the leak tightness of the first inter-liner space could not, for whatever reason, be demonstrated by a high fidelity examination in the field such as helium spectroscopy. 
         [0049]    While the foregoing description and drawings represent exemplary embodiments of the present disclosure, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope and range of equivalents of the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other forms, structures, arrangements, proportions, sizes, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. In addition, numerous variations in the methods/processes described herein may be made within the scope of the present disclosure. One skilled in the art will further appreciate that the embodiments may be used with many modifications of structure, arrangement, proportions, sizes, materials, and components and otherwise, used in the practice of the disclosure, which are particularly adapted to specific environments and operative requirements without departing from the principles described herein. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive. The appended claims should be construed broadly, to include other variants and embodiments of the disclosure, which may be made by those skilled in the art without departing from the scope and range of equivalents.