Abstract:
Apparatus for extracting useful electric or mechanical power in significant quantities from the decay heat that is produced within spent nuclear fuel storage casks. The power is used for either powering an active forced air heat removal system for the nuclear fuel storage casks, thereby increasing the thermal capacity of the casks, or for emergency nuclear plant power in the event of a station blackout. Thermoelectric generators or other heat engines are employed using the thermal gradient that exists between the spent nuclear fuel container surface and the environment surrounding the cask&#39;s components housing the nuclear fuel to produce the power.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part and claims priority to U.S. patent application Ser. No. 13/798,271, entitled “A Source Of Electricity Derived From A Spent Fuel Cask,” filed Mar. 13, 2013. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    This invention pertains generally to power sources that derive their energy from decay heat and, more particularly, from such a power source that derives its energy from a nuclear spent fuel storage cask containing spent nuclear fuel. 
         [0004]    2. Related Art 
         [0005]    Pressurized water nuclear reactors are typically refueled on an 18-month cycle. During the refueling process, a portion of the irradiated fuel assemblies within the core are removed and replaced with fresh fuel assemblies which are relocated around the core. The removed spent fuel assemblies are typically transferred under water to a separate building that houses a spent fuel pool in which these radioactive fuel assemblies are stored. The water in the spent fuel pools is deep enough to shield the radiation to an acceptable level and prevents the fuel rods within the fuel assemblies from reaching temperatures that could breach the cladding of the fuel rods, which hermetically house the radioactive fuel material and fission products. Cooling continues at least until the decay heat within the fuel assemblies is brought down to a level where the temperature of the assemblies is acceptable for dry storage. Typically, the spent fuel assemblies are stored in such pools for a period of fifteen years during which the assemblies can be cooled while they produce decay heat which decays exponentially with time. After fifteen years, the decay heat has decreased sufficiently that the assemblies can be removed from the spent fuel pool and transferred into long-term storage casks, each typically capable of holding 21 or more assemblies. These casks are generally relocated to another area on the nuclear plant site and stored indefinitely. 
         [0006]    Since the fuel assemblies continue to produce decay heat in the casks, a natural convection air flow is used to provide for heat removal. This keeps the interior cask&#39;s temperatures at a level that is suitable for the materials used. Each cask has an interior stainless steel cylindrical canister that contains the spent fuel assemblies. This canister is placed in the storage casks&#39; structural housing which is a thick reinforced cylindrical concrete shell that is lined on the inside face with stainless steel. In one such design there is an approximately 3.50 inch (8.89 cm) radial gap between the inner canister and the outer casks housing when assembled. This geometrical arrangement is shown in  FIGS. 1 and 2 .  FIG. 1  shows the casks shell  10  cut away without the inner canister installed. The casks shell  10  typically comprises three annular concrete sections, a lower segment  12 , a middle segment  14  and an upper segment  16 , that are laterally restrained by shear keys  18  and are held in position by the tie rods  20 . A steel liner  22  surrounds the interior of the segments  12 ,  14  and  16  and is capped by a thermal shield  24  and annular shield ring  26 . Support rails  28  vertically extend along the interior of the segments  12 ,  14  and  16  and guide the stainless steel canister into position and space the canister from the interior walls of the steel liner  22 . Support tubes  30  at the lower end of the central opening  42  in the outer segments  12 ,  14  and  16 , support the inner stainless steel canister  36  shown in  FIG. 2 . An air inlet  32  typically capped by a screen  34  funnels air through the lower portion of the bottom segment  12  of the concrete shell  10  through the interior of the concrete shell into the annular passage between the shell  10  and the interior cylindrical canister that fits within the central opening in the concrete outer shell  10 . The air entering through the intake  32  is exhausted through an air outlet passage  38  in the upper segment  16  of the concrete shell  10  that is capped by a screen  40 . Typically, there are multiple air intakes  32  and air outlet passages  38  circumferentially spaced around the outer cask  10 A top cover  41  is sealed by bolts  43  which extend through the cover and into the annular sealed ring  26  to secure the cover and the interior cylindrical canister  36  once filled with fuel assemblies and loaded within the central opening  42  of the concrete shell  10 . 
         [0007]      FIG. 2  shows the interior canister  36  that slides inside the outer concrete shell  10 . The inner canister  36  has an outer steel shell  44  that is closed at the lower end by a bottom end plate  46  that covers a bottom shield plug  48  which is seated over a bottom closure plate  50 . Spacer plates  52  are arranged within the inner canister shell  44  in a spaced tandem array and have substantially aligned square openings  56  into which the individual fuel assemblies are positioned. The aligned openings  56  maintain a designed spacing between fuel assemblies. The spacer plates  52  are held in position by an assembly of support rods  54  which extend therethrough around the perimeter of the spacer plates. A drain port  58  and vent port  60  span substantially the length of the canister shell  44  to evacuate water in the canister. The top of the canister  36  is closed by a top shield plug  62  which is covered by a top inner closure plate  64 . The top inner closer plate  64  includes an instrument port  66  which communicates with radiation and temperature monitors within the canister to communicate corresponding output signals to the exterior of the canister  36 . The inner canister assembly is covered by a top outer closure plate  68  fastened in place by circumferential bolts and includes a leak test port  70  for assuring a hermetical seal on the inner canister. 
         [0008]    The flow of cooling air enters the annulus at the bottom of the cask&#39;s shell  10  through the radial inlet passages  32  and the heating that incurs within the annulus between the inner canister  36  and the steel liner  22  of the outer concrete shell  10  induces a natural draft of air which is exhausted through the radial outlet passages  38  at the top of the cask. The residual decay heat from the spent fuel is thus dissipated over time to the surrounding environment. 
         [0009]    It is an object of this invention to convert the waste heat from spent nuclear fuel within a spent nuclear fuel storage cask to useful work. 
         [0010]    It is a further object of this invention to convert such waste heat to an energy source that can be used to further cool the spent fuel cask so that it can dissipate the heat from the spent fuel at an increased rate. 
         [0011]    It is an additional object of this invention to convert such waste heat to mechanical or electrical energy which can be employed as an auxiliary power source for the facility in which the cask is stored. 
       SUMMARY 
       [0012]    These and other objects are achieved by a spent nuclear fuel storage container having a canister for storing nuclear fuel and a heat engine in heat transfer relationship with the canister for converting a differential in heat between the latent heat of the stored nuclear fuel and an ambient environment, into electrical or mechanical power. In one embodiment, the spent nuclear fuel storage container includes an outer cask surrounding the canister with an annular space therebetween. An air intake extends through a lower portion of the cask, extending from outside the cask to the annular space. An air outlet extends through an upper portion of the cask, extending from the annular space to the outside of the cask. Preferably, the heat engine is in heat transfer relationship with the annular space. In one embodiment, the heat transfer relationship is implemented through a heat transfer medium to transport heat from the annular space to an exterior of the outer cask. In one such embodiment, the heat transfer medium is a heat pipe and the heat engine may be selected from a Rankine cycle engine, a Sterling cycle engine or a thermoelectric device. 
         [0013]    In still another embodiment, the heat engine is a thermoelectric device supported within the annular space on an outer surface of the inner canister that houses the nuclear fuel. Preferably, the thermoelectric device is supported at an elevation substantially between the air inlet and the air outlet. Desirably, the thermoelectric device is supported substantially midway between the air inlet and the air outlet. 
         [0014]    In still another embodiment, the heat engine has an electrical output that is connected to a coolant circulation system operable to cool a coolant. Preferably, the circulation system extends through the annular space between the outer cask and the inner canister and through the cask to the exterior thereof, with the coolant circulation system circulating a fluid coolant between an interior of the annular space and the exterior of the cask. 
         [0015]    In still another embodiment, the spent nuclear fuel storage container includes a coolant circulation system that cools the fluid within a spent fuel pool of a nuclear power plant. Desirably, the electric power forms an auxiliary power source for the nuclear plant. 
         [0016]    In another embodiment, the spent nuclear fuel storage container includes a fluid circulation system for circulating a cooling fluid over at least a portion of a circumference of the canister, with the fluid circulation system having a fluid inlet and a fluid outlet which extends through a shield cask that surrounds the canister. A fluid baffle system is provided in fluid communication with the fluid outlet which is supported on the shield cask, with the heat engine supported, at least in part, in the fluid baffle system in heat exchange relationship with the fluid exhausted from the fluid outlet. In one such embodiment the fluid baffle system is a substantially annular passage that fits around or on the shield cask. Preferably the fluid baffle system is supported from an upper portion of the shield cask and the fluid baffle system has an inlet that is substantially hermetically sealed to the fluid outlet. 
         [0017]    In yet a further embodiment the fluid circulation system has a plurality of fluid outlets circumferentially spaced around the shield cask and the fluid baffle system is in fluid communication with at least several of the fluid outlets. In the latter embodiment a perforated tube or plate is supported within the fluid baffle system in fluid communication with the fluid outlet, with the perforated tube or plate extending at least partially through the fluid baffle system for distributing the fluid over a fluid path through the baffle system. Preferably the heat engine is a plurality of thermoelectric generators that are supported through the fluid path through the fluid baffle system. Desirably, fins extend on the outside of the baffles support structure to promote heat transfer and the fluid path extends vertically in a serpentine course. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    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: 
           [0019]      FIG. 1  is an isometric view of the outer shell of a spent fuel casks partially exploded to show the top cover removed and partially in section exposing the interior thereof;  FIG. 1  also schematically shows several embodiments of the application of waste heat from the spent nuclear fuel to power various facets of a nuclear facility; 
           [0020]      FIG. 2  is an isometric view of an inner canister of a spent nuclear fuel cask partially exploded and cut away to expose the interior thereof that houses the spent nuclear fuel assemblies; 
           [0021]      FIG. 3  is a schematic of a thermoelectric module that can be used as part of the power generation system employed in one embodiment of the spent nuclear fuel cask illustrated in  FIGS. 1 and 2 ; 
           [0022]      FIG. 4  is a graphical representation of the temperature profile of the outer concrete shell and inner canister surfaces of the spent fuel cask of  FIGS. 1 and 2 ; 
           [0023]      FIG. 5  is an isometric view of a spent fuel cask showing the outer concrete shell with the inner canister partially removed; 
           [0024]      FIG. 6  a schematic view of a cross-section of part of a spent fuel outer concrete shell showing the cooling air path extending through another embodiment of this invention; and 
           [0025]      FIG. 7  is a graphical representation of power output vs. temperature differential across one example of a thermoelectric generator that can be employed with this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    This invention provides a means for converting waste heat from a spent fuel cask into electrical or mechanical power that can be used to support a multitude of functions. In one embodiment, thermoelectric generators are mounted on the outer surface of the inner canister of a spent fuel cask. The thermoelectric generators use the delta temperature difference between the inner canister housing the nuclear fuel and the air flow in an annular space between the inner canister and the outer concrete shell to produce power. Typically, commercially available thermoelectric devices will produce significant power when a delta T of 300° F. (149° C.) or better is placed across the devices. An exemplary thermoelectric device is illustrated in  FIG. 3  and is generally designated by reference character  72 . The thermoelectric device  72  generally consists of two or more elements of N and P-type doped semiconductor material  74  that are connected electrically in series and thermally in parallel. The N-type material is doped so that it will have an excess of electrons (more electrons than needed to complete a perfect molecular lattice structure) and P-type material is doped so that it will have a deficiency of electrons (fewer electrons than are necessary to complete a perfect lattice structure). The extra electrons in the N material and the “holes” resulting from the deficiency of electrons in the P material are the carriers which moves the heat energy from a heat source  76  through the thermoelectric material to a heat sink  78  which, in this case, is the annulus between the liner  22  on the inside of the concrete shell  10  and the inner canister shell  44 . The electricity that is generated by a thermoelectric module such as that shown in  FIG. 3  is proportional to the magnitude of the temperature difference between each side of the module. In accordance with this embodiment, the thermoelectric generator would be attached around the outer circumference of the inner cylindrical canister  36  in a band located approximately midway along the canister&#39;s axial height, which typically is between 75 and 125 inches (190.5 and 317.5 cm) from the bottom of the canister, i.e., approximately one fourth of the canister surface area. This surface area is noted in  FIG. 2  by reference character  80  and one such thermoelectric generator is figuratively illustrated in  FIG. 2  and designated by reference character  82 . The temperature profile within the casks for different components is given in  FIG. 4 . As can be seen, the canister  36  surface temperature in the middle elevation area is approximately 470° F. (243° C.). The air temperature will necessarily be greater than the inside of the concrete housing and can be found from an energy balance on this component. Conservatively using the total convective and radiation heat transfer lost from the outer cask surface to the atmosphere, and equating this to the convective heat transfer to the inside of the concrete housing enables an estimate of air temperature within the annulus. Using a free convection heat transfer coefficient of 2.0 B/hr-ft 2 -degree Fahrenheit, the air temperature is found to be approximately ten degrees warmer than the housing surface or a maximum of 170° F. (77° C.). Thus, 300° (149° C.) temperature difference exists between the canister shell  44  and the air stream in the central portion of the annulus between the shell  44  and the inner wall of the concrete outer shell  10 . 
         [0027]    Application of commercially available thermoelectric generator elements within this defined area will result in a power production of up to 10 kilowatts from each cask. Since the decay heat has already exponentially decayed for a minimum of fifteen years before the fuel assemblies are loaded in the casks, the remaining decay heat levels stay fairly constant, so this power is always available if needed. Once a spent fuel pool is full, each refueling offload requires three additional long-term storage casks, so a total of approximately 30 kilowatts of additional potential power is available every eighteen months, i.e., the refueling cycle. The thermoelectric generator elements  72  act like individual batteries and can be connected electrically in a combination of parallel and series arrangements to provide voltage and current levels for specific applications. This passively generated power can be used for many important things, for example, during a loss of on-site and off-site power (station blackout). Typically, during such conditions a plant must cope with only backup battery systems to power essential loads. For the AP1000®, a passive nuclear plant design offered by Westinghouse Electric Company LLC, Cranberry Township, Pa., this coping capability is at least 72 hours, and for older existing plants, the period is much shorter. The power generated from each cask can be used to provide battery charging, control room lighting, instrumentation needs and power to cool a spent fuel pool such as that designated by reference character  84 , schematically shown in  FIG. 1 , thereby extending the plant coping time under station blackout conditions. 
         [0028]    The power produced in each cask  86 , shown partially assembled in  FIG. 5  with the fuel assembly bundles  88  within the inner canister  36 , can be used to provide a forced draft of air in the annulus  90 , thereby significantly increasing the heat removal capability of the casks  86 . For this purpose, a thermoelectric generator element  82  is shown connected by an electrical lead  92  to an air blower or fan  94  that will move the air from the air intake  32  up through the annulus  90  and exhaust the air through the air outlet  38  in the upper portion of the concrete shell  10 . Alternately, the blower or fan  94  can be positioned outside the concrete shell  10  and be connected by piping to the intake  32  and outlet  38  while being driven by a thermoelectric element within the annulus  90  powered through leads that extend through the concrete outer shell  10 . Either arrangement for forcibly moving air through annulus  90  allows the fuel assemblies to be off loaded from the spent fuel pool at an earlier time and decreases the decay heat load on the spent fuel pool. This has the very positive result of reducing the cooling needs of the pool during station blackout conditions and improves the coping strategy for the plant. 
         [0029]    Alternately, a heat pipe  96  can be employed extending through the annulus  90  and through the outer concrete shell  10  to convey the heat generated in the annulus  90  or within the canister  36  to the outside where it can he employed to drive a mechanical heat engine, such as a Sterling cycle or Rankine cycle engine as figuratively illustrated, respectively, by reference characters  98  and  100  in  FIG. 1 . Either of the Sterling cycle or the Rankine cycle engines can be employed to drive the blower  94  to force air through the annulus or drive a pump  102  which can be employed to circulate spent fuel pool water  106  through a heat exchanger  104  where it can be cooled and returned to the spent fuel pool  84 . The operation of both the Rankine cycle engine and the Sterling cycle engine is more fully described in application Ser. No. 13/558,443, filed Jul. 26, 2012 (Attorney Docket No. CLS-UFS-001). 
         [0030]    In some instances it may not be practical to access the annulus between the outer concrete shield cask shell  10  and the inner canister  44  after the fuel assemblies  88  have been loaded and the cask sealed. This invention also contemplates a way to use a heat engine, such as thermoelectric generator technology to utilize the energy from the spent fuel without the need to place any hardware into the cask. 
         [0031]    Thermoelectric generator elements operate between two temperatures, as previously mentioned, and in general the performance or energy conversion efficiency will depend on the temperature difference. Using the internal canister shell surface provides a relatively large delta-T between the canister shell and cooling airstream flowing through the annulus. However, there is also a sufficient, though smaller delta-T available between the exhausted cooling air which has absorbed approximately 92% of the decay heat energy and the ambient air in the surroundings. By accepting a lower energy conversion efficiency, it is possible to still utilize thermoelectric generator technology to produce significant useful power without the need for internal cask modifications. 
         [0032]    Accordingly, this invention also envisions the placement of a baffled support structure that can be positioned over the top of the cask and supported from the robust concrete outer shield shell. The general configuration of one embodiment of this arrangement is shown in  FIG. 6 . Like reference characters are used among the several figures to denote corresponding components. For a circular cask shell  10  the baffle support structure may take the form of an annular cylinder  110  that extends around the cask and is supported from a flange  116  that rests on top of the cask and extends down to the air outlet  38  through the concrete cask shield shell  10 . The use of a gasket  118  under the support flange  116  and an elastomer seal  114  around the bottom of the support structure  110  between the support structure and the concrete shell  10  prevents leakage of exhaust airflow prior to entering the baffled region. A series of baffle plates  122  direct the exhaust flow from the air outlet  38  in a serpentine manner before being released to the atmosphere. The inside surface of the support structure walls are tined with individual thermoelectric generators  82  that are directly exposed to the heated exhaust airstream. The outside surface of the support structure walls are exposed to the cooler ambient air. The support structure material is made from an alloy with relatively high thermal conductivity such as aluminum. If needed, fins  108  are added to this outer surface to promote heat transfer between the ambient environment and the support structure. A perforated plate  112  can be provided at the inlet to the babble structure to distribute the airstream over the baffle walls. Since the air outlets  38  are typically, circumferentially spaced around the concrete cask at discrete locations the perforated plate  112  acts to distribute the airstream around the annulus within the cylindrical baffle arrangement. 
         [0033]    The amount of electrical energy that can be derived from a cask in this manner will vary with the type of thermoelectric generator used, but the performance of a representative example (Tellurex model G2-56-0375) is shown in  FIG. 7 . The air temperature leaving the cask will depend on the local heat transfer characteristics within the cask annulus and the mass flow of air. Using thermal data given in the BNFL W21Canister Storage FSAR, the exhaust temperature of the airstream leaving the cask is calculated to be about 103 degrees C. With a 25-30 degree C. ambient, the power produced from a single thermoelectric generator element is seen to be about 2.5 Watts. By utilizing the cylindrical areas shown in  FIG. 6  for mounting the thermoelectric generator elements, it is possible to generate approximately 3 kW of DC power for every 10 inches of height. As energy is extracted from the air, the temperature will decrease reducing the power output of the downstream elements. For each 3 kW of thermoelectric generator power, the airstream temperature is calculated to drop by about 10 degrees C., so the practical limit might be two areas of thermoelectric generator elements with a total power output of about 6 kW per cask. For an older plant with perhaps 20 casks on site, this represents potentially 120 kW of steady DC power that is available all the time which can be very significant during a station blackout scenario for powering instrumentation, ventilation fans, small pumps or other equipment needed to maintain plant safety. By way of comparison, the station blackout load post 72 hours for the AP1000® plant, which would use the on-site ancillary diesel powered generators is about 35 kW. 
         [0034]    Thus, the invention provides a very practical way of passively producing DC electric power at any site that has stored spent fuel casks. 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.