Patent Document

BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Example embodiments relate generally to nuclear reactors, and more particularly to a method and apparatus for a fret resistant fuel rod for a Light Water Reactor (LWR) nuclear fuel bundle. The method and apparatus may include a fret resistant layer integrated within outer layers of fuel rod cladding, using embedded ceramic particles. The particles may be integrated within surfaces of the fuel rod cladding, by melting a thin layer of the cladding and re-solidifying the cladding to capture the particles within the cladding material matrix. 
     Related Art 
     As shown in  FIG. 1 , a conventional Boiling Water Reactor (BWR) nuclear reactor fuel assembly  10  includes a channel  12  with fuel rods  14  that may provide heat energy to a nuclear reactor to heat water into steam. While much of the discussion herein is directed toward a Boiling Water Reactor (BWR) fuel assembly  10 , it should be understood that example embodiments may be applied to Light Water Reactors (LWRs) in general, including Pressurized Water Reactors (PWRs) and Canada Deuterium Uranium (CANDU) reactors. The steam is produced to cycle through steam turbines (not shown) to convert heat energy into work to ultimately produce electricity. Fuel rods  14  may be anchored in a lower tie plate  18 , and may extend through spacers  22  to varying axial elevations within the assembly  10 . For instance, full length fuel rods  14  may extend up to upper tie plate  20 , and long partial length fuel rods  14   a  may extend just below the upper tie plate  20 . Short part length fuel rods  14   b  may only extend just beyond the lowest level spacer  22 . The fuel rods  14  contain nuclear fuel pellets  16  (as shown in more detail, in  FIG. 2 ), and therefore integrity of the cladding  24  of the fuel rods  14  is critical to ensuring that the fuel  16  does not escape the confines of the fuel rods  14 . Leaking fuel that escapes the confines of the fuel assembly  10 , and migrates throughout equipment located within the BWR steam cycle, may cause costly BWR system maintenance and/or plant shutdown. 
     During operation, water and steam flowing through the fuel assembly  10  may frequently contain foreign material (debris) in the form of loose metal shavings, wires, and other materials which typically originate at reactor locations remote from the fuel rods  14 . These materials may be sufficiently hard to wear or fret the soft fuel rod material (often made from a zirconium-alloy). During reactor operation, this debris can migrate into the opening in the lower tie plate  18  and enter the fuel bundle. Debris can also enter the fuel bundle through the upper tie plate  20  during refueling operations. Once inside the bundle, debris may be entrapped by the spacers  22  where it may be maintained in a quasi-suspended state (due to fluid flow). Debris may cause cladding  24  of each fuel rod  14  to be particularly susceptible to debris fretting, whereas the debris may cyclically contact the fuel rods, imposing wear forces sufficient to penetrate the fuel rod  14  walls. Severe wear forces may also be placed on portions of the fuel rods  14  that contact spacers  22  (this is particularly the case in PWRs, where Grid to Rod Fretting, or GRE, may be prevalent). Cladding  24  wear may further be caused during fuel assembly  10  manufacturing and maintenance, as the fuel rods  14  may contact other fuel assembly  10  components during insertion (and removal) of the fuel rods  14  into (and, out of) the channel  12  of the assembly  10 . 
     Cladding  24  of fuel rods  14  is typically manufactured from a zirconium-alloy. The hostile environment of the reactor requires that structural modifications and/or material that is added to the fuel rod cladding  24  must satisfy a number of constraints. First, any wear resistant material added to the cladding must be approximately equal to or harder than the metallic debris particles found in the fuel assembly, to effectively resist abrasion from the particles. Second, any material applied to the cladding must be compatible with the thermal expansion of the cladding and form a strong bond with the cladding. Third, any material added to the cladding must be resistant to the chemical environment in the reactor, which characteristically includes hot water and steam in the case of BWRs and lithium hydride and boric acid in the case of PWRs. Fourth, the thickness of any material applied to the cladding must be relatively thin, so that the flow of water around the fuel rods is not significantly impeded. Fifth, any material added to the cladding is preferably capable of application in a process which does not require heating of the cladding tube above 400° C., to maintain the integrity of the cladding. Sixth, any material added to the fuel rod must not react with the cladding material or cause a reaction between the cladding and the environment. 
     Coatings of various forms and functions have conventionally been applied to fuel rod cladding, to provide a contiguous, dissimilar material layer to cladding to protect it from wear resistance. For example, a thin coating of an enriched boron-10 glass has been deposited on fuel rod cladding. Electroplating of fuel rod cladding has also been used, to provide a matrix metal and boron compound of, for example, nickel, iron manganese or chrome to coat the outside of the cladding. Furthermore, vapor deposition of volatized boron compounds have been applied to cladding. Lastly, ion-assisted vacuum deposition techniques, such as cathodic arc plasma deposition (CAPD), have been employed to deposit thin films on fuel rod cladding to increase wear resistance. Using each of these conventional methods, coatings or layers of wear resistant material form only a contiguous layer of protection that is not integrated within the actual cladding itself. 
     SUMMARY OF INVENTION 
     Example embodiments provide a method and/or an apparatus for providing a fret resistant fuel rod for a Light Water Reactor (LWR) nuclear fuel bundle. Specifically, a fret resistant layer may be integrated within outer layers of the actual fuel rod cladding itself. The fret resistant layer may include embedded ceramic particles with a hardness sufficient to resist wear of foreign materials that typically cause fuel rod failure. The particles may be integrated within the fuel rod cladding, by melting a thin layer of the cladding or material substantially similar to the cladding and re-solidifying it around the particles, ensuring that the particles are captured within the modified cladding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a perspective view of a conventional boiling water nuclear reactor (BWR) fuel assembly; 
         FIG. 2  is cross-sectional views of a conventional fuel rod; 
         FIG. 3  is cross-sectional views of a fuel rod, in accordance with an example embodiment; 
         FIG. 4  is a schematic depicting an Electro-Spark Discharge process, in accordance with an example embodiment; 
         FIG. 4A  is a flowchart showing the method steps of an Electro-Spark Discharge process, in accordance with an example embodiment; 
         FIGS. 5A-5F  are simplified schematics (not to scale) depicting fret resistant layers using different layer thicknesses, particles sizes, and different number densities of particles; 
         FIG. 6  is a schematic depicting Cold Spray process, in accordance with an example embodiment; and 
         FIG. 6A  is a flowchart showing the method steps of a Cold Spray process, in accordance with an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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. 
     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. 
     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.). 
     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. 
     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. 
       FIG. 3  is cross-sectional views of a fuel rod  14 , in accordance with an example embodiment. As shown in  FIG. 3 , the cladding  24  of the fuel rod  14  may include a fret resistant layer  26 , for increasing the wear resistance of the cladding  24 . The fret resistant layer  26  may include ceramic particles that are embedded into the actual cladding  24 , itself. The particles may be applied to the cladding, as described in detail herein. 
     Electro-Spark Discharge (ESD) 
       FIG. 4  is a schematic depicting an Electro-Spark Discharge (ESD) process, in accordance with an example embodiment. ESD is a process that may provide a true metallurgical bond between cladding and a wear resistant layer. The process may involve creating a voltage differential between the electrode  30  (containing wear resistant particles) and the cladding  24  of a fuel rod, in order to deposit the electrode material onto the cladding. The tip end  30   a  of the electrode may contact the cladding  24  while cyclic power pulses may be applied to the electrode  30  to create a high energy density that forms a plasma arc  34 . The heat associated with the plasma arc  34  causes the tip  30   a  of the electrode to deposit onto cladding  24 . By running the tip  30   a  along the surface of cladding  24 , a fret resistant layer  26  consisting of hardened electrode material (containing wear resistant particles) and re-hardened cladding  24  is formed above diffusion layer  36 . 
       FIG. 4A  is a flowchart showing the method steps of an Electro-Spark Discharge process, in accordance with an example embodiment. As shown in step S 40 , the electrode  30  ( FIG. 4 ) may be electrically charged (relative to the cladding  24 ). The voltage applied to the electrode  30  may be, for instance, about 150V. In step S 42 , the tip end  30   a  of the electrode is brought into contact with the cladding  24 , and the power cycled at a frequency of about 50-60 Hz, for example. Electrical discharge of a capacitor bank (applied to the electrode), ranging for instance at about 100-500 micro-Farad, may produce an arc between the electrode tip  30   a  and the cladding  24 , melting the electrode and the surface of the cladding. In step S 44 , the tip  30   a  of the electrode may then be moved along a surface of the cladding  24  (at about 0.5 inches/minute) in order to deposit the electrode on the cladding  24 . The resulting fret resistant layer  26  is a mix of the electrode material and the cladding surface material. 
     Ceramic Particles 
     Ceramic particles may be used within the electrode, to provide a fret resistant layer with the necessary hardness to resist wear. It should be understood that while this discussion of fret resistant particles is being described in the context of the ESD process, the requirements for these particles is equally applicable to the other processes of applying particles to cladding, as described herein. 
     Acceptable ceramic materials that may be used to make the fret resistant particles may include zirconium carbide or stabilized zirconia, though example embodiments are not limited to these materials. Other requirements for the fret resistant particles are as follows. 
     1. The fret resistant coating must have a hardness that prevents fretting of the cladding (typically &gt;30 Rc). 
     2. The particles must be compatible with both the base cladding material (the target material) and the applied material (the material contained in the electrode). That is to say, the particles should not cause an adverse chemical reaction or create an adverse material phase as a result of processing. 
     3. The particles must be compatible with the application process, to ensure that the particles are not damaged while being applied to the cladding. 
     4. The selection of a particle&#39;s elemental composition should include a consideration of the impact of neutron consumption and isotopic activation. Elements with a high neutron cross section may adversely affect power, while specific elements such as Zn and Co can undesirably activate into isotopes that may adversely affect personnel dosage and fuel rod storage concerns. 
     Applied Material and Fret Resistant Particles 
     The electrode may contain both fret resistant materials and an applied material. During the ESD process, the applied material and the cladding (the target material, otherwise known as the base material) may melt and re-harden to form a mixture of the applied material and the base material, capturing the dispersed fret resistant particles within the fret resistant recast layer  26  (shown in  FIG. 4 ). The diffusion barrier layer  36  between the applied material and the base material may be a thin layer (microns thick) where the two materials form a metallurgical bond. A relationship therefore exists between the characteristics of the applied material, the fret resistant particle size, and the particle density, as described herein. It should be understood that while this relationship is being described in the context of the ESD process, this relationship also applies to the other processes of applying particles to cladding, as described in this document. 
     The total thickness of the fret resistant layer, and the size and number density of the wear resistant particles within the fret resistant layer, impacts the operation of the fuel rod cladding while in operation. If the fret resistant layer is too thick, the layer may cause undesirable thermal hydraulic issues within an operating fuel bundle. If the fret resistant layer is too thick, the overall diameter of the fuel rod may also be increased to the point where the rod may not offer a proper clearance from other fuel bundle components (and, not allow adequate fluid flow around the rod, during operation). If the fret resistant layer is too thin, the ability of the layer to mitigate fretting may be compromised. Therefore, a preferred thickness of the fret resistant layer (containing base cladding material, applied material from the electrode, and fret resistant particles from the electrode) is about 0.5-2 mils. However, other thicknesses of the fret resistant layer may be used, such as a range of thicknesses of 10 mils or less, or preferably 5 mils or less, or even more preferably 3 mils or less. 
     An acceptable particle size is also a consideration in forming the fret resistant layer.  FIGS. 5A-5F  are simplified schematics (not to scale) depicting fret resistant layers  26  using different layer thicknesses, different particle  25  sizes, and different number densities of the particles  25 .  FIGS. 5A and 5B  show cladding with fret resistant layers  26   a / 26   b  having small diameter particles  25   a . As shown in  FIG. 5A , when a thickness of the fret resistant layer is thick and small diameter particles  25   a  are used, and increased number density of the particles  25   a  is required to provide an adequate particle coverage. The example embodiment of  FIG. 5B  may offer better protection against fretting, as a more thin fret resistant layer  26   b  is used with the small diameter particles  25   a  to ensure that more of the particles are located at an outer surface of the fret resistant layer  26   b  (note that approximately the same number density of particles is used in  FIGS. 5A and 5B , with a more efficient use of particles being shown in  FIG. 5B  where more of the particles are located near a surface of the fret resistant layer).  FIGS. 5C and 5D  use medium sized particles  25   b  of a similar number density, with  FIG. 5D  providing slightly more effective fret resistance (notice that a greater number of particles  25   b  are located near an outer surface of the fret resistant layer  26   d  of  FIG. 5D ).  FIGS. 5E and 5F  use large sized particle  25   c , with a more effective fret resistant layer  26   e  being shown in  FIG. 5E  ( FIG. 5F  uses a fret resistant layer  26   f  that is too thin, thereby fully exposing particles  25   c  that may detach from cladding  24  while in use). Therefore, based on the simplified schematic of  FIG. 5 , it is to be understood that the particles  25  must be small enough to remain captured in the fret resistant layer  26 , while being large enough to effectively mitigate fretting. Additionally, as a particle size  25  is reduced, the number density of the particles must be increased (especially when a thickness of the fret resistant layer  26  is also increased) to provide an effective coverage of the particles  25  within the fret resistant layer  26 . Therefore, to produce an adequate coverage of particles  25  within the fret resistant layer  26 , particles must be finely dispersed within the electrode. Particles with a diameter on the order of about 2-15 microns may preferably be used to ensure that the particles are dispersed within the electrode, as shown in Table 1 below. 
     Electrodes 
     Below is a table describing the characteristics of different suitable electrodes containing fret resistant particles. The electrodes of Table I have been chosen for the purpose of using an ESD process to apply a fret resistant layer to fuel rod cladding made of a zirconium alloy. However, it should be understood that an ESD process may be used to apply a fret resistant coating to another component other than fuel rod cladding. Additionally, the ESD process may be used to apply a fret resistant coating to another target material, besides zirconium. The core material of the electrode (i.e., the applied material) should ideally match the cladding material (the target material, which in this case is zirconium). That is to say, the core material of the electrode should not be a dissimilar material from that of the target material. To ensure that the two materials are similar, the core material (which may be an alloy) may share at least one common chemical element with the target material. For instance, in applying the electrode core materials of Table 1 (below) to zirconium cladding, the common chemical element between the two materials is zirconium (Zr). Using the common chemical element of zirconium, a range of zirconium in the electrode core material may be preferably at least 90% zirconium (by weight, but not including the weight of the entrained particles), more preferably greater than 95% zirconium, even more preferably 97% zirconium, and most preferably 98% zirconium. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Particle 
                   
                   
               
               
                   
                   
                   
                 Volume (as 
                 Particle 
                 Parti- 
               
               
                 Electrode 
                   
                   
                 compared 
                 Hardness, 
                 cle 
               
               
                 Core 
                 Electrode 
                 Particle 
                 to core 
                 kg/mm2 
                 Diam- 
               
               
                 Material 
                 Diameter 
                 Material 
                 material) 
                 [Knoop] 
                 eter 
               
               
                   
               
             
             
               
                 Zirconium 
                  1/16- 
                 Zirconium 
                 10-20% (by 
                 2400 
                 2-15 
               
               
                 Alloy 
                  3/32″ 
                 Carbide 
                 volume) 
                 [2100] 
                 microns 
               
               
                   
                   
                 (ZrC) 
                   
                   
                   
               
               
                 Zirconium 
                  1/16- 
                 Yittria 
                 10-20% (by 
                 1300 
                 2-15 
               
               
                 Alloy 
                  3/32″ 
                 Stabilized 
                 volume) 
                 [1160] 
                 microns 
               
               
                   
                   
                 Zirconia 
                   
                   
                   
               
               
                   
                   
                 (ZrO 2 Y 2 O 3 ) 
               
               
                   
               
             
          
         
       
     
     It should be understood that the example materials of Table 1 are merely examples of preferred materials that work well (due to the materials low neutron absorption) regardless of the final fret resistant layer. However, applied materials with higher neutron absorption rates may be used. To minimize this increase in parasitic neutron absorption, the thickness of fret resistant coating may be minimized. In the event of using non-preferred applied materials (that exceed a neutron absorption rate of typically associated with Zircaloy materials), thicknesses of the final fret resistant material preferably should not exceed 5 mils (preferably not to exceed 3 mils, more preferably not to exceed 2 mils, and most preferably should not exceed 1 mil). 
     Cold Spray 
     Kinetic Metallization Process 
       FIG. 6  is a schematic depicting Cold Spray process, in accordance with an example embodiment. Cold spray is a kinetic metallization process that may retain the composition and phases of the initial wear resistant particles without requiring fuel gasses or extreme electrical heating. Cold spray may be considered a subset of thermal spray processes. The process may involve compressing inert gas in a high pressure gas supply  94 , and mixing a portion of the high pressure gas with a coating powder (in powder feeder  92 ) that is entrained with fret resistant particles. A portion, or all, of the high pressure gas may be heated to temperatures of up to approximately 1,000° C. before being injected into a receiving port  82  of a cold spray gun  80 . The gun  80  may include a nozzle  84  that may restrict a flow of the pressurized, heated gas to increase a speed of the gas within a barrel  86  of the gun  80  to about 500 to 900 m/s. A discharge flow of cold spray gas  88  may discharge from gun  80 , at a distance of about 20-40 millimeters from a target cladding, to form a fret resistant layer  26  on the cladding  24 . Due to the high kinetic energy of the cold spray gas  80 , localized thermal energy may be produced that creates small micro welds between the entrained fret resistant particles and the cladding that melts cladding  24  (down to a diffusion barrier layer  36 ) to effectively fuse the particles within the fret resistant layer  26 . 
       FIG. 6A  is a flowchart showing the method steps of a Cold Spray process, in accordance with an example embodiment. As shown in method step S 100 , an inert gas may be pressurized to provide an adequate velocity for the Cold Spray process. In step S 102 , the pressurized gas may be heated to temperatures as high as 1,000° C., and injected with a coating powder (described below). In step S 104 , the gas may then be accelerated to speeds of about 500 to 900 m/s. In step S 106 , the high speed gas may be directed at cladding to produce a fret resistant layer  26  on the cladding  24 , as shown in  FIG. 6 . 
     Coating Powders 
     It should be understood that discussion of the fret resistant material thickness, particle sizes, and number density of the particles, as discussed in relation to the ESD process, are also applicable to this application method (with the understanding that the base material must be in powdered form). Below is a table describing the characteristics of different suitable coating powders. Ideally, the composition of the powder should match the target material (in this case, the target material is assumed to be fuel rod cladding, made from zirconium). 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Powder 
                   
                   
                   
                   
               
               
                   
                 Particle 
                   
                 Particle 
                   
                   
               
               
                   
                 Size 
                   
                 Volume (as 
                 Particle 
                 Parti- 
               
               
                 Coating 
                 (10-90% 
                   
                 compared 
                 Hardness, 
                 cle 
               
               
                 Powder 
                 distri- 
                 Particle 
                 to coating 
                 kg/mm2 
                 Diam- 
               
               
                 Material 
                 bution) 
                 Material 
                 powder) 
                 [Knoop] 
                 eter 
               
               
                   
               
             
             
               
                 Zirconium 
                 5-25 
                 Zirconium 
                 10-20% (by 
                 2400 
                 2-15 
               
               
                 Based 
                 microns 
                 Carbide 
                 volume) 
                 [2100] 
                 microns 
               
               
                 Alloy 
                   
                 (ZrC) 
                   
                   
                   
               
               
                 Zirconium 
                 5-25 
                 Yittria 
                 10-20% (by 
                 1300 
                 2-15 
               
               
                 Based 
                 microns 
                 Stabilized 
                 volume) 
                 [1160] 
                 microns 
               
               
                 Alloy 
                   
                 Zirconia 
                   
                   
                   
               
               
                   
                   
                 (ZrO 2 Y 2 O 3 ) 
               
               
                   
               
             
          
         
       
     
     The particle size of the powder must be small enough to be carried by the gas stream to the target cladding, and large enough to have sufficient mass to keep from melting and deforming upon impact with the cladding. Therefore, the particle size of the powder is dependent on the other process parameters, such as gas composition, temperature and velocity. 
     Similar to ESD (and Table 1), it should be understood that the example materials of Table 2 are merely examples of preferred materials that work well (due to the materials low neutron absorption) regardless of the final fret resistant layer. However, applied materials with higher neutron absorption rates may be used if only a very thin fret resistant thickness is used. In the event of using non-preferred applied materials (that exceed a neutron absorption rate typically associated with Zircaloy materials), thicknesses of the final fret resistant material preferably should not exceed 5 mils (preferably not to exceed 3 mils, more preferably not to exceed 2 mils, and most preferably should not exceed 1 mil). 
     Other Applications 
     It should be understood that other processes, besides the ESD and Cold Spray processes described above, may also be used to produce a fret resistant layer on a target material. The target material may be a component other than fuel rod cladding. The target material may be made from a material other than zirconium, or a zirconium alloy. Other such processes must ensure that an applied material entrained with fret resistant particles adheres to a thin film of the target material by using an applied material that matches the target material, thereby ensuring that the fret resistant particles are effectively captured within the target material matrix itself. 
     Locations 
     To reduce costs and increase the overall effectiveness of the methods described above, target materials do not have to be fully coated with a fret resistant layer. Instead, applications of the fret resistant layer may simply be applied to areas of fuel rods (or other components in the nuclear reactor) where debris failures occur most often. In particular, a fret resistant layer may only be applied to fuel rod cladding that is to be located near spacer grids (with the fret resistant coating being applied in locations that span from a few centimeters above spacer grid locations to a few centimeters below spacer grid locations). Because approximately 7-9 spacer locations generally exist in a typical reactor, a fret resistant layer may therefore be applied along the fuel rod in approximately 7-9 bands along the outer surface of each fuel rod. The fret resistant layer may also be applied to other areas and other components of the reactor that experience high degrees of shadow corrosion, fretting, or other such wear 
     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.

Technology Category: g