Abstract:
A process of using laser welding in the assembly of boiling water reactor fuel debris filters is disclosed. The laser welding process minimizes the distortion of the debris filter cast lower tie plate by applying minimal heat during the welding. Fixtures hold the cast lower tie plate through four degrees of motion under a constant controlled laser source during welding. The welding process also reduces the potential for stress corrosion cracking resulting from crevices in partial penetration welds that might occur in a laser welding process.

Description:
[0001]    The present invention relates to welding, and more particularly, to the combination of laser welding, special fixturing, the machined geometries and various methods being employed, to minimize the distortion of cast components as a result of the welding process. The invention minimizes the distortion of cast components and permits a change in the standard assembly sequence of machine, weld, machine again and clean to machine, clean, and finish weld components. 
       BACKGROUND OF THE INVENTION 
       [0002]    Over the years, the nuclear power industry has seen dramatic improvements in fuel designs. At present, there is a greater expectation that fuel will operate without failures. Some of the most common nuclear fuel failure mechanisms include: (1) debris fretting, (2) cladding corrosion, (3) pellet cladding interaction, and (4) failure due to manufacturing defects. One device that is used to prevent debris fretting is a Generation III Defender debris filter, which prevents the entry of debris into a nuclear reactor&#39;s fuel bundle, a problem that has previously caused fuel failures in reactors. The Defender debris filter is used in the BWR fleet, a series of boiling water nuclear reactors operating in the United States, Mexico, Japan, India, and several European countries. 
         [0003]    Gas tungsten arc welding, also known as tungsten inert gas (“TIG”) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce a weld. The weld area is protected from atmospheric contamination by a shielding gas, such as the inert gas argon. A filler metal is typically used, though some welds do not require it. A constant current welding power supply  55  produces energy, which is conducted across an arc through a column of highly ionized gas and metal vapors called plasma. 
         [0004]    TIG welding is commonly used to weld thin sections of stainless steel and light metals, such as aluminum, magnesium and copper alloys. TIG welding allows for stronger, higher quality welds; however, the process is complex and significantly slower than other welding techniques. TIG welding is also used to weld cast plates, such as stainless steel cast surfaces. However, the process can create excessive distortion in such cast plates due to the heat generated in the welding process. 
         [0005]    TIG welding has been used is in the manufacture of BWR fuel assembly lower tie plates. One of the difficulties with the use of TIG welding of cast components like the lower tie plates has been the distortion encountered when welding cast halves or machined components together in an assembly process. The distortion encountered is typically extreme enough to require final machining of the component due to the distortion. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    The present invention is directed to the use of laser welding to minimize distortion on cast stainless steel components as a result of welding. More specifically, the present invention is directed to the process of using laser welding in the assembly of boiling water reactor fuel debris filters, such as the Defender debris filter. The laser welding process minimizes the distortion of pre-machined cast surfaces on the Defender debris filter lower tie plate by applying minimal heat during the welding of a cover plate to the lower tie plate. 
         [0007]    During the laser welding process, a laser beam is applied symmetrically about a centerline between weld flanges on the cover plate and the lower tie plate. This concentration of light energy at the centerline between the cover plate weld flanges and the lower tie plate generates heat that is conducted within the joint formed between the cover plate and the lower tie plate, causing the metals from which the plates are formed to change from a solid state, into a liquid state of molten metal, and thereby, combine the centerline between the weld flanges on the cover plate and the lower tie plate. After the centerline of the two metal plates change back into a solid state, the two plates are then said to be welded together so as to form a butt welded joint between the cover plate and the lower tie plate. 
         [0008]    The welding process of the present invention utilizes a fixture designed to hold the debris filter lower tie plate through four degrees of motion under a fixed focused laser source during welding. Applying laser welding in the assembly of the lower tie plate minimizes distortion during the process of installing the debris filter within the pre-machined cast lower tie plate. The application of the laser weld subsequent to final machining of components provides a finished product that differs from typical high heat (Energy) input techniques (TIG). The techniques differ by; 1) no further machining of areas distorted by welding, 2) debris free cleaning, ensuring the exclusion of foreign material, and 3) resulting weldment geometry designed to minimize the concern of stress corrosion cracking. 
         [0009]    The present invention is also directed to the reduction of stress corrosion cracking resulting from crevices in partial penetration welds that might occur in the laser welding process. A specified minimum weld penetration along with reliefs behind the cover plate weld flange geometry, are intended to eliminate any possibility of cervices being created during the welding process at the interface between the cover plate and the lower tie plate. The resistance of the Defender weldment to stress corrosion cracking is preserved if the depth of weld penetration is equal to or greater than 70% of the weld joint. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of the principal components to be joined together and the overall layout of the Defender debris filter lower tie plate assembly. 
           [0011]      FIG. 2  is a front elevation view of the debris filter lower tie plate, showing the cover plate attached to the lower tie plate and the location of the laser welds used to attach the cover plate to the lower tie plate. 
           [0012]      FIG. 3  is a front view showing the locations of the tack welds used to hold the cover plate to the lower tie plate prior to laser welding the cover plate to the lower tie plate. 
           [0013]      FIG. 4  is a drawing of a tack weld station including a fixture for compressing the filter while holding the cover plate on the lower tie plate for purposes of tack welding the cover plate to the lower tie plate. 
           [0014]      FIG. 5A  is a drawing of the laser power supply and controls which provides laser power to the tie plate weld station. 
           [0015]      FIG. 5B  is a drawing of the laser weld station used to weld the cover plate to the lower tie plate. 
           [0016]      FIG. 6  is a picture of a tie plate pre-weld location with jog Z &amp; Y crosshairs placed in a start position for the laser welding of the cover plate to the lower tie plate. 
           [0017]      FIG. 7  is a drawing of a fixture for holding the lower tie plate through four degrees of axis motion in front of a focused laser source. 
           [0018]      FIG. 8A  is a schematic cross-sectional end view of the cover plate and lower tie plate weld joints showing the top and the bottom joints. 
           [0019]      FIG. 8B  is another schematic cross-sectional view of weld joints at one end of the cover plate and the lower tie plate. 
           [0020]      FIG. 9A  is a top-down, rear perspective view of the cover plate showing weld surfaces and weld reliefs. 
           [0021]      FIG. 9B  is a bottom-down rear perspective view of the cover plate showing weld surfaces and weld reliefs. 
           [0022]      FIG. 10  is a perspective view of the lower tie plate showing weld surfaces in the opening through which a defender debris filter assembly is inserted. 
           [0023]      FIG. 11A  is a metallographic cross section from the top horizontal weld joint. 
           [0024]      FIG. 11B  is a metallographic cross section from the bottom horizontal weld joint. 
           [0025]      FIG. 12  is a metallographic cross section showing mechanical loading by the debris filter once compressed within the cavity of the lower tie plate. 
           [0026]      FIG. 13  is a graph showing stress intensity in cover plate welds relative to penetration of the weld. 
           [0027]      FIG. 14  is a drawing showing the horizontal and vertical weld joint configurations for welding the cover plate to the lower tie plate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIG. 1  is a perspective view of the various components that make-up a Defender debris filter lower tie plate assembly  10 . The Defender debris filter lower tie plate assembly  10  consists of a machined lower tie plate  14  with a machined cast inlet  13 , a filter plate assembly  12  inserted into a rectangular cavity  25  within lower tie plate  14 , and a cover plate  16  that is welded to lower tie plate  14 . The cover plate  16  includes a finger spring pocket  17 . The lower tie plate  14  includes a plurality of fluid flow holes  15  and nuclear fuel pin holes  9 , with an opening  11  in one of its sides, through which the filter plate assembly  12  is inserted into the rectangular pocket  25  within lower tie plate  14 . The filter plate assembly  12  is a rectangular assembly comprised of a plurality of wavy stainless steel plates  18  welded together. Filter plate assembly  12  must be compressed slightly when installed within the lower tie plate  14  to prevent flow-induced vibration during reactor operations. The cover plate  16  retains the filter plate assembly  12  within the lower tie plate  14  and prevents coolant leakage from being diverted from the reactor&#39;s fuel bundle, after the welding process that seals the cover plate  16  to the lower tie plate  14 . The cover plate  16  is welded to lower tie plate  14  along two vertical weld joints  20  and two horizontal weld joints  21  extending around the perimeter of the cover plate  16 , as shown in  FIG. 2 . 
         [0029]    After the filter plate assembly  12  is inserted into tie plate  14 , cover plate  16  is fitted over the opening  11  leading to rectangular cavity  25  and tack welded preferably at two locations  22  and  24  shown in  FIG. 3 , although it should be understood that more than two tack welds can be used, if desired. The tack welds  22  and  24  are made as small as possible by developed parameters, while retaining the filter plate assembly  12  under compression within the lower tie plate  14 , but with enough strength to hold the cover plate  16  in position with respect to the lower tie plate  14  for subsequent laser welding. The tack welds  22  and  24  are fully consumed during the subsequent laser welding process. It should be noted that the tack welds are made using the TIG process. The tacks are made utilizing a precision power supply  55  noted for its stable arc starting capabilities, for producing a strong, low heat input, yet small profile tack weld, as needed for this application. 
         [0030]      FIG. 4  shows a tie plate tack welding station  50  for tack welding the cover plate  16  to the lower tie plate  14 . The tie plate tack welding station  50  is a foreign material exclusion control area designed to minimize the intrusion of foreign materials into the lower tie plate&#39;s inner cavity  25 . Tack welding station  50  includes a digital TIG tack welding power supply  55 , a manual loading and unloading clamp/ram  53 , that has a poly insulator  54  that retains the inlet end  23  of the lower tie plate  14  when clamped within the tack welding station  50 . Attached between the loading and unloading clamp  53  and the poly insulator  54  is the support plate  51  with a push rod  47  attached by a locking device  45 . A copper chill block  49  attached to a tie plate fixture  52 , that&#39;s affixed upon a manual arbor press  60  that&#39;s adjustable in the “Y” axis and load cell/sensor  62  that&#39;s adjustable in the “Z” axis, that&#39;s to be used with an alignment tool  64 . 
         [0031]    Prior to welding the weld joints between the cover plate  16  and the lower tie plate  14  are wiped with acetone or alcohol to ensure all the components a clean and contaminate free, including all work surfaces. The filter plate assembly  12  is orientated into the lower tie plate&#39;s cavity  25  and then placed into the tack weld station  50 . The copper chill block  49  that faces the lower tie plate  14  base, contains several precise locating pins that are made to fit snug into either the nuclear fuel pin holes  9  and/or the fuel flow holes  15  that are located on the surface base  31  of the lower tie plate  14 . Copper chill block  49  has a negative grounding cable that&#39;s attached from the Digital TIG tack welding power supply  55 . The lower tie plate  14  is then clamped into the copper chill block  49  by the manual loading and unloading clamp/ram  53 . The filter plate assembly  12  is inserted into the lower tie plate cavity  25  until it bottoms on the opposite wall. The preferred orientation of the filter assembly  12  within the lower tie plate cavity  25  utilizes the compressive nature of the filter assembly  12 . The push rod  47  locates the filter  12  and seats the filter within the inner cavity  25 . The push rod is locked in place using locking device  45  to avoid filter  12  slippage and proper engagement of cover plate wedge  37 . Thereafter, a fixture guide plate  56  is installed and held in place by the two palm-grip hand knobs  48 , that are attached to the top side of the lower tie plate fixture  52 . Once the fixture guide plate  56  is aligned with an edge of the tie plate&#39;s rectangular cavity  25 /opening  11 , an alignment tool  64  is then used to set a cover plate clamp  58  a predetermined distance from the guide face edge. Preferably, this distance is approximately 010″. Alignment tool  64  is placed on edge against the inner front surface of the fixture guide plate  56 , with the two palm-grip hand knobs  48  remaining loose enough for final adjustment of cover plate  16  gap clearance. Alignment tool  64  surface “A”  63  will come to rest upon horizontal weld surface  19  within cavity/opening  11 , while surfaces “B”  61  of the alignment tool  64  will come to rest upon the top surface of the fixture guide plate  56 . The alignment tool  64  is pulled against the inner surface of the fixture guide plate  56  and tightened into a final position by the two palm grip hand knobs  48 . The cover plate  16  is then installed and compressed with a manual arbor press  60  by way of the arbor press handle  57 . Before seating the cover plate  16 , a minimum force of 30 lbs must be indicated by the load cell  62 . The cover plate  16  is fully seated with additional arbor press force to minimize gap between the cover plate  16  and the tie plate  14  and to seat the cover plate  16  completely. If the load sensor  62  located on the “Z” axis reads above 30 pounds, but less than 1,000 pounds, the amount of force being used to compress the filter assembly  12  is within an acceptable range. Once cover plate  16  has been seated, the guide plate  56  is removed and the cover plate  16  is visually inspected to ensure that it is properly seated. If necessary, a small rubber mallet can be used with light taps to re-center the cover plate  16  in the lower tie plate  14  rectangular cavity/opening  11 . Preferably, the maximum gap between the cover plate  16  and the lower tie plate  14  is 0.010″ with a maximum gap on the two vertical joints  20  at 0.003″. While maintaining a load on the cover plate  16  and filter  12  with the arbor press  60  the final position of the filter  12  is inspected for proper positioning below the cover plate wedge  37 . Once this determination is made, two light tack welds  22  and  24  are made, as shown in  FIG. 3 . After tack welding, the lower tie plate assembly  10  is checked for overall final dimensions prior to laser welding using an envelope gage. 
         [0032]    The welding station  FIG. 5B  where cover plate  16  is laser welded to lower tie plate  14  is a foreign material exclusion control area. Typically, the laser welding system, in addition to the laser system  75 , would include a power supply  70 , a master control panel  72 , a main laser power switch (not shown), heat controls (not shown) and a chiller control (not shown). There would also be a computer switch (not shown) that controls the operation of the laser system  75 . In a typical welding procedure, the main power switch for the laser is turned on and then the laser que switch at the laser is also turned on. The laser control panel  72  is turned to a CNC mode. CNC is the computer numerical control that is used to control the power output and path of the laser. The chiller and heating control switches are then turned on, as are the main power switch to the lower tie plate welder and the computer is turned on. Thereafter, air, helium, vacuum (not shown) and water circulator (not shown) are turned on and a check is done to ensure that the laser lens  33  ( FIG. 14 ) of laser welding system  75  is in the center of its focus range. A program for operating the laser welding system  75  is then loaded. The lower tie plate  14  and cover  16  are positioned inside of the laser welding system  75 , whereupon the laser system  75  door  77  is closed and a switch is actuated to move the tie plate  14  and cover  16  to a pre-weld location. As shown in  FIG. 6 , jog Z&amp;Y crosshairs  80  and  82  are placed in a start position  84  between plates  14  and  16  and a verification is run prior to starting the laser welding sequence. After path verification, the weld sequence is started, during which a fixture  74 , shown in  FIG. 7 , which holds lower tie plate  14 , is moved to facilitate the welding of the cover plate  16  to the tie plate  14 . A back purging gas line  79  is attached to the copper chill block  78  that has several precise locating pins attached, that are made to provide a slip fit into either the nuclear fuel pin holes  9  and/or the fuel flow holes  15 , that are located on the surface base  31  of the lower tie plate  14 . The purge gas insures that the inner welded surfaces of the lower tie plate  14  are not contaminated during the laser welding operation when full penetration is meet. The fixture  74  is designed to hold lower tie plate  14  through four degrees of motion under a collimated laser beam  35  ( FIG. 14 ) within laser system  75  during the welding of cover plate  16  to lower tie plate  14 . 
         [0033]    The locations and configurations of the welded butt joints are shown in  FIGS. 2 ,  8 A and  8 B of the present application.  FIG. 2  is a front elevation view of the debris filter lower tie plate assembly  10  showing cover plate  16  attached to lower tie plate  14  and the location of the laser welds  20  and  21  used to attach cover plate  16  to tie plate  14 . As shown in  FIG. 2 , cover plate  16  is welded to lower tie plate  14  along two vertical weld joints  20  and two horizontal weld joints  21  extending around the perimeter of the cover plate  16 . 
         [0034]      FIG. 8A  is a schematic view of horizontal weld joints  21  at the top and the bottom of cover plate  16 , along with reliefs  28  behind weld flanges  26  ( FIGS. 9A &amp; 9B ).  FIG. 8B  is another schematic view of one of the vertical weld joints  20  at the end of cover plate  16 , again with a relief  28  behind a weld flange  26  and weld joint  20  engaging flange  26  and tie plate  14 . It should be noted that another weld flange  26  and a vertical weld joint  20  are located at the other end of cover plate  16 , again with a relief  28  behind the weld flange  26  and weld joint  20 . 
         [0035]      FIG. 9A  is a top rear perspective view of the cover plate  16  showing weld flanges  26 , weld surfaces  29  and backside relief  28 .  FIG. 9B  is a bottom rear perspective view of the cover plate  16  also showing weld flanges  26 , weld surfaces  29  and backside relief  28 .  FIG. 10  is a perspective view of the Defender debris filter lower tie plate assembly  10 , similar to  FIG. 1 , but showing weld surfaces  19  in rectangular cavity/opening  11  through which filter plate assembly  12  is inserted into the lower tie plate  14 . The weld surfaces  19  in rectangular cavity/opening  11  are recessed surfaces engaged by the weld flanges  26  of cover plate  16  when cover plate  16  is inserted into cavity opening  11  and during the welding of cover plate  16  to lower tie plate  14 . 
         [0036]    The reliefs  28  behind the cover plate weld flanges  26 , along with complete joint weld penetration are intended to eliminate any possibility of cervices at the interface between cover plate  16  and lower tie plate  14 , and thereby, reduce stress corrosion cracking resulting from crevices in partial penetration welds that might otherwise occur in the laser welding process. 
         [0037]    During the laser welding process of the present invention, the laser beam is applied symmetrically about a centerline between weld flanges  26  on cover plate  16  and lower tie plate  14  onto weld surfaces  29  on the cover plate  16  and weld surfaces  19  on the lower tie plate  14 . The focused coherent laser energy between the cover plate weld flanges  26  and the lower tie plate  14  generates heat that is conducted into weld joints  20 / 21  formed between the cover plate  16  and the lower tie plate  14 , causing the metal from which the plates are formed to change from a solid to a liquid, so as to combine the two separate liquid plate metals into one. After the two metals change back to a solid, the two plates  14  and  16  are welded together so as to form butt weld joints  20 / 21  between the two plates  14  and  16 . 
         [0038]    The present invention is also directed to the reduction of stress corrosion cracking resulting from crevices in partial penetration welds that might occur in a welding process. As noted above, complete joint weld penetration, along with reliefs  28  behind the cover plate weld flanges are intended to eliminate any possibility of cervices at the interface between the cover plate  16  and the lower tie plate  14 . 
         [0039]    Partial penetration welds, such as welds  40  and  42  shown in  FIGS. 11A and 11B , respectively, result in crevices  41  and  43  that increase the risk of stress corrosion cracking. Although the welding process of the present invention for joining the cover  16  and tie plate  14  is designed to result in full penetration welds, such penetration can be affected by mechanical loading on the welds  20  and  21  attaching cover plate  16  to lower tie plate  14 . Direct forces on the cover welds come from compression of the filter plate assembly  12  during attachment of the cover plate  16  to the tie plate  14  and from the coolant pressure differential during operation of the debris filter lower tie plate assembly  10 . The principal component of loading is the residual stress from shrinkage of the weld metal during solidification and cooling of the welds, combined with the stiffness of the cast cover plate  16  and tie plates  14 . Full penetration joint welds avoid crevices at the weld roots  44  and  46 , and, thereby, minimize the likelihood of stress corrosion cracking in creviced regions. 
         [0040]    In a preferred embodiment of the debris filter lower tie plate assembly  10 , the filter plate assembly  12  is constructed from an austenitic stainless steel. Preferably, the lower tie plate  14  is a solution annealed CF3 casting stainless steel. Preferably, the cover plate  16  and each of wavy stainless steel plates  18  forming the filter plate assembly  12  are made from solution annealed 316L stainless steel. Preferably, the lower tie plate  14  and cover plate  16  are machined after annealing without subsequent heat treatment. Preferably, the filter element  12  is re-annealed after assembly and welding of the wavy stainless steel plates  18  and before insertion of the filter plate assembly  12  into the lower tie plate  14 . 
         [0041]    Stress corrosion cracking is not an issue with respect to the lower tie plate  14 , due to the material used in its construction, i.e., cast, low-carbon stainless steel. Similarly, crevice-induced stress corrosion cracking is not an issue with the filter element  12  due to the high rate of coolant flow through the Defender debris filter lower tie plate assembly  10 . Crevice-induced stress corrosion is an issue in the cover plate  16  in the region of the laser welds  20  and  21  used to attach cover plate  16  to lower tie plate  14 , if the welds are not full penetration welds that include crevices. 
         [0042]    The stress rule index, given in equation (1), provides a means for assessing the potential effect of weld crevices on the likelihood of stress corrosion cracking. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         m 
                       
                       + 
                       
                         P 
                         b 
                       
                     
                     
                       S 
                       y 
                     
                   
                   + 
                   
                     
                       
                         Q 
                         + 
                         F 
                         + 
                         R 
                       
                       ≤ 
                       A 
                     
                     
                       
                         S 
                         y 
                       
                       + 
                       
                         0.002 
                          
                         E 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where
       A=1.0 for cast austenitic stainless steel (e.g., weld metal) and 0.7 for wrought 316L in a creviced condition,   P m =Primary membrane stress,   P b =Primary bending stress,   S y =Yield strength at temperature,   Q=Secondary stress   R=Residual stress,   E=Modulus of elasticity at temperature.       
 
         [0050]    A lack of fusion at the weld roots  44  and  46  shown in  FIGS. 11A and 11B  is a structural discontinuity that increases the stress in the adjacent material. The severest case is shown in  FIG. 11A , where the cover and lower tie plates  16  and  14  connect with no discernible gap and no discernible radius at the tip of the unfused region. Stress concentration factors for such partially fused butt welds are given by the empirically based relationship given in equation (2). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       σ 
                       max 
                     
                     σ 
                   
                   = 
                   
                     max 
                      
                     
                       [ 
                       
                         
                           C 
                           0 
                         
                         , 
                         
                           
                             C 
                             1 
                           
                           + 
                           
                             
                               C 
                               2 
                             
                              
                             
                               ( 
                               
                                 b 
                                 / 
                                 a 
                               
                               ) 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where
       a=Non-fused (crack) length,   b=Overall section width,   c 0 =4.5 (minimum concentration factor for short cracks),   c 1 =1.0,   c 2 =14.7,   σ=Average stress away from concentration,   σ max =Maximum stress at concentration.       
 
         [0058]    The stress concentration factors from equation (2) range from 4.5 to 8.3 for weld penetrations of slightly less than 100% to 50%, respectively. For reference, the stress concentration factor from elastic theory for the joint in  FIG. 11B  is 3.6. The larger values given by equation (2) are a conservative approximation that eliminates the uncertainty of crack width and tip radius from stress calculations. The stress concentration factor is applied to both normal and shear loading of the welded joint. 
         [0059]    The nature of the mechanical loading differs between the top and bottom welds  21  (horizontal) and end welds  20  (vertical) between plates  16  and  14 . That is, the forces needed to satisfy equilibrium create primarily a shear stress in the horizontal welds  20  and a tensile stress in the vertical welds  21 . In both types of welds, shrinkage during post-weld cooling creates primarily tensile stress in the weld material. 
         [0060]    The resistive force of the filter plate assembly  12  against the welded cover plate  16  varies with component dimensions. It is measured during installation of the cover plate  16  and ranges from 30 lbs. to a maximum of 1,000 lbs. The upper end of this range leads to plastic deformation of the filter element  12  within the lower tie plate inner cavity  25 . The coolant pressure difference between the fuel bundle inlet and the bypass region is approximately 10 psi. Residual welding stresses approach the yield strength of the cover plate  16  and tie plate  14  due to shrinkage of the weld metal during solidification of the weld joints and the geometry of the joints. The force due to compression of the filter element  12  and residual weld stresses relax during operation of the Defender debris filter lower tie plate assembly  10  due to thermal and irradiation-based processes. The force due to the coolant pressure differential varies with flow conditions during bundle operation, but remains throughout the operational life of the filter plate assembly  12 , i.e., in the range of 6-9 years. 
         [0061]    The loading and stresses in the weld joints  30  and  32  are shown schematically in  FIG. 12 . These simplifications are consistent with the stress rule given in equation (1), i.e., partitioning the stress into primary and secondary components. The primary components are the stress resultants needed to satisfy force equilibrium and are affected by the joint geometry and the resulting stress concentration. The secondary components are the stresses that are self-relieving. The dominant secondary component results from weld shrinkage and the stiffness of the cover and tie plates  16  and  14 . The secondary stress arises from shrinkage during cooling of the weld metal from the approximate mid-point to the annealing temperature range to room temperature, i.e., the cooling from 1070° C. to 20° C. The resulting, average thermal strain in the weld metal is slightly greater than 2%, which is approximately 10 times the strain for the onset of plastic deformation. The residual stress in the weld at the completion of assembly operations is the average yield strength of 316L at room temperature, namely, 33.8 ksi. 
         [0062]    The primary stress due to compression of the filter plate assembly  12  within the lower tie plate  14  and the residual welding stress relax due to thermal and irradiation effects. Thermal relaxation and relaxation due to the fast neutron flux in the region of the welds  20  and  21  reduces the stresses by additional amounts. 
         [0063]    The calculated stress indices are shown relative to penetration and limiting values in  FIG. 13 . Based on the stress relaxation during initial operation, weld penetration should be greater than or equal to 70% for the cover plate  16  adjacent to the weld. Although the weld metal is loaded directly based on the joint geometry shown in  FIG. 11A , the region at the edge of the weld has been found to undergo large plastic strains during welding and to be susceptible to stress corrosion cracking. Thus, it is preferred that partial fusion of the butt welds  20  and  21 , which connect the cover plate  16  and lower tie plate  14  in the debris filter lower tie plate  10 , will not affect the resistance of the weldment to stress corrosion cracking if the weld penetration is greater than or equal to 70% of the weld joint. 
         [0064]      FIG. 14  shows the laser welding used by the method of the present invention to weld the cover plate  16  to the lower tie plate  14 . As shown in  FIG. 14 , there is a laser beam  35  that is focused by a laser lens  33  to a focal point  34 , which is followed by a defocal point  36 . The laser beam is used with a power level that achieves a weld penetration greater than or equal to 70% of the weld joint. A focused beam  38  is used for horizontal weld joints  21  and a defocused beam  39  is used for vertical weld joints  20 . The laser beam  35  is defocused  39  preferably on vertical sections. 
         [0065]    The method of the present invention can be used to weld metal components other than the lower tie plate  14  and the cover plate  16  that are part of the Defender debris filter lower tie plate assembly  10 . The thickness of the particular metal components to be welded together will affect the parameters selected for the operation of the laser welder. For weld joints of a given thickness, the power level, focal length and speed for the laser beam are preferably set to result in a laser beam power density and weld speed needed to achieve sufficient weld joint penetration to preclude crevice—induced stress corrosion cracking, while minimizing the distortion of the metal pieces to be welded together so as to be within final acceptance specifications, thereby requiring no post weld machining of the metal components to be considered a final product. The inert gas flow is set to maximize cooling and minimize weld oxidation. Preferably, there is no wire brushing of the weld after completion to comply with foreign material exclusion methods. 
         [0066]    After completion of the welding, the fixture  74  holding the debris filter lower tie plate assembly  10  moves via CNC to an unload position, whereupon soot is vacuumed from the debris filter lower tie plate assembly  10  before it is removed from laser system  75 . The debris filter lower tie plate assembly  10  is unloaded and wiped to remove excess soot with a clean wipe and filtered compressed air. A wire brush or other mechanically abrasive means are never used to clean the welds  20  and  21 . Filtered compressed air is then blown through both ends of the tie plate  14  for at least 15 seconds and wiped clean with alcohol and a clean wipe. 
         [0067]    In the welding process of the present invention, the Defender debris filter lower tie plate assembly  10  and the weld area is cleaned and bagged in a foreign material exclusion area. 
         [0068]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.