Patent Publication Number: US-2013248434-A1

Title: Debris filter

Description:
TECHNICAL FIELD 
     The present invention relates to a debris filter, capable of reducing a pressure loss of coolant as it flows and capturing debris in the coolant. 
     BACKGROUND ART 
     Various debris filters for fuel assemblies have been developed for years. 
     Patent document 1 discloses “a lower tie plate assembly for a nuclear reactor including a debris catcher comprising a pair of superposed plates, and each having a plurality of holes of substantially identical size, shape and pitch, wherein the holes of one of said pair of plates are offset laterally from the holes of the other of said pair of plates”. (See claim 1 and FIGS. 10-14 of patent document 1.) 
     However, because the debris catcher of patent document 1 has the two plates superposed so that the holes of one of said pair of plates are offset from the holes of the other of said pair of plates, this arrangement just narrows the paths for coolant. Besides, when debris in a linear shape flows in the direction of the flow of the coolant, there is a probability that the debris passes through a hole in one plate toward a hole in the other plate. 
     As a method to reduce the probability that debris passes through a lower tie plate, debris filters have been improved so that coolant paths would not be linear. 
     For example, patent document 2 discloses “a fuel supporting attachment comprising a filter for catching debris included in a coolant on an upstream side of the fuel assembly, wherein the filter has a plurality of holes including at least one bend portion to a degree by which an outlet portion of a hole cannot be seen through from an inlet portion of a hole by a straight line, and the holes are arranged in a pattern of a houndstooth check”. (See claim 1, and FIGS. 3 and 5 of patent document 2.) 
     Furthermore, patent document 3 teaches “a fuel assembly comprising a foreign object filter having a pair of plate assemblies, each of which includes a lattice formed by a combination of plurality of first plates and a plurality of second plates arranged perpendicularly to the first plates, wherein each plate assembly has the first plates arranged parallel to an axis of the lower tie plate and the second plates inclined at an angle to the axis, wherein one of the plate assemblies, or a first plate assembly, is arranged closer to the fuel rods in the axial direction than the other plate assembly, or a second plate assembly; and flow paths formed between adjacent second plates run straight from the inlet to the outlet at an inclined angle to the axis; the flow paths formed in the second plate assembly are inclined to one side, whereas flow paths formed in the first plate assembly are inclined to the opposite side; the lower ends of the first plates of the first plate assembly contact the upper ends of the first plates of the second plate assembly; the outlet end, located at the outlet of one flow path, of one second plate of the second plate assembly is positioned at around the center of the inlet of one flow path of the first plate assembly; and the inlet end, located at the inlet of one flow path, of one second plate of the first plate assembly is positioned at around the center of the outlet of one flow path of the second plate assembly”. (See claim 1 and FIGS. 3-5 of patent document 3.) 
     However, a pressure loss in the flowing coolant becomes large with the bent flow paths disclosed in Documents 2 and 3. A technical idea of superimposing two plates each having a plurality of holes, such as the one disclosed in Document 1, and of further deforming the formed flow paths is disclosed, for example, in patent document 4. 
     Patent document 4 discloses “a fuel assembly having a grid in contact with the upper surface of the adapter plate of a lower nozzle having flow holes, whose outlets are subdivided with a lattice plate of the grid and which are formed so that the holes are inclined relative to the direction of flow passages formed out of the plate wherein the holes of one row and the holes of its adjacent rows are inclined in opposite directions”. (See claim 1, and FIGS. 5 and 6 of patent document 4.) 
     Also, patent document 5 teaches “a debris filter comprising curved plates arranged at equal intervals to one another, each curved plate having a middle portion offset laterally from inlet and outlet portions thereof so that no flow passage runs straight from an inlet thereof to an outlet thereof, wherein each curved plate has a face along which water flows, the surface provided with protrusions locally to subdivide the flow passages”. (See claim 1 of patent document 5.) 
     However, there was no debris filter which realized both of a sufficient reduction in the pressure loss and a satisfactory catch of debris. 
     When a fuel assembly equipped with a conventional debris filter is immersed in coolant of a nuclear reactor, pieces of debris in the shape of a needle or wire, which are carried in a stream of coolant flowing through the debris filter, are kept in the debris filter and prevented from flowing through the outlet thereof to the downstream side. However, when the circulation of coolant in the nuclear reactor is stopped, or the debris filter is taken out of the coolant, the pieces of debris that have been contained in the debris filter are discharged from the bottom of the debris filter to the outside by gravity, which results in non-removal of the debris from the nuclear reactor. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent document 1: JP H8-240680 A 
         Patent document 2: JP 2004-317522 A 
         Patent document 3: Japanese Patent No. 3977969 
         Patent document 4: JP H8-105988 A 
         Patent document 5: JP 2001-116872 A 
       
    
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     An objective to be achieved by the present invention is to provide a debris filter which ensures to capture debris in circulating coolant, yet causes only a small pressure drop in it. 
     Another objective to be achieved by the present invention is to provide a debris filter from which debris, having been caught by it, is hardly dropped or discharged from it to the outside. 
     Still another objective to be achieved by the present invention is to provide a debris filter that requires a smaller area to be fitted to. 
     A further objective to be achieved by the present invention is to provide a debris filter which requires a process of fewer steps to be produced. 
     Means to Solve the Problems 
     The present invention, or means for achieving the aforementioned objectives, provides: 
     (1) A debris filter comprising a plurality of bent plates having a same bent shape, a plurality of first plates having a same first shape, and a plurality of second plates having a same second shape, wherein
         each of the first plates is in the shape of a strip;   each of the second plates is in the shape of a strip;   each of the bent plates is composed of a first inclined plate in the shape of a strip, and a second inclined plate in the shape of a strip, wherein the first inclined plate shares a longitudinal edge with the second inclined plate; wherein   the bent plates are arranged so that the first inclined plates are parallel with each other and the second inclined plates are parallel with each other;   the bent plates and the first plates are fixed with each other so that the first plates intersect a first longitudinal edge of each first inclined plate, the first longitudinal edge facing the longitudinal edge that the first inclined plate shares with the second inclined plate;   the bent plates and the second plates are fixed with each other so that the second plates intersect a second longitudinal edge of each second inclined plate, the second longitudinal edge facing the longitudinal edge that the second inclined plate shares with the first inclined plate; and   a first inner longitudinal edge of the first plates, the first inner longitudinal edge facing an opposite second plate, does not touch a second inner longitudinal edge of the opposite second plate, the second inner longitudinal edge facing an opposite first plate.       

     (2) The debris filter according to means (1), wherein
         each of the first plates has a plurality of first inclined slots in the first inner longitudinal edge, the first slots inclined in relation to the first inner longitudinal edge;   each of the second plates has a plurality of second inclined slots in the second inner longitudinal edge, the second slots inclined in relation to the second inner longitudinal edge;   the first longitudinal edge of each bent plate is inserted into one of the first inclined slots of each first plate and fixed thereto; and   the second longitudinal edge of each bent plate is inserted into one of the second inclined slots of each second plate and fixed thereto.
 
(3) The debris filter according to means (2), wherein each of the bent plates has a plurality of first perpendicular slots in the first longitudinal edge, the first perpendicular slots formed perpendicularly to the first longitudinal edge, and a plurality of second perpendicular slots in the second longitudinal edge, the second perpendicular slots formed perpendicularly to the second longitudinal edge; and
   each bent plate is joined with the first plates in such a manner that the first perpendicular slots mesh with the first inclined slots, and each bent plate is joined with the second plates in such a manner that the second perpendicular slots mesh with the second inclined slots.       

     (4) The debris filter according to means (3), wherein the number of first perpendicular slots in each bent plate is the same as the number of first inclined slots in each first plate; the number of second perpendicular slots in each bent plate is the same as the number of second inclined slots in each second plate; and the number of bent plates is the same as the number of first inclined slots. 
     (5) The debris filter according to means (1), wherein
         each of the bent plates has a plurality of first perpendicular slots in the first longitudinal edge, the first perpendicular slots formed perpendicularly to the first longitudinal edge, and a plurality of second perpendicular slots in the second longitudinal edge, the second perpendicular slots formed perpendicularly to the second longitudinal edge;   the first inner longitudinal edge of each first plate is inserted into one of the first perpendicular slots of each bent plate and fixed thereto; and   the second inner longitudinal edge of each second plate is inserted into one of the second perpendicular slots of each bent plate and fixed thereto.       

     (6) The debris filter according to means (1), wherein the first plates are fixed to the bent plates at a first regular interval, and the second plates are fixed to the bent plates at a second regular interval, wherein the second regular interval is larger than the first regular interval. 
     (7) The debris filter according to means (6), wherein
         each of the first plates has a plurality of first inclined slots in the first inner longitudinal edge, the first slots inclined in relation to the first inner longitudinal edge;   each of the second plates has a plurality of second inclined slots in the second inner longitudinal edge, the second slots inclined in relation to the second inner longitudinal edge;   the first longitudinal edge of each bent plate is inserted into one of the first inclined slots of each first plate and fixed thereto; and   the second longitudinal edge of each bent plate is inserted into one of the second inclined slots of each second plate and fixed thereto.       

     (8) The debris filter according to means (7), wherein each of the bent plates has a plurality of first perpendicular slots in the first longitudinal edge, the first perpendicular slots formed perpendicularly to the first longitudinal edge, and a plurality of second perpendicular slots in the second longitudinal edge, the second perpendicular slots formed perpendicularly to the second longitudinal edge; and
         each bent plate is joined with the first plates in such a manner that the first perpendicular slots mesh with the first inclined slots, and each bent plate is joined with the second plates in such a manner that the second perpendicular slots mesh with the second inclined slots.       

     (9) The debris filter according to means (6), wherein
         each bent plate has a plurality of first perpendicular slots in the first longitudinal edge, the first perpendicular slots formed perpendicularly to the first longitudinal edge, and a plurality of second perpendicular slots in the second longitudinal edge, the second perpendicular slots formed perpendicularly to the second longitudinal edge;   the first inner longitudinal edge of each first plate is inserted into one of the first perpendicular slots of each bent plate and fixed thereto; and   the second inner longitudinal edge of each second plate is inserted into one of the second perpendicular slots of each bent plate and fixed thereto.       

     Advantages of the Invention 
     The debris filter of the present invention has bent flow paths formed only by the bent plates because the first inner longitudinal edge of the first plates does not touch the second inner longitudinal edge of the second plates. The sectional area of each of these bent flow paths is larger than that of each of inlet flow paths formed by the bent plates and the second plates. As a result, the bent flow paths are capable of reducing a pressure loss of coolant as it flows in them and of rectifying flow of the coolant along the bent plates. Therefore the present invention provides a debris filter experiencing only a little pressure loss in the paths through which liquid flows. The debris filter according to the present invention has an inlet portion having inlet flow paths formed by the second plates and the bent plates, and an outlet portion having outlet flow paths formed by the first plates and the bent plates, wherein the sectional area of each inlet flow path and that of each outlet flow path are small. This structure provides the debris filter with two opportunities to catch debris, which ensures the realization of the debris filter capable of infallibly capturing debris. 
     In an embodiment of the debris filter according to the present invention, the first plates are fixed to each bent plate at a first regular interval, and the second plates are fixed to each bent plate at a second regular interval, wherein the second regular interval is larger than the first regular interval. This structure keeps caught debris inside the filter between the portion formed by the first plates and the bent plates and the portion formed by the second plates and the bent plates. Therefore when the liquid circulation is stopped or the debris filter is taken out of the liquid, debris that has been caught by it is hardly dropped. 
     In an embodiment of the debris filter according to the present invention, the first and second plates and/or the bent plates have slots, and the plates are joined in such a manner that the slots receive the longitudinal edges, or alternatively the slots in the first and/or second plates engage with the slots in the bent plates, or the slots in the first and/or second plates and those in the bent plates receive each other&#39;s plates. This structure makes it possible to reduce the distance between the entrance of coolant and the exit thereof, compared with debris filters equipped with plates having no slots. Employing this structure, the present invention provides a debris filter that requires a smaller space to be installed. 
     In an embodiment of the debris filter according to the present invention, the number of first inclined slots in each first plate or that of second inclined slots in each second plate is the same as the number of bent plates, the number of first perpendicular slots in each bent plate is the same as the number of first plates, and the number of second perpendicular slots in each bent plate is the same as the number of second plates. This numerical feature liberates assemblers from a labor of checking which plate should be inserted into the slots formed in which plate. Embodying this numerical feature, the present invention provides a debris filter that requires fewer steps for producing it. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing an example of the debris filter according to the present invention. 
         FIG. 2  is a front view of the debris filter shown in  FIG. 1 . 
         FIG. 3  is a partially enlarged perspective view of the debris filter shown in  FIG. 1 . 
         FIG. 4  is a plan view showing another example of the debris filter according to the present invention. 
         FIG. 5(   a ) shows an aspect viewed from the side of the first plates of the debris filter shown in  FIG. 1 .  FIG. 5(   b ) shows an aspect viewed from the side of the second plates of the same debris filter. 
         FIG. 6  shows an aspect viewed from the side of the first plates of another example of the debris filter according to the present invention. 
         FIG. 7  is a plan view showing an embodiment of the first plates of a debris filter according to the present invention. 
         FIG. 8  is a plan view showing an embodiment of the bent plates of a debris filter according to the present invention. 
         FIG. 9  is a partially enlarged perspective view of still another example of the debris filter according to the present invention. 
         FIG. 10(   a ) shows an aspect viewed from the side of the first plates of the debris filter shown in  FIG. 9 .  FIG. 10(   b ) shows an aspect viewed from the side of the second plates of the same debris filter. 
         FIG. 11  is a plan view showing another embodiment of the first plates of a debris filter according to the present invention. 
         FIG. 12  is a plan view showing still another embodiment of the bent plates of a debris filter according to the present invention. 
         FIG. 13  is a plan view showing still another example of the debris filter according to the present invention. 
         FIG. 14  is a plan view showing a state in which pieces of debris are captured in a debris filter according to the present invention. 
     
    
    
     BEST MODE TO CARRY OUT THE INVENTION 
     We will explain an example of the debris filter according to the present invention hereinafter, referring to the figures. In  FIG. 1-3 , the example of the debris filter is viewed from the different directions.  FIG. 1  shows an aspect of the debris filter in a plane including the longitudinal face of a first plate which will be explained hereinafter, and that of the second plate corresponding to said first plate.  FIG. 2  shows an aspect of the debris filter in a plane including the longitudinal edge of a bent plate, which will be explained hereinafter.  FIG. 3  is a perspective view of the debris filter. The debris filter, which may be disposed, for example, in a rectangular part inside the lower tie plate for fuel assemblies or may be disposed at a coolant inlet in the side wall of the fuel support in place of the inlet orifice for the reactor core, is capable of capturing debris included in liquid such as coolant circulating through the fuel assemblies. 
     The debris filter  1  shown in  FIG. 1  has several bent plates  2  having a same bent shape, several first plates  3  having a same first shape, and several second plates  4  having a same second shape. 
     Each of the first plates  3  is in the shape of a strip, and each of the second plates  4  in the shape of a strip. Each of the bent plates  2  is composed of a first inclined plate  5  in the shape of a strip, and a second inclined plate  7  in the shape of a strip, wherein the first inclined plate  5  shares a longitudinal edge  6  with the second inclined plate  7 . 
     The size of each part of the first inclined plate  5 , that of each part of the second inclined plate  7 , and the angle between the first inclined plate  5  and the second inclined plate  7  may be decided suitably, depending on relevant factors such as the ability to capture debris and the pressure drop in the coolant. Furthermore, the measurement of the shorter edge of each first plate  3 , that of the longer edge thereof, and the thickness thereof are respectively the same as the measurement of the shorter edge of each second plate  4 , that of the longer edge thereof, and the thickness thereof. Therefore the production of a single type of plates in the shape of a strip will provide first plates  3  and second plates  4 , which reduce the number of steps of a process for producing a debris filter  1  of this example. It must, however, be added that the dimensions of each first plate are not necessarily the same as those of each second plate, depending on relevant factors such as the ability to capture debris and the pressure drop in the coolant. 
     The bent plates of a debris filter according to the present invention may be produced by any suitable method, as long as it produces such bent plates that are capable of achieving the objective of the present invention. Specifically, a rectangular plate having a longer edge and a shorter edge may be bent, mountain folded or valley folded, along a virtual longitudinal segment between the midpoint of one shorter edge and that of the other shorter edge. Thus a bent plate having a first inclined plate and a second inclined plate is formed. The first inclined plate and the second inclined plate share the segment as a longitudinal edge. The bent plate may be produced by bending the rectangular plate in this way. Alternatively, a longer edge of a first strip-shaped plate may be joined with a longer edge of a second strip-shaped plate by a suitable method such as welding in such a manner that the two strip-shaped plates are inclined to each other. A bent plate, having a first inclined plate and a second inclined plate that share a longitudinal edge, is thus produced. The measurement of the shorter edges of the first inclined plate of a bent plate and that of the shorter edges of the second inclined plate thereof may be the same as or different from each other. 
     In the debris filter of the present invention, the bent plates are arranged so that the first inclined plates are parallel with each other and the second inclined plates are parallel with each other. 
     It is easy to arrange all the bent plates in such a manner that the first inclined plates are parallel with each other and the second inclined plates are parallel with each other, when all the first inclined plates have a same shape, all the second inclined plates have a same shape, and all the bent plates have a same angle between the first inclined plate and the second inclined plate. 
     When all the first inclined plates have a same first shape and all the second inclined plates a same second shape, and the first inclined plate of each bent plate and the second inclined plate of the same bent plate are different from each other in size, the bent plates have to be arranged in such a manner that the first inclined plates of a same first size are parallel with each other or the second inclined plates of a same second size are parallel with each other. 
     To the bent plates  2 , whose the first inclined plates  5  are arranged parallel with each other and whose second plates  7  are arranged parallel with each other, the first plates  3  are fixed so that a first longitudinal edge  8  of each bent plate, the opposite edge of the longitudinal edge  6  of each first inclined plate  5 , crosses the first plates  3 . Also to the bent plates  2  are fixed the second plates  4 , so that a second longitudinal edge  10  of each bent plate, the opposite edge of the longitudinal edge  6  of each second inclined plate  7 , crosses the second plates  4 . 
     The first inclined plates  5  and the first plates  3  are assembled into the debris filter  1  of this embodiment so that the first inclined plates and the first plates cross each other. The bent plates  2  and the first plates  3  are fixed to each other in such a manner the first longitudinal edge  8  of each bent plate  2  intersects a first inner longitudinal edge  23  of each first plate  3 , wherein the first inner longitudinal edge  23  is the longer edge on the side of each first plate opposite the side near the second plates  4 . The second inclined plates  7  and the second plates  4  are assembled so that that the second inclined plates and the second plates cross each other. The bent plates  2  and the second plates  4  are fixed to each other in such a manner that the second longitudinal edge  10  of each bent plate  2  intersects a second inner longitudinal edge  24  of each second plate  4 , wherein the second inner longitudinal edge  24  is the longer edge on the side of each second plate opposite the side near the first plates  3 . 
     The intersection of the first longitudinal edge of each bent plate and each first plate suffices for a requirement of the present invention. As shown in  FIG. 4 , for example, bent plates  102  and first plates  103  are fixed to each other in such a manner that the first longitudinal edge  108  of each bent plate  102  crosses a first inner longitudinal edge  109  of each first plate  103 , wherein the first inner longitudinal edge  109  is the longer edge on the side of each first plate near the second plates  4 . In the same way, bent plates  102  and second plates  104  are fixed to each other in such a manner that the second longitudinal edge  110  of each bent plate  102  crosses a second inner longitudinal edge  112  of each second plate  104 , wherein the second inner longitudinal edge  112  is the longer edge on the side of each second plate near the first plates  3 . 
       FIG. 5(   a ) shows the debris filter  1  viewed from the side of the first plates  3 , whereas  FIG. 5(   b ) shows the debris filter  1  viewed from the side of the second plates  4 . As shown in  FIG. 5(   a ), this example of the debris filter  1  has the bent plates  2  and the first plates  3  assembled in such a manner that the first longitudinal edge  8  of each bent plate  2  perpendicularly intersects the first plates  3 . As shown in  FIG. 5(   b ), the bent plates  2  and the second plates  4  are also arranged in such a manner that the second longitudinal edge  10  of each bent plate  2  perpendicularly intersects the second plates  4 . The distance between the adjacent first plates  3  fixed in each bent plate  2  is the same as the distance between the adjacent second plates  4  fixed in each bent plate  2 . 
     The intersection of the first longitudinal edge of each bent plate and each first plate suffices for a requirement of this invention. As shown in  FIG. 6 , for example, bent plates  102  and first plates  103  may be arranged in such a manner that the first longitudinal edge  108  of each bent plate  102  obliquely crosses each first plate  103 . In the same way, although not shown in the figures, bent plates  102  and second plates  104  may be arranged in such a manner that the second longitudinal edge  110  of each bent plate obliquely crosses each second plate  104 . The selection between the examples of the debris filter in light of how the first longitudinal edge of each bent plate intersects the first plates and how the second longitudinal edge of each bent plate intersects the second plates, or the selection between the debris filter  1  and the debris filter  101 , should be made depending on the shape and dimensions of pieces of debris included in the coolant circulating through the fuel assemblies for which the debris filter is installed. 
     As shown in  FIGS. 1-3 , a first inner longitudinal edge  9  of each first plate  3  does not touch a second inner longitudinal edge  12  of each second plate  4 . In other words, the first plates and second plates are arranged with a space in between. When coolant flows from bottom to top in  FIG. 1 , or in the debris filter of this arrangement, a coolant inlet flow path formed by adjacent bent plates  2  and adjacent second plates  4 , and a coolant outlet flow path formed by adjacent bent plates  2  and adjacent first plates  3  have a smaller sectional area than a bent flow path formed by adjacent first inclined plates  5  and adjacent second inclined plates  7 . Therefore a debris filter  1  has portions with flow paths for coolant&#39;s passing through the debris filter  1  each of which has a smaller sectional area, and a portion with coolant flow paths each of which has a larger sectional area. 
     The flow path with a smaller sectional area through which coolant passes when the coolant flows into the debris filter  1 , or the flow path formed by adjacent second plates  4  and adjacent second inclined plates  7  of the bent plates  2  may be hereinafter called an inlet flow path  13 . The flow path with a larger sectional area, or the flow path formed only by the inclined bent plates  2  without the aid of the first plates  3  or second plates  4 , may be hereinafter called a bent flow path  14 . The flow path with a smaller sectional area through which coolant passes when the coolant flows out of the debris filter  1 , or the flow path formed by adjacent first plates  3  and adjacent second inclined plates  5  of the bent plates  2  may be hereinafter called an outlet flow path  15 . 
     The action of coolant when it passes through the debris filter  1  will be explained in the following. Coolant flows through inlet flow paths  13 , bent flow paths  14  and outlet flow paths  15  in this order. Prior to flowing into the debris filter  1 , coolant normally flows through a flow path whose sectional area is larger than that of each inlet flow path  13 , that of each bent flow path  14 , and that of each outlet flow path  15 . Therefore when coolant flows into the inlet flow paths  13 , the inlet portion has a large pressure loss. However, when coolant flows further into the bent flow paths  14  whose sectional area is smaller than that of the flow path outside the debris filter  1  yet larger than that of the inlet flow paths  13 , the pressure loss in the bent flow paths becomes much smaller than that in the inlet flow paths  13 . Coolant is not allowed to flow through the bent flow paths  14  completely freely but the flow thereof is rectified by the bent plates  2 . As a result of the rectification, coolant flows smoothly in the zone of the bent flow paths  14 , from the end of the inlet flow paths  13  to the entrance of the outlet flow paths  15 . Since coolant has been rectified in the bent flow paths  14 , the pressure loss in the outlet flow paths  15  does not become large when the coolant flows into the outlet flow paths  15  from the bent flow paths  14 . Finally, the coolant flows out of the debris filter  1  through the outlet flow paths  15 , with the pressure loss in the outlet flow paths kept small. As a result, compared with conventional debris filters that inevitably have a large pressure loss because of flow paths with a small sectional area through which coolant flows, the debris filter of the present invention  1  has a much reduced pressure loss and allows coolant to flow through it with the pressure loss kept small, due to the bent flow paths  14  formed by the bent plates  2 . 
     Furthermore the debris filter  1  has two opportunities to catch debris because it has flow paths with a small sectional area at two portions thereof: the inlet flow paths  13  and the outlet flow paths  15 . Therefore the debris filter  1  is capable of capturing debris more reliably than conventional debris filters that have flow paths with a small sectional area at one portion thereof. 
     The bent plates, the first plates, and the second plates of the debris filter according to the present invention should be made of materials that are not chemically changed by coolant, in particular, not dissolved in coolant, or not seriously damaged by debris. Examples of the materials may include metals such as stainless steel and Inconel. 
     The members of the debris filter according to the present invention may be fixed to each other in the following way: Slots are formed in the first and second plates and/or the bent plates, and then plates are inserted into the slots or the slots are engaged with the slots. The debris filter  1 , an example, shown in  FIGS. 1-3 , has slots in all of the bent plates, the first plates, and the second plates. The bent plates are combined with the first and second plates in such a way that the slots of the bent plates engage with the slots of the first and second plates, or the slots of the bent plates receive the deeper parts of the first and second plates than the depth of the inclined slots. 
     The bent plates  2  should be fixed to the first plates  3  and the second plates  4  in such a manner that the fixed bent plates, first plates, and second plates are not separated by the pressure given by coolant flowing through the fuel assemblies. For example, the connected parts may be welded to be fixed. 
     In the debris filter  101 , an example, shown in  FIG. 4 , the first longitudinal edge  108  of each bent plate  102  contacts the first inner longitudinal edge  109  of each first plate  103 , and the second longitudinal edge  110  of each bent plate  102  contacts the second inner longitudinal edge  112  of each second plate  104 . A method of fixing the bent plates  102  to the first inner longitudinal edge  109  of each first plate  103  and fixing the bent plates  102  to the second inner longitudinal edge  112  of each second plate  104  may be welding of the contact points between the edges. 
     We will explain hereinafter embodiments where slots are formed in all of the members.  FIG. 7  shows one of the first plates  3  of the debris filter  1  shown in  FIGS. 1-3 . 
     The first plate  3  has a plurality of first inclined slots  16  formed in the first inner longitudinal edge  9 , which runs along the longer side of the first plate  3 . The first slots are inclined in relation to the first inner longitudinal edge. The first plates  3  of the debris filter  1  are the same as the second plates  4  thereof in dimensions; a first plate  3  formed as shown in  FIG. 7  may be used also as a second plate  4 . When first plates  3  and second plates  4  are produced so that the former have different dimensions from the latter, a plurality of second inclined slots should be formed in the second inner longitudinal edge  12  of each second plate  4  in such a manner that the second slots are inclined in relation to the longer side of the second plate, which are not shown in  FIG. 7 . 
     When slots are formed only in the first plates  3  and the second plates  4 , the first longitudinal edge  8  of each bent plate  2  is inserted into one of the first inclined slots  16  of each first plate  3 . Each bent plate is fixed to each first plate  3  by friction force between them, and outlet flow paths  15  are thus formed. Also, the second longitudinal edge  10  of each bent plate  2  is inserted into one of the second inclined slots of each second plate  4  and fixed to it, whereby inlet flow paths  13  are formed. 
     This first way of fixing, in which first inclined slots  16  and second inclined slots are formed, and the first longitudinal edge  8  and the second longitudinal edge  10  of each bent plate  2  are respectively inserted into and fixed to the first inclined slots  16  and the second inclined slots, provides a small-size debris filter, compared with the way of fixing in which the edges having no slots are simply contacted with and fixed to each other. It is because the former way of fixing is capable of shortening the length from the entrance of the inlet flow paths to the exit of the outlet flow paths. Therefore debris filters made by this first way of fixing may be fitted to smaller installing areas. 
     One of the bent plates  2  of the debris filter  1  in  FIGS. 1-3  is shown in  FIG. 8 . 
     The bent plate  2  has a plurality of first perpendicular slots  17  formed in the first longitudinal edge  8 , the first perpendicular slots formed perpendicularly to the first longitudinal edge, and a plurality of second perpendicular slots  18  in the second longitudinal edge  10 , the second perpendicular slots formed perpendicularly to the second longitudinal edge. The number of first perpendicular slots  17  is the same as that of second perpendicular slots  18 , and the shape of each first perpendicular slot is the same as that of each second perpendicular slot. Thus, when the first plates  3  and the second plates  4  have same dimensions, just to form one type of slots in both longitudinal edges  8 ,  10  of each bent plate  2  enables the bent plate to engage with the first and second plates, which reduces the steps of the production process. 
     When slots are formed only in the bent plates  2 , the first inner longitudinal edge  9  of each first plate  3  is inserted into and fixed to one of the first perpendicular slots  17  of each bent plate  2 . Outlet flow paths  15  are thus formed. Also, the second inner longitudinal edge  12  of each second plate  4  is inserted into one of the second perpendicular slots  18  of each bent plate  2  and fixed to them. Inlet flow paths  13  are thus formed. 
     This second way of fixing, in which first perpendicular slots  17  and second perpendicular slots  18  are formed in bent plates  2 , and the first inner longitudinal edge  9  of each first plate  3  and the second inner longitudinal edge  12  of each second plate  4  are respectively inserted into and fixed to the first perpendicular slots  17  and the second perpendicular slots  18 , provides a small-size debris filter, compared with the way of fixing in which the edges having no slots are simply contacted with and fixed to each other. It is because the former way of fixing is capable of shortening the length from the entrance of the inlet flow paths to the exit of the outlet flow paths. Therefore debris filters made by this second way of fixing may be fitted to smaller installing areas. 
     Another way of forming slots in the bent plates and/or the first and second plates may be forming slots in all of the bent plates and the first and second plates. In this third way, the bent plates and the first plates are combined with and fixed to each other in such a manner that the first perpendicular slots engage with the first inclined slots, and the bent plates and the second plates are combined with and fixed to each other in such a manner that the second perpendicular slots engage with the second inclined slots. 
     This third way of fixing, as well as the first and second ways, is capable of shortening the length from the entrance of the inlet flow paths to the exit of the outlet flow paths, compared with the way of fixing where the edges having no slots are simply contacted with and fixed to each other. Therefore this third way of fixing provides a small-size debris filter that may be fitted to smaller installing areas. 
     When all of the first and second plates and the bent plates have slots, preferable should be the embodiment where the number of first perpendicular slots in each bent plate is the same as the number of first plates, the number of second perpendicular slots in each bent plate is the same as the number of second plates, and the number of bent plates is the same as the number of first inclined slots in each first plate and/or that of second inclined slots in each second plate. This embodiment, since it enables manufacturers to form as many slots as the corresponding plates, makes it possible to make first plates and second plates both in a same size at one production and to simplify the process of producing a debris filter. 
     Another example of the debris filter according to the present invention is shown in  FIG. 9 . The debris filter  201  shown in  FIG. 9  is different from the debris filter  1  shown in  FIGS. 1-3  in the arrangement of the first and second plates. Specifically, as shown in  FIG. 9 , the distance between adjacent second plates  204 , fixed to bent plates  202 , is larger than that between adjacent first plates  203 , also fixed to the bent plates  202 . On the other hand, as shown in  FIG. 3 , the distance between adjacent second plates  4 , fixed to the bent plates  2 , is the same as that between adjacent first plates  3 , also fixed to the bent plates  2 . 
       FIG. 10(   a ) shows the debris filter  201  viewed from the side of the first plates  203 , whereas  FIG. 10(   b ) shows the same debris filter viewed from the side of the second plates  204 . As shown in  FIG. 10(   a ), the bent plates  202  and the first plates  203  are arranged in such a manner that the first longitudinal edge  208  of each bent plate  202  perpendicularly intersects the first plates  203 . As shown in  FIG. 10(   b ), the bent plates  202  and the second plates  204  are also arranged in such a manner that the second longitudinal edge  210  of each bent plate  202  perpendicularly intersects the second plates  204 . 
     As mentioned hereinbefore, the distance between adjacent second plates  204  on the bent plates  202 , shown in  FIG. 10(   b ), is larger than that between adjacent first plates  203  on the same bent plates  202 , shown in  FIG. 10(   a ). For convenience of explanation, the flow path formed by adjacent second plates  204  and the second inclined plates  207  of adjacent bent plates  202  may be hereinafter called an inlet flow path  213 , the flow path formed by adjacent first plates  203  and the first inclined plates  205  of adjacent bent plates  202  may be hereinafter called an outlet flow path  215 . The area of the section of the inlet flow path  213  perpendicular to the direction of the coolant flow is larger than that of the section of the outlet flow path  215  perpendicular to the direction of the coolant flow. In other words, a second lattice formed by the second plates  204  and second longitudinal edges  210  of the bent plates  202 , shown in  FIG. 10(   b ), has a smaller reticular density than a first lattice formed by the first plates  203  and first longitudinal edges  208  of the bent plates  202 , shown in  FIG. 10(   a ), has. 
     In the debris filter of this example, the sectional area of each inlet flow path  213  is larger than that of each outlet flow path  215 . In other words, the reticular density of the first lattice comprised of the inlet flow paths  213  is smaller than that of the second lattice comprised of the outlet flow paths  215 . Therefore pieces of debris with such a size that the first plates  203  do not allow them to flow through are not caught by the second plates  204 . Such pieces of debris pass through the second plates  204 , flow into bent flow paths  214 , which are formed by the bent plates, and are finally caught by the first plates  203 . When the flow of coolant is stopped in this situation, or when the fuel assemblies with this debris filter are taken out of coolant, the pieces of debris captured within the bent flow paths  214  are prevented from being discharged from the debris filter  201 . As a result, debris that has fallen outside the debris filter  201 , for example onto the bottom of the nuclear reactor, is prevented from flowing into the fuel assemblies when circulation of the coolant is restarted. 
     The manners of fixing the bent plates  202  to the first plates  203  and the second plates  204  may be the same as the manners which have been explained with the debris filter shown in  FIGS. 1-3 . For example, slots are formed in the first and second plates  203 ,  204  and/or the bent plates  202 , and then plates are inserted into the slots or the slots are engaged with the slots. 
       FIG. 11  shows one of the first plates  203  of the debris filter  201  shown in  FIGS. 9 and 10 , and  FIG. 12  one of the bent plates  202 . The first plate  203  has a plurality of first inclined slots  216  formed in a first inner longitudinal edge  209 , which runs along the longer side of the first plate  203 . The first slots are inclined in relation to the first inner longitudinal edge. The first plates  203  are the same as the second plates  204  thereof in dimensions; a first plate  203  formed as shown in  FIG. 11  may be used also as a second plate  204 . As shown in  FIG. 12 , the bent plate  202  has a plurality of first perpendicular slots  217  formed in a first longitudinal edge  208  at first regular intervals, the first perpendicular slots formed perpendicularly to the first longitudinal edge, and a plurality of second perpendicular slots  18  in a second longitudinal edge  210  at second regular intervals, the second perpendicular slots formed perpendicularly to the second longitudinal edge. The debris filter  201  of this example has the plurality of first perpendicular slots  217  formed in each first longitudinal edge  208 , the number of which is larger than the number of second perpendicular slots  218  formed in each second longitudinal edge  210 . Thus the first plates  203  are fixed to each bent plate  202  at first regular intervals, the second plates  204  are fixed to each bent plate  202  at second regular intervals, and the second distance at which adjacent second plates  204  are fixed to each bent plate  202  is larger than the first distance at which adjacent first plates  203  are fixed to each bent plate  202 . 
     A still another example of the debris filter according to the present invention is shown in  FIG. 13 . The debris filter  301  shown in  FIG. 13  is different from the debris filter  1  shown in  FIG. 1  in the following respects: Firstly, each of the bent plates  302  shown in  FIG. 13  is comprised of a first inclined plate  305 , a second inclined plate  307 , and a third inclined plate  319 , whereas each of the bent plates  202  shown in  FIG. 9  is comprised of a first inclined plate  205  and a second inclined plate  207 . Secondly, the third inclined plates  319  shown in  FIG. 13  extend parallel with the direction of coolant flow from the inlet flow paths to the outlet flow paths of the debris filter  301 , whereas the second inclined plates  207  shown in  FIG. 9  extend obliquely to the direction of coolant flow from the inlet flow paths to the outlet flow paths of the debris filter  201 . Thirdly, the width of each second plate  304 , or the length of the shorter edge along the shorter side thereof, shown in  FIG. 13  is larger than that of each second plate  204  shown in  FIG. 9 . 
     In the debris filter  201  shown in  FIG. 9 , the first plates  203  and the second plates  204  have a same shape and same dimensions. Furthermore, the number of first inclined slots  216  formed in each first inner longitudinal edge  209 , running along the longer side of each first plate  203 , and the depth of each first inclined slot are respectively the same as the number of second inclined slots formed in each second inner longitudinal edge  212 , running along the longer side of each second plate  204 , and the depth of each second inclined slot. 
     On the other hand, in the debris filter  301  shown in  FIG. 13 , the width of each second plate  304  is smaller than the total of the width, or the length of the shorter edge along the shorter side, of each third inclined plate  319  and the width, or the length along the shorter side, of each second inclined plate  307 , and is larger than the width of each third inclined plate  319 . In other general embodiments, the width of each second plate  304 , or the length of shorter edge thereof, may be equal with or shorter than the width of the third inclined plate  319 . 
     Liquid such as coolant is allowed to flow into the debris filter  301  shown in  FIG. 13  from the free end of the third inclined plates  319 . The liquid is introduced into divided spaces formed by adjacent third inclined plates  319  and adjacent second plates  304 , and rectified in the divided spaces. It is introduced into and further rectified in the spaces formed by adjacent second inclined plates  307  and adjacent second plates  304 . Then, the liquid flows into inner spaces between first inner longitudinal edges  309  of the aligned first plates  303  and inner longitudinal edges  312  of the second plates  304 , the spaces formed by the aligned bent plates  302 . The liquid is subsequently introduced into divided flow paths formed by adjacent first inclined plates  305  and adjacent first plates  303 , and is finally allowed to flow out of the debris filter  301 . 
     We will explain liquid flow in the debris filter in more detail, referring to  FIG. 14 . A piece of debris  320  in the shape of a needle or a piece of wire included in liquid, such as coolant, flows into a space formed by the third inclined plates  319  and the second plates  304 , wherein the piece of debris is in a position generally perpendicularly to the second plates  304  and the bent plate  302 . Because the third inclined plates  319  run from bottom to top in the sheet including  FIG. 13 , the piece of debris in the shape of a needle, together with the liquid, tends to easily come into the debris filter  301 . Then, the piece of debris in the shape of a needle flows into an obliquely arranged flow path formed by adjacent second inclined plates  307  and adjacent second plates  304 , which obliquely arranged flow path communicates with the flow path formed by adjacent bent plates  302 , located between the inner longitudinal edges  309  of the first plates  303  and the inner longitudinal edges  312  of the second plates  304 . Until this point of the travel the piece of debris in the shape of a needle has become inclined  321  in relation to the direction of liquid flow. This position is caused by turbulent flow created by complicated flow paths in the debris filter  301 . The inclined piece of debris becomes further inclined  322  so that it also turns oblique to the longitudinal side of the bent plates  302 . The piece of debris in the position  321  or  322  is not able to pass through the outlet flow path  315  formed by adjacent first plates  303  and first bent plates  302 , because the sectional area of each outlet flow path  315  formed by adjacent first plates  303  and adjacent bent plates  302  is smaller than that of each inlet flow path  314  formed by adjacent second plates  304  and adjacent bent plates  302 . 
     When the flow of coolant passing through the debris filter is stopped in this situation, or when this debris filter is taken out of the liquid, it is difficult for such pieces of debris caught within the bent flow paths formed by the bent plates  302  between the inner longitudinal edges  309  of the first plates and the inner longitudinal edges  312  of the second plates  304 , and for those caught within the flow paths formed by the second plates  304  and the bent plates  302  to leave the debris filter  301  and to flow away to the outside. 
     The manners of fixing the bent plates  302  to the first plates  303  and the second plates  304  may be the same as the manners which have been explained with the debris filter shown in  FIGS. 1-3 . For example, slots are formed in the first and second plates  303 ,  304  and/or the bent plates  302 , and then plates are inserted into the slots or the slots are engaged with the slots. 
     EXPLANATION OF REFERENCE SIGNS 
     
         
           1 ,  101 ,  201 ,  301 : debris filter 
           2 ,  102 ,  202 ,  302 : bent plate 
           3 ,  103 ,  203 ,  303 : first plate 
           4 ,  104 ,  204 ,  304 : second plate 
           5 ,  105 ,  205 ,  305 : first inclined plate 
           6 ,  106 ,  206 ,  306 ,  9 ,  109 ,  209 ,  309 ,  12 ,  112 ,  212 ,  312 ,  23 ,  123 ,  24 ,  124 : longitudinal edge 
           7 ,  107 ,  207 ,  307 : second inclined plate 
           8 ,  108 ,  208 ,  308 : first longitudinal edge 
           10 ,  110 ,  210 ,  310 : second longitudinal edge 
           13 ,  113 ,  213 ,  313 : inlet flow path 
           14 ,  114 ,  214 ,  314 : bent flow path 
           15 ,  115 ,  215 ,  315 : outlet flow path 
           16 ,  216 : first inclined slot 
           17 ,  217 : first perpendicular slot 
           18 ,  218 : second perpendicular slot 
           319 : third inclined plate 
           320 : a piece of debris in the shape of a needle 
           321 : the piece of debris in the shape of a needle inclined in relation to the direction of liquid flow 
           322 : the inclined piece of debris in the shape of a needle also turning oblique to the longitudinal side of the bent plates