Abstract:
A strainer for an emergency core cooling system (ECCS) in a nuclear power plant comprises a perforated strainer element that is immersed in a reservoir of cooling water, which is drawn through the strainer element into the emergency core cooling system. The side of the strainer element in contact with the cooling water has a contoured configuration for disrupting the formation of a flat bed of fibrous material that can trap small particulate material intended to pass through the strainer element. Incorporating this strainer element into an ECCS strainer enables the strainer to be made more compact, because the debris bed need not be spread over an unduly large area to prevent excessive head loss from the debris load in the event of a reactor loss of coolant accident. The strainer also incorporates a modular construction that uses individual strainer disc modules. Each disc module includes a perforated first disc part having a central opening and a perforated second disc part also having a central opening. The first and second disc parts fit together to form an interior space with facing perforated major surfaces and an axial opening, and connecting tubes between the discs place the axial openings in fluid communication. The entire assembly is secured together by tie rods that hold the discs together with the connecting tubes compressed between them.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 60/570,802, filed May 14, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a suction strainer to remove entrained solids from the cooling liquid in a nuclear reactor, and more particularly, to a suction strainer of modular construction with a contoured surface configuration that reduces head loss across the strainer in the presence of liquids with entrained debris. 
     2. Description of Related Art 
     A nuclear power plant typically includes an emergency core cooling system (ECCS) that circulates large quantities of cooling water to critical reactor areas in the event of accidents. A boiling water reactor (BWR) commonly draws water from one or more reservoirs, known as suppression pools, in the event of a loss of coolant accident (LOCA). Water is pumped from the suppression pool to the reactor core and then circulated back to the suppression pool. A LOCA can involve failure of reactor components that introduce large quantities of solid matter into the cooling water, which entrains the solids and carries them back to the suppression pool. For example, if a LOCA results from the rupture of a high pressure pipe, quantities of thermal insulation, concrete, paint chips and other debris can be entrained in the cooling water. A pressurized water reactor (PWR) after a LOCA typically draws cooling water from a reactor water storage tank (RWST), and then after a signal, shuts off the flow from the RWST and recirculates this water through the reactor. That is, a PWR has a containment area that is dry until it is flooded by the occurrence of an accident, and the ECCS uses pump connected to a sump in the containment area to circulate the water through the reactor. Nevertheless, the water that is pumped in the event of a PWR accident will also usually contain entrained solids that typically includes insulation, paint chips, and particulates. In other words, in both types of reactor, cooling water is drawn from a reservoir and pumped to the reactor core, and entrained solids can impair cooling and damage the ECCS pumps if permitted to circulate with the water. 
     As a result, strainers are typically placed in the coolant flow path upstream of the pumps, usually by immersing them in the cooling water reservoir. It is critical that these strainers be able to remove unacceptably large solids without unduly retarding the flow of coolant; in other words, the pressure (head) loss across the strainer must be kept to a minimum. Strainers are commonly mounted to pipes that are part of the ECCS and that extend into the suppression pool (BWR) or sump (PWR), and the ECCS pumps draw water through the strainers and introduce it to the reactor core. There has been considerable effort expended on the design of strainers to decrease head loss across the strainer for the design coolant flow. These strainers typically include a series of stacked perforated hollow discs (BWRs) or flat perforated plates (PWRs) and a central core through which water is drawn by the ECCS pump. The perforated discs prevent debris larger than a given size from passing the strainer perforations and reaching the pumps. An example of a particularly effective strainer design is the present inventors&#39; U.S. Pat. No. 5,759,399, which is assigned to the same assignee as the present invention and is incorporated herein by reference (as discussed in detail below). 
     Large amounts of fibrous material can enter the circulating coolant water in the event of a reactor accident. This fibrous material, which originates with reactor pipe or component insulation that is damaged and enters the ECCS coolant stream in the event of a LOCA, accumulates on the strainer surfaces and captures fine particulate matter in the flow. The resulting fibrous debris bed on the strainer surfaces can quickly block the flow through the strainer, even though the trapped particulates may be small enough to pass through the strainer perforations. Heretofore, this flow blockage effect has been addressed by making the strainer larger, the goal being to distribute the trapped debris over more area, reduce the velocity through the debris bed, and thus reduce the head loss across the strainer as a whole. This is, however, an imperfect solution, both because the available space in a reactor for suction strainers is usually limited, and because larger strainers are more costly. Accordingly, it is possible that the expected debris load after a LOCA can dictate strainers that are too large for the space allotted for them in the containment area. Moreover, larger strainers are more difficult to work with and thus more costly to install. 
     Prior art ECCS strainers have also been constructed in ways that can make them somewhat expensive to fabricate. As a result of all of the above factors, it has proven difficult to reduce the costs of strainers for a nuclear power plant ECCS to any meaningful degree and to provide strainers that fit within the space constraints. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an ECCS strainer that overcomes the above disadvantages of the prior art. 
     In accordance with a first aspect of the invention, a strainer for an emergency core cooling system for a nuclear power plant comprises a strainer element for immersion in a reservoir of cooling water utilized by the emergency core cooling system, the strainer element having at least one major surface with perforations therethrough; and a suction opening operatively connected to an internal side of the major surface for drawing cooling water into the emergency core cooling system through the perforations from an external side of the major surface in contact with the cooling water, wherein at least a portion of the external side of the major surface includes a contoured configuration for disrupting the formation of a flat bed of material that can trap particulate material small enough to pass through the perforations. 
     More specifically, the contoured configuration according to this aspect of the invention can assume a variety of embodiments. It can comprise a wire mesh cloth covering the strainer element. It can also comprise a plurality of protrusions, which may be substantially uniform and raised above the external side of the major surface in a regular repeating pattern and may also be substantially hemispherical in cross section. The contoured configuration can further comprise a plurality of substantially uniform depressions alternating in the pattern with the protrusions, which depressions are substantially hemispherical. Yet another embodiment of the contoured configuration comprises a plurality of substantially similar corrugations in the strainer element. 
     In accordance with another aspect of the invention, a modular strainer comprises a plurality of hollow strainer discs disposed in a stack along an axis with a major surface of one disc facing a major surface of an adjacent disc, each disc including a first disc part with perforations therethrough and having an opening and a second disc part with perforations therethrough and having an opening, wherein the first and second disc parts fit together to form an interior space with facing perforated major surfaces and an opening formed at the axis by the openings in the disc parts, a plurality of connecting tubes placing the openings of the discs in fluid communication, at least one structural member securing the strainer discs together and holding the connecting tubes in place between the strainer discs, and a pipe in fluid communication with the opening of the strainer disc at one end of the stack. In one embodiment of the invention, the axis is centrally located in the discs. 
     In a modular strainer in accordance with this aspect of the invention, each strainer disc can include a core flow regulator extending between the first and second disc parts for regulating fluid flow from the interior space to an axis of the strainer. In addition, the core flow regulator may comprise a tubular core boss secured in the openings of the disc parts and having at least one aperture placing the interior space in fluid communication with a strainer core formed by the tubular core bosses and connecting tubes. In a particularly advantageous arrangement, the aperture is smaller in the core boss in strainer discs further from the one end of the stack than in an adjacent strainer disc closer to the one end of the stack. The axis can also be located generally in the center of the discs. 
     Moreover, a modular strainer in accordance with this aspect of the invention may further incorporate the first aspect of the invention, wherein the facing major surfaces of adjacent discs essentially comprise strainer elements with contoured configurations as discussed above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects of the invention will be better understood from the detailed description of its preferred embodiments which follows below, when taken in conjunction with the accompanying drawings, in which like numerals refer to like features throughout. The following is a brief identification of the drawing figures used in the accompanying detailed description. 
         FIG. 1  is a schematic side view of a completed modular ECCS strainer having a contoured surface in accordance with an embodiment of the invention that reduces head loss across the strainer in the presence of fibrous and particulate materials in the ECCS flow. 
         FIG. 2  is an exploded side view of a portion of an ECCS strainer such as that shown in  FIG. 1 , constructed as a modular unit in accordance with another embodiment of the invention. 
         FIG. 3  is a schematic plan view of a portion of a strainer with a contoured surface in accordance with another preferred embodiment of the invention. 
         FIG. 4  depicts a section taken at line  4 - 4  of the strainer in  FIG. 3 . 
         FIG. 5  is a cross section of a contoured surface in accordance with yet another embodiment of the invention. 
         FIG. 6  is a exploded perspective view of a two-part modular strainer disc according to another aspect of the invention. 
         FIG. 7  is a perspective view of part of a pressure water reactor containment area with strainers according to the present invention. 
         FIG. 8  is a schematic illustration of a test set up used to perform tests on a working example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  schematically illustrate a stacked-disc ECCS strainer  10  in accordance with a preferred embodiment of the invention. The strainer shown in  FIG. 1  includes a series of hollow discs  12   a ,  12   b ,  12   c ,  12   d ,  12   e ,  12   f ,  12   g  and  12   h  with their major surfaces in mutually facing relationship. The disc configuration and construction is described in detail below in connection with  FIG. 3 , but suffice it to say here that each disc  12  has central opening that is in placed in fluid communication with the central opening of an adjacent disc through one of a plurality of connecting tubes  14   a ,  14   b ,  14   c ,  14   d ,  14   e ,  14   f  and  14   g . The discs  12  and the tubes  14  have perforated walls in accordance with conventional strainer design. That is, the disc and connecting tube surfaces are perforated with openings large enough to permit the passage of fluid but small enough to strain particulate material with diameters less than the design specification of the strainer. Typically, the perforations are on the order of ⅛ in. across, although in some instances they can be as small as 0.04 to 0.08 in., and are circular for ease of fabrication. While the construction of the strainer  10  is unique, the general principles underlying its operation are the same as those set forth in the discussion regarding the general operation and application of nuclear reactor suction strainers in the aforementioned U.S. Pat. No. 5,759,399, which discussion is incorporated by reference herein as if set out in full. 
       FIG. 2  illustrates in more detail the modular nature of the design of the strainer  10  of this embodiment of the invention. In that regard, each disc  12  is essentially self-contained and has a tubular core boss  16  extending through the disc. As noted above, the plural hollow discs  12  are assembled with the external sides of their major surfaces  18  in mutually facing relationship. The ends  20  of each core boss  16  protrude from the opposite major faces of each disc  12 . The connecting tube  14  between each pair of discs  12  fits snugly over the protruding ends  20  of the facing core bosses in the adjacent discs. When the desired number of discs  12  are assembled with the connecting tubes  14  therebetween, the assembly is held together by fasteners  22  at the ends of tie rods  24  that extend through holes in the discs  12  at their corners (see  FIG. 6 ). The present embodiment contemplates using tie rods that are threaded at their ends and, as the fasteners  22 , nuts that are screwed down tightly against the outside surfaces of the end discs  12   a  and  12   h  to hold all of the discs  12  and connecting tubes  14  in place by compression. It will be appreciated that spacers between the discs  12  may be necessary to support the compressive forces exerted by the fasteners. These spacers may take any form, but conveniently they are tubes that fit over the tie rods between the discs  12  to positively establish a predetermined inter-disc spacing and prevent the discs from deforming under the compressive forces applied by the fasteners  22 . The assembly can also include optional stiffener beams  26 , shown in phantom in  FIG. 1 , to impart further rigidity to the assembly. The stiffener beams will be typically welded to the discs&#39; peripheral surfaces, but other means of fastening can be used in accordance with the requirements of a particular application. Thus, while the strainer discs  12  and connecting tubes  14  are by themselves not sufficiently strong to support the strainer, the entire unit, when secured together with the structural tie rods  24  and the optional stiffener beams  26 , is structurally self-supporting. 
     The strainer  10  further includes a suction pipe  28  welded to the protruding end of the core boss (not shown) extending through the disc  12   h . In use, a flange  30  on the pipe  28  attaches to a cooperating flange on a pipe (not shown) leading to an ECCS pump, as described in U.S. Pat. No. 5,759,399. That is, the pipe  28  serves as a suction opening operatively connected to the interior of the immersed hollow strainer discs to draw cooling water external to the discs through the strainer perforations and into the ECCS. The other end of the strainer  10 , formed by the outside major surface  12   a   1  of the end disc  12   a , is closed; that is, the core boss for that disc does not extend through the disc surface  12   a   1 . In another configuration, the core boss extending through the surface  12   a   1  can be closed by a plate (not shown) perforated with holes having the same size and pattern as the perforations in the remainder of the surface of the disc  12   a.    
     In addition to the modular construction of the strainer  10 , another feature that comprises an important aspect of the present invention is the contoured configuration of the major surfaces  18  of the discs  12 . As explained in U.S. Pat. No. 5,759,399, a design issue for nuclear reactor strainers is maintaining proper fluid flow through the strainer in the presence of large amounts of debris. That patent proposes tailoring the strainer dimensions in a way that evens out the distribution of the debris over the surface area of the strainer, thereby maximizing strainer capacity. That approach has proved very effective in improving strainer performance, but the present invention can provide similar performance with a smaller strainer, thus utilizing available reactor space more efficiently. 
     To repeat a point made earlier, a nuclear reactor strainer must filter debris that can cause a very high head loss when captured by the strainer. The debris entrained in the ECCS flow typically contains large amounts of fibrous debris (from insulation destroyed in the course of the reactor accident) and also a range of sizes of particulates from other sources (paint chips, other insulation, oxide particles, etc.). The strainer perforations are sized to prohibit the passage of large material, although sufficiently small particulate material may pass through the strainer. However, fibrous debris beds trap particulates that would otherwise pass through the strainer. For some debris compositions, particles that are small enough to pass through the strainer perforations can be trapped by a thin layer of the fibrous debris that forms on the strainer surfaces. That is, the strainer becomes effectively coated with a thin layer of fibers that trap particulates, and the fiber with the trapped particulates prevents water from passing through this debris bed. This thin fibrous/particulate debris bed has a very high head loss (that is, it is very resistant to flow through it), since the characteristic area of the openings in the debris bed become extremely small. Known ECCS strainers all use flat perforated plates as strainer elements, as shown in prior art such as U.S. Pat. No. 5,696,801 and U.S. Pat. No. 5,935,439. 
     The strainer  10  incorporates a feature to alleviate that problem. As seen in  FIG. 1 , the major surface of each strainer disc  12  in the strainer  10  is contoured. The purpose of this contoured surface is to prevent fibrous debris from laying flat on the strainer surfaces, thereby to disrupt the formation of a flat bed of fibrous material that can trap small particulates and reduce the resulting severe head loss in flat plate strainers.  FIG. 1  illustrates a preferred manner of contouring the strainer surfaces. In this embodiment a screen mesh  32  is attached to the major surface of each disc  12 . The screen mesh  32  is a woven wire mesh cloth similar in construction to a window screen (although the screen mesh used with a strainer in a nuclear reactor is typically made of wire thicker than that used in a window screen with a coarser weave). The wire mesh cloth  32  is attached to the strainer discs  12  using suitable fasteners, such as rivets (not shown), at locations spaced around the disc surfaces. The wire mesh cloth can also be spot welded as needed to provide a secure attachment to the strainer disc surfaces. In another embodiment the wire mesh cloth can be bent around the discs, and/or spaced from the disc surface by spacers distributed on the disc surface. A possible configuration in that regard would hold the wire mesh cloth about 0.25 in. from the disc surface. 
     The exact dimensions of the screen mesh  32  for a particular application can be determined empirically depending on the type of debris expected to be encountered in the reactor installation incorporating the strainer  10 . The general dimensions of a wire mesh cloth in accordance with this aspect of the invention are suggested by head loss tests run with a wire mesh cloth made from 0.135 in. diameter wire having mesh openings ¼ in. square and a wire mesh cloth with 0.120 in. diameter wire with mesh openings ⅜ in. square. These particular tests revealed that debris beds on the order of 10 mm thick, or five times thicker than the debris beds for a strainer with flat perforated strainer plates, still permitted water to pass easily through the plates. In the tests referred to here, wire mesh cloth having thicker wire and smaller mesh openings tended to exhibit less head loss than wire mesh cloth with larger openings. Those skilled in the art will appreciate that the wire mesh cloth configuration for optimum performance with a particular type of debris composition (that is, types and relative amounts of different debris components) may be determined by testing using simulated operating conditions and debris compositions expected to be encountered in actual operation. An exemplary protocol for such testing is discussed below in connection with a specific working example of the invention. (Those tests are slightly different from the tests referred to just above, in which the pressure across an sample strainer element was provided by a 16 ft. high column of water in a 6 in. diameter round pipe.) 
       FIGS. 3 and 4  illustrate a contoured strainer surface according to an alternate embodiment of the invention.  FIG. 3  depicts in plan view a portion of a strainer surface  12 ′, such as a major surface of one of the discs  12  of the strainer  10 . Being drawn to a larger scale than  FIGS. 1 and 2 ,  FIG. 3  shows more clearly the perforations  40  in the strainer surface. (It will be understood that the surfaces of the discs  12  and the connecting tubes  14  are both perforated in this same manner, as suggested more schematically in  FIGS. 1 and 2 .) The surface treatment in  FIG. 3  comprises a plurality of protrusions  42  that project from the surface of the disc against the direction of flow F through the strainer. The protrusions  42  are circular in planform, with a diameter D. Circular protrusions are easy to manufacture because they can be stamped with a suitable machine tool into a flat plate of an appropriate thickness t, which is typically the same dimension as the hole diameter, to form a strainer surface such as that depicted in  FIG. 3 . It is preferable that the tooling simultaneously form the perforations and the dimples, since punching the perforations and then dimpling the plate will likely deform the perforations, the diameters of which must be maintained within given tolerances. The protrusions  42  are generally hemispherical in cross section with a height A (the diameter of the hemisphere, or approximately D), with adjacent protrusions along a line being spaced at a pitch P. Those skilled in the art will appreciate that the protrusions can have other shapes and be spaced in other patterns. Ranges of typical protrusion dimensions are A=⅛ in. to ¾ in. and P=0.5 in. to 1.5 in., with preferred values for one particular debris combination that was tested being A=⅜ in. and P=1 in. However, as explained above, the exact values of A and P for a particular nuclear reactor application may be determined by appropriate testing. 
     Other modifications of this contour configuration are also possible. For example, the protrusions  42  could instead be dimples or depressions recessed into the disc surface in the direction of the flow (that is, oriented as if the flow were approaching the disc in the opposite direction from arrow F in  FIG. 4 ). In addition, the surface could be comprised of a pattern of both recessed dimples and raised protrusions.  FIG. 5  illustrates another contoured surface configuration that was tested and found to reduce the head loss across the strainer.  FIG. 5  is a cross section of a corrugated disc  12 ″ with a pitch P measured from peak to peak between corrugations. As with the embodiment depicted in  FIGS. 3 and 4 , a corrugated disc can be stamped from a flat blank using a suitable machine tool. For the reasons explained above, it is preferable to use tooling that forms the perforations and corrugations simultaneously, unless holes are punched in specific locations to account for the deformation of the holes. The height or magnitude of each corrugation is A. Tests were run on an exemplary corrugated disc in which P=½ in. and A=¼ in. As with the tests referred to above in connection with the wire mesh cloth surface treatment, the tests on this particular corrugated strainer element also showed that debris beds on the order of 10 mm thick, or five times thicker than the debris beds for a strainer with flat perforated strainer plates, permitted water to pass easily through the corrugated plates. Again, the exact corrugation configuration will have to be determined by testing under simulated operating conditions and debris compositions. 
     An aspect common to all of these contoured surface embodiments is that they prevent entrained fibrous material, that is, material comprising thread-like fibers, from laying flat on the strainer surface. While not wishing to be limited to any particular theory for why the present invention with a contoured strainer surface reduces head loss as compared to a flat surface, it is believed that preventing the fibers from laying flat on those portions of the strainer surface with perforations forces openings between the fibers to remain sufficiently large to prevent trapping of very small particulates. The surface contours of the embodiments depicted in the above figures all satisfy that criterion, in that they disrupt the formation of a flat bed of fibrous material. 
       FIG. 6  is an exploded view of one of the modular discs  12  incorporated into the strainer  10 . The disc  12  comprises a first part  50  with perforations on all of its surfaces. The first disc part  50  has a major surface  50   1  and flanges  52 ,  54 ,  56  and  58  that are integral with the major surface  50   1 . The disc  12  is stamped from a generally rectangular perforated blank with the corners cut out so that when the periphery of the blank is bent up to form the flanges  52 ,  54 ,  56  and  58 , they form seams where their edges meet. Three of the seams  62 ,  64  and  66  are shown in the figure; the fourth is hidden by a part to be described, and the seams are welded to form the body of the first disc part  50 . A tic-rod boss  68 ,  70 ,  72 ,  74  is welded in place in each corner of the disc body. The tie-rod bosses reinforce openings (not shown) in the major surface  50   1  that accept the tie rods  24  described above in connection with  FIG. 1 . The first disc part  50  also includes support spacers  76 ,  78 ,  80  and  82 . These spacers comprise small studs welded to the inside of the major surface  50   1  for a purpose described below. The core boss  16  is also welded into an opening  84  in the major surface. As described above in connection with  FIG. 2 , the core boss  16  protrudes slightly beyond the outside of the surface  50   1 . The core boss  16  includes a slot  86 , also for a purpose described below in connection with the operation of the strainer  10 . 
     The disc  12  also includes a second disc part  90 , which can be conveniently fabricated in the same manner as the first disc part  50 . In fact, the second disc part  90  is almost identical to the first disc part body. The main difference, other than omission of the bosses  68 ,  70 ,  72  and  74 , and the studs  76 ,  18 ,  80  and  82 , is that the second disc part  90  is slightly larger than the first disc part  50 , so that the flanges of the second disc part (only flanges  92  and  94  are seen in the drawing) fit snugly over the flanges  52 ,  54 ,  56  and  58  of the first part. Although the perforations in the overlapping flanges may not line up exactly, this does not impair strainer performance because the edges of the discs formed by the flanges comprise a very small percentage of the total surface area of the strainer. In fact, in am alternative construction the flanges are not perforated, since that may facilitate fabrication of the disc parts. 
     Tie-rod openings  96 ,  98 ,  100  and  102  at the corners of the major surface  90   1  of the second disc part  90  mate with the tie-rod bosses  68 ,  70 ,  72  and  74 , and the core boss  16  passes through an opening  104  in the major surface  90   1 . The edges of the flanges  52 ,  54 ,  56  and  58  bear against the inside of the surface  90   1  to establish the distance between the surfaces  50   1  and  90   1 . The ends of the studs  76 ,  78 ,  80  and  80  also bear against the inside of the surface  90   1  to maintain that distance constant when there is a pressure differential between the inside and outside of the disc during operation. The edges and studs, and the core boss  16 , are dimensioned so that when the two disc parts  50  and  90  are assembled, the proximal end of the core boss passes through the opening  104  and protrudes slightly beyond the outside of the surface  90   1 , as described above in connection with  FIG. 2 . The disc parts  50  and  90  are secured together by welding the core boss  16  to the peripheral edge of the opening  104 . It is generally unnecessary to further secure the two disc parts  50  and  90  together, because the tension placed on the through rods  24  secures the discs together at their peripheries. 
     The advantages of this modular construction will be immediately apparent. The discs  12  can readily be made in a variety of configurations to fit different applications. For example, although the major surfaces of the disc parts  50  and  90  are rectangular, they can easily be made trapezoidal to fit into the space available in the nuclear reactor where the strainer will be installed. The discs can, in fact, be any desired shape, whether polygonal or not or whether regular or not, and can be assembled into a strainer. In addition, it should be understood that the fabrication techniques described above in connection with  FIG. 6  are exemplary only. Other forming methods and fastening techniques can be employed to provide a modular strainer in accordance with the present invention. In one variation, the flanges of the disc part  90  can fit inside the flanges of the disc part  50  (although the tie-rod bosses in that case must be slightly spaced from the disc part  50  flanges). 
     Furthermore, this strainer construction permits incorporation of the advantages of the strainer described in U.S. Pat. No. 5,759,399. That is, the strainer design described in that patent tailors the size and shape of the strainer discs and central core to distribute the flow through the strainer. The present invention provides a fabrication approach that facilitates tailoring the strainer disc sizes and shapes, while also regulating the core flow through the strainer. It will be apparent to those skilled in the art that the strainer described herein functions in accordance with the general operational principles discussed in U.S. Pat. No. 5,759,399, which discussion is incorporated herein by reference, in that cooling water is drawn by the ECCS pumps into the interiors of the discs  12 , into the core bosses  16  through the slots  86 , and out of the strainer through the suction opening provided by the pipe  28 . 
     In the present invention, the slot  86  in each core boss  16  provides an aperture that acts as a core flow regulator in accordance with the discussion in U.S. Pat. No. 5,759,399. That patent describes a technique for regulating the core flow by changing the diameter of the central core along the strainer axis. By changing the size of the aperture from disc to disc, the core flow regulation discussed in that patent is achieved. In line with that discussion, in the present embodiment the slots  86  will get progressively larger for discs  12  more distant from the strainer outlet at the pipe  28  (see  FIG. 1 ). That is, the width (circumferential extent) of the slot in the disc  12   h  will be larger than the width of the slot in the disc  12   g , the width of the slot in the disc  12   g  will be larger than the width of the slot in disc  12   f , etc. 
     Those skilled in the art will realize that flow regulation structure other than a slot in a core boss can be used for this function. For example, plural slots or discrete holes can be used, and instead of changing the size of a single slot, greater numbers of slots or holes can be used from disc to disc. In the illustrated embodiment, the core boss is a circular cylinder, but it can also have other cross sectional shapes and still function to regulate the flow from the disc&#39;s interior space into the strainer core at the central axis. For example, the core boss can be elliptical, square, rectangular, or any regular or irregular shape (polygonal or not), depending on the demands of the strainer installation. Nor do all of the core tubes or discs have to be the same shape. It will also be appreciated that the modular construction described herein will facilitate fabrication of stacked disc strainers with varying size discs, as described in U.S. Pat. No. 5,759,399. All of the features and operational characteristics described in that patent and mentioned in this description are incorporated by reference as if described in full herein. 
       FIG. 7  is a perspective view of part of a PWR containment area  100 . When an accident occurs, the normally dry containment area floods with water, which thus forms a reservoir from which water is drawn to cool the reactor core. A plurality of strainers  10   a ,  10   b ,  10   c ,  10   d ,  10   e ,  10   f ,  10   g ,  10   h ,  10   i ,  10   j ,  10   k ,  10   l ,  10   m ,  10   n ,  10   o , and  10   p  are disposed in the containment area behind a shield wall  102 . The strainers are connected by pipes  104 ,  106 ,  108  and  110  to ECCS pumps (not shown) below the floor  112  of the containment area. The strainers  10   a ,  10   b ,  10   c ,  10   d  and  10   e  are connected in series to the pipe  104 . Likewise, the strainers  10   f ,  10   g  and  10   h  are connected in series to the pipe  106 ; the strainers  10   i ,  10   j  and  10   k  are connected in series to the pipe  108 , and the end discs of the strainers  10   h  and  10   i  are connected by the pipe; and the strainers  10   l ,  10   m ,  10   n ,  10   o  and  10   p  are connected in series to the pipe  110 . It will be appreciated that the end discs  12   a  of the strainers  10   b ,  10   c ,  10   d  and  10   e  will have core tubes configured to accept a pipe similar to the suction pipe  28  attached to the disc  12   h  shown in  FIG. 1 . This is the case for all of the strainers connected to pipes at both ends of the strainer, and this design variation from one strainer to the next is readily incorporated because of the strainer&#39;s modular design. Another aspect of the enhanced versatility of the modular strainer construction according to the invention is also readily apparent in  FIG. 7 , which shows different size strainers at different locations in the containment area depending on the size constraints at a particular location. For example, the strainer  10   h  is shorter than other strainers because the reactor design specifications call for it to fit into a smaller envelope. In addition, the discs comprising the strainers need not be rectangular shape as shown in  FIG. 6 , as illustrated by the trapezoidal discs shown in  FIG. 7 . 
     In operation, the containment area  100  of the PWR shown in  FIG. 7  floods in the event of a LOCA, thus forming a cooling water reservoir. The ECCS pumps apply suction to the pipes  104 ,  106 ,  108  and  110 , which in turn draw water from the containment area through the strainers  10 . After a LOCA, the water in the containment area  110  will normally be filled with debris in the form of fibrous elements from destroyed insulation, fine particulate matter and other debris such as paint chips. In accordance with the principles discussed above, and in U.S. Pat. No. 5,759,399, the core flow in the strainers is regulated to distribute the debris over the surface area of the individual strainers by the action of the flow regulating slots  86  therein. Furthermore, by incorporating contoured surfaces into the discs in accordance with the invention, the head loss through the strainers is greatly reduced in order to provide adequate coolant flow. 
     WORKING EXAMPLE 
       FIG. 8  schematically illustrates a test set up  200  that validates the concepts underlying the contoured strainer surface aspect of the present invention, and suggests a test protocol for determining the parameters of a contoured surface in accordance with the present invention for a given reactor application. A tank  202  simulates a reactor coolant reservoir. In the tests, materials simulating the debris encountered in a LOCA were introduced into the tank  202 . An electrically powered propeller functions as a stirrer  204  to maintain the test debris in suspension in the test liquid in the tank  202 . A pump array  206 , simulating a reactor ECCS pump, draws water through a test strainer rig TS and circulates it back to the tank  202  through an adjustable control valve  208  that enables regulation of the fluid flow rate. The control valve outlet is introduced into a generally cylindrical baffle  209  at the bottom of the tank to inhibit the establishment of a flow pattern between the valve outlet in the tank  202  and the test strainer rig TS. Instrumentation includes a Rosemount 1151 DP differential pressure transducer  210  connected to a Sensotec GMA display  212 . The display provides a voltage signal, indicative of the pressure differential sensed by the pressure transducer, to a Dataq DI-220 12-bit analog-to-digital converter  214 . The resulting digital signal is introduced to a data acquisition program stored on a computer (not shown). The tank  202  is approximately 7 ft. in diameter and 30 in. deep. The pump array includes three Hayward® pumps, each having a maximum flow rate of approximately 100 gallons per minute. 
     The test strainer rig TS comprised two semicircular hollow discs  302 ,  304  with their major surfaces parallel to each other. (The disc  304  is beneath the disc  302  and in fact cannot be seen in  FIG. 8 .) The facing major surfaces of the discs are perforated to simulate the perforated surfaces of a nuclear reactor strainer, such as the strainer  10  shown above in  FIG. 1 . The discs are mounted with a 56 mm gap between their parallel facing surfaces. The discs have an outside diameter of 3 ft. and a central semicircular cutout with a diameter of 10.75 in. A suction pipe  305  is placed in communication with the hollow interiors of both discs, with flow being permitted to enter the core tube at the gap between the discs  302  and  304  through perforations as discussed above with reference to connecting tubes  14 . The three pumps of the pump array  206  are connected to a pipe (not shown) communicating with the pipe  305  to draw water from the tank  202 , into the hollow interiors of the discs  302  and  304 , and into the pipe  305 . The non-facing major surfaces of the discs are solid (that is, not perforated) to force fluid to flow into the gap between the discs, through the perforations, and into the suction pipe, thus simulating the flow between facing perforated disc surfaces in a strainer installed in a reactor. The high pressure end of the pressure transducer  210  is attached to the tank wall, and the low pressure end is located to sense the pressure in the pipe connected to the pipe  305 , so that the pressure differential signal from the pressure transducer represents the pressure drop across the perforated discs. The water was at room temperature. 
     The tank was filled to a depth of 27 inches and a mixture of mineral wool and calcium silicate insulation was tested in a mass ratio of 1.3 calcium silicate to mineral wool. A simulated debris load was prepared in accordance with procedures established by the Boiling Water Reactor Owners Group in which the mineral wool was shredded using a leaf shredder and the calcium silicate was ground by hand into a fine powder of small particulates. All the debris was soaked to ensure it could be entrained into the strainer. Testing was conducted to compare the performance of a perforated plate with protrusions to that of a flat perforated plate with different amounts of debris on the strainer. In a series of tests, three different amounts of the debris mixture, representing thin, medium, and thick debris loads, were tested on discs  302   304  with both a flat perforated surface and a surface with protrusions. These tests corresponded to 0.75 lb., 1.5 lb., and 2.25 lb. of mineral wool with a corresponding amount of calcium silicate. The effect of the protrusions significantly reduced head loss, under identical conditions in all cases. The ratio of the head loss for the perforated plate with protrusions compared to the head loss for the flat perforated plate was 0.56, 0.19, and 0.5 for the three cases, respectively. Thus, the protrusions reduced the head loss across the strainer by a factor of about 2-5. 
     SUMMARY 
     Those skilled in the art will readily recognize that only selected preferred embodiments of the invention have been depicted and described, and it will be understood that various changes and modifications can be made other than those specifically mentioned above without departing from the spirit and scope of the invention, which is defined solely by the claims that follow.