Nuclear fuel assembly lower tie-plate and method of its assembling

A fuel assembly lower tie-plate with better performance in capturing linearly elongated foreign substances. The lower tie-plate has a screening plate positioned below the network section in the lower tie-plate cavity. The screening plate is arranged substantially horizontally so that the lower tie-plate cavity is divided into upper and lower parts by the screening plate. Tubular filters are attached to the screening plate so that the tubular filters have openings below and above the screening plate. Top ends of the tubular filters are closed, and the openings above the screening plate are formed in side walls of the tubular filters. The lower tie-plate may be assembled by combining the screening plate, the network section and the nozzle section together, after the tubular filters have been attached to the screening plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2001-390855, filed on Dec. 25, 2001; the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention is related generally to a lower tie-plate of a nuclear fuel assembly for a light water reactor, and more particularly to a lower tie-plate with better performance in blocking or suppressing flow of foreign substances entrained by the coolant into the fuel portion, in addition to a method of assembling it.

A conventional fuel assembly for a boiling water reactor has a channel box1with a substantially square-shaped cross-section, as shown in FIG.1. The channel box1contains a plurality of fuel rods2and at least one water rod3arranged in a square lattice. An upper tie-plate4and a lower tie-plate5are attached to the upper and lower ends of the fuel assembly, respectively. A plurality of spacers6are attached to the water rod3with specified axial intervals there between, so that the fuel rods2may be held in a lattice, although only one of the spacers6is shown in FIG.1.

The lower tie-plate5has a network section7for holding the fuel rods2and the water rod3directly, and a nozzle section8which extends downward from peripheral portion of the network section7, so that a lower tie-plate cavity9may be formed surrounded by the nozzle section8below the network section7. A lower tie-plate inlet opening10is formed at the lower end of the nozzle section8.

Each of the fuel rods2has a cladding tube loaded with a plurality of fuel pellets (not shown), and a lower end plug11for closing the lower end of the cladding tube. Lower part of the lower end plug11is formed in a slim circular cylindrical rod which is inserted through an insertion-hole13formed in the network section7of the lower tie-plate, so that the lower end plug11of the fuel rod is held there.

The water rod3is a hollow metal pipe, and has inlet holes26slightly above the lower tie-plate5and outlet holes27slightly below the upper tie-plate4in the channel box1. The coolant flows into the water rod3through the inlet holes26in liquid phase, flows upward in the water rod3remained in liquid phase, and flows out through the outlet holes27.

The water rod3has a lower end plug12at its bottom end. The lower end plug12of the water rod3is similar to the lower end plug11of the fuel rod and has a slim circular cylindrical rod shape which is inserted through an insertion hole13formed in the network section7of the lower tie-plate, so that the lower end plug12of the water rod is supported.

The network section7of the lower tie-plate has the insertion holes13for receiving and supporting the lower end plugs11and12as described above, as well as through-holes (not shown) for coolant passes there through between adjacent lower end plugs11and12.

The coolant15flows into the lower tie-plate cavity9through the lower tie-plated inlet opening10, passes through the through-holes in the network section7, flows around the fuel rods2and the water rod3in the channel box1, and then, flows out of the fuel assembly through the upper tie-plate4.

The nozzle section8of the lower tie-plate has one or more small leakage holes17on its sides, so that a small part of the coolant15coming to the lower tie-plate cavity9flows out of the channel box1.

Some fuel assemblies with high performance developed recently have filters for preventing foreign substances from entering the fuel assembly. For example, a lower tie-plate design has a network section with through-holes of about 5 mm in diameters which are smaller than those of conventional designs so that the resistance to the flow or the pressure loss may be increased. Such a design may enhance core stability and also may function as a filter for foreign substances.

The foreign substances which may be expected to enter the fuel assemblies may include small metal wastes remained in the reactor primary containment system during the plant construction, metal brush pieces which have broken off during equipment cleansing and broken pieces which may be results of equipment breaches. The foreign substances may be in various shapes including plates, spiral wires and straight wires.

FIG. 2shows a prior art lower tie-plate having a filter function for foreign substances (See Japanese Patent Disclosure Hei 7-306284). As shown inFIG. 2, lower parts of the lower end plugs11and12of the fuel rods2and the water rod3, respectively, penetrate the insertion-holes13of the network section7of the lower tie-plate5. A screening plate20for filtering foreign substances is disposed below the network section7and arranged substantially horizontally across the lower tie-plate cavity9. The screening plate20has many small holes24for allowing coolant to flow through while blocking foreign substances, as well as the through-holes21and22for the lower end plugs of the fuel rods2and the water rod3, respectively, penetrate.

Referring toFIG. 2, the coolant15flows into the lower tie-plate cavity9through the lower tie-plate inlet opening10, passes through the small holes24in the screening plate20, and then, passes through the through-holes in the network section7into the area around the fuel rods2and the water rod3within the channel box1. At this time, most of the foreign substances may be prevented from flowing into the channel box1, since they would not pass through the small holes24in the screening plate20.

The prior-art lower tie-plate described above could prevent the foreign substances which had reached the inlet area of the core from flowing into the core at a certain probability. However, straight and slim foreign substances might possibly pass through the small holes24in the screening plate20and the through-holes in the network section7when the foreign substances were carried in a position vertically elongated along the flow direction, because the small holes24in the screening plate20and the through-holes in the network section7are aligned substantially linearly upward.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a nuclear fuel assembly lower tie-plate improved in performance of blocking especially straight and slim foreign substances. It is another object of the present invention to provide a method of assembling a nuclear fuel assembly lower tie-plate improved in performance of blocking especially straight and slim foreign substances.

There has been provided, in accordance with an aspect of the present invention, a nuclear fuel assembly lower tie-plate comprising: a network section for holding lower parts of fuel rods loaded with nuclear fuel so that the fuel rods are held in a lattice with spaces there between; a nozzle section extending downward from peripheral part of the network section for forming a lower tie-plate cavity below the network section, the nozzle section having an inlet opening at its lower end; a screening plate positioned below the network section in the lower tie-plate cavity, the screening plate being arranged substantially horizontally so that the lower tie-plate cavity is divided into upper and lower parts by the screening plate; and a plurality of tubular filters attached to the screening plate so that the tubular filters each has openings below and above the screening plate; wherein top ends of the tubular filters are closed, and the openings above the screening plate are formed in side walls of the tubular filters.

There has also been provided, in accordance with another aspect of the present invention, a method for assembling a nuclear fuel assembly lower tie-plate, the lower tie-plate comprising: a network section for holding lower parts of fuel rods loaded with nuclear fuel so that the fuel rods are held in a lattice with spaces there between; a nozzle section extending downward from peripheral part of the network section for forming a lower tie-plate cavity below the network section, the nozzle section having an inlet opening at its lower end; a screening plate positioned below the network section in the lower tie-plate cavity, the screening plate being arranged substantially horizontally so that the lower tie-plate cavity is divided into upper and lower parts by the screening plate; and a plurality of tubular filters attached to the screening plate so that the tubular filters each has openings below and above the screening plate; wherein top ends of the tubular filters are closed; the method comprising: a first step of attaching the tubular filters to the screening plate; and a second step of combining the screening plate, the network section and the nozzle section together, after the first step.

There has also been provided, in accordance with yet another aspect of the present invention, a nuclear fuel assembly lower tie-plate comprising: a network section for holding lower parts of fuel rods loaded with nuclear fuel so that the fuel rods are held in a lattice with spaces there between; a nozzle section extending downward from peripheral part of the network section for forming a lower tie-plate cavity below the network section, the nozzle section having an inlet opening at its lower end; a screening plate positioned below the network section in the lower tie-plate cavity, the screening plate being arranged substantially horizontally so that the lower tie-plate cavity is divided into upper and lower parts by the screening plate; and a plurality of tubular filters attached to the screening plate, the tubular filters each having openings below and above the screening plate; wherein: at least part of the fuel rods have lower end plugs including rod portions extending downward; upper parts of the plurality of tubular filters are positioned surrounding the rod portions, and gaps between the upper portions of the tubular filters and the rod portions are substantially closed; and the openings above the screening plate are disposed mainly on side walls of the tubular filters.

There has also been provided, in accordance with yet another aspect of the present invention, a method of assembling a nuclear fuel assembly lower tie-plate, the lower tie-plate comprising: a network section for holding lower parts of fuel rods loaded with nuclear fuel so that the fuel rods are held in a lattice with spaces there between; a nozzle section extending downward from peripheral part of the network section for forming a lower tie-plate cavity below the network section, the nozzle section having an inlet opening at its lower end; a screening plate positioned below the network section in the lower tie-plate cavity, the screening plate being arranged substantially horizontally so that the lower tie-plate cavity is divided into upper and lower parts by the screening plate; and a plurality of tubular filters attached to the screening plate, the tubular filters each having openings below and above the screening plate; wherein: at least part of the fuel rods have lower end plugs including rod portions extending downward; upper parts of the plurality of tubular filters are positioned surrounding the rod portions, and gaps between the upper portions of the tubular filters and the rod portions are substantially closed; and the openings above the screening plate are disposed mainly on side walls of the tubular filters; the method comprising: a first step of fixing the plurality of tubular filters to the screening plate; and a second step of combining the screening plate between the network section and the nozzle section, after the first step.

There has also been provided, in accordance with yet another aspect of the present invention, a method of assembling a nuclear fuel assembly lower tie-plate, the lower tie-plate comprising: a network section for holding lower parts of fuel rods loaded with nuclear fuel so that the fuel rods are held in a lattice with spaces there between; a nozzle section extending downward from peripheral part of the network section for forming a lower tie-plate cavity below the network section, the nozzle section having an inlet opening at its lower end; a screening plate positioned below the network section in the lower tie-plate cavity, the screening plate being arranged substantially horizontally so that the lower tie-plate cavity is divided into upper and lower parts by the screening plate; and a plurality of tubular filters attached to the screening plate, the tubular filters each having openings below and above the screening plate; wherein: at least part of the fuel rods have lower end plugs including rod portions extending downward; upper parts of the plurality of tubular filters are positioned surrounding the rod portions, and gaps between the upper portions of the tubular filters and the rod portions are substantially closed; and the openings above the screening plate are disposed mainly on side walls of the tubular filters; the method comprising: a first step of disposing the screening plate between the network section and the nozzle section; and a second step of inserting the plurality of tubular filters through the inlet opening of the nozzle section and attaching the plurality of tubular filters to the screening plate, after the first step.

DETAILED DESCRIPTION OF THE INVENTION

In the following description and also in the above description of background of the invention, like reference numerals represent like elements, and redundant description may be omitted.

Now the first embodiment of a lower tie-plate according to the present invention is described referring toFIGS. 4 through 7. The lower tie-plate of this embodiment is not formed as a single unit like the prior-art lower tie-plate. It is rather formed by combining separate parts of a network section7, a nozzle section8and a horizontal flat plate of screening plate30between them by welding, for example. Many tubular filters32penetrate and are attached vertically to the screening plate30. InFIG. 4, the channel box1(SeeFIG. 1) is eliminated for illustrative simplicity.

Each of the tubular filters32is shaped in a hollow circular cylinder, and its top end33is closed while its bottom end34is open as shown in FIG.5. Each of the tubular filters32may have a diameter of 5 mm and a height of 50 mm, for example. Many small holes36are formed in upper part of the side wall of each of the filters32. The diameters of the small holes36may be about 2 mm, for example.

Each of the filters32penetrates the screening plate30at one of screening-plate bores200and is fixed so that the screening plate30may be positioned between the top bottom33and the bottom end34of the filter32, and all of th small holes36may be positioned above the screening plate30, as shown in FIG.6.

As shown inFIG. 7, the screening plate30has tubular filters32which are attached to the screening plate30and arranged in a lattice, as well as holes38and40for the lower end plugs11and12of the fuel rods2and water rods3, respectively, to penetrate. The screening-plate bores200for attaching the tubular filters32to the screening plate30have diameters substantially equal to the outer diameters of the filters32so as to prevent foreign substances (not shown) in the coolant from passing through the gaps between the screening-plate bores200and the filters32. The holes38and40have diameters slightly larger than the outer diameters of the lower end plugs11and12, respectively, so as to prevent foreign substances in the coolant from passing through the gaps between the holes38and40in the screening plate30and the lower end plugs11and12.

The holes38and40are aligned vertically with the lower end plugs11and12or they are arranged in the same horizontal positions with the fuel rods2and the water rods3. The tubular filters32are positioned at the centers between mutually adjacent lower end plugs11and12.

When the lower tie-plate of this embodiment is assembled, the holes38and40for the lower end plugs11and12, respectively, to penetrate the screening plate30and the screening-plate bores200for the tubular filters32to penetrate the screening plate30are formed first. Then, the filters32are fixed in the screening-plate bores200by welding, for example. Then, the screening plate30with the filters32are disposed between the network section7and the nozzle section8, and they are integrated together by welding, for example.

The lower tie-plate of the prior art shown inFIG. 2was formed as a single unit including the network section7, the nozzle section8and the screening plate30by casting. Therefore, it was difficult to attach tubular filters32to the screening plate30. On the other hand, the lower tie-plate of the present embodiment is assembled after the network section7, the nozzle section8and the screening plate30with the filters32are formed separately. Therefore, the tubular filters32can be easily attached to the screening plate30before the screening plate30is assembled with the network section7and the nozzle section8.

In operation of this embodiment, coolant15entraining foreign substances flows into the lower tie-plate cavity9through the lower tie-plate inlet opening10formed at the bottom end of the nozzle section8, as shown in FIG.4. The coolant15flows into the tubular filters32through the lower ends34of the filters32, and flows out through the small holes36, as best shown in FIG.6. Since the flow direction changes in the filters32, foreign substances in shapes of wires of 20 to 30 mm length, for example, would be trapped in the filters32. Foreign substances in shapes of small plates would be trapped at the bottom ends34of the filters32.

The coolant15which has come out of the filters32through the small holes36goes upward through the flow passages142between the lower end plugs11and12in the network section7, and then, to the area around the fuel rods2and the water rods3.

Some coolant would flow upward through the gaps between the holes38and40in the screening plate30and the lower end plugs11and12which penetrate the holes38and40, but foreign substances would not pass through them because the gaps are small.

The small holes36can be in any shapes including ellipses, semi-circles, crosses, stars, crescent shapes and polygons as well as circles. The tubular filters32can be in shapes of polygonal cylinders, circular cones, polygonal cones, serial combinations of circular cylinders and circular cones, or serial combinations of circular cylinders and polygonal cones, for example.

Now other embodiments utilizing alternative tubular filters are disclosed referring toFIGS. 8to18with which the filters32of the first embodiment can be replaced. The other features of the lower tie-plate are similar to those of the first embodiment.

FIG. 8shows a tubular filter42of the second embodiment. This filter42has small holes36not only in the upper part but in the whole part of the side wall. The other features are similar to those of the filter32of the first embodiment. When the filter42is attached to the screening plate30as shown inFIG. 7, the small holes36are positioned not only above but also below the screening plate30. Therefore, coolant would flow into the filter42not only through the open lower end34but also through the small holes36below the screening plate30. Thus, probability of local flow blockage in the filter42due to the foreign substances in the coolant can be lowered.

FIG. 9shows a tubular filter44of the third embodiment. This filter44has a top end46shaped in a circular cone or a dome which is upwardly tapered. The other features are similar to those of the filter32of the first embodiment. According to this embodiment, the upward flow passages for the coolant which has flown out of the filter44through the small holes36expand gradually toward the network section7of the lower tie-plate, so that the flow resistance can be reduced.

FIG. 10shows a tubular filter48of the fourth embodiment, which is a modification of the third embodiment shown in FIG.9. The filter48has a step50on the side wall below which a larger-diameter tubular portion52is formed. The upper section above the step50is shaped similar to the filter44of the third embodiment and has top end46shaped in a circular cone or a dome which is upwardly tapered. Many small holes36are disposed on the side wall above the step50. The bottom end34of the larger-diameter tubular portion52is open while the side wall of the larger-diameter tubular portion52is closed.

This filter48is easily positioned in the screening plate30due to the step50. The filter48may be inserted into the screening-plate bore200upwardly from the bottom, and it is fixed to the screening plate30by welding, for example, when the step50in the filter48contacts the bottom surface of the screening plate30.

FIG. 11shows a tubular filter54of the fifth embodiment, which is a modification of the third embodiment shown in FIG.9. The bottom end56of the filter54is closed. The filter54has many small holes36on its whole side wall not only in the upper part but also in the lower part. When the filter54is inserted in the screening-plate bore200and fixed to the screening plate30, the small holes36are positioned above and below the screening plate30similarly to the second embodiment shown in FIG.8.

In operation of this embodiment, the coolant15flows into the lower tie-plate cavity9through the lower tie-plate inlet opening10at the lower end of the nozzle section8, then it flows into the tubular filter54through the small holes36in the side wall of the filter54below the screening plate30, and then it flows out through the small holes36above the screening plate30.

Since the bottom end56of the filter54is closed in this embodiment, the foreign substances in the coolant would remain in the filter54. Thus, the foreign substances may be taken out of the reactor vessel (not shown) in a relatively high probability when the fuel assembly is taken out. If the bottom end were open, the foreign substances trapped in the filter would fall back due to gravity when the coolant flow had stopped, and they would possibly fall down through the lower tie-plate inlet opening10to the lower plenum or bottom of the reactor pressure vessel to be accumulated there.

FIG. 12shows a tubular filter58of the sixth embodiment, which is a combination of the fourth embodiment shown in FIG.10and the fifth embodiment shown in FIG.11. The filter58has a step50on the side wall below which a larger-diameter tubular portion52is formed as in the fourth embodiment. The portion above the step50of this embodiment is similar to that of the filter54of the fifth embodiment, and has a top end46shaped in a circular cone or a dome which is upwardly tapered. The bottom end56of the larger-diameter tubular portion52is closed similarly to that of the fifth embodiment. The side wall of the larger-diameter tubular portion52has many small holes60.

The small holes60in the larger-diameter tubular portion52is positioned below the screening plate30when the filter58is attached to the screening plate30so that the step50contacts the bottom surface of the screening plate30, similarly to the fourth embodiment.

FIG. 13shows a tubular filter62of the seventh embodiment, which is similar to the filter32of the first embodiment shown inFIGS. 5 and 6, except that a horizontal annular collar64is attached to the outer surface of the side wall of the filter62between the top end33and the lower end34, by welding, for example. According to this embodiment, since the annular collar64is attached to the filter62, the filter62can be easily positioned on the screening plate30, and can be firmly fixed to the screening plate30by welding the annular collar64and the screening plate30together, for example.

FIG. 14shows a tubular filter66of the eighth embodiment, which has the horizontal annular collar64of the seventh embodiment shown inFIG. 13on the side wall of the tubular filter44of the third embodiment shown in FIG.9. This embodiment has both advantages of the third and the seventh embodiments although the production cost may be higher.

FIG. 15Ashows a tubular filter68of the ninth embodiment, which is similar to the filter32of the first embodiment shown inFIGS. 5 and 6, except that the filter68is not inserted in the screening-plate bore200but is disposed on the upper surface of the screening plate30over the screening-plate bore200. A bottom body70shown inFIG. 15Bis disposed on the bottom surface of the screening plate30at the screening-plate bore200. The bottom body70has a horizontal annular collar72and a tubular part74which extends upward from the inner ridge of the annular collar72. The upper part of the tubular part74is tapered upward and the top end of it is open.

When the tubular filter68is attached to the screening plate30, the tubular part74of the bottom body70is inserted from the bottom into the screening-plate bore200so that the annular collar72can contact the bottom surface of the screening plate30. Then, the annular collar72is fixed to the bottom surface of the screening plate30by welding, for example. At that time, the tubular part74extrudes upward above the upper surface of the screening plate30. Then, the tubular filter68is disposed over the tubular part74on the upper surface of the screening plate30, and is fixed by welding, for example.

According to this embodiment, the tubular filter68can be easily positioned and firmly fixed to the screening plate30.

FIG. 16Ashows a tubular filter76of the tenth embodiment, which is similar to the tubular filter44of the third embodiment shown inFIG. 9except that the side wall has a star-shaped small hole78as well as circular small holes36. The tubular filter76is positioned fixed on the upper surface of the screening plate30by the bottom body70as shown inFIG. 16B, similarly to the ninth embodiment shown inFIGS. 15Aand15B.

FIG. 17shows a tubular filter80of the ninth embodiment, which is similar to the fourth embodiment shown inFIG. 10with exceptions described below. The inner space of the large-diameter tubular portion52below the level of the step50, which corresponds to the level of the screening plate30when the filter80is attached to the screening plate30, is tapered from the bottom (inlet) end34toward the upper (exit) end82. The diameters of the inner space may be 10 mm at the bottom end34and 6 mm at the upper end82, for example.

A large circular side hole84, the diameter of which may be 5 mm, for example, is formed in the side wall slightly above the step50. In addition, many small holes36, the diameter of which may be 2 mm, for example, are formed in the side wall above the large circular side hole84. A horizontal annular collar85is attached to the inner surface of the side wall between the large circular side hole84and the small holes36. An inner tapered tube86, which is upwardly tapered, is attached to the annular collar85, so that the bottom end of the tube86fits the central opening of the annular collar85.

The flow passage in the filter80above the step50may have a diameter of 5 mm, the central opening of the annular collar85may have a diameter of 4 mm, and the top open end of the inner tapered tube86may have a diameter of 3 mm, for example.

In operation, the coolant flows upward into the filter80through the bottom end34and the top end82of the larger-diameter cylindrical portion52. Since the flow passage in the large-diameter tubular portion52tapers upward, a jet is formed. Therefore, the wire-shaped foreign substances in the coolant would go upward and is accelerated in the inner tapered tube86, instead of going out horizontally through the large circular side hole84.

At the same time, small plate-shaped foreign substances in the coolant would be blocked at the bottom end34of the filter80. If a large volume of foreign substances emerged, the foreign substances would go out of the filter80through the large circular side hole84, and probability of flow blockage would be reduced.

FIG. 18shows a tubular filter88of the twelfth embodiment, which is similar to the eleventh embodiment shown inFIG. 17with exceptions described below. The tubular filter88has a horizontal annular collar90outside of the side wall instead of the annular collar85inside of the side wall (FIG.17). An inner tapered tube86is disposed within the side wall so that the flow passage in the tube86may be tapered upward from the level of the annular collar90, similarly to the eleventh embodiment.

According to this embodiment, coolant flow contracts smoothly upward passing the step50into the inner tapered tube86without abrupt contraction at the annular collar90, resulting in smoother flow than the eleventh embodiment. The inner tapered tube86is attached to the annular collar90before the filter88is assembled. The tubular side wall is divided into two parts, the parts above and below the annular collar90. The annular collar90with the inner tapered tube86is disposed between the two parts of the tubular side wall, and then, they are combined together.

This embodiment has small star-shaped holes78on the side wall instead of the circular small holes36(FIG.17). The function of the small star-shaped holes78is substantially the same as the function of the small circular-shaped holes36.

FIG. 19shows a lower tie-plate of the thirteenth embodiment which is similar to the first embodiment shown inFIG. 4with exceptions described below. Portions of the lower end plugs92and94of the fuel rods and the water rods, respectively, below the network section7are hollow, and flow passages are formed in them where foreign substances may be trapped. Thus, the foreign substances may be trapped not only at the tubular filters attached to the screening plate30similarly to the first embodiment, but also at the lower end plugs92and94.

Alternatively, separate filters may be attached to the lower end plugs92and94instead of forming filters within the lower end plugs92and94themselves by forming flow passages in them.

In addition, any combinations of the second to the twelfth embodiments described above and the lower end plugs92and94with functions of trapping foreign substances can be possible.

FIG. 20shows a lower tie-plate of the fourteenth embodiment which is similar to the thirteenth embodiment shown inFIG. 19with exceptions described below. The lower ends of the tubular filters32are connected to each other with connecting rods96.FIG. 20also show that the tubular filters32are connected to the lower end plugs94via support bodies202. Alternatively, the filters32may be connected to the lower end plugs92. According to this embodiment, the filters32are firmly fixed, and then, the number of welding points between the filters32and the screening plate30can be reduced. In addition, even if some of the filters32broke into pieces, the pieces would be prevented or suppressed from flowing with the coolant.

Alternatively, only one of the means may be employed—connecting the lower ends of the filters32to each other with the connecting rods96, or supporting the filters32with the lower end plugs92and/or94.

In addition, the feature of connecting the filters to each other with connecting rods can be applied to any of the first to the thirteenth embodiments described above.

FIGS. 21 through 25show a lower tie-plate of the fifteenth embodiment according to the present invention. Tubular filters132are disposed on the screening plate30so that each one of the filters132may surround lower part of one of the lower end plugs11or12which penetrates the screening plate30. The screening plate30has openings135and each one of the filters132is disposed over one of the openings135each, as shown in FIG.22. The gap between the filter132and the opening135of the screening plate30is small enough to prevent the foreign substances in the coolant from passing through. There are not any openings in the screening plate30except the openings135.

The screening plate30may have a thickness of 2 mm, for example. The filter132may have a diameter of 12 mm and a height of 20 mm, for example.

The bottom of the filter132is open, and the filter132is disposed over the opening135of the screening plate30to cover the opening135. The filter132has a top plate136which has a circular hole137for the rod portion of one of the lower end plugs11or12to penetrate. The diameter of the hole137may be 7.2 mm, for example, and the gap between the hole137and the lower end plug11or12is small enough to block the foreign substances in the coolant.

The side wall138of the filter132has a plurality of small holes139. The holes139may be circles with diameters of about 2 mm, for example. Alternatively, the holes139may have shapes other than circles, such as polygons or star shapes (not shown).

The central axis of the circular tubular filter132is positioned to align with the central axis of the central axis of the lower end plug11or12, as shown inFIG. 25, for example. A lower end plug11of a fuel rod is shown inFIGS. 22,23and24as an example, but the structure would be almost the same if the lower end plug11of a fuel rod is replaced with a lower end plug12of a water rod.

In assembling the lower tie-plate, the filters132are attached to the screening plate30first, as shown in FIG.23. Subsequently, the screening plate30is disposed between the network section7and the nozzle section8, and then, they are combined together by welding, for example.

In operation, if a wire-shaped foreign substance (not shown) of a length of 20-30 mm, for example, flows into the lower tie-plate cavity9through the lower tie-plate inlet opening10with coolant, the foreign substance would flow upward into the filter132through the opening135in the screening plate30. The flow of the coolant15changes direction to horizontal in the filter132and goes out to the space above the screening plate30through the holes139. At that time, the foreign substances would probably remain in the filter132because it would be difficult for the wire-shaped foreign substances to change directions. The foreign substances of small plates would also be trapped in the filter132.

Furthermore even if a large volume of foreign substances emerged due to any malfunction or breach of a plant facility, required coolant flow could be secured. In addition, the foreign substances could be taken out of the reactor vessel with the fuel assemblies when the fuel assemblies are taken out. The filters themselves are reliable in structure and it would be prevented that broken parts of the filters become part of the foreign substances flowing into the channel box of the fuel assembly.

FIGS. 26 through 28show a lower tie-plate of the sixteenth embodiment. Tubular filters165are similar to the filters132of the fifteenth embodiment shown inFIGS. 21 through 25, but the filters165are disposed so that they are held penetrating the openings166of the screening plate30halfway, instead of being disposed on the upper surface of the screening plate30at the openings135.

The lower tie-plate of this embodiment is formed by combining separate parts of the network section7, the nozzle section8and the screening plate30between them, by welding, for example, like the lower tie-plates of the embodiments described above. Then, the filters165are inserted into the lower tie-plate cavity9through the inlet opening10of the nozzle section8to attach them to the screening plate30.

The filters165form groups of filters165, wherein each group includes a plurality of filters165combined together with one or more connecting rods167which extend horizontally. In the embodiment shown inFIGS. 26 through 28, the filters165form nine groups of filters165, and each of the groups has two to ten filters165, most of the groups being arranged in 3×3 lattices, as best shown in FIG.27. The number of groups of filters165, the number of the filters165contained in each group and the shapes of the groups are not limited, but each group should be slim enough to be inserted into the lower tie-plate cavity9through the inlet opening10of the nozzle section8.

The bottom surface of the screening plate30in this embodiment has grooves168for receiving the connecting rods167. Each of the grooves168is formed between two projections169. The grooves168may be formed all locations corresponding to the connecting rods167, but the number of the grooves168can be less than that of the connecting rods167as shown inFIG. 28if all of the groups of the filters165can be securely held by the grooves168.

According to this embodiment, the lower tie-plate can be assembled by inserting the filters165after the rest parts other than the filters165have been assembled. In addition, the filters165which have captured foreign substances can be replaced with new filters, when the fuel assemblies are replaced.

In addition, a plurality of filters165are combined together by the connecting rods167to form the groups of the filters165before the filters165are inserted through the inlet opening10of the nozzle section8to attach them to the screening plate30. Thus, labor cost for attaching the filters165can be reduced compared to a method of attaching each of the filters165to the screening plate30separately. That is because the work of combining the plurality of filters165together with the connecting rods167is a relatively easy work which can be done in a wide open space while the work of attaching the filters165to the screening plate30through the inlet opening10of the nozzle section8is relatively difficult work.

As a modified version of the sixteenth embodiment described above, the filters165may be attached to the screening plate30separately without using the connecting rods167. In this case, the bottom surface of the screening plate30should have concavities and/or convexities (not shown) to hold each of the filters165instead of the grooves168for receiving the connecting rods167.

As another modified version of the sixteenth embodiment described above, the network section7, the screening plate30and the nozzle section8may be formed in a single unit by casting, for example, instead of being formed by combining separate parts with welding, for example. This can be accomplished because the filters165are inserted and attached to the screening plate30after the rest part of the lower tie-plate is formed. Thus, the process of forming the lower tie-plate can be the same as that of the prior art except the step of attaching filters165.

In the process of forming the lower tie-plate of the fifteenth or the sixteenth embodiment described above, the filters can be in any shapes including circular cylinders, tapered cylinders and polygonal cylinders.

In addition, any of the filters of the first to sixteenth embodiments including their modified versions can be combined in a single lower tie-plate.

Numerous modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that, within the scope of the appended claims, the present invention can be practiced in a manner other than as specifically described herein.