Fuel assembly for a pressurized water nuclear reactor

Abstract

To further improve agitation mixing performance of a coolant in a fuel assembly.SOLUTION: The thimble tube and the fuel rod are inserted through each cell of the support grid between the upper nozzle and the lower nozzle, and during operation, a coolant for cooling the fuel rod flows through the cell in a vertical direction, the support grid is composed of a lowermost support grid, an uppermost support grid, and intermediate support grids arranged between the lowermost support grid and the uppermost support grid at a predetermined interval, and each of the intermediate support grids is provided with a flow condition changing means for changing a flow direction of the coolant from a vertical direction to a direction in which a horizontal flow is given in an upper part of the intermediate support grid during operation. In the fuel assembly for the pressurized water reactor, in at least a pair of adjacent intermediate support grids, a flow condition changing means in one intermediate support grid and a flow condition changing means in the other intermediate support grid are constituted so as to form flows in mutually different directions.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a fuel assembly for a pressurized water reactor (PWR). [Background technology]

[0002] In a PWR core, coolant flows from bottom to top around each fuel rod inserted in a loaded fuel assembly to remove heat generated by the nuclear reaction of the fuel.

[0003] If the heat flux from the fuel rods becomes excessive, boiling bubbles that form near the surface of the fuel rods, which are the heat transfer surface to the coolant, may grow and form stable localized boiling vapor films along the heat transfer surface, resulting in a transition from nucleate boiling to film boiling. This phenomenon is called "DNB" (Departure from Nucleate Boiling). When DNB occurs, the heat transfer surface is covered with a vapor film, which rapidly reduces the heat transfer efficiency and significantly reduces the heat removal capacity from the fuel rods. This results in a rapid temperature rise at the heat transfer surface, which, in the worst case, could lead to the failure of the fuel rods.

[0004] Therefore, conventionally, among the multiple support grids arranged at a predetermined interval in the vertical direction in the fuel assembly to support the inserted fuel rods, blade sections (mixing blades) for mixing the coolant have been provided on the intermediate support grids, excluding the uppermost support grid located at the upper end and the lowermost support grid located at the lower end, to promote stirring and mixing of the coolant, uniformize the temperature distribution of the coolant within the fuel assembly, and suppress the occurrence of DNB (for example, Patent Document 1).

[0005] Alternatively, a mixing grid, which is specialized solely for promoting the mixing of the coolant, is installed in the fuel assembly in addition to the support grid, thereby promoting the stirring and mixing of the coolant and suppressing the occurrence of DNB (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-28772 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-232726 Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, there has been a strong demand for nuclear power generation due to factors such as rising fuel prices caused by uneven distribution of fossil resources and the impact of carbon dioxide emissions on global warming, and it is therefore extremely desirable to make maximum use of existing reactors as a means of making effective use of nuclear power. Examples of making maximum use of existing reactors include increasing output power and lengthening the operating cycle. Increasing output power means an increase in the output per fuel assembly. Furthermore, lengthening the operating cycle means an increase in the number of fuel assemblies replaced per cycle, which places greater constraints on fuel arrangement within the reactor. In either case, it is conceivable that the fuel assemblies will be subjected to thermally more severe conditions than currently assumed operating conditions.

[0008] Thus, in order to operate nuclear reactors more safely and economically in the future, it is necessary to maintain and improve the safety margin against the occurrence of DNB even under more severe thermal conditions than presently. Therefore, it is desirable to further improve the stirring and mixing performance of the coolant in the fuel assembly compared to the technologies of Patent Documents 1 and 2.

[0009] Furthermore, when the safety margin against DNB occurrence is improved by improving the mixing performance through the addition of a support grid or a mixing grid, as in Patent Document 2, the coolant pressure loss increases. In domestic PWR cores, fuel assemblies of different designs are mixed and operated, and in order to ensure the compatibility of these fuel assemblies, it is necessary to suppress the increase in coolant pressure loss so that the pressure loss characteristics do not change significantly from those of conventional fuel assemblies.

[0010] Therefore, an object of the present invention is to provide a fuel assembly for a pressurized water reactor that can maintain and improve the safety margin against the occurrence of DNB even under severe thermal conditions, and that can further improve the stirring and mixing performance of the coolant in the fuel assembly so that the pressure loss of the coolant does not increase compared to conventional fuel assemblies. [Means for solving the problem]

[0011] As a result of extensive research into solving the above problems, the present inventors have found that the above problems can be solved by the invention described below, and have completed the present invention.

[0012] The invention described in claim 1 is A fuel assembly for a pressurized water reactor, in which a plurality of thimble tubes and fuel rods are inserted into each cell of a support grid having a plurality of cells between an upper nozzle and a lower nozzle, and in which a coolant for cooling the fuel rods flows vertically through the cells from the lower nozzle to the upper nozzle during operation, the support grid is composed of a lowest support grid, a highest support grid, and a plurality of intermediate support grids arranged at predetermined intervals between the lowest support grid and the highest support grid, Each of the intermediate support grids is provided with a flow condition changing means for changing the flow direction of the coolant from a vertical direction to a horizontal flow direction above the intermediate support grid during operation, A fuel assembly for a pressurized water reactor is characterized in that, in at least one pair of adjacent intermediate support grids, the flow condition changing means in one intermediate support grid and the flow condition changing means in the other intermediate support grid are configured to form flows in different directions.

[0013] The invention described in claim 2 is the flow condition changing means is arranged above each cell of the intermediate support grid through which the fuel rods are inserted, and is composed of a vane portion inclined at a predetermined angle; 2. The fuel assembly for a pressurized water reactor according to claim 1, characterized in that, depending on the arrangement position and inclination direction of the blade portions in each cell, during operation, the flow direction of the coolant above the intermediate support grid changes from a vertical direction to a direction in which a horizontal flow is applied.

[0014] The invention described in claim 3 is 3. The fuel assembly for a pressurized water reactor according to claim 2, wherein two or more of the intermediate support grids, each having the same arrangement position and inclination direction of the blade portion arranged at the top of each cell, are arranged in different directions from each other, thereby providing different horizontal flows from each other.

[0015] The invention described in claim 4 is 4. The fuel assembly for a pressurized water reactor according to claim 3, wherein a plurality of the intermediate support grids are arranged in directions that are different from each other by 90 degrees. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a fuel assembly for a pressurized water reactor, which can maintain and improve the safety margin against the occurrence of DNB even under severe thermal conditions, and which can further improve the stirring and mixing performance of the coolant in the fuel assembly so that the pressure loss of the coolant does not increase compared to conventional fuel assemblies. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic side view illustrating a conventional fuel assembly. [Figure 2] FIG. 2 is a schematic perspective view illustrating a support grid. [Figure 3] FIG. 10 is a diagram illustrating an example of a change in the flow condition of a coolant in a conventional fuel assembly. [Figure 4] FIG. 1 is a schematic perspective view of a fuel assembly according to an embodiment of the present invention. [Figure 5]FIG. 2 is a diagram illustrating an example of a change in the flow condition of a coolant in a fuel assembly according to an embodiment of the present invention. [Figure 6] FIG. 10 is a diagram illustrating another example of an intermediate support grid in a fuel assembly according to an embodiment of the present invention. [Figure 7] FIG. 1 is a diagram illustrating the relationship between the position of a fuel assembly and TNU (coolant temperature non-uniformity). [Figure 8] FIG. 2 is a diagram illustrating the relationship between the position of a fuel assembly and the surface temperature of a fuel rod. DETAILED DESCRIPTION OF THE INVENTION

[0018] [1] Overview of the present invention First, an outline of a fuel assembly for a pressurized water reactor (PWR fuel assembly) according to the present invention will be described.

[0019] In the PWR fuel assembly according to the present invention, a plurality of thimble tubes and fuel rods are inserted into each cell of a support grid having a plurality of cells between an upper nozzle and a lower nozzle, and coolant for cooling the fuel rods flows vertically through each cell of the support grid from the lower nozzle to the upper nozzle during operation. Each of the plurality of intermediate support grids among the support grids is provided with a flow condition changing means above the intermediate support grid that changes the coolant flow direction from vertical to a horizontal flow direction during reactor operation.

[0020] Furthermore, in the present invention, in at least one pair of adjacent intermediate support grids, the flow condition change means in one intermediate support grid and the flow condition change means in the other intermediate support grid are configured to form flows in different directions from each other.

[0021] With this configuration, by forming flows in different directions, as will be described later, the coolant flowing from the lower nozzle to the upper nozzle can be stirred and mixed in a more complex manner, thereby further improving the stirring and mixing performance of the coolant in the fuel assembly and maintaining and improving the safety margin against the occurrence of DNB even under severe thermal conditions.

[0022] [2] Specific embodiments The present invention will be specifically described below with reference to the drawings.

[0023] 1. Conventional PWR fuel assembly First, a conventional PWR fuel assembly will be described. Note that, here, a fuel assembly (17x17 type fuel assembly) having seven intermediate support grids with cells formed in 17 rows and 17 columns is used as an example for the description, but other types such as 14x14 type fuel assemblies and 15x15 type fuel assemblies can also be considered in the same way, with only the number of cells in the intermediate support grid and the number of intermediate support grids being different.

[0024] (1)Fuel assembly Fig. 1 is a schematic side view illustrating a conventional fuel assembly, and Fig. 2 is a schematic perspective view illustrating a support grid. Note that in Fig. 1, some of the illustrations on the left side of the page are omitted.

[0025] In Fig. 1, 10 is a fuel assembly, 20 is a fuel rod, 22 is a thimble tube that guides control rods, etc., 30 is a bottom nozzle, 35 is an upper nozzle, 40 is an intermediate support grid, 44 is a bottom support grid, and 45 is a top support grid. Also, in Fig. 2, 1 is a blade portion (mixing blade) as a flow condition changing means, 2 is a cell that constitutes the intermediate support grid, 46 is a grid plate provided on the intermediate support grid, 48 is a dimple that holds the inserted fuel rod 20, and 49 is a spring that holds the inserted fuel rod 20.

[0026] As shown in FIG. 1, a conventional fuel assembly 10 includes a bottom support grid 44, an intermediate support grid 40, and a top support grid 45, arranged at equal intervals between a bottom nozzle 30 and an upper nozzle 35, in that order from the bottom nozzle 30 side, and the periphery of each grid is covered by a frame (not shown). A predetermined number of thimble tubes 22 and a predetermined number of fuel rods 20 are inserted into cells 2 formed in the bottom support grid 44, the intermediate support grid 40, and the top support grid 45. As shown in FIG. 2, each support grid 40, 44, and 45 is formed by a grid plate 46, which defines rectangular cells 2 when viewed vertically. The fuel rods 20 are inserted into the cells 2 and held in place by springs 49 and dimples 48.

[0027] By flowing coolant vertically upward within the space of the cell 2 from the lower nozzle 30 to the upper nozzle 35 of the fuel assembly 10, the inserted fuel rods 20 are cooled and heat is removed.

[0028] 2, blades 1 are provided on the upper portions of the grid plates 46 of the cells 2 as flow condition changing means. The blades 1 are arranged at a predetermined angle, for example, 1 to 89° with respect to the flow direction of the coolant that flows vertically within the cells of the intermediate support grid, and are formed to a size and shape that does not interfere with the fuel rods 20. When the coolant flowing vertically from below hits the blades 1, the flow direction (flow condition) of the coolant changes from the vertical direction to a direction in which a horizontal flow is added, and then the coolant flows into the next intermediate support grid 40 and moves upward.

[0029] At this time, the coolant whose flow pattern has been changed is stirred and mixed before flowing into the next intermediate support grid 40, so that the coolant flowing upward can efficiently cool and remove heat from the fuel rods 20.

[0030] Although the mixing performance can be improved by increasing the size of each blade section, if the size is too large, the pressure loss of the coolant flowing inside the fuel assembly increases. Therefore, it is preferable to set the size appropriately, taking into consideration the balance between mixing performance and pressure loss.

[0031] (2) Coolant flow direction The change in the coolant flow direction at the intermediate support grid 40 will now be described.

[0032] Figure 3 is a diagram illustrating an example of a change in the coolant flow pattern in a conventional fuel assembly, and is a top view of 3 rows and 3 columns (9 cells) extracted from the 17 rows and 17 columns of cells in the intermediate support grid 40. The intermediate support grid 40 is equipped with blade portions 1a, 1b, 1c, and 1d as flow pattern changing means. In Figure 3, 20 is a fuel rod, and the arrows indicate the coolant flow direction (downward right and upward left) after the flow pattern change.

[0033] Here, the intermediate support grid 40 is made up of two types of cells with different arrangements of blades: one is cell A having blades 1a and 1b, and the other is cell B having blades 1c and 1d.

[0034] In cell A, the lower end of blade 1a is connected to the upper end of lattice plate 46 on the upper side of each cell and is inclined at a predetermined angle. This allows the coolant flow direction to be changed from the vertical direction to the direction in which a horizontal flow toward the lower right is applied (downward and right direction) as indicated by the arrow. Furthermore, the lower end of blade 1b is connected to the upper end of lattice plate 46 on the lower side of each cell and is inclined at a predetermined angle. This allows the coolant flow direction to be changed from the vertical direction to the direction in which a horizontal flow toward the upper left is applied (upper and left direction) as indicated by the arrow.

[0035] On the other hand, in cell B, blade 1c is connected to the upper end of lattice plate 46 on the right side of each cell and is inclined at a predetermined angle. This changes the flow direction of the coolant from the vertical direction to the direction in which a horizontal flow toward the upper left, as indicated by the arrow (upper left direction), is applied. Also, blade 1d is connected to the upper end of lattice plate 46 on the left side of each cell and is inclined at a predetermined angle. This changes the flow direction of the coolant from the vertical direction to the direction in which a horizontal flow toward the lower right, as indicated by the arrow, is applied (lower right direction).

[0036] As a result, as shown in Figure 3, for the entire intermediate support grid 40, coolant flows toward the bottom right due to the blade portion 1a of cell A and the blade portion 1d of cell B and coolant flows toward the top left due to the blade portion 1b of cell A and the blade portion 1c of cell B are alternately formed, and these flows are stirred and mixed in the space above the intermediate support grid 40.

[0037] In the conventional fuel assembly 10, the same intermediate support grids 40 are used for all seven stages of intermediate support grids. As a result, the fuel is uniformly stirred and mixed in all the intermediate support grids 40, which may result in insufficient cooling and heat removal of the fuel rods, in view of safer and more economical operation of nuclear reactors in the future.

[0038] 2. PWR fuel assembly according to this embodiment (1)Fuel assembly Fig. 4 is a schematic perspective view illustrating a PWR fuel assembly according to this embodiment. In the fuel assembly 10 shown in Fig. 4, two types of intermediate support grids 40 and 41 having different flow condition changing means are arranged alternately. The intermediate support grid 40 is the same as the intermediate support grid in the conventional fuel assembly shown in Fig. 1.

[0039] In the intermediate support grids 40 and 41, each flow condition changing means is composed of a blade portion arranged at the upper edge of each cell of the intermediate support grid, as in the conventional case, but by changing the arrangement position and inclination direction of the blade portion in each cell, the flow direction of each coolant in the intermediate support grids 40 and 41 can be changed to two or more different directions from the vertical direction.

[0040] In this case, by arranging two intermediate support grids with the same vane positions in different directions, for example, rotated by 90° from each other, it is possible to impart different horizontal flows to each other. In this case, the fuel assembly can be assembled using only intermediate support grids with the same vane positions and inclination directions, making it possible to provide a fuel assembly efficiently and at low cost.

[0041] In FIG. 4, the fuel assembly 10 is constructed using two types of intermediate support grids 40 and 41, but the fuel assembly may also be constructed using three or more types of intermediate support grids provided with flow condition changing means that create flows in different directions.

[0042] In addition, in Figure 4, two types of intermediate support grids 40, 41 are arranged alternately, but this is not limited to this, and it is sufficient that the flow condition changing means in at least one pair of adjacent intermediate support grids is configured to impart different horizontal flows to each other, and for example, there may be some locations where the same intermediate support grids are attached consecutively.

[0043] Furthermore, the flow condition changing means may be configured by a member other than the above-mentioned blade portion.

[0044] (2) Coolant flow direction The flow direction of the coolant in the first intermediate support grid 40 is as shown in Fig. 3. The flow direction of the coolant in the second intermediate support grid 41 will be described below.

[0045] 5 is a diagram illustrating an example of a change in the coolant flow condition in the fuel assembly according to this embodiment, and is a top view of 3 rows and 3 columns (9 cells) extracted from the 17 rows and 17 columns of cells in the second intermediate support grid 41, as in FIG. 3. As shown in FIG. 5, the second intermediate support grid 41 is arranged by rotating the first intermediate support grid 40 shown in FIG. 3 by 90 degrees around a vertical axis. Therefore, in the intermediate support grid 41, the coolant flow is changed to a direction (upper right and lower left) rotated by 90 degrees from the direction (lower right and upper left) of the change in the coolant flow caused by the intermediate support grid 40 shown in FIG. 3.

[0046] As described above, the horizontal flow of the coolant imparted to the first intermediate support grid 40 and the second intermediate support grid 41 is different, so the flow of the coolant passing through each support grid becomes more complex, allowing the coolant to be stirred and mixed more thoroughly than before, thereby efficiently cooling and removing heat from the fuel rods.

[0047] Furthermore, if the number of blade sections (mixing blades) is the same as in the conventional case where only a single type of support grid is used, the pressure loss of the coolant due to the fuel assembly does not increase compared to the conventional case and remains at the same level.

[0048] (3) Variations In this embodiment, the intermediate support grids are not limited to the intermediate support grids 40 and 41 shown in Figure 4, but may be modified examples such as the third intermediate support grid 42 shown in Figure 6(a) or the fourth intermediate support grid 43 shown in Figure 6(b).

[0049] (a) Third intermediate support grid 42 Fig. 6(a) is a partial top view of the third intermediate support grid 42. As shown in Fig. 6(a), the third intermediate support grid 42 corresponds to the first intermediate support grid 40, except that the positions of the blades 1c and 1d in cell B are changed and the blades 1a, 1b, 1c, and 1d are arranged adjacent to each other at the intersections of the grid plates 46 to form flow condition changing means. This makes it possible to generate more complex changes in the flow conditions at the intersections of the grid plates 46.

[0050] (b) Fourth intermediate support grid 43 FIG. 6(b) is a partial top view of the fourth intermediate support grid 43. As shown in FIG. 6(b), in the fourth intermediate support grid 43, the blades 1a and 1b are arranged in the same manner as in cell A of the first intermediate support grid 40, while the blades 1c and 1d are arranged in the same manner as in cell B of the second intermediate support grid 41. Four blades 1a, 1b, 1c, and 1d are provided in all cells to form a flow condition changing means. This allows for more complex flow condition changes. In the case of the fourth intermediate support grid, it is preferable to minimize the difference in pressure loss between different support grids by, for example, reducing the size of the blades (mixing blades).

[0051] (4) Evaluation of the coolant mixing performance Although thermal hydraulic tests can be conducted to evaluate the stirring and mixing performance of the intermediate support grid of a fuel assembly, CAE (Computer Aided Engineering) analysis such as CFD (Computational Fluid Dynamics) analysis can easily and inexpensively quantify the effect of the design of the intermediate support grid of a fuel assembly on the flow of coolant. Quantitative performance evaluation of the present invention can also be easily performed using CAE. [Example]

[0052] [Example 1 and Comparative Example 1] An analysis was performed using CFD analysis technology to improve the coolant stirring and mixing performance for a fuel assembly (Example 1) shown in FIG. 4 equipped with the first intermediate support grid 40 shown in FIG. 3 and the second intermediate support grid 41 shown in FIG. 5, which have different blade arrangements, and a fuel assembly (Comparative Example 1) shown in FIG. 1 equipped with only the intermediate support grid 40 shown in FIG. 3. In Example 1, the first intermediate support grids 40 and the second intermediate support grids 41 were arranged such that, counting from the bottom (upstream of the coolant flow) of the seven intermediate support grids, the first intermediate support grids 40 (four grids) were arranged at odd-numbered positions, and the second intermediate support grids 41 (three grids) were arranged at even-numbered positions. Note that the outer surfaces of the lower grid plates 46 in FIGS. 3 and 5 (the lowest grid plates including those not shown) were arranged on the same side of the fuel assembly, serving as the reference plane shown in FIG. 4.

[0053] The analysis conditions are as follows: (1) Horizontal size of fuel assembly: Size of fuel rods arranged evenly in a 5x5 (5 rows, 5 columns) pattern (2) Axial size of fuel assembly: Approximately 4 m (3) Number of intermediate support grids: 7 (4) Distance between intermediate support grids: Equal (5) Number of fuel rods: 25 (of which approximately half are high-power fuel rods and the rest are low-power fuel rods)

[0054] The coolant stirring and mixing performance was evaluated using the following two indices. (1) Thermal Non-Uniformity (TNU) (2) Surface temperature of high-power fuel rods (fuel rod surface temperature)

[0055] TNU is defined by the following equation (1): The smaller the TNU value, the more uniform the temperature across the cross section and the more the coolant is stirred and mixed.

[0056] TNU=[(1 / A)Σ i {(T i -T ave ) 2 A i}] 1 / 2 (1)

[0057] In addition, in formula (1), T ave =Σ i {(T i A i ) / A} (2) A=Σ i A i (3) and A: Cross-sectional area of ​​the coolant in the horizontal cross section of the fuel assembly T ave : Average coolant temperature in a horizontal cross section of the fuel assembly A i : Cross-sectional area of ​​any mesh of coolant in a horizontal cross section of a fuel assembly T i : Temperature of any mesh of coolant in a horizontal cross section of a fuel assembly is.

[0058] Figure 7 is a diagram explaining the relationship between the position of the fuel assembly and TNU (coolant temperature non-uniformity), and shows the analysis results (changes) of TNU at a position (height) above the uppermost intermediate support grid. In Figure 7, the horizontal axis represents the height position along the fuel rod, with upstream representing the lower side and downstream representing the upper side, and the support grid position represents the height of the upper end of the uppermost intermediate support grid. The analysis results of Example 1 are represented by "●" and the analysis results of Comparative Example 1 (conventional design) are represented by "+".

[0059] 7, the TNU of Example 1 is smaller than that of Comparative Example 1 (conventional design) at all positions (heights). This confirms that the layout design of the intermediate support grid of the fuel assembly of Example 1 is superior in the stirring and mixing performance of the coolant compared to the fuel assembly of Comparative Example 1.

[0060] Next, Figure 8 is a diagram explaining the relationship between the position of the fuel assembly and the fuel rod surface temperature, and shows the analysis results (changes) of the fuel rod surface temperature (fuel rod surface temperature) at a position (height) downstream from the bottom end of the fuel rod. In Figure 8, the horizontal axis represents the height position along the fuel rod, with upstream representing the lower side and downstream representing the upper side, and the support grid position represents the height of the upper end of the uppermost intermediate support grid. The analysis results of Example 1 are represented by "●" and the analysis results of Comparative Example 1 (conventional design) are represented by "+".

[0061] 8, the fuel rod surface temperature in Example 1 is smaller than that in Comparative Example 1 (conventional design) at all positions (heights). This confirms that the support grid layout design of the fuel assembly in Example 1 has better coolant stirring and mixing performance than the fuel assembly in Comparative Example 1.

[0062] Generally, the surface temperature of the fuel rods increases from the upstream side to the downstream side due to heat generation from the fuel rods, but in Figure 8, it temporarily drops in the area above the upper end of the intermediate support grid. This is thought to indicate that the stirring and mixing effect of the intermediate support grid makes the coolant temperature uniform and allows for effective cooling. [Explanation of symbols]

[0063] 1, 1a to 1d: Blade section 2 cells 10 Fuel assembly 20 fuel rods 22 Thimble pipe 30 Lower Nozzle 35 Upper nozzle 40 First intermediate support grid 41 Second intermediate support grid 42 Third intermediate support grid 43 Fourth intermediate support grid 44 Bottom support grid 45 Top support grid 46 Lattice plate 48 Dimples 49 Spring

Claims

1. A fuel assembly for a pressurized water reactor, in which a plurality of thimble tubes and fuel rods are inserted into each cell of a support grid having a plurality of cells between an upper nozzle and a lower nozzle, and in which a coolant for cooling the fuel rods flows vertically through the cells from the lower nozzle to the upper nozzle during operation, the support grid is composed of a lowest support grid, a highest support grid, and a plurality of intermediate support grids arranged at predetermined intervals between the lowest support grid and the highest support grid, Each of the intermediate support grids is provided with a flow condition changing means for changing the flow direction of the coolant from a vertical direction to a horizontal flow direction above the intermediate support grid during operation, A fuel assembly for a pressurized water reactor, characterized in that, in at least one pair of adjacent intermediate support grids, the flow condition changing means of one intermediate support grid and the flow condition changing means of the other intermediate support grid are configured to form flows in different directions from each other.

2. the flow condition changing means is arranged above each cell of the intermediate support grid through which the fuel rods are inserted, and is composed of a vane portion inclined at a predetermined angle; 2. The fuel assembly for a pressurized water reactor according to claim 1, wherein the arrangement position and inclination direction of the blade portions in each cell are such that, during operation, the flow direction of the coolant above the intermediate support grid changes from a vertical direction to a direction in which a horizontal flow is applied.

3. 3. The fuel assembly for a pressurized water reactor according to claim 2, wherein two or more of the intermediate support grids having the same arrangement position and inclination direction of the blade portions arranged at the upper part of each cell are arranged in different directions from each other, thereby providing different horizontal flows from each other.

4. 4. The fuel assembly for a pressurized water nuclear reactor according to claim 3, wherein a plurality of said intermediate support grids are arranged in directions different from each other by 90 degrees.

Images