Nuclear fuel assemblies with improved seismic performance and control rod value

Abstract

To provide a nuclear fuel assembly suitable for a Small Modular Reactor (SMR).SOLUTION: A nuclear fuel assembly comprises: a spacer grid (110) in which 17×17 grid cells are provided; a plurality of fuel rods (120) arranged in the grid cells of the spacer grid (110); guide tubes (130) arranged and fixed in the grid cells of the spacer grid (110); and an upper end fitting (140) and a lower end fitting (150) fixed to an upper end and a lower end of the guide tubes (130), respectively. In the nuclear fuel assembly, eight guide tubes (130) are arranged in main axis directions (Ixx)(Iyy) with respect to a central cell of the spacer grid (110), four guide tubes are arranged in a diagonal direction (Ixy), and sixteen guide tubes are arranged in non-diagonal directions (Ixxy)(Ixyy).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a nuclear fuel assembly suitable for a small modular reactor (SMR). [Background technology]

[0002] The core of a typical light water reactor includes a number of fuel assemblies, each of which includes a number of long fuel rods and a number of guide tubes in the form of tubes used to guide the control rods during operation. The guide tubes are fixed to upper and lower end fixed bodies of the fuel assembly to form a framework of the fuel assembly.

[0003] Control rods contain a material that absorbs the neutrons produced during the fission process, and traditionally materials with a high neutron capture cross section, such as boron carbide (B4C), hafnium (Hf) or silver-indium-cadmium (Ag-In-Cd), have been used.

[0004] The fuel assemblies of a typical light water reactor, including the APR1400, a model nuclear power plant in Korea, are the same regardless of whether they are in the control rod position or the non-control rod position inside the core, and all fuel assemblies have guide tubes even when used in a core position where there are no guided control rods, and the guide tubes are used to form the fuel assemblies into a single framework.

[0005] A typical nuclear fuel assembly is configured with n of 17 × 17, with a total of 289 lattice cells, of which 264 lattice cells are used to place fuel rods, 24 lattice cells are used to place guide tubes, and the remaining lattice cell is located in the center of the support lattice and is used to place instrumentation tubes.

[0006] Meanwhile, development is underway on SMRs, which dramatically reduce the size and output of conventional reactors. Theoretically, their smaller size makes it easier to adjust output and cool the reactor. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-1994-0003796 (Publication Date: 1994.05.03) [Patent Document 2] Korean Patent Publication No. 10-1992-0007739 (Publication Date: 1992.09.16) Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to provide a nuclear fuel assembly suitable for a small modular reactor (SMR). [Means for solving the problem]

[0009] To achieve the above object, the nuclear fuel assembly according to the present invention includes a support lattice having 17x17 lattice cells, a plurality of fuel rods arranged in the lattice cells of the support lattice, guide tubes arranged and fixed in the lattice cells of the support lattice, and upper end fixtures and lower end fixtures fixed to the upper and lower ends of the guide tubes, respectively, and is characterized in that, based on the central cell of the support lattice, eight guide tubes are arranged in the main axis direction (Ixx)(Iyy), four guide tubes are arranged in the diagonal direction (Ixy), and sixteen guide tubes are arranged in the non-diagonal direction (Ixxy)(Ixyy).

[0010] Preferably, the guide tubes are arranged at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6) in the major axis direction.

[0011] Preferably, the guide tubes are arranged diagonally at (5,5), (-5,-5), (-5,5), and (5,-5).

[0012] Preferably, the guide tubes are arranged in a non-diagonal manner at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4), and (3,-6).

[0013] Next, the support grid assembly according to the present invention includes a support grid having 17 x 17 grid cells, and sleeves fixed within the grid cells of the support grid and assembled with guide tubes, characterized in that eight sleeves are arranged in the main axis direction (Ixx) (Iyy), four sleeves are arranged in the diagonal direction (Ixy), and 16 sleeves are arranged in the off-diagonal direction (Ixxy) (Ixyy) based on the central cell of the support grid.

[0014] Preferably, the sleeves are axially arranged at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6).

[0015] Preferably, the sleeves are arranged diagonally at (5,5), (-5,-5), (-5,5), and (5,-5).

[0016] Preferably, the sleeves are arranged in a non-diagonal manner at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4), and (3,-6). [Effects of the Invention]

[0017] The nuclear fuel assembly according to the present invention includes a support lattice having 17x17 lattice cells, a plurality of fuel rods arranged in the lattice cells of the support lattice, guide tubes arranged and fixed in the lattice cells of the support lattice, and upper end fixtures and lower end fixtures fixed to the upper and lower ends of the guide tubes, respectively. The guide tubes are arranged in the main axis direction (Ixx)(Iyy) with respect to the central cell of the support lattice as a reference, with eight guide tubes arranged in the main axis direction (Ixx)(Iyy), four guide tubes arranged in the diagonal direction (Ixy), and 16 guide tubes arranged in the off-diagonal direction (Ixxy)(Ixyy), for a total of 28 guide tubes. This has the following effects of improving control rod worth and seismic performance.

[0018] 1) Improvement of control rod worth for boron-free core operation Conventional commercial pressurized water reactors are operated with their control rods fully withdrawn for most of their operation, with excess reactivity controlled by water-soluble boron and burnable absorber rods. In contrast, recent developments in small modular reactors have focused on boron-free cores, which require the insertion of control rods from the beginning of the operating cycle. Excess reactivity, controlled by water-soluble boron and burnable absorber rods in existing commercial reactors, can be controlled solely by enriched Gd burnable absorber rods in SMR cores, and the remaining excess reactivity (below 1000 pcm) can be controlled to a critical state (keff = 1.0) by inserting adjustable control rods. Therefore, the present invention satisfies the requirement for subcriticality through improved control rod worth.

[0019] 2) Improved seismic performance The present invention has the effect of increasing the number of guide tubes fixed to the upper end fixed body and the lower end fixed body, thereby increasing the bending rigidity of the nuclear fuel assembly skeleton and increasing the natural frequency, thereby reducing the displacement of the nuclear fuel assembly at the same energy level, and reducing the generated load, thereby improving seismic resistance. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a front view of a nuclear fuel assembly according to an embodiment of the present invention. [Figure 2] FIG. 2 is a plan view illustrating a fuel rod placement pattern of a support grid assembly according to an embodiment of the present invention. [Figure 3] FIG. 2 is a plan view of a control rod assembly according to an embodiment of the present invention. [Figure 4] FIG. 1 is a graph showing the subcriticality evaluation results under the control rod 24 pin reference ARI and N-1 conditions, which shows the evaluation results under all control rod insertion conditions (N-1) except for one control rod with the maximum control rod worth. [Figure 5] FIG. 1 is a graph showing the subcriticality evaluation results under the control rod 28 pin reference ARI and N-1 conditions, which shows the evaluation results under all control rod insertion conditions (N-1) except for one control rod with the maximum control rod worth. DETAILED DESCRIPTION OF THE INVENTION

[0021] The specific structural or functional descriptions presented in accordance with the embodiments of the present invention are merely exemplary for the purpose of describing embodiments in accordance with the inventive concepts, which may be embodied in various forms, and should not be construed as being limited to the embodiments set forth herein, but should be understood to include all modifications, equivalents, or alternatives falling within the spirit and scope of the present invention.

[0022] Meanwhile, the terms used in this specification are merely used to describe specific embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly dictates otherwise. The terms "comprise" or "have" used in this specification are intended to specify the presence of implemented features, numbers, steps, operations, components, parts, or combinations thereof, and should not be understood to preclude the presence or possibility of adding one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0023] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0024] FIG. 1 is a front view of a support grid assembly according to an embodiment of the present invention, with the fuel rods omitted.

[0025] Referring to FIG. 1, the nuclear fuel assembly 100 of this embodiment includes a support grid 110, guide tubes 130 arranged and fixed within the lattice cells of the support grid 110, and upper end fixtures 140 and lower end fixtures 150 fixed to the upper and lower ends of the guide tubes 130, respectively. The fuel rods are supported by the support grid 110 and arranged axially parallel to the guide tubes 130.

[0026] The upper end fixture 140 may be provided with a coil spring or a leaf spring to provide a pressing force against axial movement of the nuclear fuel assembly, and the lower end fixture 150 may be provided with a nozzle through which cooling water flows.

[0027] The support grid 110 is formed by assembling a number of lattice plates (straps) horizontally and vertically to form a lattice shape, and one fuel rod or guide tube is placed inside each square lattice cell partitioned by the lattice plates.

[0028] The support grid 110 has dimples and grid springs provided in the grid cells into which the fuel rods are inserted to elastically support the fuel rods, and may be fixed via sleeves 111 in the grid cells into which the guide tubes 130 are inserted. While FIG. 1 illustrates the support grid 110 being provided with the sleeves 111 and assembled with the guide tubes 130, the support grid 110 and the guide tubes 130 may also be fixed by direct welding. Such a support grid 110 is typically fabricated by sheet metal processing and welding processes, but may also be fabricated using a known metal 3D printing device and is not limited to a specific manufacturing method.

[0029] Preferably, the support grid 110 has 17 x 17 grid cells, and the guide tubes 130 are arranged at specific positions within the grid cells of the support grid 110, making a total of 28 of them. An embodiment of this will now be described in detail.

[0030] 2 is a plan view showing the arrangement pattern of fuel rods in a support grid assembly according to an embodiment of the present invention. To facilitate understanding, the horizontal direction of the support grid 110 is the x-axis, the vertical direction is the y-axis, and the position of a specific grid cell is displayed in a two-axis coordinate system (x, y), with the coordinates of the center cell being (0, 0).

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[0031] Referring to FIG. 2, the support grid 110 is made up of 17×17 grid cells, with an instrument tube located in the center cell (0,0), a total of 28 guide tubes arranged at specific positions, and fuel rods 120 arranged in the remaining grid cells, although the fuel rods are omitted from FIG. 2.

[0032] For purposes of describing the present invention, the principal axis direction refers to the horizontal (Ixx) and vertical (Iyy) directions of the support grid 110 relative to the center cell (0,0), the on-diagonal direction refers to the 45° (135°) diagonal direction (Ixy), and the off-diagonal direction refers to the diagonal direction (Ixxy) (Ixyy) other than 45°. The principal axis direction, diagonal direction, and off-diagonal direction include both the positive (+) direction and the negative (-) direction.

[0033] Preferably, eight guide tubes are arranged in the main axis direction (Ixx) (Iyy) with respect to the central cell (0,0) of the support grid 110, four are arranged in the diagonal direction (Ixy), and 16 are arranged in the off-diagonal direction (Ixxy) (Ixyy), for a total of 28 guide tubes provided in the support grid 110. Meanwhile, as described above, the arrangement position of each guide tube in the support grid may be such that the guide tube is directly fixed, or a sleeve may be provided and the guide tube may be fixed via the sleeve.

[0034] Preferably, the off-diagonal directions (Ixxy) and (Ixyy) are arranged at equal angles (θ) with respect to the diagonal direction (Ixy), and include a first off-diagonal direction (Ixxy) biased to the x-axis and a second off-diagonal direction (Ixyy) biased to the y-axis. In this embodiment, the first off-diagonal direction (Ixxy) and the second off-diagonal direction (Ixyy) each form an included angle (θ) of 18° with the diagonal direction (Ixy).

[0035] Preferably, the guide tubes are arranged at (6,0), (3,0), (-3,0), (-6,0) in the horizontal direction (Ixx) and at (0,6), (0,3), (0,-3), (0,-6) in the vertical direction (Iyy).

[0036] Preferably, the guide tubes are arranged in diagonal directions (Ixy) at (5,5), (-5,-5), (-5,5), (5,-5).

[0037] Preferably, the guide tubes are arranged in the first off-diagonal direction (Ixxy) at {(6,3), (4,2), (-4,-2), (-6,-3)}, {(-6,3), (-4,2), (4,-2), (6,-3)} and in the second off-diagonal direction (Ixyy) at {(3,6), (2,4), (-2,-4), (-3,-6)}, {(-3,6), (-2,4), (2,-4), (3,-6)}.

[0038] FIG. 3 is a plan view of a control rod assembly according to an embodiment of the present invention.

[0039] As shown in FIG. 3, a control rod assembly 200 according to an embodiment of the present invention includes a cylindrical spider body 210, a plurality of spider vanes 220 extending radially from the spider body 210, and spider fingers 230 provided on the spider vanes 220 to fix guide tubes, and each spider finger 230 corresponds to one of the 28 guide tubes described above, and the control rod 210 is assembled thereto.

[0040] Thus, the present invention allows for the insertion of 28 control rods at specific locations in a support grid having 17x17 grid cells, thereby improving boron-free operation and seismic performance.

[0041] Specifically, from the perspective of fuel assembly design, it was confirmed that applying 28 control rods to a fuel assembly with a 17x17 lattice cell support lattice increases control rod worth by 17%, reduces fuel loading by 1.5%, and improves seismic performance. It was also confirmed that the reactivity is relatively higher and the core operating cycle length is increased compared to a fuel assembly using conventional 24-pin control rods.

[0042] Additionally, the subcriticality assessment results for the 24-pin control rod system showed that the effective multiplication factor (keff) was below 0.95 under ARI conditions and below 0.99 under N-1 conditions, fulfilling the subcriticality conditions with little margin (see Figure 4). The subcriticality assessment results for the 28-pin control rod system under conservative conditions showed that the effective multiplication factor (keff) was 0.98 under N-1 conditions, further securing margin (see Figure 5), further strengthening the safety of the shutdown margin and providing four spaces for a Top-Mounted In-Core Instrumentation Nozzle (TM-ICI) or control rod assembly compared to the 24-pin system.

[0043] Secondly, from the perspective of mechanical design, the 28-pin control rods are designed to increase control rod value in relation to boron-free operation, and have the advantage of facilitating the accommodation of TM-ICI and the insertion / removal of control rods. In conventional nuclear power plants, core reactivity was adjusted by diluting the toxic substance boron in the coolant, but this could have adverse effects such as crud deposition not only on the nuclear fuel but also on the main components of the core. In this invention, the boron-free operation of small modular reactors requires adjustment of core reactivity only using control rods, and therefore the control rod value of this invention is increased compared to conventional systems, making it effective for boron-free operation.

[0044] Furthermore, the present invention can enhance the mechanical properties of the framework of the nuclear fuel assembly by increasing the number of guide tubes fixed to the upper end fixed body and the lower end fixed body.

[0045] The present invention as described above is not limited to the above-described embodiments and the accompanying drawings, and it will be apparent to those skilled in the art to which the present invention pertains that various substitutions, modifications and changes are possible within the scope of the technical idea of ​​the present invention. [Explanation of symbols]

[0046] 100 Nuclear fuel assembly 110 Support grid 111 Sleeve 120 fuel rods 130 Guide tube 140 Upper end fixed body 150 Lower end fixing body

Claims

1. A nuclear fuel assembly including a support grid having 17 x 17 grid cells, a plurality of fuel rods arranged in the grid cells of the support grid, guide tubes arranged and fixed in the grid cells of the support grid, and an upper end fixture and a lower end fixture fixed to the upper and lower ends of the guide tubes, The nuclear fuel assembly, wherein eight of the guide tubes are arranged in the main axis direction (Ixx) (Iyy) with respect to the central cell of the support grid, four of the guide tubes are arranged in the diagonal direction (Ixy), and sixteen of the guide tubes are arranged in the non-diagonal direction (Ixxy) (Ixyy).

2. 2. The nuclear fuel assembly of claim 1, wherein the guide tubes are arranged in the major axis direction at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6), where x in (x,y) represents the horizontal lattice cell position of the center cell (0,0) and y represents the vertical lattice cell position of the center cell (0,0).

3. 2. The nuclear fuel assembly of claim 1, wherein the guide tubes are arranged diagonally at (5,5), (-5,-5), (-5,5), and (5,-5) (wherein in (x,y), x represents the horizontal lattice cell position of the center cell (0,0) and y represents the vertical lattice cell position of the center cell (0,0)).

4. 2. The nuclear fuel assembly of claim 1, wherein the guide tubes are arranged in a non-diagonal direction at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4), and (3,-6), where x in (x,y) represents the horizontal lattice cell position of the center cell (0,0) and y represents the vertical lattice cell position of the center cell (0,0).

5. A support grid assembly including a support grid having 17x17 grid cells and a sleeve fixed in the grid cell of the support grid and assembled with a guide tube, A support grid assembly, characterized in that eight of the sleeves are arranged in the main axis direction (Ixx) (Iyy), four are arranged in the diagonal direction (Ixy), and sixteen are arranged in the non-diagonal direction (Ixxy) (Ixyy) based on the central cell of the support grid.

6. 6. The support grid assembly of claim 5, wherein the sleeves are arranged along the major axes at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6), where x in (x,y) represents the horizontal grid cell position of the center cell (0,0) and y represents the vertical grid cell position of the center cell (0,0).

7. 6. The support grid assembly of claim 5, wherein the sleeves are diagonally positioned at (5,5), (-5,-5), (-5,5), and (5,-5), where x in (x,y) represents the horizontal grid cell position of center cell (0,0) and y represents the vertical grid cell position of center cell (0,0).

8. 6. The support grid assembly of claim 5, wherein the sleeves are arranged in off-diagonal directions at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4), and (3,-6), where x in (x,y) represents the horizontal grid cell position of center cell (0,0) and y represents the vertical grid cell position of center cell (0,0).

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