Nuclear fuel assembly with improved seismic performance and control rod worth

Abstract

The present invention relates to a nuclear fuel assembly suitable for a small modular reactor (SMR), the nuclear fuel assembly comprising: a spacer grid (110) having 17x17 grid cells; 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 the upper end and the lower end of the guide tubes (130), respectively, wherein the guide tubes (130) are characterized in that eight guide tubes are arranged in the main axis directions (Ixx)(Iyy) with respect to the central cell of the spacer grid (110), four guide tubes are arranged in the diagonal direction (Ixy), and sixteen guide tubes are arranged in the non-diagonal directions (Ixxy)(Ixyy).

Description

Nuclear fuel assembly with improved seismic performance and control rods

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

[0002] The core of a typical light-water reactor (LWR) contains multiple fuel assemblies, each of which contains a number of long fuel rods and a number of guide tubes, which are tubular structures used to guide the control rods during their movement. The guide tubes are secured to the upper and lower mounting members of the fuel assembly, forming the skeleton of the fuel assembly.

[0003] Control rods contain materials that absorb neutrons generated during fission. Traditionally, materials with high neutron capture cross sections, such as boron carbide (B4C), hafnium, or silver-indium-cadmium, have been used.

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

[0005] A typical nuclear fuel assembly consists of a 17×17 grid with a total of 289 grid cells, of which 264 grid cells contain fuel rods, 24 grid cells contain guide tubes, and the remaining one grid cell is located in the center of the grid and contains instrumentation tubes.

[0006] Meanwhile, development is underway on SMRs that dramatically reduce the size and output of traditional reactors, and theoretically have the advantage of being smaller in size, making output control and reactor cooling easier.

[0007] [Prior Art Literature]

[0008] [Patent Document]

[0009] Patent Document 1: Korean Patent Gazette No. 10-1994-0003796 (Publication Date: May 3, 1994)

[0010] Patent Document 2: Korean Patent Gazette No. 10-1992-0007739 (Publication Date: September 16, 1992)

[0011] The present invention seeks to provide a nuclear fuel assembly suitable for a small modular reactor (SMR).

[0012] In order to achieve these purposes, a nuclear fuel assembly according to the present invention comprises a support grid having 17×17 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 fixing body and a lower fixing body fixed to the upper and lower ends of the guide tubes, wherein eight of the guide tubes 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) with respect to the center cell of the support grid.

[0013] Preferably, the guide tube is arranged at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6) in the direction of the main axis.

[0014] Preferably, the guide tubes are arranged diagonally at (5,5), (-5,-5), (-5,5), and (5,-5).

[0015] Preferably, the guide tubes are arranged in the 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).

[0016] Next, the support grid assembly according to the present invention is a support grid assembly including a support grid having 17×17 grid cells, and a sleeve fixed within the grid cells of the support grid and assembled with a guide tube, wherein the sleeves are characterized in that 8 are arranged in the main axis direction (Ixx)(Iyy), 4 are arranged in the diagonal direction (Ixy), and 16 are arranged in the non-diagonal direction (Ixxy)(Ixyy) based on the center cell of the support grid.

[0017] Preferably, the sleeve is positioned at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6) in the direction of the main axis.

[0018] Preferably, the sleeves are arranged diagonally at (5,5), (-5,-5), (-5,5), and (5,-5).

[0019] Preferably, the sleeves are arranged non-diagonally 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).

[0020] A nuclear fuel assembly according to the present invention comprises a support grid having 17×17 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 fixing body and a lower fixing body fixed to the upper and lower ends of the guide tubes, wherein the guide tubes are arranged in a number of eight in a main axis direction (Ixx)(Iyy) with respect to the center cell of the support grid, four in a diagonal direction (Ixy), and 16 in a non-diagonal direction (Ixxy)(Ixyy), for a total of 28 guide tubes, thereby having the following effects of improving control rod worth and seismic performance.

[0021] 1) Improvement of control rods for boron-free core operation

[0022] A typical commercial pressurized water reactor operates with the control rods fully withdrawn for most of its life, and the excess reactivity is controlled by soluble boric acid and combustible absorber rods. On the other hand, the recent development of small modular reactors is proceeding with a boric acid-free core as the top priority, and therefore requires control rod insertion from the beginning of the cycle. The excess reactivity, which was controlled by soluble boric acid and combustible absorber rods in existing commercial reactors, can be controlled only by the enriched Gd combustible absorber rods in the SMR core, and the remaining excess reactivity (less than 1000 pcm) can be controlled to a critical (keff = 1.0) state by inserting a modulating control rod. Therefore, the present invention satisfies the subcriticality requirement by improving the control rod value.

[0023] 2) Improved seismic performance

[0024] The present invention increases the number of guide tubes fixed to the upper and lower fixing bodies, thereby increasing the bending rigidity of the skeleton of the nuclear fuel assembly, increasing the natural frequency, reducing the displacement of the nuclear fuel assembly at the same energy level, and also reducing the load generated, thereby enhancing the seismic performance.

[0025] Figure 1 is a front view of a nuclear fuel assembly according to an embodiment of the present invention.

[0026] FIG. 2 is a plan view showing the arrangement pattern of fuel rods of a support grid assembly according to an embodiment of the present invention.

[0027] Figure 3 is a plan view of a control rod assembly according to an embodiment of the present invention.

[0028] Figure 4 is a graph showing the results of the subcriticality evaluation under ARI and N-1 conditions based on 24 control rod pins, and is the evaluation result under all control rod insertion conditions except for one control rod with the largest control rod value (N-1).

[0029] Figure 5 is a graph showing the results of the subcriticality evaluation under ARI and N-1 conditions based on 28 control rod pins, and is the evaluation result under all control rod insertion conditions excluding the control rod with the largest control rod value (N-1).

[0030] The specific structural and functional descriptions presented in the embodiments of the present invention are merely illustrative for the purpose of explaining embodiments according to the concept of the present invention, and embodiments according to the concept of the present invention may be implemented in various forms. Furthermore, they should not be construed as being limited to the embodiments described herein, but should be understood to include all modifications, equivalents, and substitutes included within the spirit and technical scope of the present invention.

[0031] Meanwhile, the terminology used in this specification is only used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. It should be understood that the terms "comprises" or "has" in this specification are intended to specify the presence of implemented features, numbers, steps, operations, components, parts, or combinations thereof, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0032] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

[0033] Figure 1 is a front view of a support grid assembly according to an embodiment of the present invention, with the fuel rods omitted.

[0034] Referring to FIG. 1, the nuclear fuel assembly (100) of the present embodiment includes a support grid (110), guide tubes (130) arranged and fixed within the lattice cells of the support grid (110), and upper fixing members (140) and lower fixing members (150) respectively fixed to the upper and lower ends of the guide tubes (130), and the fuel rods are supported on the support grid (110) and arranged axially parallel to the guide tubes (130).

[0035] The upper fixing body (140) may be provided with a coil spring or a plate spring to provide a pressing force for the axial movement of the nuclear fuel assembly, and the lower fixing body (150) may be provided with a nozzle through which cooling water is introduced.

[0036] The support grid (110) is assembled to have a lattice shape by assembling a plurality of grid plates (straps) in the horizontal and vertical directions, and one fuel rod or guide tube is placed inside each square grid cell divided by the grid plates.

[0037] The support grid (110) is provided with dimples and grid springs within the grid cell into which the fuel rod is inserted to elastically support the fuel rod, and can be fixed within the grid cell into which the guide tube (130) is inserted via a sleeve (111). Meanwhile, in FIG. 1, the support grid (110) is provided with a sleeve (111) and is assembled with the guide tube (130), but the support grid (110) and the guide tube (130) may be fixed by direct welding. The support grid (110) is generally manufactured by sheet metal processing and welding processes, but can be manufactured using a known metal 3D printing device and is not limited to a specific manufacturing method.

[0038] Preferably, the support grid (120) is provided with 17×17 grid cells, and the guide tube (130) is arranged at a unique position among the grid cells of the support grid (110), and is characterized by being composed of a total of 28, and an embodiment thereof will be described in detail below.

[0039] FIG. 2 is a plan view showing the arrangement pattern of fuel rods of a support grid assembly according to an embodiment of the present invention. To help understanding, the horizontal direction of the support grid (110) is the x-axis, the vertical direction is the y-axis, and the location of a specific grid cell is indicated in a two-axis coordinate system of (x, y), and the coordinate of the center cell is indicated as (0, 0). In FIG. 2 is displayed as -1.

[0040] Referring to Fig. 2, the support grid (110) is composed of 17×17 grid cells, the center cell (0,0) is where the measurement tube is located, and a total of 28 guide tubes are placed at specific locations, and fuel rods (120) are placed in the remaining grid cells, but the fuel rods are omitted in Fig. 2.

[0041] In the description of the present invention, the principal axis direction means the horizontal direction (Ixx) and vertical direction (Iyy) of the support grid (110) with respect to the center cell (0,0), the on-diagonal direction means the diagonal direction (Ixy) of 45° (135°), and the off-diagonal direction means the diagonal direction (Ixxy) (Ixyy) other than 45°. The principal axis direction, the diagonal direction, and the off-diagonal direction include both positive (+) and negative (-) directions.

[0042] Preferably, 8 guide tubes are arranged in the main axis direction (Ixx)(Iyy) based on the center cell (0,0) of the support grid (110), 4 are arranged in the diagonal direction (Ixy), and 16 are arranged in the non-diagonal direction (Ixxy)(Ixyy), so that a total of 28 guide tubes are provided in the support grid (110). Meanwhile, as described above, each guide tube arrangement position of the support grid may be directly fixed to the guide tube, or a sleeve may be provided so that the guide tube may be fixed via the sleeve.

[0043] Preferably, the off-diagonal direction (Ixxy) (Ixyy) is arranged at an equal angle (θ) with respect to the diagonal direction (Ixy) and includes a first off-diagonal direction (Ixxy) biased toward the x-axis and a second off-diagonal direction (Ixyy) biased toward the y-axis. In the present embodiment, the first off-diagonal direction (Ixxy) and the second off-diagonal direction (Ixyy) each have an angle (θ) of 18° with the diagonal direction (Ixy).

[0044] 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).

[0045] Preferably, the guide tubes are positioned at (5,5), (-5,-5), (-5,5), (5,-5) in the diagonal direction (Ixy).

[0046] Preferably, the guide tubes are arranged in the first off-diagonal direction (Ixxy) as {(6,3), (4,2), (-4,-2), (-6,-3)}, {(-6,3), (-4,2), (4,-2), (6,-3)}, and in the second off-diagonal direction (Ixyy) as {(3,6), (2,4), (-2,-4), (-3,-6)}, {(-3,6), (-2,4), (2,-4), (3,-6)}.

[0047] Figure 3 is a plan view of a control rod assembly according to an embodiment of the present invention.

[0048] As illustrated 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 the 28 guide tubes described above, and the control rod (210) is assembled thereto.

[0049] In this way, the present invention can improve boron-free operation and seismic performance by allowing 28 control rods to be inserted at specific locations in a support grid having 17×17 grid cells.

[0050] Specifically, from the design perspective of the nuclear fuel assembly, it was confirmed that by applying 28 control rods to the nuclear fuel assembly of the 17×17 grid cell support grid, the control rod worth increased by 17%, the nuclear fuel loading amount decreased by 1.5%, and there was an effect of improving the seismic performance. In addition, it was confirmed that the reactivity was relatively high and the core cycle length also increased compared to the nuclear fuel assembly that adopted the conventional 24-pin control rod.

[0051] In addition, in the subcriticality evaluation results based on 24 control rod pins, the effective multiplication factor (keff) was evaluated to be 0.95 or less under the ARI condition and 0.99 or less under the N-1 condition, which means that the subcriticality condition was satisfied with almost no margin (see Fig. 3). Under conservative conditions, the subcriticality evaluation results based on 28 control rod pins were evaluated to be 0.98 under the N-1 condition, which means that the margin was secured more (see Fig. 4), and therefore the safety of the stoppage margin was further strengthened, and four additional holes for the Top-Mounted In-Core Instrumentation Nozzle (TM-ICI) or the control rod assembly can be secured compared to 24 control rod pins.

[0052] Next, from a mechanical design perspective, the 28-pin control rod is intended to increase the control rod value in relation to boron-free operation, which has the effect of facilitating TM-ICI acceptance and control rod loading / withdrawal. In conventional nuclear power plants, core reactivity was controlled by diluting boron, a toxic substance, in the coolant, but this can cause adverse effects such as crud deposition not only on nuclear fuel but also on major core components. In the present invention, core reactivity control is required using only control rods for boron-free operation of a small modular reactor, and therefore, the present invention is effective for boron-free operation by increasing the control rod value compared to the past.

[0053] In addition, the present invention can enhance the mechanical characteristics of the skeleton of a nuclear fuel assembly by increasing the number of guide tubes fixed to the upper and lower fixing bodies.

[0054] The present invention described above is not limited to the above-described embodiments and the attached drawings, and it will be apparent to a person skilled in the art to which the present invention pertains that various substitutions, modifications, and changes are possible within a scope that does not depart from the technical spirit of the present invention.

[0055] [Explanation of symbols]

[0056] 100: Nuclear fuel assembly 110: Support grid

[0057] 111: Sleeve 120: Fuel rod

[0058] 130: Guide tube 140: Upper fixing body

[0059] 150: Lower fixed body

Claims

In a nuclear fuel assembly including a support grid having 1.17×17 grid cells, a plurality of fuel rods arranged within the grid cells of the support grid, a guide tube arranged and fixed within the grid cells of the support grid, and an upper fixing body and a lower fixing body fixed to the upper and lower ends of the guide tube, respectively, A nuclear fuel assembly characterized in that the above guide tubes are arranged in a number of eight in the main axis direction (Ixx)(Iyy), four in the diagonal direction (Ixy), and 16 in the non-diagonal direction (Ixxy)(Ixyy) based on the center cell of the support grid.

2. A nuclear fuel assembly according to claim 1, characterized in that 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 main axis direction. However, in (x,y), x is the horizontal grid cell position of the center cell (0,0), and y is the vertical grid cell position of the center cell (0,0).

3. A nuclear fuel assembly according to claim 1, characterized in that the guide tubes are arranged diagonally at (5,5), (-5,-5), (-5,5), and (5,-5). However, in (x,y), x is the horizontal grid cell position of the center cell (0,0), and y is the vertical grid cell position of the center cell (0,0).

4. A nuclear fuel assembly according to claim 1, characterized in that 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). However, in (x,y), x is the horizontal grid cell position of the center cell (0,0), and y is the vertical grid cell position of the center cell (0,0). In a support grid assembly including a support grid having grid cells of 5.17×17 and a sleeve fixed within the grid cells 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 16 are arranged in the non-diagonal direction (Ixxy)(Ixyy) based on the center cell of the support grid.

6. A support grid assembly in accordance with claim 5, wherein the sleeves are arranged at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6) in the main axis direction. However, in (x,y), x is the horizontal grid cell position of the center cell (0,0), and y is the vertical grid cell position of the center cell (0,0).

7. A support grid assembly in accordance with claim 5, characterized in that the sleeves are arranged diagonally at (5,5), (-5,-5), (-5,5), and (5,-5). However, in (x,y), x is the horizontal grid cell position of the center cell (0,0), and y is the vertical grid cell position of the center cell (0,0).

8. In the fifth paragraph, the support grid assembly is characterized in that the sleeves are arranged in the 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). However, in (x,y), x is the horizontal grid cell position of the center cell (0,0), and y is the vertical grid cell position of the center cell (0,0).

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