JP6516659B2 - Simulated pellet, simulated fuel rod, and simulated fuel assembly - Google Patents

Description

  The present invention relates to simulated pellets, simulated fuel rods, and simulated fuel assemblies.

  Casks in which fuel assemblies are stored are required to be evaluated for safety when dropped. In the case of safety evaluation, it is necessary to set the deformation or damage state of the fuel assembly at the time of cask drop. As one method for setting the deformation or failure state of the fuel assembly, there is a confirmation method by a drop test using a simulated fuel assembly that simulates the fuel assembly. Here, there are many cases where it is difficult to conduct a drop test of a fuel assembly containing radioactive substances from the viewpoint of limitations of the test site, etc. Therefore, a simulated fuel assembly simulating a fuel assembly is prepared, and the simulated fuel assembly is prepared. A drop test of the body is performed to obtain data necessary for the evaluation.

  The simulated fuel assembly is an assembly of simulated fuel rods that simulates actual fuel rods. The simulated fuel rods are those in which simulated pellets simulating actual nuclear fuel pellets are disposed inside a fuel cladding tube (see, for example, Patent Document 1). In the simulated fuel assembly, although simulated pellets are used instead of nuclear fuel pellets, the same members as actual fuel assemblies are used as components other than nuclear fuel pellets including fuel cladding. The use of simulated pellets alleviates restrictions on the test site.

  In the conventional drop test, in order to make the weight of the simulated pellet equal to that of the actual nuclear fuel pellet, the simulated pellet having the same level of density as that of the actual nuclear fuel pellet is used. As a material of the simulated pellet, a lead alloy having a large specific gravity and easy forming is often used. By adjusting the composition of the lead alloy, simulated pellets having the same level of density as nuclear fuel pellets are produced.

Japanese Patent Laid-Open No. 2000-039494

  An actual fuel rod mainly has a fuel cladding tube and nuclear fuel pellets disposed inside the fuel cladding tube. When the fuel rod is dropped, the impact of the drop causes the fuel rod to be bent and the inner surface of the fuel cladding tube comes in contact with the nuclear fuel pellet. The occurrence of contact between the fuel cladding and the nuclear fuel pellets affects the structural integrity of the fuel cladding. And the bending deformation behavior of the fuel rod at the time of falling is influenced by the rigidity of the nuclear fuel pellet disposed inside the fuel cladding tube.

  Therefore, in order to simulate the actual drop test of the fuel assembly, it is necessary to make the simulated pellet not only the density but also the rigidity the same level as the nuclear fuel pellet. Since the simulated pellet consisting of lead alloy has extremely low rigidity compared to nuclear fuel pellets, the drop test of the simulated fuel assembly constructed using the simulated pellet shows a large amount of deformation compared to the actual fuel rods. It may be the amount of deformation.

  An object of the present invention is to provide simulated pellets, simulated fuel rods, and simulated fuel assemblies capable of simulating actual fuel assembly drop tests.

  The present invention is a simulated pellet that simulates a nuclear fuel pellet disposed inside a fuel cladding, the outer diameter and length dimensions being equal to the nuclear fuel pellet, having an outer surface in contact with the inner surface of the fuel cladding, An outer cylinder portion formed of a first material, and an inner core portion disposed inside the outer cylinder portion and formed of a second material, the outer cylinder portion being higher than the inner core portion The inner core portion provides a simulated pellet that is rigid and has a higher specific gravity than the outer cylinder portion.

  According to the present invention, the simulated pellet has a two-layer structure consisting of the outer cylindrical portion of the first material and the inner core portion of the second material, using a high rigidity material as the first material, and a high specific gravity material as the second material. The use of the present invention provides simulated pellets having the same level of stiffness and density as actual nuclear fuel pellets. In the drop test of a simulated fuel assembly constructed using simulated pellets, in order to simulate the drop test of an actual fuel assembly, it is necessary to use simulated pellets of the same level as nuclear fuel pellets as well as density. There is. The outer cylinder portion in contact with the fuel cladding tube is formed of a high rigidity first material at the same level as the rigidity of the nuclear fuel pellets, and the inner core portion not in contact with the fuel cladding tube is formed of a second material of high specific gravity By making the total weight (average density) of the simulated pellets the same as the total weight of the nuclear fuel pellets, it is possible to simulate the drop test of the fuel assembly and to appropriately evaluate the actual behavior of the fuel rods. it can. Also, by making the simulated pellet into a two-layer structure, the first and second materials are selected according to the contents of the test, and the volume ratio (weight ratio) between the outer cylinder and the inner core is adjusted. By doing so, the simulated pellet can obtain the necessary rigidity and overall weight.

  In the present invention, it is preferable that the outer cylinder part has the same rigidity as the nuclear fuel pellet or higher rigidity than the nuclear fuel pellet, and the inner core part has a higher specific gravity than the nuclear fuel pellet.

  By making the outer cylinder more rigid than the nuclear fuel pellets, the fuel cladding can be damaged more than the actual nuclear fuel pellets do to the fuel cladding, and the fuel cladding integrity under severe test conditions It can be evaluated. In addition, by making the outer cylinder part the same rigidity as the nuclear fuel pellets, the fuel cladding can be damaged to the same level as the damage of the actual nuclear fuel pellets to the fuel cladding, and the nuclear fuel pellets contact the fuel cladding It is possible to reproduce the behavior of the fuel rod when the By making the inner core portion have a specific gravity higher than that of the nuclear fuel pellets, it is possible to simulate the weight of the nuclear fuel pellets as the whole simulated pellet.

  In the present invention, it is preferable that the outer cylinder part contains a ceramic, and the inner core part contains tungsten.

  By forming the outer cylinder portion with ceramics, the same level of rigidity as the nuclear fuel pellet can be obtained. In addition, since the inner core portion is formed of tungsten having a high specific gravity, the simulated pellet can obtain the same total weight as the nuclear fuel pellet.

  In the present invention, the first material includes any one of alumina, forsterite, zirconia, silicon nitride, silicon carbide, fluorine phlogopite, carbon steel, stainless steel, alloy steel, tungsten, and tungsten carbide, The second material preferably contains any one of tungsten, tungsten carbide, lead, lead alloy, gold, silver, platinum and tantalum.

  These first materials have the same rigidity as the nuclear fuel pellet or higher rigidity than the nuclear fuel pellet. Also, these second materials have a higher specific gravity than nuclear fuel pellets. Therefore, by using these first and second materials, the rigidity and overall weight of the nuclear fuel pellet can be reproduced.

In the present invention, the average density is preferably adjusted to 10 [g / cm ³ ] or more and 11 [g / cm ³ ] or less.

The theoretical density of uranium dioxide is said to be about 10.96 [g / cm ³ ]. On the other hand, the actual nuclear fuel pellet is a polycrystalline sintered body having a grain size of 5 μm to 10 μm, and its actual density is usually 95% to 97% of the theoretical density. It is said that there is. Therefore, the average density of the simulated pellets (the total density of the outer cylinder part and the inner core part combined) is 10 [g / cm ³ ] or more and 11 [g / cm ³ ] or less according to the actual density of nuclear fuel pellets. Is preferred.

  In the present invention, the outer cylinder portion is preferably a single member made of the first material.

  In the drop test, the inner surface of the fuel cladding tube and the outer cylinder contact with each other, and a force is applied to the outer cylinder. In the case where the outer cylindrical portion is composed of a plurality of members, in the drop test, there is a high possibility that the member connecting portion of the outer cylindrical portion may be cracked or the outer cylindrical portion may be damaged starting from the member connecting portion. By forming the outer cylinder portion as a single member, cracking and breakage at the connection portion of the outer cylinder member which does not occur in actual nuclear fuel pellets in the drop test can be suppressed.

  The present invention provides a simulated fuel rod comprising a fuel cladding tube and the above-described simulated pellet disposed inside the fuel cladding tube.

  According to the present invention, it is possible to simulate an actual fuel assembly drop test or an actual fuel rod drop test.

  In the present invention, a plurality of the simulated pellets having the same outer shape are provided inside the fuel cladding tube, and a plurality of the simulated pellets are a first simulation pellet and an end portion of the fuel cladding tube than the first simulation pellet. It is preferable that the second simulated pellet is disposed in the second simulated pellet and the volume of the outer cylinder portion is larger than that of the first simulated pellet.

  In a drop test in which the end is dropped downward from the center of the simulated fuel rod, the end of the simulated fuel rod is bent and deformed to a greater extent than the center. The first simulated pellet and the second simulated pellet have the same outer shape, and the volume of the outer cylinder of the second simulated pellet is larger than the volume of the outer cylinder of the first simulated pellet. That is, the proportion of the outer cylinder portion of the second simulated pellet is larger than that of the first simulated pellet. Therefore, in terms of rigidity, the second simulated pellet is closer to the actual nuclear fuel pellet than the first simulated pellet. By placing a second simulated pellet having the same level of rigidity as the actual nuclear fuel pellet at the end of the fuel cladding tube that undergoes significant bending deformation in the drop test, the simulated fuel rod is subjected to bending deformation behavior of the fuel rod when falling Can be reproduced more faithfully. On the other hand, with regard to the overall weight, the first simulated pellet is larger than the second simulated pellet. Therefore, the actual weight of the fuel rod can be reproduced as the whole simulated fuel rod.

  The present invention comprises a fuel cladding tube and a plurality of simulated pellets disposed inside the fuel cladding tube and simulating nuclear fuel pellets, the plurality of simulated pellets having the same outer shape, a first material and a second material. It is formed of at least one of the materials, the first material is higher in rigidity than the second material, and the second material is higher in specific gravity than the first material, and among the plurality of simulated pellets, The volume occupied by the first material in the simulated pellet disposed at the end of the fuel cladding tube is larger than the volume occupied by the first material in the simulated pellet disposed at the central portion of the fuel cladding tube. Provide simulated fuel rods.

  According to the present invention, since the fuel rod is composed of at least two types of simulated pellets of different stiffness and specific gravity, the same level of rigidity as the specific part of the actual fuel rod and the same overall weight of the actual fuel rod A simulated fuel rod is provided. Therefore, it is possible to simulate an actual fuel assembly drop test or an actual fuel rod drop test. In addition, by using at least two types of simulated pellets, simulation can be performed by selecting the first and second simulated pellets according to the content of the test or adjusting the ratio of the first and second simulated pellets. The fuel rods can provide the necessary rigidity and overall weight.

  In the present invention, it is preferable that the simulated pellet disposed at the end is formed of only the first material, and the simulated pellet disposed at the central portion is formed of only the second material.

  In a drop test in which the end is dropped downward from the center of the simulated fuel rod, the end of the simulated fuel rod is bent and deformed to a greater extent than the center. The simulated pellet disposed at the end is formed of only the first material with high rigidity, and in terms of rigidity, the simulated pellet disposed at the end is more than the simulated pellet disposed at the central portion, It approximates to actual nuclear fuel pellets. By placing the simulated pellet, which has the same level of rigidity as the actual nuclear fuel pellet, at the end of the fuel cladding tube that undergoes large bending deformation in the drop test, the simulated fuel rod can make the bending deformation behavior of the fuel rod during falling more It can be faithfully reproduced. On the other hand, the simulated pellet disposed in the central portion is formed of only the second material of high specific gravity, and the actual weight of the fuel rod can be reproduced as the entire simulated fuel rod. In addition, since the simulated pellet is formed of only the first material or the second material alone, the manufacturing cost of the simulated pellet is suppressed.

  In the present invention, of the plurality of simulated pellets, at least one of the simulated pellets has an outer surface in contact with the inner surface of the fuel cladding tube, and an outer cylinder portion formed of the first material; And an inner core portion disposed inside the portion and formed of the second material.

  The simulated pellet having the outer cylindrical portion formed of the first material and the inner core portion formed of the second material provides the simulated pellet having the same level of rigidity and density as the actual nuclear fuel pellet. Thus, the simulated fuel rods can reproduce the actual fuel rod stiffness and overall weight.

  The present invention provides a simulated fuel assembly comprising a plurality of the simulated fuel rods described above.

  According to the present invention, it is possible to simulate the actual fuel assembly drop test, to grasp the actual behavior of the fuel rods and fuel assemblies, and to appropriately evaluate the safety of the cask.

  According to the present invention, there are provided simulated pellets, simulated fuel rods and simulated fuel assemblies capable of simulating the actual fuel assembly drop test.

FIG. 1 is a block diagram showing an example of a simulated fuel assembly according to the first embodiment. FIG. 2 is a cross-sectional view showing an example of a simulated fuel assembly according to the first embodiment. FIG. 3 is a view showing an example of a simulated fuel rod according to the first embodiment. FIG. 4 is a side sectional view showing an example of the simulated pellet according to the first embodiment. FIG. 5 is a plan view showing an example of the simulated pellet according to the first embodiment. FIG. 6 is a view schematically showing an example of the behavior of the simulated fuel rod at the time of the drop test according to the first embodiment. FIG. 7 is a view schematically showing an example of the behavior of the simulated fuel rod at the time of the drop test according to the first embodiment. FIG. 8 is a view schematically showing an example of a simulated fuel rod according to a second embodiment. FIG. 9 is a view showing a modification of the simulated pellet. FIG. 10 is a view showing a modified example of the simulated pellet. FIG. 11 is a view showing a modified example of the simulated pellet. FIG. 12 is a view schematically showing an example of a simulated fuel rod according to a third embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited thereto. The components of each embodiment described below can be combined as appropriate. In addition, some components may not be used.

First Embodiment
The first embodiment will be described. FIG. 1 is a configuration view schematically showing an example of a simulated fuel assembly 1 according to the present embodiment. FIG. 2 is a cross-sectional view of the simulated fuel assembly 1 according to the present embodiment. As shown in FIGS. 1 and 2, in the simulated fuel assembly 1, a plurality of simulated fuel rods 2, a control rod guide tube 4 into which the control rods 3 are inserted, and a detector 5 for in-core instrumentation are inserted. A guide tube 6 for in-core instrumentation and a grid 7 for bundling these are provided. The simulated fuel assembly 1 further includes an upper nozzle 8 provided at the upper part in the axial direction of the simulated fuel rod 2 and a lower nozzle 9 provided at the lower part in the axial direction. The upper nozzle 8 and the lower nozzle 9 fix the upper end and the lower end of the control rod guide pipe 4 and the in-core instrumentation guide pipe 6 in the axial direction.

  FIG. 3 is a view showing an example of the simulated fuel rod 2 according to the present embodiment, and a diagram showing a part of the simulated fuel rod 2 in a cutaway manner. The simulated fuel rod 2 comprises a cylindrical fuel cladding tube 10 and a plurality of simulated pellets 20 disposed inside the fuel cladding tube 10 and simulating nuclear fuel pellets. A plurality of simulated pellets 20 are disposed in the axial direction of the fuel cladding tube 10 inside the fuel cladding tube 10. The outer shapes of the plurality of simulated pellets 20 are equal. In addition, the simulated fuel rod 2 has a spring 11 for suppressing the simulated pellet 20 disposed on the uppermost side, an upper end plug 12 for closing the upper end of the fuel cladding tube 10, and a lower end plug for blocking the lower end of the fuel cladding 10 It has 13 and. The spring 11 prevents movement of the simulated pellet 20 during transportation or handling.

  FIG. 4 is a side sectional view showing an example of the simulated pellet 20 according to the present embodiment. FIG. 5 is a plan view showing an example of the simulated pellet 20 according to the present embodiment. The simulated pellet 20 simulates a nuclear fuel pellet disposed inside the fuel cladding tube 10. The nuclear fuel pellet is formed into a cylindrical shape after compression molding of a powder of an atomic material such as uranium dioxide, sintering in a hydrogen atmosphere or in a mixed atmosphere of hydrogen and nitrogen. That is, the nuclear fuel pellet is a ceramic body. The simulated pellet 20 is manufactured such that its weight is at the same level as that of the nuclear fuel pellet, and the rigidity of the portion in contact with the inner surface of the fuel cladding tube 10 is at the same level as that of the nuclear fuel pellet.

  The simulated pellet 20 has an outer surface 20S in contact with the inner surface of the fuel cladding tube 10, and is disposed inside the outer cylinder portion 21 formed of the first material and the outer cylinder portion 21 and formed of the second material And an inner core portion 22. The outer cylindrical portion 21 is a single member made of a first material. The inner core portion 22 is a single member made of the second material. The outer cylindrical portion 21 has a cylindrical shape, and the inner core portion 22 has a cylindrical shape. The center of the outer cylinder 21 and the center of the inner core 22 coincide with each other. The simulated pellet 20 has a two-layer concentric structure.

  The outer cylindrical portion 21 is more rigid than the inner core portion 22. The inner core portion 22 has a higher specific gravity than the outer cylinder portion 21. The outer cylindrical portion 21 has the same rigidity as the nuclear fuel pellet. The inner core portion 22 has a higher specific gravity than nuclear fuel pellets.

  Examples of the first material forming the outer cylinder portion 21 include at least one of alumina, forsterite, zirconia, silicon nitride, silicon carbide, fluorine phlogopite, carbon steel, stainless steel, alloy steel, tungsten, and tungsten carbide. .

  Examples of the second material for forming the inner core portion 22 include at least one of tungsten, tungsten carbide, lead, a lead alloy, gold, silver, platinum, and tantalum.

  In the present embodiment, the outer cylinder portion 21 is a ceramic body of any one of alumina, forsterite, zirconia, silicon nitride, silicon carbide, and fluorine phlogopite among the first materials. The inner core portion 22 is formed of tungsten or tungsten carbide.

  The outer cylindrical portion 21 is manufactured by forming a cylindrical ceramic body of a first material and drilling a central portion of the ceramic body with a drill. Alternatively, the outer cylindrical portion 21 may be manufactured by molding into a cylindrical shape using a mold without using a drill. After the cylindrical outer cylindrical portion 21 is manufactured, a simulated pellet 20 is manufactured by fitting a cylindrical inner core portion 22 made of the second material into the inner side of the outer cylindrical portion 21. The outer cylindrical portion 21 and the inner core portion 22 may be fixed by utilizing a thermal expansion difference between the outer cylindrical portion 21 and the inner core portion 22 such as shrink fitting or cold fitting, or using an adhesive. It may be fixed.

The theoretical density of uranium dioxide is said to be about 10.96 [g / cm ³ ]. On the other hand, the actual nuclear fuel pellet is a polycrystalline sintered body having a grain size of 5 μm to 10 μm, and the actual density thereof is usually 95% to 97% of the theoretical density. It is said that Therefore, the density of the nuclear fuel pellets is 10.412 [g / cm ³ ] or more and 10.6312 [g / cm ³ ] or less.

The density of the second material is greater than the density of nuclear fuel pellets. For example, the density of tungsten is 19.3 [g / cm ³ ], the density of tungsten carbide is 15.63 [g / cm ³ ], and the density of lead is 11.36 [g / cm ^3]. ], The density of gold is 19.32 [g / cm ³ ], the density of silver is 10.49 [g / cm ³ ], the density of platinum is 21.45 [g / cm]. ³ ], and the density of tantalum is 16.6 [g / cm ³ ].

On the other hand, the density of the first material is smaller than the density of nuclear fuel pellets. In the present embodiment, the outer cylinder made of the first material so that the average density of the simulated pellets 20 (the total density of the outer cylinder 21 and the inner core 22 combined) is the same level as the density of nuclear fuel pellets. The ratio (volume ratio) of the portion 21 and the inner core portion 22 made of the second material is adjusted. In the present embodiment, the ratio between the outer cylindrical portion 21 and the inner core portion 22 is adjusted such that the average density of the simulated pellet 20 is 10 g / cm ³ or more and 11 g / cm ³ or less. Ru.

  Next, an example of a drop test of the simulated fuel assembly 1 constructed using the simulated pellets 20 will be described. In the present embodiment, a drop test is performed in which the lower end portion of the simulated fuel assembly 1 (the simulated fuel rod 2) is dropped downward with respect to the central portion of the simulated fuel assembly 1 (simulated fuel rod 2). That is, in the drop test, as shown by the arrow in FIG. 1, the simulated fuel assembly 1 drops with the lower nozzle 9 disposed below the upper nozzle 8.

  FIG. 6 is a view schematically showing the behavior of the simulated fuel rod 2 in the drop test of the simulated fuel assembly 1. The impact of the drop causes at least a portion of the simulated fuel rod 2 to be bent and deformed. As shown in FIG. 6, when the simulated fuel rod 2 falls with the lower end of the simulated fuel rod 2 directed downward from the central portion, the lower end is larger than the central portion of the simulated fuel rod 2 It bends and deforms.

  FIG. 7 is a view schematically showing the relationship between the fuel cladding tube 10 and the simulated pellet 20 when the simulated fuel rod 2 is bent and deformed. When the fuel cladding tube 10 bends due to the drop, the relative position between the fuel cladding tube 10 and the simulated pellet 20 changes, and the contact force with a part (particularly the corner) of the outer surface 20S of the simulated pellet 20 increases. The fuel cladding tube 10 is damaged from the simulated pellet 20.

  In the present embodiment, the outer cylinder portion 21 having the same level of rigidity as the nuclear fuel pellet is provided with the outer surface 20S of the simulated pellet 20 in contact with the inner surface of the fuel cladding tube 10. In other words, of the simulated pellet 20, the outer cylinder portion 21 having the same level of rigidity as the nuclear fuel pellet contacts the inner surface of the fuel cladding tube 10.

  It is considered that the bending deformation behavior of the actual fuel rod at the time of the drop is influenced by the rigidity of the nuclear fuel pellet inside the fuel cladding tube 10. In the present embodiment, the portion of the simulated pellet 20 in contact with the inner surface of the fuel cladding tube 10 is as rigid as the nuclear fuel pellet, so the bending deformation behavior of the actual fuel rod when dropped is Can be reproduced using.

  Further, since the rigidity of the outer cylinder portion 21 is the same level (same rigidity) as the rigidity of the nuclear fuel pellet, the fuel cladding tube 10 can be damaged to the same level as the damage of the actual nuclear fuel pellet to the fuel cladding tube 10 The behavior of the fuel rod when the nuclear fuel pellet contacts the fuel cladding tube 10 can be reproduced.

  As described above, according to the present embodiment, the simulated pellet 20 has a two-layer structure including the outer cylindrical portion 21 of the first material and the inner core portion 22 of the second material, and the high rigidity material is used as the first material. By using a high specific gravity material as the second material, it is possible to provide simulated pellets 20 having the same level of rigidity and density as actual nuclear fuel pellets.

  By forming the outer cylinder portion 21 in contact with the fuel cladding tube 10 with the first material of the same level of rigidity as the rigidity of the nuclear fuel pellets, the bending deformation behavior of the fuel rod at the time of falling is simulated using the simulated fuel rod 2 It can be reproduced. In addition, the fuel cladding tube 10 can be damaged to the same level as the actual nuclear fuel pellet damages the fuel cladding tube 10, and the behavior of the fuel rod when the nuclear fuel pellets contact the fuel cladding tube 10 can be reproduced Can.

  The specific gravity of the first material is less than the specific gravity of the nuclear fuel pellets. By forming the inner core portion 22 not in contact with the fuel cladding tube 10 with the second material of high specific gravity, the overall weight (average density) of the simulated pellet 20 can be made the same level as the overall weight of the nuclear fuel pellet. This makes it possible to reproduce nuclear fuel pellets not only in terms of rigidity but also in terms of weight.

  Therefore, actual fuel rod and fuel assembly drop tests can be simulated, actual fuel rod and fuel assembly behavior can be grasped, and cask safety evaluation can be appropriately performed.

  Further, by making the simulated pellet 20 into a two-layer structure, the first and second materials can be selected according to the contents of the test, and the ratio (weight ratio, volume ratio) between the outer cylindrical portion 21 and the inner core portion 22 The simulated pellet 20 can obtain the required rigidity and overall weight by adjusting the

  Further, in the present embodiment, the outer cylinder portion 21 contains a ceramic of any one of alumina, forsterite, zirconia, silicon nitride, silicon carbide, and fluorine phlogopite among the first materials, and the inner core portion 22 Contains tungsten. By forming the outer cylinder portion 21 with a ceramic, the same level of rigidity as the nuclear fuel pellet can be obtained. In addition, since the inner core portion 22 is formed of tungsten having a high specific gravity, the simulated pellet 20 can obtain the same total weight as the nuclear fuel pellet.

  Further, in the present embodiment, the outer cylinder portion 21 is a single member made of the first material. As described with reference to FIG. 6, in the drop test, the contact force between the inner surface of the fuel cladding tube 10 and the outer cylinder portion 21 increases. When the outer cylindrical portion 21 is formed of a plurality of members, in the drop test, there is a possibility that a crack may occur in the member connecting portion of the outer cylindrical portion 21 or the outer cylindrical portion 21 may be damaged starting from the member connecting portion. Get higher. By forming the outer cylindrical portion 21 as a single member, it is possible to suppress cracking or breakage in the member connection portion in the outer cylindrical portion 21 which does not occur in actual nuclear fuel pellets in the drop test.

  In the present embodiment, the outer cylinder portion 21 has the same rigidity as the nuclear fuel pellet. The outer cylindrical portion 21 may be more rigid than nuclear fuel pellets. By making the outer cylindrical portion 21 more rigid than nuclear fuel pellets, the fuel cladding tube 10 can be damaged more than the actual nuclear fuel pellets damage the fuel cladding tube 10, and the fuel cladding tube under severe test conditions 10 can be evaluated. Further, even if the rigidity of the inner core portion 22 is extremely low, the rigidity of the nuclear fuel pellet as the whole simulated pellet 20 can be simulated by making the outer cylinder portion 21 higher in rigidity than the nuclear fuel pellet.

Second Embodiment
The second embodiment will be described. In the present embodiment, an example will be described in which the volume ratio between the outer cylindrical portion 21 and the inner core portion 22 differs for each simulated pellet 20.

  FIG. 8 is a view schematically showing an example of the simulated fuel rod 2 according to the present embodiment. Similar to the first embodiment described above, a plurality of simulated pellets 20 having the same outer shape are provided in the axial direction of the simulated fuel rod 2 inside the fuel cladding tube 10.

  The simulated pellet 20 disposed at the center of the fuel cladding tube 10 is referred to as a first simulated pellet 20A and the simulated pellet 20 disposed at the lower end is referred to as a second simulated pellet 20B in the axial direction of the simulated fuel rod 2 In this case, the volume of the outer cylinder portion 21 of the second simulated pellet 20B is larger than the volume of the outer cylinder portion 21 of the first simulated pellet 20A. In other words, the proportion (volume) of the outer cylinder portion 21 (first material) is larger in the second simulated pellet 20B than in the first simulated pellet 20A. Therefore, with respect to the rigidity, the second simulated pellet 20B is closer to the actual nuclear fuel pellet than the first simulated pellet 20A.

  As described above, in the drop test in which the end portion of the simulated fuel rod 2 is dropped downward from the central portion of the simulated fuel rod 2, the end portion of the simulated fuel rod 2 is bent and deformed larger than the central portion. By placing the second simulated pellet 20B having the same level of rigidity as the actual nuclear fuel pellet at the lower end of the fuel cladding tube 10 that undergoes significant bending deformation in the drop test, the simulated fuel rod 2 can The bending deformation behavior can be faithfully reproduced. On the other hand, regarding the weight, the first simulated pellet 20A is larger than the second simulated pellet 20B. The specific gravity of the first material is smaller than the specific gravity of the nuclear fuel pellets, and the specific gravity of the second material is larger than the specific gravity of the nuclear fuel pellets, so the proportion of the first material of the second simulated pellets 20B is increased. By increasing the proportion of the second material of the pellet 20A, it is possible to reproduce the actual weight of the fuel rod as the whole simulated fuel rod 2.

(Modification of the first and second embodiments)
In the first and second embodiments described above, the outer cylinder portion 21 has a cylindrical shape, and the inner core portion 22 has a cylindrical shape and has a two-layer concentric structure. The inner core portion 22 may have a cylindrical shape. In addition, as shown in FIG. 9, the outer cylindrical portion 21 may have a cylindrical portion 211 and a bottom portion 212. Further, as shown in FIG. 10, the outer cylindrical portion 21 may have a cylindrical portion 211, a bottom portion 212, and a top portion 213 having an opening 213K. In the example shown in FIG. 10, the inner core portion 22 is manufactured by casting the second material into the outer cylindrical portion 21 through the opening 213K. Moreover, as shown in FIG. 11, two outer cylinder parts 21 may be provided.

Third Embodiment
A third embodiment will be described. In the present embodiment, an example will be described in which a simulated pellet formed only of the first material and a simulated pellet formed only of the second material are disposed in the fuel cladding tube 10.

  FIG. 12 is a view schematically showing an example of the simulated fuel rod 2 according to the present embodiment. Similar to the first and second embodiments described above, a plurality of simulated pellets 20 having the same outer shape are provided in the axial direction of the simulated fuel rod 2 inside the fuel cladding tube 10.

  The simulated pellet 20 disposed at the center of the fuel cladding tube 10 is referred to as a third simulated pellet 20C and the simulated pellet 20 disposed at the lower end is referred to as a fourth simulated pellet 20D with respect to the axial direction of the simulated fuel rod 2 In the case, the fourth simulated pellet 20D disposed at the lower end portion is formed only of the first material, and the third simulated pellet 20C disposed at the central portion is formed only of the second material.

  Regarding the rigidity, the fourth simulated pellet 20D disposed at the lower end portion is closer to the actual nuclear fuel pellet than the third simulated pellet 20C disposed at the central portion. By placing the fourth simulated pellet 20D having the same level of rigidity as the actual nuclear fuel pellet at the lower end of the fuel cladding tube 10 that undergoes significant bending deformation in the drop test, the simulated fuel rod 10 is The bending deformation behavior can be faithfully reproduced. On the other hand, the third simulated pellet 20C disposed in the central portion is formed of only the second material of high specific gravity, and the entire weight of the fuel rod can be reproduced as the entire simulated fuel rod 2. Moreover, the manufacturing cost of the simulation pellet 20 is suppressed by the simulation pellet 20 being formed only by the 1st material or only the 2nd material.

  In each embodiment described above, the simulated fuel assembly 1 may simulate a spent fuel assembly or may simulate a new fuel assembly.

  In each of the embodiments described above, a plurality of simulated pellets having the same outer diameter and length dimension are provided inside the fuel cladding tube 10. A plurality of simulated pellets having different outer diameters and length dimensions may be disposed on the fuel cladding tube 10. For example, a simulated pellet having a large outer diameter is disposed at the end of the simulated fuel rod 2 having a large deformation at the time of falling, and a simulation having a small outer diameter at the central portion of the simulated fuel rod 2 having a smaller deformation at the time of falling Pellets may be placed. The simulated pellet having a large outer diameter has a smaller gap with the inner surface of the fuel cladding tube 10 than the simulated pellet having a small outer diameter, and contacts the inner surface of the fuel cladding tube 10 with a small deformation of the simulated fuel rod 2. Damage to the cladding tube 10 is large. Therefore, it is preferable that the simulated pellet having a large outer diameter be disposed at the end of the fuel cladding tube 10 having a large deformation when dropped. On the other hand, from the viewpoint of ease of production, a simulated pellet with a large gap with the inner surface of the fuel cladding tube 10 and a small outer diameter is disposed at the central portion of the simulated fuel rod 2 with a small deformation at the time of dropping. Is preferred.

DESCRIPTION OF SYMBOLS 1 Simulated fuel assembly 2 Simulated fuel rod 3 Control rod 4 Control rod guide tube 5 In-core instrumentation detector 6 In-core instrumentation guide tube 7 Grid 8 Upper nozzle 9 Lower nozzle 10 Fuel cladding tube 11 Spring 12 Upper end plug 13 Lower end plug 20 simulated pellet 20A first simulated pellet 20B second simulated pellet 20C third simulated pellet 20D fourth simulated pellet 20S outer surface 211 cylindrical portion 212 bottom portion 213 top portion 213K opening

Claims (12)

A simulated pellet that simulates a nuclear fuel pellet disposed inside a fuel cladding tube, comprising:
An outer cylinder portion having an outer surface in contact with the inner surface of the fuel cladding tube and formed of a first material;
An inner core portion disposed inside the outer cylinder portion and formed of a second material;
Equipped with
The outer cylinder portion is more rigid than the inner core portion,
The inner core portion has a higher specific gravity than the outer cylinder portion.
Simulated pellet. The outer cylindrical portion has the same rigidity as that of the nuclear fuel pellet or higher rigidity than that of the nuclear fuel pellet,
The inner core portion has a higher specific gravity than the nuclear fuel pellet.
The simulated pellet according to claim 1. The outer cylinder portion contains a ceramic,
The inner core portion comprises tungsten,
The simulated pellet according to claim 1 or 2. The first material includes any one of alumina, forsterite, zirconia, silicon nitride, silicon carbide, fluorine phlogopite, carbon steel, stainless steel, alloy steel, tungsten, and tungsten carbide,
The second material includes any one of tungsten, tungsten carbide, lead, lead alloy, gold, silver, platinum and tantalum.
The simulated pellet according to claim 1 or 2. The average density is adjusted to 10 [g / cm ³ ] or more and 11 [g / cm ³ ] or less,
The simulated pellet according to any one of claims 1 to 4. The outer cylindrical portion is a single member made of the first material.
The simulated pellet according to any one of claims 1 to 5. Fuel cladding tube,
The simulated pellet according to any one of claims 1 to 6, which is disposed inside the fuel cladding tube.
Simulated fuel rods comprising: A plurality of the simulated pellets having the same outer shape are provided inside the fuel cladding tube,
A plurality of the simulated pellets and a second simulated pellet are disposed at the end of the fuel cladding tube than the first simulated pellet and the first simulated pellet, and the volume of the outer tube portion is larger than the first simulated pellet And including
The simulated fuel rod according to claim 7. Fuel cladding tube,
A plurality of simulated pellets disposed inside the fuel cladding and simulating nuclear fuel pellets;
Equipped with
The plurality of simulated pellets have the same outer shape, and are formed of at least one of the first material and the second material,
The first material is more rigid than the second material,
The second material has a higher specific gravity than the first material,
The volume occupied by the first material in the simulated pellet disposed at the end of the fuel cladding tube among the plurality of simulated pellets is the first volume of the simulated pellet disposed at the central portion of the fuel cladding tube. Greater than the volume occupied by the material,
Simulated fuel rods. The simulated pellet disposed at the end is formed of only the first material,
The simulated pellet disposed in the central portion is formed of only the second material,
The simulated fuel rod according to claim 9. Among the plurality of simulated pellets, at least one of the simulated pellets is
An outer cylinder portion having an outer surface in contact with the inner surface of the fuel cladding tube and formed of the first material;
An inner core portion disposed inside the outer cylinder portion and formed of the second material;
Have
A simulated fuel rod according to claim 9 or 10.   A simulated fuel assembly comprising a plurality of the simulated fuel rods according to any one of claims 7 to 11.

Images