JP2016200465A - Fuel rod and fuel assembly - Google Patents

Abstract

A fuel assembly having sufficient manufacturability even when using a SiC / SiC composite and having compatibility with an existing nuclear reactor is obtained. According to an embodiment, a fuel assembly includes a plurality of fuel rods 200 arranged in a grid, an upper structural member, and a lower structural member. Each of the fuel rods 200 includes a plurality of segment elements 210 arranged in series and a connecting portion 300 that connects the segment elements 210 with a predetermined range of bending rigidity. The segment element 210 includes a nuclear fuel 211, a cladding tube 212 made of a silicon carbide fiber reinforced composite, an upper end plug 213 made of a silicon carbide fiber reinforced composite, and a lower end plug 214 made of a silicon carbide fiber reinforced composite. . [Selection] Figure 2

Description

  Embodiments described herein relate generally to a fuel rod and a fuel assembly including the same.

  The fuel assembly is configured such that a nuclear fuel material such as uranium dioxide is enclosed in a fuel cladding tube to form a fuel rod, and the fuel rods are arranged in a grid. The fuel cladding tube is a container for storing fuel pellets such as uranium. The fuel cladding tube contains the fission product gas (FP gas) generated by the nuclear reaction of the fuel so as not to leak to the outside, and isolates the fuel and the coolant so that the coolant does not enter and contact the fuel. It is provided for.

  In light water reactors, zirconium alloys have conventionally been used as a material for fuel cladding tubes. This is because there is little loss due to neutron absorption for sustaining the nuclear reaction, excellent corrosion resistance in high-temperature and high-pressure water, and high thermal conductivity.

  As mentioned above, metal materials such as zirconium alloys have been mainly used in light water reactor reactor structures, but these metal materials are oxidized by high-temperature steam and metal-water reaction to generate hydrogen. . For example, assuming that the nuclear reactor reaches a serious event such as power loss due to a large-scale natural disaster, the nuclear reactor is first shut down by inserting a control rod. However, if the cooling facility for the core does not operate, there is a possibility that the reactor will become hot due to the decay heat caused by fission products in the fuel.

  When such a situation is reached, hydrogen or reaction heat may be generated by the above-described metal-water reaction. As hydrogen generation progressed, the hydrogen concentration in the containment vessel increased, and depending on the conditions, hydrogen combustion could impair the integrity of the containment vessel. From these points, in order to further enhance the safety of the nuclear reactor even in the event of a serious accident, the fuel cladding tube, which occupies a large portion of the structure in the reactor, is reinforced with a nonmetallic material, such as silicon carbide long fiber, for example. Research and development for applying composite materials (SiC / SiC composite materials) are being conducted.

  The SiC / SiC composite material is a material in which the matrix is silicon carbide, which is reinforced with long silicon carbide fibers, and has improved the low toughness that has been a problem in conventional ceramics. The reason for the improvement is that the composite material has a large number of matrix-fiber interfaces.

  The crack generated in the matrix propagates in the matrix and reaches the long fibers. The crack that reaches the long fibers stops once at the interface between the matrix and the long fibers. The crack propagates while changing direction here and propagating along the interface between the matrix and the long fibers. As described above, the propagation direction of the crack is not linear but progresses while deflecting in a specific direction, so that the propagation distance becomes long. In addition, due to the cross-linking effect and drawing effect of the fibers, stress relaxation that does not occur in monolithic ceramics also occurs, and the crack growth rate slows down. By improving crack toughness by such crack deflection, cross-linking effect, and drawing, it is possible to drastically improve rapid fracture, which is the greatest drawback of monolithic ceramics, and the resulting decrease in reliability.

  Other reasons for applying the silicon carbide long fiber composite material include: (1) the silicon carbide long fiber composite material has no hydrogen absorption / embrittlement compared to the zirconium alloy, and (2) the thermal neutron absorption cross section is small. Absorption of thermal neutrons is small, (3) Material deterioration due to neutrons is small, (4) High temperature strength is excellent, (5) Corrosion rate in water is small, (6) Light and excellent in earthquake resistance , And is expected to withstand to some extent even in serious accidents.

Japanese Patent Laid-Open No. 6-18686 JP 2012-233734 A

  As described above, many ceramic materials such as silicon carbide have the advantage of being stable at high temperatures, but have the disadvantage of poor workability compared to metals because they do not have malleability or ductility. From this point, when manufacturing a long tube, the probability that a defect will occur per unit length, even if it is assumed that there is no difference in manufacturability due to product scale, i.e. length. Is constant, in the manufacturing stage, the longer the length of the tube, the higher the probability that a defect exists in one tube. For this reason, there is a problem that the yield of the production of the fuel cladding tube may be remarkably reduced by simply replacing the material of the metal fuel cladding tube of the current fuel assembly with the SiC / SiC composite material. .

  On the other hand, if the form of nuclear fuel is completely different from the current fuel assembly with an emphasis on manufacturability of SiC / SiC composites, it will be difficult to load and operate existing nuclear reactors. It becomes difficult to utilize the accumulated design, operation, and management technologies, and it is necessary to accumulate these technologies from scratch. If the fuel using the SiC / SiC composite is similar to the conventional fuel that can be loaded into an existing nuclear reactor and operated, it can be loaded and operated to generate hydrogen and hydrogen due to overheating during a serious accident. Oxidation heat generation can be suppressed, and the operation of existing nuclear reactors can be enhanced. In addition, it is possible to utilize the accumulated design, operation and management technologies with conventional fuels.

  In the case of a fuel assembly with a certain number of fuel cladding tubes, if there is a large vibration such as an earthquake, in the case of a pressurized water LWR (PWR) in the gap between the fuel assemblies in the case of a boiling water LWR (BWR) The control rod is inserted almost instantaneously into the control rod guide tube in the fuel assembly. Until the control rod is fully inserted, the fuel assembly must withstand large amplitude deformations. In a fuel assembly made of a zirconium alloy, the fuel cladding tube, which is a long and integral one, maintains its soundness even when it is deformed. On the other hand, in the SiC / SiC composite material, it is necessary to take into consideration that the boundary between the fiber and the matrix may be peeled off due to the energy of fracture even if it does not break.

  In order to increase safety when the core is overheated, such as in an accident, a fuel assembly using a SiC / SiC composite material as a fuel member uses a long cladding tube as in conventional fuels so that it can be loaded into an existing furnace. When it comprised, the subject that difficulty arises in the point of the manufacturability of a long cladding tube occurred. Also, if a fuel assembly in which a certain number of fuel cladding tubes are bundled is subject to large vibrations such as earthquakes, the SiC / SiC composite material, due to its characteristics, will generate minute cracks inside the cladding tube, even if it does not break. However, there is a problem that the leakage of fission product gas is concerned.

  Therefore, an embodiment of the present invention has been made to solve the above-described problem, and an object thereof is to obtain a fuel assembly having sufficient manufacturability even when a SiC / SiC composite material is used.

  In order to achieve the above-described object, the fuel assembly according to the present embodiment supports a plurality of fuel rods extending in a vertical direction parallel to each other and arranged in a grid in the horizontal direction, and upper ends of the plurality of fuel rods. An upper structural member that supports the lower ends of the plurality of fuel rods, each of the fuel rods extending in a vertical direction, and each of the fuel rods storing the nuclear fuel and extending in a vertical direction and having an upper end and A cladding tube made of a silicon carbide fiber reinforced composite material having an open lower end, an upper end plug made of a silicon carbide fiber reinforced composite material that blocks an open portion at the upper end of the cladding tube, and an opening portion at the lower end of the cladding tube A lower end plug made of a silicon carbide fiber reinforced composite material, and a plurality of segment elements arranged in series in the vertical direction, and the segment elements adjacent to each other vertically are connected with a predetermined range of bending rigidity. Connecting part , Characterized by including the.

  In order to achieve the above-described object, a fuel rod according to the present embodiment is made of a nuclear fuel that extends in the vertical direction, and a silicon carbide fiber reinforced composite material that houses the nuclear fuel and extends in the vertical direction and is open at the upper and lower ends. An upper end plug made of a silicon carbide fiber reinforced composite material that closes an open portion at the upper end of the cladding tube, and a lower end plug made of a silicon carbide fiber reinforced composite material that closes the open portion at the lower end. It is characterized by comprising a plurality of segment elements arranged in series in the vertical direction and a connecting portion for connecting the segment elements adjacent to each other with a predetermined range of bending rigidity.

  According to the embodiment of the present invention, a fuel assembly having sufficient manufacturability can be obtained even when a SiC / SiC composite material is used.

It is a longitudinal cross-sectional view which shows the structure of the fuel assembly which concerns on 1st Embodiment. It is a longitudinal cross-sectional view which shows notionally the structure of the fuel rod which concerns on 1st Embodiment. It is a longitudinal cross-sectional view which shows the structure of the connection part between the fuel rods concerning 1st Embodiment. It is a longitudinal cross-sectional view which shows the mutual relationship between the segment fuel in the connection part position between the segment elements of the fuel rod which concerns on 1st Embodiment. It is a longitudinal cross-sectional view which shows the kind of fuel rod which concerns on 1st Embodiment. It is a horizontal sectional view showing an example of the arrangement of the fuel rods of the fuel assembly according to the first embodiment. It is a longitudinal cross-sectional view which shows the comparison with the hollow fuel in a fuel rod, (a) shows the case of a hollow fuel, (b) shows the case of a solid fuel. It is the horizontal arrangement | sequence figure and longitudinal cross-sectional view which show the arrangement | sequence of the fuel rod of the fuel assembly which concerns on 1st Embodiment, (a) is one type of segment elements, (b) is 3 types, (c) is 5 types. This is the case. It is a graph which compares the relative output of the axial direction in each case of FIG. It is a longitudinal cross-sectional view which shows the structure of the connection part between the fuel rods concerning 2nd Embodiment. It is a longitudinal cross-sectional view which shows the structure of the modification of the connection part between the fuel rods concerning 2nd Embodiment. It is a perspective view which shows the structure of the joint spacer which is a connection part between the fuel rods of the fuel assembly which concerns on 3rd Embodiment. It is a longitudinal cross-sectional view which shows the state of the coupling | bonding in the joint spacer which is a connection part between the fuel rods of the fuel assembly which concerns on 3rd Embodiment, (a) is before insertion, (b) is during insertion, (c ) Indicates the combined state after insertion. It is a perspective view which shows the structure of the joint spacer which is a connection part between the fuel rods of the fuel assembly which concerns on 4th Embodiment.

  Hereinafter, fuel rods and fuel assemblies according to embodiments of the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

[First Embodiment]
FIG. 1 is a longitudinal sectional view showing a configuration of a fuel assembly 100 according to the first embodiment. Although FIG. 1 shows a case of a BWR type fuel assembly, it can also be applied to PWR and other reactor types. The following description will be made mainly using the case of the BWR type.

  The fuel assembly 100 includes a plurality of fuel rods 200, a fuel spacer 101, a lower tie plate 102, an upper tie plate 103, and a channel box 104.

  The plurality of fuel rods 200 extend in the vertical direction, and are arranged in a square lattice parallel to each other. In addition, although a square lattice shape is common, it is not limited to this, A triangular lattice etc. may be sufficient.

  The lower ends of the plurality of fuel rods 200 are bound and supported by a lower tie plate 102 constituting a lower structural member in a BWR type fuel assembly. Further, the upper ends of the plurality of fuel rods 200 are bound and supported by the upper tie plate 103 constituting the upper structural member in the BWR type fuel assembly. Note that there may be a partial fuel rod that is not connected to the upper tie plate 103 in part. In the PWR type fuel assembly, the lower ends of the plurality of fuel rods 200 are supported by a lower nozzle as a lower structural member, and the upper ends of the plurality of fuel rods 200 are supported by an upper nozzle as an upper structural member. A plurality of fuel rods 200 are not bundled like a BWR type fuel assembly.

  Furthermore, fuel spacers 101 are provided at a plurality of locations along the vertical direction between the upper and lower ends of the plurality of fuel rods 200. The fuel spacer 101 suppresses the horizontal movement of each fuel rod 200.

  The channel box 104 has a plurality of fuel rods 200 arranged in a lattice shape by the lower tie plate 102, the fuel spacer 101, and the upper tie plate 103, and the side surfaces from the lower tie plate 102 to the upper tie plate 103 are displayed. It is provided to cover.

  FIG. 2 is a longitudinal sectional view conceptually showing the structure of the fuel rod 200. One fuel rod 200 has a plurality of segment elements 210. The segment elements 210 are connected so as to form a line in the vertical direction.

  Each segment element 210 has a cladding tube 212, an upper end plug 213, a lower end plug 214, and a nuclear fuel 211. Each material of the cladding tube 212, the upper end plug 213, and the lower end plug 214 is a silicon carbide fiber reinforced composite material (SiC / SiC composite material) reinforced with silicon carbide long fibers. The material is not limited to the SiC / SiC composite material, and may be a material other than the SiC / SiC composite material as long as the material has sufficient stability under high temperature conditions.

  The cladding tube 212 is, for example, a cylindrical tube that is open at the top and bottom. An upper end plug 213 is attached to an open portion at the upper end of the cladding tube 212, and the cladding tube 212 and the upper end plug 213 are hermetically sealed by, for example, brazing. In addition, a lower end plug 214 is attached to the open portion at the lower end of the cladding tube 212, and the cladding tube 212 and the lower end plug 214 are similarly hermetically sealed by, for example, brazing.

  Nuclear fuel 211 is accommodated in a sealed space formed by the cladding tube 212, the upper end plug 213, and the lower end plug 214. The nuclear fuel 211 is, for example, uranium dioxide pellets enriched with uranium 235, but is not limited thereto.

  A plenum 216 is formed in the upper space not occupied by the nuclear fuel 211. As combustion of the nuclear fuel 211 progresses, the plenum 216 increases the gas pressure in the sealed space by adding fission products such as krypton and xenon, which are fission products, to the initially filled gas such as helium gas. However, it is a space for keeping this gas pressure within an appropriate range. The plenum 216 is not limited to the upper part in the sealed space. For example, it may be formed in the lower part or the middle part in the vertical direction.

  A connecting portion 300 is provided between the segment elements 210 adjacent to each other in the vertical direction. The connecting part 300 connects the segment elements 210 with a predetermined bending rigidity. Here, the bending rigidity is not the rigidity in the vertical direction but the rigidity related to the deformation in the bending direction, that is, the direction deviating from the vertical axis along which the fuel rod 200 extends.

  As will be described later, there is an upper limit value for the bending rigidity of the connecting portion 300, which is set lower than at least the bending rigidity of the segment element 210. In the segment element 210, the cladding tube 212, the upper end plug 213, and the lower end plug 214, which are the structures, are made of a SiC / SiC composite material, and have extremely high rigidity. For this reason, the bending rigidity of the fuel rod 200 is substantially determined by the bending rigidity of the plurality of connecting portions 300 provided in the vertical direction. In the case of BWR, the bending rigidity of the fuel assembly 100 is mainly a combined rigidity of the channel box 104 and the plurality of fuel rods 200 housed therein. In the case of PWR, it is mainly the combined rigidity of the plurality of fuel rods 200.

  For example, when the rigidity of the connecting portion 300 is too small, the overall bending rigidity of the fuel assembly 100 is reduced. As a result, when an earthquake occurs, the amount of deformation in the bending direction of the fuel rod 200 of the fuel assembly 100 increases. When the displacement in the individual bending direction is further added to the average displacement in the individual fuel rods 200, the deformation in the bending direction of the fuel rods 200 is further increased. Therefore, even in the PWR type in which the control rods are inserted between the fuel assemblies, the increase in the bending displacement of the fuel rods in the fuel assemblies is not a preferable direction for the insertability of the control rod clusters.

  As described above, the rigidity of the connecting portion 300 depends on the condition that the displacement in the bending direction of the fuel assembly 100 or the displacement in the bending direction of the fuel rod 200 does not hinder the insertion property of the control rod in the case of PWR. There is a lower limit.

  On the other hand, when the rigidity of the connecting portion 300 is too high, the connecting portion 300 cannot absorb the load due to the acceleration during the earthquake, and a bending load is applied to the segment element 210. In particular, the cladding tube 212 which is a structure of the segment element 210 and is a SiC / SiC composite material may cause a minute crack inside to cause fission product gas leakage when an excessive bending load is applied. . Therefore, there is an upper limit for the rigidity of the connecting portion 300 for the purpose of preventing an excessive bending load from acting on the cladding tube 212.

  As described above, the rigidity of the connecting portion 300 is a value between the lower limit value from the viewpoint of insertability of the control rod and the upper limit value from the viewpoint of ensuring the soundness of the cladding tube 212, whichever condition Therefore, it is necessary to take a value within a range that can ensure an appropriate margin. The upper limit value and the lower limit value can be determined after evaluation based on the structure of the fuel assembly 100 including the number of connecting portions 300.

  FIG. 3 is a longitudinal sectional view showing the configuration of the connecting portion 300 between the fuel rods 200. In the segment element 210, nuclear fuel 211 is accommodated in the cladding tube 212. In the example of FIG. 3, the nuclear fuel 211 is, for example, a pellet. The plenum 216 is provided with a plenum spring 215 that suppresses nuclear fuel 211 and suppresses vibration of the nuclear fuel 211.

  The connecting part 300 includes a ball 311 and a connecting cylinder 312. Also, a recess 214a formed in the lower end plug 214 of the segment element 210 adjacent to the ball 311 above the ball 311 and an upper end plug 213 of the segment element 210 adjacent to the ball 311 below the ball 311. The recess 213a constitutes the connecting part 300. The shape of the ball 311 is a spherical shape or an elliptical rotating body. In this way, the connecting portion 300 is an indirect connecting portion, that is, an indirect connecting portion, in which two segments 210 adjacent to each other in the vertical direction are indirectly connected via the connecting cylinder 312 or the like. .

  The recess 214a and the recess 213a face each other vertically to form a space corresponding to a spherical or elliptical rotating body, and the ball 311 is accommodated in this space. A gap exists between the upper end of the upper end plug 213 and the lower end of the lower end plug 214 with the ball 311 sandwiched, and the upper end plug 213 and the lower end plug 214 sandwich the ball 311 relatively. It has a rotatable angle range.

  Each of the upper end portion of the upper end plug 213 and the lower end portion of the lower end plug 214 has a cylindrical shape. The connection tube 312 has a cylindrical shape with the top and bottom open, and is provided so as to surround the outer periphery of the upper end portion of the upper end plug 213 and the lower end portion of the lower end plug 214 with the ball 311 interposed therebetween. The connecting cylinder 312 is attached to the upper end portion of the upper end plug 213 and the lower end portion of the lower end plug 214, respectively, and connects the segment elements 210 adjacent to each other vertically.

  For example, the connection tube 312 and the upper end plug 213 are coupled to each other by forming a hole (not shown) penetrating the connection tube 312 and the upper end portion of the upper end plug 213, and a pin (not shown). ). In this case, a sufficient gap is formed between the hole formed in the upper end portion of the upper end plug 213 and the pin. The connection tube 312 and the lower end portion of the lower end plug 214 are similarly coupled.

  A predetermined gap is formed between the inner surface of the connection tube 312, the outer surface of the upper end portion of the upper end plug 213, and the outer surface of the lower end portion of the lower end plug 214. The segment elements 210 adjacent to each other vertically are deformed in the bending direction within the range of the gap. That is, the deformation of the fuel rod 200 in the bending direction is controlled by this gap.

  It should be noted that there are three or more in the circumferential direction between the inner surface of the connection tube 312 and the outer surface of the upper end portion of the upper end plug 213 and between the inner surface of the connection tube 312 and the outer surface of the lower end portion of the lower end plug 214. The number of springs (not shown) may be provided at equal intervals. By adjusting the location where the spring is provided and the rigidity of the spring, etc., the bending rigidity of the fuel rod 200, and hence the fuel assembly 100, can be adjusted. Alternatively, the connection tube 312 itself may be a spring material.

  FIG. 4 is a longitudinal sectional view showing the mutual relationship between the fuel rods 200 at the position of the connecting portion 300 between the segment elements 210 of the fuel assembly 100. As described above, each of the plurality of fuel rods 200 has the segment elements 210 having the nuclear fuel 211 connected to each other via the connecting portion 300 in the vertical direction. Since the nuclear fuel 211 is not stored in the connecting portion 300, the vertical distribution of the nuclear fuel is interrupted in each connecting portion 300.

  In the arrangement of the fuel rods 200 shown in FIG. 4, the height positions of the connecting portions 300 in the fuel rods 200 are different. Therefore, as a whole of the fuel assembly 100, nuclear fuel always exists in any fuel rod 200 at any height position.

  FIG. 5 is a longitudinal sectional view showing the types of fuel rods. In the figure, A, B, C, D, and E represent the types of fuel rods 200, and their configurations are displayed below each. In FIG. 5, the hatched portion is the portion of the segment element 210 that contains the nuclear fuel 211, and the white portion is the portion that does not contain the nuclear fuel 211 including the connecting portion 300. In the vertical direction, the number of the white parts is one and the number of the hatched parts is four. As a rule, the length of the part containing the nuclear fuel 211 is assumed to be about four times the length of the part not containing the nuclear fuel. This is because. Only the arrangement in the vertical direction is different, and each of A to E is shifted downward one by one. When A, B, C, D, and E are compared in the height direction, nuclear fuel is always present at any height.

  FIG. 6 is a horizontal sectional view showing an example of the arrangement of the fuel rods 200. In the 10 × 10 array in FIG. 6, the fuel rods of the same type do not exist around any fuel rod. In this arrangement, there are always five types of fuel in any row seen in the horizontal direction in the figure or any row seen in the vertical direction in the figure. That is, in such an arrangement, a portion having nuclear fuel is present at any height in both the vertical and horizontal cross sections in the figure.

  When the segment element 210 is used, the neutron absorption by the nuclear fuel material such as uranium is reduced in the fuel rod 200 adjacent to the region where the nuclear fuel 211 does not exist, such as the connecting portion 300 and the plenum 216. For this reason, an output spike in which the local output of this portion of the adjacent fuel rod 200 increases may occur. Since the operation limit value for maintaining the soundness of the fuel is determined by the maximum linear power density, if there is a local output spike, it is necessary to reduce the average output of that portion, and the combustion efficiency of the fuel decreases.

  In order to prevent output spikes, there is a method of arranging a neutron absorber in the plenum part, but using this method when segment fuel constitutes the entire core, a large amount of neutron absorber is placed in the core. This will significantly reduce the neutron economy. With the configuration as shown in FIG. 6, output spikes can be suppressed by dispersing and arranging regions that do not contain nuclear fuel material and reducing non-uniform neutron absorption.

  Here, when the segment elements 210 other than the upper end portion and the lower end portion of the fuel rod 200 are viewed in the vertical direction, the length of the portion where the nuclear fuel 211 exists and the other portions, that is, the upper end plug 213, the lower end plug 214, The approximate ratio of the length of the plenum 216 and the combined portion of the connecting portion 300 is n: 1 as shown in FIG. For example, in the case shown in FIG. 5, since the number of hatched blocks is 4 for one white block, n = 4. In this case, since (n + 1) = 5, if there are five types of fuel rods shifted from each other in the vertical direction by 1/5 of the vertical pitch of the segment elements 210, as shown in FIG. In this case, the nuclear fuel 211 is present, and the occurrence of spikes can be suppressed.

  In the case of a 10 × 10 array, since 10 is a multiple of 5, five types of fuel rods 200 can be evenly distributed in any direction as shown in FIG. In this way, the occurrence of spikes can be further suppressed by further suppressing non-homogeneity in the fuel assembly 100.

  If the type of the fuel rod 200 is set according to the value of n, the number of segment elements 210 provided at the upper end and the lower end of the fuel rod 200 increases as n increases. For this reason, the size of n is desired to be an appropriate size. For this purpose, it is necessary to adjust the length of the plenum 216 in the sealed space.

  FIG. 7 is a longitudinal sectional view showing a comparison with the hollow fuel in the fuel rod 200, where (a) shows a hollow fuel and (b) shows a solid fuel. (A) Since the nuclear fuel 211a is hollow, the plenum 216a in the segment element 210a in the case of solid fuel is surrounded by the nuclear fuel 211a and the volume in the range where there is no nuclear fuel 211a above the sealed space. It becomes the sum of the volume of the center part.

  Therefore, compared with the segment element 210b which has the plenum 216b with the solid nuclear fuel 211b, the length of the part without the nuclear fuel in the sealed space can be shortened. Thus, the value of n can be adjusted by introducing hollow fuel.

  FIG. 8 is a horizontal arrangement diagram and a longitudinal sectional view showing the arrangement of fuel rods. Case 1 shown in (a) shows one kind of segment element, case 2 shown in (b) shows three kinds, and (c) shows. Case 3 is a case of five types. In the case of the three types of case 2 in FIG. 8B, the arrangement is such that three types of fuel rods are present at all heights in any elevation. Further, in the case of the five types of case 3 in FIG. 8C, the arrangement is such that there are five types of fuel rods at all heights in any elevation. For convenience of explanation, the case of a 5 × 5 array is shown.

  FIG. 9 is a graph comparing the relative output in the axial direction in each case of FIG. The horizontal axis is the order of the blocks counted from the lower side in FIG. The vertical axis is the relative output. The fuel rod 200 of Case 1 has no nuclear fuel in the 11th to 15th blocks counting from the bottom, and the relative value cannot be calculated, so this region has no curve.

  As shown in FIG. 9, in Case 1, the relative output is large at the boundary between the region with the intermediate nuclear fuel and the region without the intermediate nuclear fuel. That is, the local peak is large. Case 2 is greatly relaxed. In case 3 where the number of fuel rods 200 is increased to five, the relative output is further flattened. Thus, the effect of flattening the relative output is obtained by the method of the present embodiment in which the types of the fuel rods 200 are increased and placed at positions shifted from each other.

  As described above, according to the present embodiment, it is not necessary to manufacture a long tube by using a short segment element instead of a long fuel rod. Due to the short length, the fuel assembly 100 can be configured using a tube with good manufacturability. Further, the fuel assembly 100 is configured by arranging the columnar fuel rods 200 in a lattice shape, and thus it is similar to the existing fuel, and can be loaded into the existing furnace.

  In addition, by appropriately designing the structure of the connecting portion 300 between the segment elements 210, the fuel rod 200 can be bent without being damaged, so that the bending of the segment element 210 itself is minimized and the entire fuel rod is bent. You will be able to Accordingly, even when a vibration is applied due to an earthquake or the like and a bending load is applied to the fuel assembly 100, the load applied to the SiC / SiC composite material can be reduced by bending the connecting portion 300. As a result, the possibility of cracks occurring in the cladding tube 212 can be minimized, and the possibility of leakage of FP gas and the like can be minimized.

  Furthermore, even if a crack occurs, the amount of fission products in the segment element 210 is small, so the effect is minimal.

  As described above, according to the present embodiment, sufficient manufacturability is obtained even when the SiC / SiC composite material is used. In addition, it has structural similarity to the conventional metal cladding tube fuel assembly to the extent that it can be loaded into existing reactors, and is compatible with existing reactors.

[Second Embodiment]
FIG. 10 is a longitudinal sectional view showing the configuration of the connecting portion 300a between the fuel rods 200 according to the second embodiment. Of the fuel rods 200 adjacent to each other in the vertical direction, an internal thread portion 214b is formed in the lower end plug 214 of the upper fuel rod 200. The upper end plug 213 of the lower fuel rod 200 is formed with a male thread portion 213b.

  The male screw portion 213b and the female screw portion 214b are screwed together. The male screw portion 213b and the female screw portion 214b constitute a connecting portion 300a. A male thread portion may be formed in the lower end plug 214 of the upper fuel rod 200, and a female thread portion may be formed in the upper end plug 213 of the lower fuel rod 200. Thus, the connecting portion 300a is a direct connecting portion, that is, a direct connecting portion, in which two segments 210 adjacent to each other in the vertical direction are directly connected by screwing at their respective end portions. ing.

  In the second embodiment configured as described above, each connecting portion 300a has one screw portion, so that the rigidity of the fuel rod 200 is reduced by this amount. Thereby, the adjustment function of the bending rigidity of the fuel rod 200 is provided.

  FIG. 11 is a longitudinal sectional view showing a configuration of a modified example of the connecting portion between the fuel rods. In the connecting portion 300 b of this modification, an intermediate screw portion 313 is provided between the lower end plug 214 of the upper fuel rod 200 and the upper end plug 213 of the lower fuel rod 200.

  The lower end plug 214 of the upper fuel rod 200 is formed with a male thread portion 214c. The upper end plug 213 of the lower fuel rod 200 is formed with a male thread portion 213b. The intermediate screw portion 313 has a cylindrical shape whose outer shape extends vertically, and female screw portions 313b are formed at the upper end and the lower end, respectively. A female screw portion 313b at the upper end of the intermediate screw portion 313 and a male screw portion 214c formed in the lower end plug 214 of the upper fuel rod 200 are screwed together. In addition, the female screw portion 313b at the lower end of the intermediate screw portion 313 and the male screw portion 213b formed in the upper end plug 213 of the lower fuel rod 200 are screwed together.

  A female thread portion is formed at the lower end plug 214 of the upper fuel rod 200 and an upper end plug 213 of the lower fuel rod 200, and male thread portions are formed at the upper end and lower end of the intermediate thread portion 313, respectively. Also good. As described above, the connecting portion 300b is an indirect connecting portion in which two segments 210 adjacent to each other in the vertical direction are indirectly connected via the intermediate screw portion 313.

  In this modified example configured as described above, since the connecting portion 300b has two screw portions, the rigidity of the fuel rod 200 further decreases. Further, by combining the coupling method according to the second embodiment and the coupling method according to the modification, it is possible to ensure intermediate rigidity between the two.

[Third Embodiment]
FIG. 12 is a perspective view showing a configuration of a joint spacer 410 which is a connecting portion 300 between fuel rods of the fuel assembly according to the third embodiment. The joint spacer 410 includes a lattice plate 411 combined in a square lattice shape and a leaf spring 412 attached to the lattice plate 411. The leaf springs 412 are attached to the four inner surfaces of each lattice. It may be attached to only two surfaces adjacent to each other.

  A lower end plug 214 of the segment element 210 is inserted into each grid from above. When the lower end plug 214 is inserted downward, the leaf spring 412 slides downward at the lower end of the plate spring 412 to form a space in which the lower end plug 214 is inserted.

  FIGS. 13A and 13B are longitudinal sectional views showing a coupling state at a joint spacer 410 which is a connecting portion between fuel rods of the fuel assembly according to the third embodiment, where FIG. 13A is before insertion, and FIG. During insertion, (c) shows the combined state after insertion. The lower end plug 214 and the cladding tube 212 form a stepped portion 214d. That is, the outer diameter of the portion of the lower end plug 214 adjacent to the cladding tube 212 is larger than the outer diameter of the cladding tube 212. Further, a tapered portion 214f is formed at the tip of the lower end plug 214 so as to be easily inserted.

  During the insertion of FIG. 13B, the leaf spring 412 is pressed against the lattice plate 411. In the stage of FIG. 13C, the leaf spring 412 is restored, and the stepped portion 214d is restrained from moving upward by the leaf spring 412.

  The above is the configuration of the upper portion of the joint spacer 410. The same configuration is applied to the lower part of the joint spacer 410. Therefore, the upper end plug 213 of the segment element 210 adjacent to the lower side is similarly coupled.

  If all the segment elements 210 are coupled to the joint spacer 410 at the same height, the output distribution of the fuel assembly 100 is not flattened. For this reason, the fuel rods 200 are grouped, and the joint spacer 410 is made to correspond to each group. In the case of the segment element 210 of the non-corresponding group of fuel rods 200, the cladding tube 212 is opposed to the leaf spring 412 of the joint spacer 410.

  As described above, according to the present embodiment, the joint spacer 410 which is also the connecting portion 300 is provided between the segment elements 210 adjacent to each other above and below the fuel rod 200, and the segment elements 210 are coupled vertically. The fuel rod 200 is also restrained in the horizontal direction.

  Since the joint spacer 410 restrains the fuel rod 200 via the leaf spring 412, the bending rigidity of the fuel rod 200 can be adjusted appropriately.

[Fourth Embodiment]
FIG. 14 is a perspective view illustrating a configuration of a joint spacer that is a connecting portion between fuel rods of the fuel assembly according to the fourth embodiment. This embodiment is a modification of the third embodiment. In the fourth embodiment, instead of the leaf spring 412 of the third embodiment, cylindrical springs 413 are provided at all corner portions in each lattice. It may be provided only at two corners adjacent to each other in each lattice. Here, the cylindrical spring 413 is a part of the circumferential angle range of a part of the cylinder at each corner.

  The lower end plug 214 contacts the circumferential center of the cylindrical spring 413 during the insertion process. The cylindrical spring 413 has high rigidity with respect to deformation from the central portion. Therefore, by using the joint spacer 420 as the connecting portion in the fourth embodiment, a connecting portion having higher rigidity than that in the third embodiment can be realized, and the rigidity adjustment range of the fuel assembly 100 can be realized. Can be further expanded.

[Other Embodiments]
As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. For example, in the first embodiment, the case of the fuel assembly of BWR is shown in FIG. 1, but it is not limited to BWR. The present invention can also be applied to the case of a PWR not provided with a channel box, and the necessity for a predetermined rigidity of the connecting portion is particularly required in a PWR in which a neutron absorber is inserted into a fuel assembly.

  Moreover, you may combine the characteristic of each embodiment. Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 100 ... Fuel assembly, 101 ... Fuel spacer, 102 ... Lower tie plate (lower structural member), 103 ... Upper tie plate (upper structural member), 104 ... Channel box, 200 ... Fuel rod, 210, 210a, 210b ... Segment 211, 211a, 211b ... nuclear fuel, 212 ... cladding tube, 213 ... upper end plug, 213a ... recessed portion, 213b ... male screw portion, 214 ... lower end plug, 214a ... recessed portion, 214b ... female screw portion, 214c ... Threaded portion, 214d ... Stepped portion, 214f ... Tapered portion, 215 ... Plenum spring, 216, 216a, 216b ... Plenum, 300 ... Connecting portion (first indirect connecting portion), 300a ... Connecting portion (direct connecting portion), 300b: Connecting portion (second indirect connecting portion), 311 ... Ball, 312 ... Connecting cylinder, 313 ... Intermediate screw portion, 313 ... female screw portion, 410, 420 ... joint spacer, 411 ... grid plate, 412 ... plate spring, 413 ... cylindrical spring

Claims (9)

A plurality of fuel rods extending in parallel to each other in the vertical direction and arranged in a grid in the horizontal direction;
An upper structural member that supports upper ends of the plurality of fuel rods;
A lower structural member supporting lower ends of the plurality of fuel rods;
With
Each of the fuel rods
A nuclear fuel extending in the vertical direction, a cladding tube made of a silicon carbide fiber reinforced composite material containing the nuclear fuel and extending in the vertical direction and having an upper end and a lower end opened, and a silicon carbide fiber blocking the open portion at the upper end of the cladding tube A plurality of segment elements having an upper end plug made of a reinforced composite material and a lower end plug made of a silicon carbide fiber reinforced composite material that closes the opening at the lower end of the cladding tube and arranged in series in the vertical direction. ,
A connecting portion that connects the segment elements adjacent to each other vertically with a predetermined range of bending rigidity;
A fuel assembly comprising: A recess is formed on the upper surface of the upper end plug and the lower surface of the lower end plug,
The connecting portion is
A ball housed between the recesses formed in each of the lower end plug and the upper end plug and facing each other;
A connecting tube provided so as to cover the periphery of the distal end portion of the lower end plug and the distal end portion of the upper end plug so as to constrain both the lower end plug and the upper end plug;
The fuel assembly according to claim 1, further comprising a first indirect coupling portion having the following. On the upper part of the upper end plug and the lower part of the lower end plug adjacent to each other, a male screw or a female screw that can be engaged with each other is formed,
The fuel assembly according to claim 1, wherein the connecting portion includes a direct connecting portion coupled by the male screw and the female screw. Each of the upper end plug and the lower end plug adjacent to each other is formed with a male screw or a female screw,
The said connection part contains the 2nd indirect connection part which has the intermediate | middle thread part by which the internal thread or external thread which can be combined with the said external thread or internal thread was formed in the upper end and the lower end. Item 4. The fuel assembly according to any one of Items 3.   5. The joint portion according to claim 1, wherein the connecting portion includes a joint spacer that restrains the plurality of fuel rods in a horizontal direction at an intermediate position in a vertical direction of the plurality of fuel rods. The fuel assembly as described.   The fuel assembly according to claim 5, wherein the joint spacer restrains the segment element in a vertical direction using a spring, a screw, or a fitting shape.   The nuclear fuel is present in at least one segment element in any horizontal cross section in the vertical direction except for the upper structural member and the lower structural member. The fuel assembly as described. All of the segment elements that are not located at the upper and lower ends of the fuel rods have the same length,
The fuel rod has a vertical length of the segment element when n is an integer equal to or greater than 1 and the ratio of the length of the portion where the nuclear fuel is present in the vertical direction of the segment element to the other portion is approximated to n: 1. The fuel assembly according to any one of claims 1 to 7, wherein the fuel assembly has (n + 1) types in which the position in the direction is shifted by 1 / (n + 1) of the length of the segment element. . A nuclear fuel extending in the vertical direction, a cladding tube made of a silicon carbide fiber reinforced composite material containing the nuclear fuel and extending in the vertical direction and having an upper end and a lower end opened, and a silicon carbide fiber blocking the open portion at the upper end of the cladding tube A plurality of segment elements having an upper end plug made of a reinforced composite and a lower end plug made of a silicon carbide fiber reinforced composite that closes the opening of the lower end and arranged in series in the vertical direction;
A connecting portion that connects the segment elements adjacent to each other vertically with a predetermined range of bending rigidity;
A fuel rod comprising:

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