JP2015025690A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel assembly used in a boiling water type nuclear power plant that prevents molten debris from spreading during severe accidents.SOLUTION: A fuel assembly 1 comprises fuel rods 3, a bottom tie plate 6, and channel boxes 2. Surfaces of the bottom tie plate and the channel boxes are made of materials having melting points higher than those of cladding tubes of the fuel rods, and therefore molten debris is prevented from spreading outside the fuel assembly even in the event of a severe accident and damage to members outside a reactor core can be limited. In other words, the surfaces of the bottom tie plate and the channel boxes are made of the materials having high melting points that cannot be damaged even in a high-temperature state in which the cladding tubes of the fuel rods melt, and therefore the molten debris is held in a capacity defined by the bottom tie plate and the channel boxes and is prevented from spreading outside the fuel assembly.

Description

  The present invention relates to a fuel assembly used in a boiling water nuclear power plant, and more particularly to a fuel assembly having components of a high heat resistant material.

  The fuel assembly is held between the upper and lower tie plates together with the upper and lower tie plates that hold both ends of the fuel rods containing the fuel pellets, the channel box that surrounds the fuel rods, and the fuel rods. It is comprised by members, such as a water rod. Conventionally, these components are generally composed of members having a relatively low melting point, such as zirconium alloys and stainless steel.

  By the way, when a severe accident event occurs that greatly exceeds the design standard event that the reactor assumed in advance for some reason, the temperature of the core becomes very high and the core melts. Under such an event, the estimated maximum temperature is higher than the melting point of 2500 to 2800 ° C. such as zirconium alloy and stainless steel, so the lower tie plate and channel box melt together with the fuel rod cladding tube, and the fuel rod Molten debris such as fuel pellets, zirconium oxide, and molten components held inside the reactor diffuses out of the reactor core, making it difficult to remove the fuel member in the reactor dismantling process after severe accidents. There was a problem. For this reason, a technique for preventing the diffusion of molten debris during a severe accident has been demanded.

  As a method for solving such a technical problem, it can be considered that the cladding tube of the fuel rod is made of a material having a high melting point. However, fuel rods are not only heat resistant, but can withstand high pressure, have high thermal conductivity, high chemical stability, high mechanical strength such as local stress, Various performances such as a small neutron absorption cross section are required, so it is not easy to find a material that can withstand these high temperatures and withstand high temperatures during severe accident events. There wasn't.

JP 2002-90489 A

  Constructing the lower tie plate of the fuel assembly and the surface of the channel box with a material having a higher melting point than the cladding of the fuel rod, so that molten debris diffuses outside the fuel assembly even under severe accident events To prevent damage to members outside the core. That is, the surface of the lower tie plate and the channel box is defined by the lower tie plate and the channel box by being made of a high melting point material that can withstand high temperature conditions in which the fuel rod cladding tube melts. The molten debris is retained in the capacity, and the molten debris is prevented from diffusing outside the fuel assembly. Thereby, the difficulty of handling in the processing after severe accident can be improved. Also, conventionally used fuel rods made of zirconium alloy can be used as they are for the fuel rods constituting the fuel assembly.

  Furthermore, it is desirable that the lower tie plate and the channel box are combined. That is, when the lower tie plate and the channel box are integrally formed to form a container, it is easy to handle the molten debris held inside the container. Here, it is desirable that the material constituting the lower tie plate and the material constituting the surface of the channel box are the same material.

  Further, the upper tie plate and the lower tie plate of the fuel assembly are joined by at least one coupling member of the channel box and the coupling rod, and the surface of the coupling member also has a higher melting point than the cladding tube of the fuel rod. It is desirable that it is made of a material having the same. Here, the connecting rod is a rod-shaped structure that is disposed between the upper tie plate and the lower tie plate in the channel box and has a melting point higher than that of the fuel rod. The connecting rod is preferably a water rod, but a part of the fuel rod may be replaced with a rod-like structure whose surface is coated with a high melting point material. The fuel assembly can be handled as a unit by combining the two with a high-melting-point member arranged between the upper and lower tie plates, improving the handling difficulty in processing after severe accidents. be able to. Here, the material of the coupling member is preferably the same material as the material constituting the lower tie plate and the channel box.

  Furthermore, it is desirable that the high melting point material constituting the surface of the lower tie plate or the channel box is a ceramic matrix composite material, a ceramic, a metal alloy, or a combination thereof. More specifically, a material selected from the group consisting of a SiC / SiC composite material, zirconia, and tungsten is desirable. This is because these materials have not only a high melting point but also various performances required for the fuel assembly.

  Furthermore, it is desirable to dispose a closed structure in which a plurality of openings are formed of materials having different melting points, close to the lattice plane of the lower tie plate. In other words, when the temperature is high during severe accidents, a material having a low melting point out of the material constituting the opening is melted to partially close the opening, thereby securing a flow path of the cooling water flowing through the opening. However, it is possible to prevent diffusion of molten debris smaller than the size of the opening before melting.

  As the blocking member described above, the debris filter disposed near the lower tie plate is made of a material having a different melting point, so that the same member can serve as both the debris filter during normal operation and the blocking member during severe accident. .

It is a schematic block diagram of the fuel assembly which is one Example which concerns on this invention. It is a schematic block diagram of the lower tie plate vicinity of one Example which concerns on this invention. It is a perspective view of the lower tie plate vicinity of one Example which concerns on this invention. It is a schematic block diagram of the lower tie plate vicinity of another Example which concerns on this invention. It is a schematic block diagram of the fuel assembly of another Example which concerns on this invention.

  An embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 is a schematic configuration diagram showing the entire fuel assembly 1, and FIG. 2 is a schematic configuration diagram in the vicinity of the lower tie plate 6. The fuel assembly 1 includes a water rod 4, a plurality of fuel rods 3 arranged in a lattice around the water rod 4, each having a cladding tube that holds fuel pellets therein, the fuel rod 3, and the water rod 4 includes an upper tie plate 5 having a lattice surface into which the upper end plugs of 4 are inserted, and a lattice surface into which the lower end plugs of the fuel rod 3 and the water rod 4 are inserted. A lower tie plate 6 having a conical portion seated on a metal fitting, a support grid 7 for bundling and supporting a plurality of fuel rods 3 at a position between an upper end and a lower end of the fuel rod 3, and a plurality of fuel rods 3 and a support A rectangular tube-shaped channel box 2 covering the periphery of the grid 7 and a handle portion 8 coupled to the upper tie plate 5 are provided.

  The upper tie plate 5 and the lower tie plate 6 are coupled via a coupling rod or a channel box 2. The connecting rod in this embodiment is a water rod 4. With this configuration, the handle portion 8 is grasped by the power plant equipment or the like, so that the fuel assembly 1 can be integrally loaded into the core and taken out from the core.

  The cladding tube of the fuel rod 3 is made of a zirconium alloy. For this reason, when it becomes a high temperature state at the time of a severe accident, it may melt | dissolve and it becomes impossible to hold | maintain an internal fuel pellet.

  On the other hand, the surfaces of the channel box 2 and the lower tie plate 6 are made of a SiC / SiC composite material which is a kind of ceramic matrix composite material. The melting point of the SiC / SiC composite material is very high at about 2700 ° C., and it does not melt even when a severe accident occurs, and maintains a bucket shape with the channel box 2 as the side and the lower tie plate 6 as the bottom. Can do. For this reason, molten debris such as a molten uranium oxide falling from the fuel rod 3, an oxide of zirconium, and a molten constituent member can be held inside the bucket-shaped container.

  Moreover, the member (water rod 4 etc.) which joins the upper tie plate 5 and the lower tie plate 6 and the surface of the upper tie plate 5 are also made of SiC / SiC composite material. Therefore, even when a severe accident event occurs, the fuel assembly 1 can maintain the integrated configuration.

  By the way, as shown in FIG. 3, the lower tie plate 6 is formed with a fuel rod pin hole 11 into which a lower end plug of the fuel rod 3 is inserted and a cooling hole 10 for flowing cooling water. For this reason, debris smaller than these holes passes through the holes 10 and 11. For this reason, the debris filter 9 is disposed close to the lower surface of the lattice plane of the lower tie plate 6 so that the debris that has passed through does not diffuse outside the fuel assembly.

  The debris filter 9 is also composed of a SiC / SiC composite material so as not to melt during severe accidents. At this time, the entire debris filter 9 may be composed of a SiC / SiC composite material. However, in this embodiment, two materials (SiC / SiC composite materials) having different melting points are used in order to further improve the effect of preventing diffusion of molten debris. And stainless steel).

  That is, the lattice plate constituting the debris filter 9 is alternately made up of a first lattice plate 9a made of a SiC / SiC composite material having a high melting point and a second lattice plate 9b made of stainless steel having a low melting point. It is arranged and configured. A large number of openings 12 through which cooling water flows are defined by the first grid plate 9a and the second grid plate 9b. When the temperature inside the fuel assembly 1 rises during a severe accident, the second lattice plate 9b having a low melting point is melted, but the first lattice plate 9a having a high melting point is not melted and maintains its shape. For this reason, the opening 12 is partially blocked by the melted second grid plate 9b, and the opening area becomes smaller than before the severe accident event occurs. Therefore, it is possible to hold the cooling water flow path even during a severe accident while the combustion aggregate 1 holds even smaller molten debris.

  The configuration of the fuel assembly 1 has been described above. However, those skilled in the art can easily conceive that the design can be appropriately changed according to the configuration and usage of the fuel assembly 1. For example, as a material constituting the surface of the channel box 2, the water rod 4, the upper tie plate 5, the lower tie plate 6, the closing member 9, etc., in addition to the ceramic matrix composite material described above, zirconia (melting point is about Materials higher than the melting point of the cladding tube of the fuel rod 3, such as ceramics such as 2700 ° C., metal alloys such as tungsten (melting point is about 3400 ° C.), and combinations thereof, can be used as appropriate.

  Moreover, you may comprise not only the surface of the channel box 2 and the lower tie plate 6 but the whole member including the inside with the same material. Furthermore, the materials constituting the surfaces of the channel box 2, the water rod 4, the upper tie plate 5, the lower tie plate 6, and the like do not have to be the same material, and the respective members may be made of different materials.

  In addition, the closing member 9 may not be used as a debris filter, and a member having a function of partially closing the opening when the temperature of the fuel assembly 1 rises may be separately provided. Further, the closing member 9 may be disposed in the vicinity of the upper surface of the lattice surface of the lower tie plate 6 as shown in FIG.

  Further, the connecting rod is composed of a rod-like structure having the same outer shape as that of the fuel rod 3 whose surface is coated with a material having a higher melting point than the material constituting the surface of the fuel rod 3. The part may be replaced with such a connecting rod. FIG. 5 is a schematic configuration diagram of an embodiment in which the second fuel rod from the outside is replaced with the connecting rod 13. In order to avoid duplication of description, members having the same functions as those in FIG. 1 are denoted by the same reference numerals. Since the connecting rods 13 shown in black have substantially the same outer shape as the fuel rods 3, they can be replaced with the fuel rods 3 without changing the lattice planes of the upper tie plate 5 and the lower tie plate 6. it can. Since the surface of the connecting rod 13 is composed of a SiC / SiC composite material, even if the surrounding fuel rod 3 and the water rod 4 are melted due to a high temperature during a severe accident, the connecting rod 13 is not melted. The fuel assembly 1 can be maintained in an integrated configuration while maintaining a state where it is combined with the tie plate 5 and the lower tie plate 6. Further, by filling the inside of the connecting rod 13 with fuel pellets, it is also possible to serve as the fuel rod 3. In addition, the material which comprises the coupling rod 13 is not restricted to a SiC / SiC composite material, It can comprise with the ceramic matrix composite material mentioned above, ceramics, a metal alloy, or these combination. Further, the arrangement of the connecting rod 13 (position to replace the fuel rod 3) can be changed as appropriate.

1 Fuel assembly 2 Channel box 3 Fuel rod (cladding tube)
4 Water rod 5 Upper tie plate 6 Lower tie plate 7 Support grid 8 Handle part 9 Closure structure (debris filter)
9a Lattice plate made of high melting point member 9b Lattice plate made of low melting point member 10 Cooling hole 11 Fuel rod pin hole 12 Opening portion 13 Connecting rod

Claims (10)

A fuel assembly comprising a fuel rod, a lower tie plate, and a channel box,
The surface of the lower tie plate and the surface of the channel box are made of a material having a melting point higher than the cladding of the fuel rod;
A fuel assembly characterized by   The fuel assembly according to claim 1, wherein the lower tie plate and the channel box are combined.   The fuel assembly according to claim 2, wherein the surface of the lower tie plate and the surface of the channel box are made of the same material. An upper tie plate and a connecting rod;
The upper tie plate and the lower tie plate are coupled by at least one member of the channel box and the coupling rod;
The surface of the at least one member is made of a material having a higher melting point than the cladding of the fuel rod;
The fuel assembly according to any one of claims 1 to 3, wherein:   The fuel assembly according to claim 4, wherein the surface of the at least one member is made of the same material as the surface of the lower tie plate or the surface of the channel box.   The fuel assembly according to claim 4, wherein the connecting rod is a water rod.   The fuel assembly according to any one of claims 1 to 6, wherein the material having a higher melting point than the cladding tube of the fuel rod is a ceramic matrix composite material, a ceramic, a metal alloy, or a combination thereof. body.   7. The material according to claim 1, wherein the material having a higher melting point than the cladding of the fuel rod is a material selected from the group consisting of a SiC / SiC composite material, zirconia, and tungsten. The fuel assembly as described. Further comprising a closing structure that is disposed in proximity to the lattice plane of the lower tie plate and has a plurality of openings formed of materials having different melting points;
The closing structure is configured such that one of the materials having different melting points is melted to partially close the opening.
The fuel assembly according to any one of claims 1 to 8, wherein   The fuel assembly according to claim 9, wherein the blocking member is a debris filter.