JPH0968585A - Fuel assembly for reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a fuel rod from being broken by a foreign matter carried together with a reactor coolant. SOLUTION: A foreign matter catching filter 20 having the following structure is provided between the lower nozzle of a fuel assembly and a bottom part support grid. Namely, it is formed of a plurality of short cylindrical pipes 23, 25, 27 aligned to each of fuel rods 5, arranged so as to surround the lower end plugs 8 of the fuel rods 5, and mutually connected, and the peripheral surfaces of the cylindrical pipes 25, 27 situated on the outermost edge are notched.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used for combustion in a nuclear reactor, and more particularly to a structure for preventing foreign substances accompanying a coolant from flowing into a fuel rod portion of the fuel assembly.

[0002]

2. Description of the Related Art Nuclear fuel used for combustion in a nuclear reactor, for example, a pressurized water reactor, takes the form of a fuel assembly having a structure in which a large number of nuclear fuel rods are bundled. This will be described with reference to FIG. 7. An upper nozzle 1 and a lower nozzle 2 having a large number of coolant flow passage holes are connected by a plurality of control rod guide tubes 4, to which a plurality of support grids 3 are attached. Are spaced apart from one another and form a skeletal structure. The support grid 3 is
An assembly of a number of thin strips or straps intersecting each other at right angles to define a plurality of rectangular cross-section openings or cells into which control rod guide tubes 4 are fixedly inserted. A large number of fuel rods 5 inserted one by one into the cells into which the control rod guide tubes 4 do not fit are elastically supported to form a fuel assembly 10. The connecting structure of the control rod guide tube 4 and the lower nozzle 2 described above is shown in FIG.
A cylindrical insert 6 is inserted into a predetermined cell of the lowermost support grid 3a of the support grid 3, a control rod guide tube 4 extends through the insert, and a mounting screw 7 inserted into the lower nozzle 2 is inserted. The lower rod 2 and the control rod guide pipe 4 are fastened to each other by being screwed into the plug at the lower end of the control rod guide pipe 4. The lower end of the cladding tube 5a of the fuel rod 5 has a solid lower end plug 8
Closed at, which extends downwardly from the lowermost support grid 3a.

[0003]

The fuel assembly 10 thus constructed is used by being arranged adjacent to each other on the lower core plate in the reactor vessel, and light water as a coolant is used as the lower core plate. To rise and cool the fuel rods 5 of the fuel assembly 10. Since light water circulates in the coolant circulation system that passes through the core composed of a large number of fuel assemblies 10, it is rare that it is accompanied by foreign matter, but it is stochastic. In that case, when foreign matter is caught in the gap between the support grid 3 and the fuel rod 5 and stays there, it vibrates due to the light water flow,
It is feared that the cladding 5a of the fuel rod 5 will be damaged by fretting. Conventionally, as a measure against foreign matter intrusion, a structure has been adopted in which the diameter of the coolant passage hole of the lower nozzle 2 is made small so that foreign matter does not pass through the lower nozzle 2. However, if the diameter of the coolant flow path hole of the lower nozzle 2 is reduced, the flow path resistance of each flow path hole generally increases, so
The number of coolant passage holes is increased to prevent an increase in pressure drop. For this reason, there arises a problem that the manufacturing cost such as the processing cost for forming the coolant passage hole increases. Therefore, it is an object of the present invention to provide a fuel assembly for a nuclear reactor which does not need to change the basic structure of the fuel assembly and has a structure that is low in cost and excellent in foreign matter capturing efficiency. .

[0004]

In order to achieve the above object, according to the present invention, an upper nozzle and a lower nozzle which face each other, extend between both nozzles in a parallel relationship, and both ends are connected to both nozzles. A plurality of control rod guide tubes and a plurality of support grids attached to the control rod guide tubes at intervals in the longitudinal direction to form a skeletal structure,
In a fuel assembly in which fuel rods each having a cladding tube whose both ends are closed by end plugs are inserted and supported in one of a plurality of support cells of the support grid of the skeletal structure,
A plurality of short circular tubes aligned with each of the fuel rods and surrounding the lower end plugs of the fuel rods and connected to each other;
The foreign matter trapping filter formed by cutting out the outer surface of the short circular tube located at the outermost edge of the short circular tube is interposed between the lowermost support grid and the lower nozzle.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the same parts, including the above-mentioned conventional technique, are designated by the same reference numerals to facilitate understanding. The skeleton structure of the fuel assembly and the structure and arrangement of the fuel rods 5 inserted into the cells of the supporting grid 3 thereof are the same as those of the conventional one (FIG. 7). The structure and arrangement of the foreign matter trapping filter 20 which is a characteristic part of the present invention are shown in FIGS. As in the conventional case, the lowermost support grid 3
The control rod guide tube 4 and the fuel rod 5 are inserted through a. FIG. 2 shows one quadrant of the plane cross section. A cylindrical insert 6 is inserted into the lowermost support grid 3a,
The lower end of the control rod guide tube 4 is inserted into this as in the conventional case,
It is fastened to the lower nozzle 2 by a mounting screw (not shown). The fuel rod 5 has a cladding tube 5a in which fuel pellets (not shown) are stacked and filled, and the lower end thereof is a lower end plug 8
It is sealed by. The lower end plug 8 is located below the lowermost support grid 3a. The foreign matter trapping filter 2 is provided between the lower nozzle 2 and the lowermost support grid 3a.
0 is provided. The foreign matter capture filter 20 is welded 2
It is fixed to the insert 6 by means of 1, the detailed structure of which is shown in FIGS.

FIG. 3 shows one quadrant of the plan view of the foreign matter trapping filter 20, which corresponds to FIG. 2, but is a circular pipe 23 having a perfect circular cross section, a circular pipe 25 having a semicircular cross section, and a quarter. A circular pipe 27 having one circular cross section (the shapes of the circular pipes 25 and 27 are not necessarily called circular pipes and can be called arc-shaped sectional members, but they are called circular pipes for the purpose of the present invention.) The central axis of is aligned with the fuel rod 5 shown in FIG. That is, the inter-axis distance, that is, the pitch of the circular tubes 23, 25, 27 is equal to the pitch of the cells of the lowermost support grid 3a and is in contact with each other without overlapping.
The outer diameter is substantially equal to the pitch of the cells of the support grid 3a. Also, as is clear from comparing FIG. 2 and FIG. 3, the dead space between the circular pipes 23 corresponds to the position of the control rod guide pipe 4, and the circular pipe 25 having a semicircular cross section is the maximum. It corresponds to the outer fuel rod 5. Such circular pipes 23, 25, 27 are connected to each other by welding 29 located at the upper and lower ends as shown in FIG. 4, and form the foreign substance trapping filter 20 having an integrated structure. Returning to FIG. 1, the upper end of the foreign matter trapping filter 20 is close to the lower end of the lowermost support grid 3a, and the upper end of the circular pipe 23 surrounds the lower end plug 8 of the fuel rod 5, so that both A narrow gap is defined between them.

Next, the foreign substance trapping filter 20 as described above is used.
The state of foreign matter capture when a fuel assembly having the above is loaded into a reactor vessel will be described. Light water, which is the coolant that has flowed into the reactor vessel, flows down in the annular descent space outside the reactor core, reverses in the lower plenum, and rises through the lower core plate. Then, the lower nozzle 2 having a coolant passage hole of a normal size
Flow through. As shown in FIG. 5, the light water passing therethrough separately flows in the annular gap C between the circular pipe 23 of the foreign matter trapping filter 20 and the fuel rod 5 and the ginkgo leaf-shaped space S between the circular pipes 23. When foreign material A is waked in the light water passing through the former,
Due to its size, the foreign matter A is trapped in a shape as shown in FIG. In this case, the foreign matter A is in contact with the fuel rod 5, but since the contact location is the outer surface of the solid lower end plug 8, the cladding tube 5a will not be damaged even if fretting occurs, and the fuel inside There is no through hole leading to the pellet filling space.
In the latter case, when the foreign matter B is accompanied by the light water flowing through the ginkgo leaf space S, it is trapped between the circular pipe 23 and the lowermost support grid 3a as shown in FIG. 6, but in this case, the foreign matter B vibrates. However, since there is no contact with the cladding tube 5a of the fuel rod 5, no problematic damage will occur.

[0008]

As described above, according to the present invention,
The foreign matter trapping filter composed of a substantially short circular tube
Since it is easy to process, it can be manufactured at low cost, and it defines the foreign material trapping space isolated from the fuel rod cladding tube while allowing thermal expansion and irradiation growth of the fuel rod.
It is possible to reliably prevent fretting damage to the fuel rod cladding tube due to foreign matter. Furthermore, since the circular tube of the foreign matter trapping filter can be manufactured with a thin wall, it does not increase the pressure drop of the coolant flow passing therethrough, and the outer edge of the circular tube is notched, so that the fuel rod It is easily accessible to detect irradiation growth.

[Brief description of drawings]

FIG. 1 is a partially cutaway partial side view showing an essential part of an embodiment of the present invention.

FIG. 2 is a partial plan sectional view of the embodiment.

FIG. 3 is a one-quadrant plan view showing a main part of the embodiment.

FIG. 4 is a side view of the foreign matter trapping filter 20 corresponding to FIG.

FIG. 5 is an operation explanatory view of the embodiment.

FIG. 6 is a perspective view for explaining the operation of the embodiment.

FIG. 7 is a partially cutaway perspective view showing an example of a fuel assembly to which the present invention is applied.

FIG. 8 is a partial side view showing the structure of a conventional fuel assembly.

[Explanation of symbols]

 1 Upper Nozzle 2 Lower Nozzle 3 Support Grid 3a Bottom Support Grid 4 Control Rod Guide Tube 5 Fuel Rod 5a Cladding Tube 8 Lower End Plug 10 Fuel Assembly 20 Foreign Material Capture Filter 23 Circular Tube 25 Circular Tube 27 Circular Tube A, B Foreign Material

Claims (1)

[Claims] 1. An upper nozzle and a lower nozzle facing each other, a plurality of control rod guide pipes extending between the both nozzles in a parallel relationship with each other, and both ends thereof connected to the both nozzles, and a longitudinal direction of the control rod guide pipe. A plurality of support grids of the skeletal structure, each of which has a skeleton structure formed of a plurality of support grids attached at intervals in a direction, and has a cladding tube whose both ends are closed by end plugs. In the fuel assembly that is inserted into and supported by one of the support cells, a foreign matter trapping filter interposed between the lowermost support grid and the lower nozzle is aligned with each of the fuel rods. It is composed of a plurality of short circular tubes arranged around the lower end plug of the fuel rod and connected to each other, and the outer surface of the short circular tube located at the outermost edge of the short circular tubes is cut out. Characteristic for reactor Fuel assembly.