JPH09318778A - Pwr fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily fix the lowest support grid for PWR fuel assembly without applying swaging. SOLUTION: A sleeve 8 for inserting the lower part of a thimble tube lower end plug 7 in is extended downward from the thimble cell 6 of the lowest support grid 3', a screw part 10 is formed on the inner circumference surface of the sleeve 8 and a screw part 1.1 screwing in the screw part 10 of the sleeve is formed at the lower outer circumference surface of the lower end plug 7. In the state that the both screw parts 10 and 11 of the sleeve 8 and the lower end plug 7 are coupled, they are fit in a back facing part 13 recessed around the thimble screw hole 12 of a lower nozzle 5 and the lower end plug 7 is fixed in the lower nozzle 5 with a thimble screw 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PWR fuel assembly used in a pressurized water reactor, and more particularly, to a fixing structure of a lowermost support grid positioned so as to be in contact with a lower nozzle.

[0002]

2. Description of the Related Art A fuel assembly used in a pressurized water reactor has a large number of fuel rods 1 arranged in parallel as shown in FIG.
In addition, the thimble tube 2 is mixed and supported by a plurality of support grids 3, a fuel portion, an upper nozzle 4 and a lower nozzle 5 fixed to the thimble tube 2, and a lowermost support grid 3'is formed. In order to enhance the effect of capturing foreign matter in the coolant, it is lowered so as to contact the upper surface of the lower nozzle 5 and fixed to the thimble tube 2. In the method of fixing the lowermost support grid 3 ', since there is almost no gap between the support grid 3' and the lower nozzle 5, as shown in FIG.
The sleeve 15 is welded above the thimble cell (part X),
The sleeve 15 and the thimble tube 2 are integrated by swaging (expanding tube) processing (Y portion).

[0003]

However, in the above-mentioned conventional fixing structure of the lowermost support grid, the swaging process requires time and labor, which hinders the rationalization of the fuel assembly, and the swage portion is attached to the thimble tube. There is a problem that the thimble tube is shortened at this portion due to the formation of the above. Therefore, it was necessary to adjust the length of the thimble tube which varies due to the swaging process by using a bushing or the like.

The present invention addresses the above situation and by finding a new construction for the fixing structure of the lowermost support grid,
The purpose is to solve the above problems.

[0005]

Means for Solving the Problems That is, the feature of the present invention which meets the above-mentioned object is that in the PWR fuel assembly in which the lowermost support grid is fixed so as to contact the lower nozzle as described above, the lowermost support is provided. A sleeve for inserting the lower part of the lower end plug of the thimble tube is extended downward from the thimble cell of the lattice, and a thread portion is formed on the inner peripheral surface of the sleeve, and a screw of the sleeve is formed on the lower outer peripheral surface of the lower end plug. Forming a threaded portion to be screwed into the lower end plug, these sleeves and the lower end plug are screwed into the threaded portion, and fitted into the counterbore portion recessed around the thimble screw hole of the lower nozzle, the lower end The stopper is fixed to the lower nozzle with a thimble screw.

[0006]

In the fuel assembly of the present invention, the threaded portion of the sleeve integrated with the lowermost support grid and the threaded portion of the thimble tube lower end plug are screwed together, and these are screwed together to form the sleeve. It is an integral part of the thimble tube and the bottom support grid is easily fixed in place by this sleeve and thimble tube connection without the need for swaging.

Further, by omitting the swaging process, it is possible to eliminate the variation in the length of the thimble tube due to this process.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Referring to the accompanying drawings,
An embodiment of the present invention will be described.

FIG. 1 is a partially enlarged sectional view showing a PWR fuel assembly according to an embodiment of the present invention from a lowermost support grid to a lower nozzle plate, and FIG. 2 is an exploded view of FIG. 1 in which the lower nozzle plate and the thimble screw are omitted. A perspective view and FIG. 3 are front views showing the entire fuel assembly.

In the fuel assembly of the above embodiment, as shown in FIG. 3, thimble tubes 2 are mixed into a large number of fuel rods 1 and are bundled by a plurality of support grids 3. It has an overall basic structure in which the upper part is fixed to the upper nozzle 4 and the lower part of the thimble tube 2 is fixed to the lower nozzle 5. The lowermost support grid 3 'is lowered until it almost or completely contacts the lower nozzle 5, and is fixed at that position.

On the other hand, in the embodiment of the present invention, in the fuel assembly as described above, as shown in FIGS. 1 and 2, the thimble tube 2 is located below the thimble cell 6 of the lowermost support grid 3 '.
The sleeve 8 into which the lower part of the lower end plug 7 of FIG. The sleeve 8 is joined by welding to a flap 9 integrally extending from the cell side wall 3a.

A female screw portion 10 is formed on the lower inner peripheral surface of the sleeve 8, and a male screw portion 11 which is screwed with the female screw portion 10 is formed on the lower outer peripheral surface of the lower end plug 7. There is.

As shown in FIG. 1, the sleeve 8 and the lower end plug 7 surround the thimble screw hole 12 on the upper surface of the lower nozzle (plate) 5 with the screw portions 10 and 11 screwed together. The lower end plug 7 is fixed to the lower nozzle 5 by a thimble screw 14 screwed from below.

In this embodiment of the present invention,
The fixed structure of the lowermost support grid 3'as described above is adopted at all positions of the thimble tube 2.

Therefore, in the fuel assembly according to the embodiment of the present invention, the male screw portion 11 of the thimble tube lower end plug 7 is screwed into the female screw portion 10 of the sleeve 8 and these are screw-coupled to each other to form the sleeve 8. The thimble tube 2 is integrated with the thimble tube 2, and the connection between the sleeve 8 and the thimble tube 2 allows the lowermost support grid to be easily fixed in place in the fuel assembly without swaging.

Furthermore, by eliminating the swaging process, it is possible to eliminate the variation in the length of the thimble tube 2 caused by this process.

[0017]

As described above, in the PWR fuel assembly of the present invention, the sleeve for inserting the lower portion of the thimble tube lower end plug is extended downward from the thimble cell of the lowermost support grid, and A threaded portion is formed on the peripheral surface, and a threaded portion that engages with the threaded portion of the sleeve is formed on the lower outer peripheral surface of the lower end plug, and the sleeve and the lower end plug are screwed together. Then, insert it into the counterbore that is recessed around the thimble screw hole of the lower nozzle,
The lower end plug is fixed to the lower nozzle by a thimble screw, and the sleeve and thimble pipe are formed by screwing together the sleeve integrated with the lowermost support grid and the threaded portions of the lower end plug of the thimble pipe. The bottom support grid can be easily fixed without the need for conventional swaging, and the complicated work associated with the swaging can be omitted. As a result, the fuel assembling work is simplified, and the remarkable effects are achieved that the process is shortened and the exposure during the assembling work is reduced. Furthermore, by eliminating the swaging process, it is possible to eliminate the variation in the length of the thimble tube caused by this process.

[Brief description of drawings]

FIG. 1 is a partially enlarged cross-sectional view showing a portion from a lowermost support grid to a lower nozzle plate of a PWR fuel assembly according to an embodiment of the present invention.

FIG. 2 is an exploded perspective view of FIG. 1 in which a lower nozzle plate and a thimble screw are omitted.

FIG. 3 is a front view showing the entire fuel assembly.

FIG. 4 is a partially enlarged perspective view showing a conventional fixing structure of a lowermost support grid.

[Explanation of symbols]

 1 Fuel Rod 2 Thimble Tube 3 Support Grid 3'Bottom Support Grid 3a Cell Side Wall 4 Upper Nozzle 5 Lower Nozzle 6 Thimble Cell 7 Lower End Plug 8 Sleeve 9 Flap 10 Female Threaded Part 11 Male Threaded Part 12 Thimble Screw Hole 13 Counterbore Part 14 Thimble screw

Claims (1)

[Claims] 1. In a PWR fuel assembly in which the lowermost support grid is fixed so as to contact the lower nozzle, a sleeve for inserting the lower part of the thimble tube lower end plug is extended downward from the thimble cell of the lowermost support grid. And forming a threaded portion on the inner peripheral surface of the sleeve, and forming a threaded portion that engages with the threaded portion of the sleeve on the lower outer peripheral surface of the lower end plug. A PWR fuel assembly characterized in that the threaded portions are screwed together, fitted into a counterbore portion recessed around the thimble screw hole of the lower nozzle, and the lower end plug is fixed to the lower nozzle by a thimble screw. .