JPH11142567A - Upper support pin of fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid impinging of a fuel assembly on an upper core plate in a pressurized water reactor even with degradation of the function of push spring of fuel assembly. SOLUTION: An upper support pin 30 of a fuel assembly has an axis 31 fixed projecting downward an upper core plate 3 positioning above fuel assemblies constituting a core, a pin body 35 fitted slidably outside the lower end 31a of the axis 31, and a compressed coil spring 37 inserted inside the pin body 35 contacting the lower end surface. Here, the pin body 35 is inserted in the upper nozzle 11 of a fuel assembly and pushed downward.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an upper support pin for fixing a fuel assembly constituting a core in a nuclear reactor vessel at a proper position and appropriately holding the fuel assembly.

[0002]

2. Description of the Related Art In a nuclear reactor, for example, a pressurized water reactor, a large number of fuel assemblies constituting a core are positioned on lower core plates via supporting pins, while an upper core plate located above the core is The fuel assembly is pressed and held downward via a spring, and the fuel assembly is positioned in the lateral direction via a support pin. Referring to FIG. 3, the fuel assembly 10 includes an upper nozzle 11, a lower nozzle 13, a plurality of control rod guide tubes 15 connecting the upper nozzle 11 and the lower nozzle 13, and extending apart from each other. And a plurality of fuel rods 1 inserted into the grid openings of the support grid 17.
9. Such a fuel assembly 10
It is placed on the lower core plate 1 in the reactor vessel, and its horizontal position is regulated by support pins (not shown).

The positioning of the upper nozzle 11 and the upper core plate 3 is performed by the support pins 21 in the same manner as the lower nozzle 13 and the lower core plate 1. However,
Since the fuel assembly 10 has neutron irradiation growth due to the operation of the nuclear reactor and has a thermal expansion difference between related members, etc., there is a gap between the upper end surface of the upper nozzle 11 of the fuel assembly 10 and the upper core plate 3. A predetermined gap is secured, and the fuel assembly 10 is securely held downward using a leaf spring 24 (shown in FIG. 4) provided on the upper surface of the upper nozzle 11. In FIG. 4, a positioning hole 25 at a diagonal position of the upper nozzle 11 receives the support pin 21. However, the upper nozzle 11 receives the support pin 21 in the positioning hole 25, and the leaf spring 23 The contact state is shown enlarged in FIG. Leaf spring 2
Numeral 3 absorbs a dimensional change of the gap d due to neutron irradiation growth, a difference in thermal expansion, and the like without generating excessive stress, and also prevents the fuel assembly 10 from floating due to a coolant flow. The support pins 21 extend through the upper core plate 3 and are fastened to the upper core plate 3 by nuts 27 screwed to upper ends.

[0004]

However, since the leaf spring itself is located near the core and is in contact with the high-temperature and high-pressure coolant, the spring force may be lost due to deterioration of the material. In this case, the coolant flowing upward in the core may lift the fuel assembly and come into contact with the upper core plate to cause a problem. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an upper support pin that can prevent the fuel assembly from coming into contact with the upper core plate even if the pressure spring attached to the upper nozzle of the fuel assembly loses its function.

[0005]

According to the present invention, an upper support pin of a fuel assembly in a reactor core is located above a fuel assembly constituting the core. A shaft protruding downward and fixed to the upper core plate, and a pin body slidably fitted outside the lower end of the shaft,
A biasing means such as a spring is provided inside the pin body in contact with the lower end surface of the shaft, and the pin body is inserted into the upper nozzle of the fuel assembly and pressed downward. It is composed of The biasing means may be interposed not in the pin body but in a shaft configured as a hollow shaft.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings. It is to be noted that the same portions are denoted by the same reference numerals throughout the drawings including the drawings related to the above-described conventional structure. First, referring to FIG. 1, the shaft 31 of the upper support pin 30 according to the present invention extends vertically through the upper core 3 and is securely fastened by a nut 33 screwed to the upper end. The pin body 35 slidably fitted to the lower end portion 31a of the shaft 31 has a shell-shaped lower end, and the fuel assembly 10
In the positioning hole 25 of the upper nozzle 11. And the lower end is an end plate 11a provided with a large number of coolant through holes.
Is in contact with the concave surface of. A hollow portion 35a for receiving the lower end portion 31a of the shaft 31 is provided inside the pin body 35.
Is formed, and a biasing means, that is, a compression coil spring 37 is interposed between the bottom surface of the hollow portion 35a and the lower end surface of the shaft 31.

According to the upper support pin 30 having the above structure, the shaft 31 is securely fixed to the upper core plate 3 and the pin body 35 is fitted in the positioning hole 25 of the upper nozzle 11. The fuel assembly 10 is securely positioned at a fixed position, and restricts lateral displacement. Further, the pin body 35 is moved by the compression coil spring 37 to the end plate 11a of the upper nozzle 11.
, The restoring force due to the elastic deformation acts on the upper nozzle 11 and thus the fuel assembly 10.
Therefore, even if a leaf spring (not shown) is pulled down and the spring force is reduced, the fuel assembly 1 is compressed by the compression coil spring 37.
0 is pressed downward to prevent contact with the upper core plate 3.

Next, a modified embodiment in which a compression coil spring is provided not on the pin body but on the shaft portion will be described. Referring to FIG. 2, a hole having a relatively large diameter is formed in the upper core plate 103, and a hollow shaft or a cylinder 131 of the upper support pin 130 is penetrated and similarly fixed by a nut 133. The upper end 135 a of the pin body 135 having the same lower end as the pin body 35 is slidably fitted inside the cylinder 131. A hollow portion 135b shorter than the aforementioned hollow portion 35a is formed inside the upper portion of the pin body 135, and a compression coil spring 137 is interposed between the hollow portion 135b and the internal space of the cylinder 131. When such a structure of the upper support pin 130 is compared with the above-described upper support pin 30, as will be easily understood by those skilled in the art, the compression coil spring 137 pushes down the fuel assembly 10 in a backward manner, and furthermore, The pin body 135 regulates the lateral displacement, and has the same effect.

[0009]

As described above, according to the present invention,
The upper support pin is configured to be expandable and contractable from a fixed shaft portion and a relatively displaceable pin body, and the pin body is pressed against the upper nozzle of the fuel assembly by biasing means interposed therebetween. As a result, the upper support pin not only has the function of restricting lateral displacement, but also has the function of pushing down the fuel assembly as a precautionary measure, preventing the fuel assembly from rising even if the function of the main pressing spring is lost. can do.

[Brief description of the drawings]

FIG. 1 is an overall sectional view showing an embodiment of the present invention.

FIG. 2 is an overall sectional view showing a modified embodiment of the present invention.

FIG. 3 is an elevation view showing an example of a fuel assembly supported by an upper support pin according to the present invention.

FIG. 4 is a conceptual perspective view showing an enlarged part of the fuel assembly.

FIG. 5 is a cross-sectional view showing the structure of a conventional support pin.

[Explanation of symbols]

 Reference Signs List 3 upper core plate 10 fuel assembly 11 upper nozzle 11a end plate 30 upper support pin 31 shaft 31a lower end portion 33 nut 35 pin body 35a hollow portion 37 compression coil spring 103 upper core plate 130 upper support pin 131 cylinder 133 nut 135 pin body 135b Hollow part 137 Compression coil spring

Claims (2)

[Claims] A shaft protruding downward and fixed to an upper core plate located above a fuel assembly constituting a core; and a pin main body slidably fitted outside a coaxial lower end portion. Biasing means interposed in contact with the lower end surface of the shaft inside the pin body, wherein the pin body is inserted through the upper nozzle of the fuel assembly and pressed downward. Upper support pin of the fuel assembly to be replaced. 2. A hollow shaft extending vertically in an upper core plate located above a fuel assembly constituting a reactor core, and an upper end portion is slidably fitted in the hollow shaft and is provided below the hollow shaft. And a biasing means interposed in the hollow shaft and biasing the pin body downward. The pin body is inserted through the upper nozzle of the fuel assembly and pressed downward. An upper support pin of the fuel assembly, wherein the upper support pin is provided.