JPH1082877A - Plenum spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plenum spring that can prevent a pellet side spring final end from coming into contact with the inner surface of a covered pipe in the plenum spring pressing a fuel pellet directly without interposing a pressing plate. SOLUTION: The pellet side spring final end 2 is positioned more on the radially inside than the periphery of an effective winding part 1 of the plenum spring. The final end 2 is, thereby, prevented from coming into contact with the inner surface of a covered pipe. The final end 2 is provided with a contact winding part of one turn or more with the same outer diameter as the effective winding part 1 and with axially adjacent winding parts welded to each other, and the final end 2 is fixed to the winding part directly above it so as to further avoid the contact of the final end 2 with the inner surface of the covered pipe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plenum spring for pressing a fuel pellet in a cladding tube of a fuel rod constituting a fuel assembly for a nuclear reactor. The present invention relates to a plenum spring for pressing.

[0002]

2. Description of the Related Art Generally, a plurality of combustion rods constituting a fuel assembly for a nuclear reactor are formed by filling a long cladding tube with a large number of fuel pellets. FIG. 5 is a longitudinal sectional view showing an example of a conventional fuel rod. This is because a predetermined number of fuel pellets 102 such as uranium dioxide (UO ₂ ) are filled in a zirconium alloy cladding tube 101, and the upper end plug 103 welded to the top end of the cladding tube 101 and the upper end of the fuel pellet 102 are formed. The fuel pellet 102 is pressed and fixed to the lower end plug 105 side by a plenum spring 104 interposed therebetween.

[0003] The plenum spring 104 cuts the spring into a required length in a spiral wound state, and polishes the end face so that the lower end face on the fuel pellet pressing side is perpendicular to the axis of the spring. The fuel pellet holding plate 106 is welded to the end face.

[0004]

In recent years, while there has been a demand for rationalization of the molding process of fuel, it is necessary to reduce the number of parts by omitting the fuel pellet holding plate and to omit the welding step with the plenum spring. Are being considered.

However, if a conventional plenum spring is used as it is without providing a holding plate,
The final end of the spring on the pellet side comes into contact with the inner surface of the cladding tube and locally applies stress, which may cause stress corrosion cracking.

This is due to the fact that the outer diameter of the coil at the final end remains the same as the effective winding portion and the clearance with the inner surface of the cladding tube is small. Causes for actually pressing the final end against the inner surface of the cladding tube include expansion in the outer radial direction when the plenum spring is compressed, bending of the spring when the plenum spring is inserted into the cladding tube, and high-temperature pellets. It is considered that the residual strain is released due to the contact.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a plenum spring that directly presses a fuel pellet without using a holding plate, and that can prevent the inner end of the cladding tube from coming into contact with the final end of the pellet side spring. And

[0008]

In order to achieve the above object, in the plenum spring according to the present invention, fuel pellets are formed in cladding tubes of a plurality of fuel rods constituting a fuel assembly for a nuclear reactor. A plenum spring for pressing downward, wherein the final end of the pellet-side spring is located radially inward of the outer periphery of the effective winding portion of the plenum spring.

According to a second aspect of the present invention, in the plenum spring according to the first aspect, the final end of the pellet-side spring has the same outer diameter as the effective winding portion of the plenum spring in the axial direction. Has one or more close winding portions in which the winding portions adjacent to each other are weld-stopped.

According to the first aspect of the present invention, the final spring end of the pellet side of the plenum spring is located radially inward of the outer periphery of the effective winding portion. It becomes difficult to contact. Therefore, the application of local stress to the cladding, which can cause stress corrosion cracking, can be suppressed as compared with the related art.

Further, as described in claim 2, the final end of the pellet-side spring is brought into close contact with one or more turns in which the windings adjacent to each other in the axial direction and having the same outer diameter as the effective winding of the plenum spring are stopped. With the winding, the final winding is welded to the upper winding, so that the final end of the spring is fixed and compressed when the plenum spring is inserted into the cladding tube. When the outer end expands or bends, the final end hardly contacts the inner surface of the cladding tube.

[0012]

FIG. 1 shows a plenum spring according to an embodiment of the present invention. FIG. 1 is a plan view of the plenum spring 1 as viewed from the final end 2 side. As can be seen from the drawing, in this plenum spring 1, the final end 2 has just the wire diameter of the spring with respect to the effective winding outer diameter of 10 mm. It is bent so as to be positioned 1.5 mm inward in the radial direction. Therefore, it is difficult for the final end 2 to contact the inner surface of the cladding tube.

FIG. 2 shows a plenum spring according to another embodiment. FIG. 2A is a plan view of the plenum spring 11 as viewed from the final end 12 side, and the plenum spring 11 is bent until the final end 12 is located substantially at the axial center of the spring. As shown in FIG. 2 (b), a normal fuel pellet has
In order to improve PCI (pellet-cladding tube interaction) characteristics, a dish-shaped depression is provided to absorb expansion in a high-temperature region in the center of the pellet.

The plenum spring 11 according to the present embodiment.
Then, when disposed on the fuel pellet in the cladding tube, the final end 12 located substantially at the center of the spring axis is located within the recess of the fuel pellet end face 13. Therefore,
The final end 12 is more difficult to move toward the inner wall of the cladding tube.

FIG. 3 shows another embodiment in which the final winding portion has a smaller diameter than the effective winding portion. FIG.
(A) shows that the final turn 22 of the plenum spring 21 has an outer diameter of 6 mm smaller than the inner diameter of the effective winding portion of 7 mm.
The final end 23 is located radially inward of the outer periphery of the effective winding portion.

FIG. 3 (b) shows a plenum spring having an effective winding diameter portion 20 provided at the lower end thereof with an end winding portion 24 which is reduced in diameter to several turns in a conical shape. FIG. 3 (c) shows, at the lower end of the effective winding diameter portion 20 of the plenum spring, several cylindrical end winding portions 25 having an outer diameter of 5 mm smaller than the inner diameter of the effective winding portion of 7 mm. 3 (b) and 3 (c), the final ends of each of the plenum springs are located substantially near the center axis of the spring and within the recesses of the end surfaces of the fuel pellets. It becomes difficult to move toward the inner surface of the cladding tube, and contact with the inner surface of the cladding tube is suppressed.

FIG. 4 shows, as another embodiment, a plenum spring 30 having, at the lower end, a two-turn tightly wound portion 31 in which winding portions adjacent to each other in the axial direction and having the same outer diameter as the effective winding portion are welded together. Is shown. In this method, two turns at the lower end are brought into close contact with each other, and the contact surface is spot-welded at two points (32). In this case, since the final end 33 is fixed to the winding portion immediately above it, even when the plenum spring 30 swells or bends, the final end 3 extends and jumps out of the effective winding portion to the outer periphery, thereby covering the plenum. It does not touch the inner surface of the pipe.

In the plenum spring 30 shown in FIG. 4, the diameter of the effective winding portion is the same as that of the effective winding portion up to the final end 33 of the tightly wound portion 31. However, as shown in FIG. May be bent. Further, the welding is not limited to the two-point spot welding, but may be of course three or more spots or a wide range of welding.

[0019]

As described above, since the present invention makes it difficult for the final end of the plenum spring to come into contact with the inner surface of the cladding tube, it has conventionally been necessary to apply local stress to the cladding tube which may cause stress corrosion cracking. There is an effect that it can be suppressed in comparison.

[Brief description of the drawings]

FIG. 1 is a plan view of a final end side of a plenum spring according to an embodiment of the present invention.

FIG. 2 is an explanatory view showing a plenum spring according to another embodiment of the present invention, wherein (a) is a plan view on the final end side,
(B) is a side view showing the shape of the fuel pellet.

FIGS. 3A and 3B are explanatory views showing a plenum spring according to another embodiment of the present invention, in which FIG. 3A is a plan view of a final end side of a plenum spring having one end turn, and FIG.
Is a side view of a plenum spring with a conical end turn,
(C) is a side view of the plenum spring having a cylindrical end turn.

FIGS. 4A and 4B are explanatory views showing a plenum spring according to another embodiment of the present invention, wherein FIG. 4A is a side view and FIG.

FIG. 5 is a longitudinal sectional view showing a fuel rod equipped with a conventional plenum spring.

[Explanation of symbols]

1, 11, 20, 21, 30: Plenum spring effective winding portion 2, 12, 23, 33: Final end 13: Fuel pellet end surface 22: Final winding 24: Conical end winding portion 25: Cylindrical end winding portion 31 : Closely wound part 32: Spot welded part

Claims (2)

[Claims] 1. A plenum spring for pressing a fuel pellet downward in a cladding tube of a plurality of fuel rods constituting a fuel assembly for a nuclear reactor, wherein a final end of the pellet-side spring has an effective winding portion of the plenum spring. A plenum spring that is located radially inward from the outer periphery. 2. The method according to claim 1, wherein the last end of the pellet-side spring has at least one close-contact winding portion in which winding portions adjacent to each other in the axial direction and having the same outer diameter as the effective winding portion of the plenum spring are welded together. The plenum spring according to claim 1.