JPH0293398A - Spacer for fuel assembly - Google Patents

Abstract

PURPOSE:To mitigate an increment of a thrusting force caused by a creeping of a channel box by providing protrusions at places close to corner parts of a spacer and by diminishing gap spaces between the inner surface of the channel box and upper ends of the protrusions at those parts. CONSTITUTION:Protrusions 7 are tapped out at an upper and a lower parts of both left and right ends of a spacer 5 side surface which are close to its corner parts. By utilizing this sort of spacer, a collision of an inner surface of the channel box with an outer surface of the spacer is apt to occur at a place of protrusion parts which are provided closely to the corner part, when a vibration of a fuel channel generates. However, with the protrusions, a gap space between upper parts of the protrusions and the inner surface of the channel box is set to be a narrow space and also an expansion of the channel box accompanied by combustion is limited; therefore, an increment of a thrusting force can be mitigated and a rupture of the fuel assembly can be prevented.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to spacers for fuel assemblies, particularly for boiling water nuclear reactors (
This invention relates to improvements to spacers that hold fuel elements at equal intervals in a fuel assembly for BWR.

(Prior art) A BWR fuel assembly basically consists of a large number of fuel rods (1) and one or two walk rods (water channels) (2) in the center, as shown in Fig. 4, with their lower ends connected to the lower type. spaced parallel to each other by the plate (4), the upper end by the upper tie plate (3), and the middle part in the longitudinal direction by a plurality of square lattice spacers (5) as shown in the fifth circle. The entire structure is covered by a square channel box (6), and the length of the fuel rod (1) to be inserted is 4 m, which is approximately the entire length of the fuel assembly.
Long fuel rods with a length of
It is called 7x7 fuel, 8x8 fuel, 9x9fi fuel, etc.

By the way, in the BWR fuel assembly as described above, there is usually a gap between the spacer (5) and the channel box (6).
It has a minimum gap of approximately 0.3 fences, and during an earthquake, when the fuel assembly vibrates in the lateral direction, the channel box (6) and spacer (5) come into contact, generating an impact force. .

Figure 6 shows the relationship between gap width and impact force.
As the gap becomes smaller, the impact force decreases. However, due to the fit between the channel box and the spacer, it is actually inconvenient to narrow the gap further than it currently is.

Moreover, during combustion in the furnace, the pressure on the inside of the channel box is higher than on the outside, so it is known that the pressure difference between the inside and outside causes it to bulge outward, causing a creep phenomenon, as shown in Figure 7. The aforementioned gap widens and the impact force during an earthquake increases.

Of course, conventional spacers are designed to withstand the increased impact force caused by this increased gap, but in order to increase the margin, they are designed to withstand the increased impact force even when the burnup is further increased. Improvements in the structure of the reservoir that alleviates the force itself are necessary and equally sought after.

(Problems to be Solved by the Invention) The present invention deals with the above-mentioned actual situation and conducts studies from various aspects focusing on the creep phenomenon of the channel box in order to alleviate the increase in impact force caused by the increase in the gap. As a result, the objective was to alleviate the impact force on the spacer part that occurs during an earthquake by improving the side surface shape of the spacer.

(Means for Solving the Problems) That is, the feature of the configuration of the present invention that satisfies the above object is that in the fuel assembly used in the BWR, a projection is formed on the spacer, particularly at or near the side corner. The purpose of this feature is to reduce the increase in impact force caused by the creep phenomenon of the channel box during an earthquake.

Here, the reason why the protrusion is provided particularly at or near the side corner of the spacer is based on the creep phenomenon of the channel box shown in FIG. That is, as is clear from the figure, the channel box tends to become round in cross section because it is expanded by pressure from the inside. In other words, the displacement at the center of the sides of the square is the largest, and the displacement is smaller at the portions near the corners. Therefore, in order to reduce the impact force, it is most effective to provide the spacer at the corner where the gap between the spacer and the channel box is the smallest.

In the second and third aspects of the present invention, the position of the corner of the side surface is specifically specified to the corner cell or the water tub closest to the corner.

(Function) By using a spacer with the above configuration, when the fuel assembly is shaken, collision between the inner surface of the channel box and the outer surface of the spacer will begin to occur at the protrusions provided near the corners; Since is a protrusion, the gap between its upper end and the inner surface of the channel box is set small, and even if the channel box expands due to combustion, it does not expand much, so it is possible to alleviate the increase in impact force, and the ijV inertia Prevent damage to the fuel assembly due to increase.

(Example) Hereinafter, the present invention will be explained in detail based on the attached page.

1 to 3 show each side of a spacer according to the present invention, and in each figure (5) indicates a spacer.

In the first embodiment, protrusions (7), which constitute the essential part of the present invention, are punched out on the upper and lower sides of both left and right ends near the corners on the side surface of the spacer (5).

Of course, the height of this protrusion (7) is appropriately set based on the creep phenomenon of the channel box.

Similarly, in order to obtain the same effect, the above protrusion (7
) is shown in Figure 2. Further, FIG. 3 shows a water tub (8) provided with the protrusion (7).

The height of each of these protrusions is set in consideration of the creep phenomenon of the channel box, and the structure is such that it can alleviate the increase in the gap caused by this phenomenon.

The above-mentioned sides are representative examples of the present invention, and in addition to the above, the present invention can also be used to increase the impact force as long as it has the same effect as the above protrusion and reduces the gap between the spacer and the channel box. It is possible to alleviate the

(Effects of the Invention) As described above, the present invention provides a protrusion near the corner of the side surface of the spacer and reduces the gap between the upper end of the protrusion at the core and the inner surface of the channel box, thereby reducing the creep phenomenon of the channel box. Even if the cross section of the spacer tends to be rounded due to the movement, the displacement is small near the corners, so the gap between the upper end of the protrusion and the inner surface of the channel box does not widen much, and therefore the spacer and The impact force caused by the contact between the channel boxes is sufficiently alleviated, and among the impact forces generated on the BWR fuel assembly spacer during an earthquake, the impact force due to the creep phenomenon of the channel boxes that occurs during combustion in the furnace increases. It is possible to reliably reduce the risk of damage and ensure the safety of the fuel assembly.

Moreover, since the impact force is alleviated as described above, restrictions on impact resistance in spacer design are relaxed,
This has the effect of increasing the design margin of the spacer itself and providing a stable fuel assembly.

Note that the spacer according to claim 2.3 exhibits the above-mentioned effects most favorably and is suitable.

[Brief explanation of drawings]

1 to 3 are schematic side views showing each embodiment of a spacer according to the present invention, FIG. 4 is a partially omitted schematic cross-sectional view showing a general configuration example of a BWR fuel assembly, and FIG. 5 (A) (υ) is a plan view and a side view showing a conventional spacer structure, FIG. 6 is a chart showing the relationship between gap width and impact force, and FIG. 7 is a schematic diagram showing the creep phenomenon of a channel box. (5)...Spacer. (6)...Channel box. (7)...protrusion. (8)...Water tub. Patent Applicant Nuclear Fuel Industry Co., Ltd. Representative Patent Attorney Yasuzuki Miyamoto Leap Figure 5 (A) Figure 6 Figure 7 Gasoir Width

Claims (1)

[Claims] 1. A spacer used in a fuel assembly for a boiling water reactor, in which a protrusion is provided at or near the corner of the side surface to increase impact force during an earthquake against the creep phenomenon of a channel box. A spacer for a fuel assembly, characterized in that the spacer is made to alleviate the 2. The spacer for a fuel assembly according to claim 1, wherein a projection is provided at a corner cell portion on a side surface of the spacer. 3. The spacer for a fuel assembly according to claim 1, wherein a projection is provided on the water tab closest to the corner of the side surface of the spacer.