JPH03128483A - Fuel assembly - Google Patents

Abstract

PURPOSE:To improve the cooling performance of fuel rods by supplying liquid drops which flow together with a steam flow to the surfaces of circumferential fuel rods by a blade member for steam flow stirring. CONSTITUTION:The blade member 33b for steam stirring is provided for some of spacers 25, constituting the assembly, which are arranged right above short- sized fuel rods 22 at a cell part 33 among spacers arranged above the short-sized fuel rods 22. Namely, a column member 33a which is raised from the lower side part of the cell part 33 is provided and the blade member 33b is provided on it. Further, the blade member 33b is embedded by welding its lower part in spacers 25 corresponding to the position of the short-sized fuel rods 22. Consequently, the steam flow which flows up vertically is stirred in the annular flow area above the assembly to add a lateral speed element, so the liquid drops which flow together with the steam flow can be supplied to the surfaces of the circumferential fuel rods 22. Therefore, the cooling performance of the fuel rods 22 can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fuel assembly for a boiling water nuclear reactor, and particularly to an improvement in a fuel assembly having short fuel rods.

"Prior Art" Conventionally, fuel assemblies for this type of boiling water reactor are generally shown in FIGS. 6 and 7. That is, a large number of fuel rods 2 surrounded by a longitudinally elongated channel box 1 are vertically provided, and both ends of the fuel rods 2 are supported by an upper tie plate 3 and a lower tie plate 4.

The middle portion of the fuel rod 2 is held by spacers 5 at a plurality of locations at different heights.

A plurality of spacers 5 are fitted at appropriate intervals in the vertical direction, and as shown in FIG.
The spacer frame 6 is provided with a spacer frame 6 formed in the shape of a round cell, and the spacer frame 6 is provided with a spring member 7 that presses the fuel rod 2. Furthermore, a projecting piece 5b is formed on the upper part of the peripheral frame 5a to direct the steam flow inward.

As shown in FIG. 7, a plurality of water rods 8 are supported inside the fuel assembly, and the coolant flows through the water rods 8 from the bottom to the top, and flows from the top to the entire fuel rod 2. It's becoming more dispersed.

Further, the fuel rod 2 contains fuel pellets inside and is surrounded by a cladding tube on the outside. As shown in FIG. 9, this fuel rod 2 includes a plurality of short fuel rods 2a supported by a lower tie plate 4 and a lower spacer 5.
It is included. In the example shown in Fig. 9, among the large number of fuel rods 2 arranged in a 9x9 arrangement, this is done in a square lattice pattern or in a dense or dense manner, one row inside the outermost fuel rod 2, and every second fuel rod 2. They are arranged in a grid pattern.

Note that FIG. 10 shows another conventional example, in which the spacer 5
is configured to have lattice-like cells.

During nuclear reactor operation, the coolant becomes a two-phase flow containing steam due to heat generated from nuclear fission in the fuel rods 2, and also serves as a moderator for fast neutrons. The fuel rods 2 prevent fuel pellets and fission products from being heated even in a transient state caused by a single operator error or a single failure of reactor internal equipment or reactor control equipment. It is designed to be held inside the cladding tube. During normal operation, the vapor void ratio of the coolant reaches approximately 70% in the upper region of the fuel rods 2. Further, the flow pattern of the two-phase flow of water and steam becomes an annular flow state, and the fuel rod 2 is cooled by the liquid film flowing on the surface. In this case, the surface temperature of the cladding tube is kept constant near the saturation temperature of the coolant.

The short fuel rods are supported at their lower ends by the lower tie plate 4, and a coolant flow region partially free of the fuel rods 2 is formed in the upper part. In this region, the flow is usually annular, so the liquid film flow that rises on the surface of the short fuel rod is separated into the vapor flow at the top and becomes a droplet flow. Furthermore, since the cross-sectional area of the flow path is wider in this region than in the region surrounded by other normal fuel rods 2, the steam flow becomes relatively high speed and many droplets flow. Since droplets generally do not contribute to cooling the surface of the fuel rod 2, it is considered effective to supply the droplets to the surface of the fuel rod 2.

[Problems to be Solved by the Invention] However, in the spacer 5 in the conventional fuel assembly described above, if the coolant were to be excessively heated, there was a possibility that the thickness of the liquid film would reach zero.

In this case, a dry-out condition occurred on the surface of the fuel rod 2, and the heat transfer coefficient between the cladding tube and the coolant decreased, resulting in an increase in the surface temperature of the cladding tube.

Furthermore, if the fuel rod continues to operate at high temperatures for a long period of time, the cladding tube may deteriorate, which may cause problems such as a decline in the integrity of the fuel rods 2. In addition, for the spacer 5 located at the top of the short fuel rod, from the viewpoint of reducing pressure loss, it is possible to effectively reduce the This could lead to problems such as not being able to cool properly.

The present invention was made in view of the above circumstances, and provides effective cooling by supplying droplets flowing along with the steam flow to the surface of the surrounding fuel rods using a vapor flow stirring blade member. The present invention provides a fuel assembly capable of producing effects.

"Means for Solving the Problems" In order to achieve the above-mentioned object, a fuel assembly according to the present invention supports the upper and lower ends of a large number of vertically arranged fuel rods by an upper tie plate and a lower tie plate, and The middle part of the rod is held by spacers at multiple locations at different heights, and the outer periphery is surrounded by a channel box.
Furthermore, in a fuel assembly for a boiling water reactor having a plurality of short fuel rods supported by a lower tie plate and a lower spacer, the shorter spacer is located higher than the short fuel rods. The fuel rod is characterized in that a vane member for stirring the vapor flow is provided in the cell portion located directly above the fuel rod.

"Operation" According to the fuel assembly according to the present invention, a vane member for stirring vapor flow is provided in the cell portion located directly above the short fuel rod among the spacers arranged at a higher place than the short fuel rod. Therefore, it becomes possible to supply droplets flowing along with this vapor flow to the surfaces of the surrounding fuel rods. This makes it possible to thicken the liquid film flowing on the surface of the fuel rod and prevent dryout, thereby improving the cooling performance of the fuel rod and increasing the integrity of the fuel rod. It will also be possible to effectively expand the operating range of fuel rods and improve fuel economy.

"Example" Hereinafter, an example of the present invention will be described with reference to FIGS. 1 to 3. The overall configuration of the fuel assembly is substantially the same as that shown in FIG. 6, so the same figure is also referred to (numerals in parentheses in FIG. 6 indicate this embodiment).

In this embodiment, a large number of fuel rods 22 are vertically provided surrounded by a longitudinally elongated channel box 21, and both ends of the fuel rods 22 are supported by an upper tie plate 23 and a lower tie plate 24. The middle portion of the fuel rod 22 is held by spacers 25 at a plurality of locations at different heights a.

A plurality of spacers 25 are fitted at appropriate intervals in the vertical direction, and a space frame 26 formed in a round cell shape and a fuel rod 2
2 and a spring member 27 that presses the spring member 2. Inside this spacer 25, a water rod 28 is provided.
A plurality of water rods 28 are provided in the vertical center, and the coolant flows from the bottom to the top through the water rods 28 and is dispersed throughout the fuel rods 22 from the top. The fuel rod 22 contains fuel pellets inside and is surrounded by a cladding tube. These fuel assemblies have a plurality of short fuel rods supported by a large number of fuel rods 22, a lower tie plate 24, and a spacer 25 disposed below.

In this device, in the cell portion 33 of the spacer 25 placed at a higher location than the short fuel rods, a vapor flow stirring layer @ member 3 is placed directly above the short fuel rods.
3b is provided. That is, as shown in FIGS. 2 and 3, this cell portion 33 is provided with a support member 33a rising from the lower part, and a blade member 33b is provided on the upper part of the support support member 33a.
is formed. Furthermore, as shown in FIG. 4, the blade member 33b is welded at its lower part and buried in the spacer 25 corresponding to the position of the short fuel rod in order to stir the steam flow. The blade member 33b is formed by assembling vertically long plate-like bodies in a cross shape, and is folded so as to be rotationally symmetrical to each other by combining the notches formed at the upper part of the axis.

Also, the outer corner of the folded part is cut diagonally. The length of the blade is widened to such an extent that it does not come into contact with surrounding fuel rods 22.

In this case, by cutting the blade member 33b, the fuel rod 2
The vapor flow and droplet flow flowing into the blade member 33b from the lower part of the blade member 33b are given more velocity components outward with respect to the rotation axis. That is, by providing a horizontal vortex component, the amount of droplets reattached to the surface of the fuel rod 22 is increased. Furthermore, the blade part t43
The amount of re-adhesion to the surface of the fuel rod 22 is increased by giving a velocity component along the direction of the blade part to the colliding droplet 3b when it leaves the tip of the blade member 33b again. It looks like this. As a result, fuel rod 2
It becomes possible to suppress thinning of the droplet flow flowing on the surface of 2.

According to the embodiments described above, in the annular flow region at the top of the fuel assembly, the vertically upward steam flow is stirred, and a lateral velocity element is added, so that the steam flow accompanies this steam flow. Droplets can be delivered to the surface of the surrounding fuel rods. This increases the thickness of the liquid film flowing on the surface of the fuel rod and prevents dryout, thereby improving the cooling performance of the fuel rod and increasing the integrity of the fuel rod. It will also be possible to effectively expand the operating range of the vehicle and improve fuel economy.

In the above-mentioned embodiments, the present invention was applied to a round cell-shaped spacer, but a spacer having a lattice-shaped cell as shown in FIG. 4, or a combination of a lattice and a round cell as shown in FIG. It is also possible to apply the present invention to a spacer.

Further, in the embodiment described above, the blade portion of the blade member 33b is
However, this is a powerful example of an effective configuration, and the number of blades is not limited to this and can be changed in various ways.

"Effects of the Invention" As explained above, according to the fuel assembly according to the present invention, the ratio of droplets adhering to the surface of the fuel rod by the blade part of the blade member is increased, so that It is possible to prevent deterioration of the cladding tube and provide an effective cooling effect, which in turn makes it possible to effectively expand the operating range of the fuel rod and improve fuel economy.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the fuel assembly according to the present invention, and is a plan view of a spacer, FIG. 2 is an enlarged view of the vane member in the spacer shown in FIG. 1, and FIG. 4 and 5 are respectively plan views showing other embodiments of the present invention, FIG. 6 is an overall configuration diagram showing a conventional example, and FIG. 7 is a schematic plan view of FIG. 6, and FIG. 8 is a plan view showing the spacer of the fuel assembly shown in FIG. 6, FIG. 9 is a view showing the arrangement of short fuel rods, and FIG. 1O is a plan view showing another conventional example. 21...Channel box, 22...
・Fuel rod, 23... Upper tie plate, 24.
...Lower tie plate, 25...Spacer, 6...Spacer frame, 27...Spring member, 8...Water rod, 3o...
...Claying tube, 1...Outer frame, 32...
...Protrusion piece, 3...Cell part, 33a...
- Support member, 3b...Blade member. Applicant Japan Atomic Energy Corporation Representative Patent Attorney Ume Yamanouchi 1 Figure 2 Petori 2 Figure High 3 Figure: 55 37 Figure 7 High School 4 Figure 27 60 and bd High School 6 Figure 5 Figure Atsushi 8 Figure B

Claims (1)

[Claims] The upper and lower ends of a large number of vertically arranged fuel rods are supported by an upper tie plate and a lower tie plate, the intermediate portions of the fuel rods are held at different heights by spacers at multiple locations, and the outer periphery is surrounded by a channel box. ,
Further, in a fuel assembly for a boiling water reactor having a plurality of short fuel rods supported by the lower tie plate and the spacer disposed on the lower side, the fuel assembly is further arranged higher than the short fuel rods. A fuel assembly characterized in that a vane member for stirring vapor flow is provided in a cell portion of the spacer located directly above the short fuel rod.