JPH04339296A - Fuel assembly - Google Patents

Abstract

PURPOSE:To improve limit thermal output by urging lateral transport within a cross-section of a fuel assembly and by intending uniformation of radial hydraulic distribution. CONSTITUTION:In a fuel assembly, a plurality of fuel rods are arranged in square grids shape by spacers 3 and also a channel box 6 forming a square flow path which surrounds each the fuel rod 1, provided. At least two groove parts 10 are provided at each side of inner surfaces of the channel box 6, but no groove part is provided at areas from the minimum gap parts between the fuel rod 1 which is positioned at corner parts of the channel box 6, to the corner parts.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for use in a light water-moderated nuclear reactor with improved thermal critical output.

0003

2. Description of the Related Art A fuel assembly used in a light water-moderated nuclear reactor, such as a boiling water reactor, has a structure shown in FIGS. 7 and 8. Note that FIG. 7 is a partially cutaway perspective view of the fuel assembly, and FIG. 8 is an enlarged cross-sectional view of FIG. 7. That is, the fuel assembly consists of fuel rods 1 arranged vertically and horizontally in an 8 x 8 square grid, two of which are replaced with water rods 2, and spaced apart by fuel spacers 3, with an upper tie plate 4 and a lower tie plate. rate 5
Both the upper and lower ends are fixed, and the outer side is the channel box 6.
It consists of things surrounded by.

The coolant flowing inside the channel box 6 enters from the lower tie plate 5 and passes through the channel box 6.
As it rises inside, it is heated by the fuel rods 1, boils, becomes a gas-liquid two-phase flow consisting of a liquid phase and a gas phase, and flows out from the upper tie plate 4. At that time, the gas phase is mainly at the fuel rod 1
A portion of the liquid phase flows along with the gas phase, and a portion of the liquid phase flows on the surface of the fuel rod 1. When the liquid phase flowing on the surface of the fuel rod 1 decreases, the surface heat transfer coefficient of the fuel rod 1 decreases (that is, the boiling transition starts), and overheating may occur. In order to prevent such an overheating state (limit output) on the surface of the fuel rod 1 from occurring, means for increasing the limit output has been proposed from the following viewpoints. (a) Supply of liquid phase to the fuel rod surface by mixing coolant. (b) Reduction of heat flux per unit surface area of the fuel rod.

[0005] Based on this knowledge, in the design of a fuel assembly corresponding to item (a), the ratio of coolant used for cooling the fuel rods is increased to increase the cooling efficiency, which contributes to the cooling of the fuel rods. It has been proposed to direct the liquid phase flowing on the inner surface of the channel box 6, which is not in use, to the fuel rods 1. That is, as disclosed in, for example, Japanese Unexamined Patent Publication No. 2-44289, a plurality of tapered grooves 7 called flow trippers, which are partially enlarged in FIG. 9, are formed on the inner surface of the channel box 6. . These tapered grooves 7 have the effect of increasing the proportion of coolant used for cooling the fuel rods 1 by imparting a lateral velocity to the liquid phase 8 flowing on the inner surface of the channel box 6. In the figure, code 12
indicates the axial coolant flow. Note that there are the following two points regarding the mixing effect of the coolant in the fuel assembly. (1) Lateral transport due to average flow: forced lateral flow to the coolant (2) Lateral transport due to turbulence mechanism inherent in three-dimensional flow

[0006] In addition, in the fuel assembly corresponding to the above item (b), it has been devised that the diameter of the fuel rod 1 is reduced to increase the surface area of the fuel rod 1, thereby reducing the heat flux per unit surface area. A design has been proposed in which the number of fuel rods is increased as shown in ×9.

On the other hand, the mechanism of the effect of the fuel spacer 3 on the boiling transition state is as shown in FIG.
It is known that the surface liquid film 8 of the upstream fuel rod 1 is ruptured by the fuel spacer 3, and based on this knowledge, the following measures have been taken in designing the fuel spacer. That is, in order to suppress the breakage of the liquid film on the surface of the fuel rod by reducing the flow obstruction, the end face of the spacer member is processed into a blade shape.

Although FIG. 10 shows an example in which the fuel rod 1 is overturned, it actually stands vertically, and the liquid film 8 adhering to the surface of the fuel rod 1 is broken from the fuel spacer 3. It conceptually shows the state in which

[0009]

[Problem to be solved by the invention] JP-A-2-44289
Although the float tripper described in the above publication has an effect corresponding to the above item (1), it is not intended to have a positive effect on the effect described in the above item (2).

The distribution of the liquid phase in the fuel assembly is shown by the curve a in FIG.
The liquid phase that is not carried by the longitudinal vortex 9 as shown in FIG. 10, which is caused by the turbulence mechanism inherent in the three-dimensional flow shown in FIG. 10, increases near the bisector of each side of the channel box 6. It is considered effective to direct the liquid layer flowing on the inner surface of the channel box 6 in this portion to cooling the fuel rods 1 in order to increase the cooling effect of the coolant.

[0011]However, although the channel box in the conventional fuel assembly is considered to be a simple box or to have a change in wall thickness and shape in the axial direction, it contributes to improving the thermal hydraulic properties of the fuel assembly. There are issues that are not structured yet.

The present invention has been made to solve the above-mentioned problems, and aims to improve the inner surface of the channel box, promote lateral transport within its cross section, and uniformize the distribution of thermal hydraulic power in the radial direction. It is an object of the present invention to provide a fuel assembly that can improve thermal limit output. [Structure of the invention]

[0013]

[Means for Solving the Problems] The present invention provides a fuel tank in which a plurality of fuel rods are arranged in a square lattice shape using fuel spacers, and a channel box forming a square flow path surrounding each of the fuel rods is provided. In the assembly, at least two grooves are provided on each side of the inner surface of the channel box, and no grooves are provided in the corner from the minimum gap between the fuel rod located at the corner of the channel box and the channel box. It is characterized by

[0014]

[Operation] Considering the turbulent flow mechanism peculiar to square flow channels, a plurality of grooves are provided on the inner surface of each side of the channel box at positions that promote transport by lateral secondary flow. These grooves widen the channel width of the secondary flow, and by not providing grooves on the inner surface of the channel box in areas that are not involved in lateral transport, the accumulation of liquid phase on the inner surface of the channel box is suppressed. The widening of the secondary flow channel promotes radial transport, and the liquid phase flowing on the inner surface of the channel box is carried toward the axial center of the fuel rod bundle, improving cooling efficiency and increasing thermal critical power. .

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a fuel assembly according to the present invention will be described with reference to FIGS. 1 to 4. Since the gist of the present invention is to improve the inner surface of the channel box in the fuel assembly, FIG. 1 shows a cross section of the channel box, and FIG. 2 shows a longitudinal section taken along line A-A' in FIG. Since the overall structure is substantially the same as that shown in FIG. 7, FIG. 7 will be referred to.

In FIG. 1, reference numeral 1 indicates a fuel rod, 2 a water rod, and 6 a channel box. 10
is a tapered groove formed on the inner surface of the channel box 6, which becomes gradually thinner from the bottom. At least two grooves 10 are provided on each side of the inner surface of the channel box 6, but no grooves are provided in the corners from the minimum gap between the fuel rod 1 located at the corner and the channel box 6. . The fuel rods 1 are arranged in an 8×8 square grid. The grooves 10 provided on the inner surface of the channel box 6 are designed to promote secondary flow from the corners peculiar to a square channel toward the center of the channel along the sides, as shown by curve b in FIG. promotes lateral mixing of the liquid phase flowing on the inner surfaces of the fuel rods 1 and increases the proportion of coolant in the fuel rods 1. Therefore, as shown by curve a in FIG. 4, the thermal limit output of the present invention is increased compared to curve b of the conventional example. In addition,
FIG. 3 shows only a cross section of the channel box 6, with the fuel rod 1 in FIG. 1 removed.

Next, the operation of the fuel assembly according to the present invention will be explained. In a channel made of a square channel box, a turbulent flow mechanism generates a secondary flow that roughly moves from the center of the channel toward the corner and returns from the corner along the sides to the center of the channel. Among the droplets that flow with the gas phase, those that adhere near the bisector of each side of the channel box due to turbulent mixing are not transported to the center of the channel by lateral transport due to this secondary flow. accumulates in
A distribution occurs in which the liquid phase increases near the bisector of each side of the channel box. The areas where the liquid phase is relatively large on the inner surface of the channel box are the corner area surrounded by the fuel rods and the channel box, and the area of the fuel assembly where the fuel rods are arranged in a square lattice in n rows and x n columns, where n is an even number. Near the bisector of each side of the channel box, there are two rods sandwiching the bisector of each side of the channel box 6, which is a fuel assembly in which fuel rods are arranged in n rows x n columns, where n is an odd number, in a square lattice. It is an open area surrounded by the outermost fuel rods and channel box. Therefore, in order to promote the lateral transport by the above-mentioned secondary flow, and in particular to transport the liquid phase adhering near the bisector of each side of the channel box to the center of the channel, the corners of the channel box 6 and each side of the channel box 6 are A groove 10 is provided between the bisector of the channel box 6 and the outermost fuel rod, and the width of the lateral flow path between the channel box 6 and the outermost fuel rod is increased so that the secondary flow flows from the corner to the vicinity of the bisector of each side. The coolant flowing on the inner surface of the channel box is carried to the center of the flow path. Therefore, the radial liquid phase distribution becomes more uniform, and the cooling efficiency of the coolant increases. There are two reasons why the channel width is not widened by not providing the groove portion 10 in the channel surrounded by the fuel rods 1 and the channel box 6 at the corner portions. (1) Since creep deformation occurs in the corner portions of the channel box 6 due to neutron irradiation, the wall thickness cannot be reduced in terms of strength. (2) Since there is a relatively large amount of liquid phase near the corner fuel rods, expanding the flow path area by providing grooves has the opposite effect on making the liquid phase distribution uniform.

Furthermore, as the axial mass velocity distribution increases in the flow path area at the groove position, the velocity of the coolant colliding with the fuel spacer decreases, so that the coolant adheres to the surface of the fuel rod, which determines the boiling transition initiation state. The strength of liquid film rupture is reduced.

In the first embodiment, two grooves 10 each having a U-shaped cross section are provided on each side of the channel box 6.
FIG. 5 shows a second embodiment in which a groove portion 11 having a concave curved surface shape is provided inside the channel box. Further, FIG. 6 shows a third embodiment of the present invention for a fuel assembly in which fuel rods are arranged in a 9×9 square lattice. In FIG. 6, since there is a minimum gap between the channel box 6 and the outermost fuel rod 1 near the bisector of each side of the channel box, the groove 10 that widens the width of the lateral flow path for promoting secondary flow is , are provided at positions that do not include the diameter dimension of the outermost fuel rod 1 existing near the bisector of each side of the channel box 6. In addition, in each example, the groove portion 10
The liquid phase flow along the inner surface of the channel box 6 is given a transverse velocity in the groove 10, and a part of it separates and flows toward the fuel rod.

The embodiments of the present invention are summarized as follows. (1) In a fuel assembly in which a plurality of fuel rods are arranged in a square lattice shape by fuel spacers and has a groove on the inner surface of a channel box forming a square flow path surrounding the fuel rods, the groove shall be provided at two or more locations on each side, and shall not be provided on the corner side from the minimum gap between the corner fuel rod and the channel box. (2) The fuel rods of the fuel assembly are n rows x n, where n is an even number.
The grooves are arranged in a square grid pattern, and the grooves are centered on the bisector of each side of the channel box, and are within the length of the gap between the two outermost fuel rods closest to the bisector of each side. Things that aren't there. (3) The fuel rods of the fuel assembly are n rows x n, where n is an odd number.
The channel box is arranged in a square grid pattern, and the groove is centered on the bisector of each side of the channel box, and is not within the length of the diameter of the outermost fuel rod closest to the bisector of each side. (4) Grooves shall be provided at two or more locations between the fuel spacers, including one immediately upstream of the fuel spacer. (5) The cross-sectional shape of the groove shall be U-shaped or concave toward the inside of the channel box.

[0021]

[Effects of the Invention] According to the present invention, the accumulation of liquid phase on the inner surface of the channel box is suppressed, lateral transport is promoted by widening the secondary flow passage, and the distribution of thermal hydraulic force in the radial direction is made uniform. Thermal limit output is improved by

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a first embodiment of a fuel assembly according to the present invention.

FIG. 2 is a longitudinal cross-sectional view taken along line AA' in FIG. 1;

FIG. 3 is a cross-sectional view for explaining the action of the fuel assembly in FIG. 1;

FIG. 4 is a curve diagram showing a comparison between the relationship between the limit output and the flow rate in the example of the present invention and the conventional example.

FIG. 5 is a cross-sectional view showing a second embodiment of the fuel assembly according to the present invention.

FIG. 6 is a cross-sectional view showing a third embodiment of the fuel assembly according to the present invention.

FIG. 7 is a partially cutaway perspective view of a conventional fuel assembly.

FIG. 8 is a cross-sectional view of the fuel assembly in FIG. 7.

FIG. 9 is an elevational view partially showing another example of a conventional fuel assembly.

FIG. 10 is a conceptual diagram showing the state of liquid film adhesion on the surface of the fuel rod in FIG. 7;

[Explanation of symbols]

1...Fuel rod, 2...Water rod, 3...Fuel spacer,
4... Upper tie plate, 5... Lower tie plate, 6... Channel box, 7... Tapered groove, 8... Liquid film, 9... Vertical vortex, 10, 11... Groove portion, 12... Axial flow of coolant.

Claims (1)

[Claims] 1. A fuel assembly comprising a channel box in which a plurality of fuel rods are arranged in a square lattice shape by fuel spacers and that forms a square flow path surrounding each of the fuel rods, wherein the channel box A fuel assembly characterized in that at least two grooves are provided on each side of the inner surface of the channel box, and no grooves are provided in the corners from the minimum gap between the fuel rod located at the corner of the channel box and the channel box. .