JPH11174183A - Fuel spacer and fuel assembly - Google Patents

Abstract

(57) Abstract: A fuel spacer and a fuel having a simple structure, in which a part of a liquid film existing on an inner wall of a channel box is directed to a fuel rod to improve a limit output and increase pressure loss little. Provide an aggregate. A bathtub having a longitudinal direction orthogonal to a main flow of a coolant is formed on a side surface of a corner portion of a side band constituting a fuel spacer,
The longitudinal length of the bathtub 14 is characterized by extending over a plurality of fuel rods 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fuel assembly for a boiling water reactor, and more particularly to a fuel spacer for holding a plurality of fuel rods at predetermined positions.

[0002]

2. Description of the Related Art A plurality of fuel rods are disposed in a fuel assembly for a conventional boiling water reactor, and both ends thereof are held by upper and lower tie plates. Further, a plurality of fuel spacers are arranged between the upper and lower tie plates to suppress the fuel rods from bending due to the bending of the fuel rods and the flow of the coolant, thereby holding the fuel rods at appropriate positions.

[0003] By the way, many types of fuel spacers have been proposed in the past, but there is a round cell type spacer 1 shown in FIGS. 11 and 12 to improve the limit output of the fuel assembly. The spacer 1 is composed of cylindrical cells 11 arranged in a base shape. The fuel rods 3 are arranged in the cells 11, are pressed and supported in the cells 11, and keep the distance between the fuel rods constant. is there. On the side surface of the side band 13, a bathtub 14 for keeping a space between the channel box 2 and the spacer 1 is formed, and on the upper end portion, a flow tab 12 serving as a guide when the channel box 2 is inserted is formed.

[0004]

In normal operation of a boiling water reactor, coolant flows through the gaps between the fuel rods, removing heat from the fuel rods, and a vapor-water two-phase flow downstream of the boiling point. It flows through the fuel assembly in a state. In particular, in the upper part of the fuel assembly, as shown in FIG. 13, a part of the liquid phase is a liquid film 5 flowing on the surface of the fuel rod 3, a liquid film 4 flowing on the inner wall surface of the channel box 2, and the rest is a vapor A flow mode exists as droplets 6 therein. As the output of the fuel assembly increases, the liquid film 5 of the fuel rod 3 no longer covers the surface, and the fuel rod 3 is exposed to steam. This phenomenon is called boiling transition,
The aggregate output when this phenomenon occurs is the limit output.

[0005] In recent years, in response to the need to improve the economics of nuclear power generation, the development of high burn-up fuels for the purpose of reducing fuel cycle costs has been progressing. In order to increase the burnup of fuel, it is important to improve thermal-hydraulic characteristics, in particular, to improve marginal power characteristics, and to increase the thermal margin of the reactor core.

Incidentally, in order to increase the critical output of the fuel assembly, the liquid film 5 flowing along the surface of the fuel rod 3 in a region where the coolant becomes a two-phase flow of steam-water is made thicker. That is, the liquid phase (water, water droplets) is positively supplied to the fuel rods 3 to form more liquid film flows on the surface of the fuel rods 3.
In particular, the fuel rods 3 near the corners of the channel box 2
Due to thermal severeness, the above-mentioned boiling transition phenomenon easily occurs,
It is desired to suppress the boiling transition phenomenon near the corner.

Therefore, the fuel rod does not contribute to cooling.
Japanese Patent Application Laid-Open No. 7-43486 discloses a patent in which a relatively thick liquid film flowing on the surface of a channel box is peeled off by a spacer and directed to a fuel rod. According to this known example, an opening is provided in a bathtub formed in a side band of a spacer closest to an inner wall of a channel box, a liquid film flowing on the surface of the channel box is taken in from the opening, and a part of the liquid film is removed from a fuel rod. It is intended to be redirected to.

However, if a bathtub which is an obstacle to the flow of the coolant is formed all over the side band, not only will the pressure loss increase, but if the contact area between the channel box and the bathtub is large, the spacer in the channel box will be increased. May not move smoothly,
The number of bathtubs that can be closest to the inner wall of the channel box is limited.

An object of the present invention is to provide a fuel spacer which has a simple structure, a part of the liquid film 4 flowing on the inner wall of the channel box 2 is directed to the fuel rods 3 so as to improve the limit output and to increase the pressure loss little. And a fuel assembly.

[0010]

In order to achieve the above-mentioned object, the present invention provides a structure in which a side surface of a side band 13 which constitutes a fuel spacer has a direction perpendicular to a main flow of coolant. Forming a bathtub 14 having a longitudinal direction,
The longitudinal length of the bathtub 14 is characterized by extending over a plurality of fuel rods 3.

That is, according to the present invention, the side band 13
Due to the bathtub 14 formed on the side surface of the corner portion, the coolant flow path suddenly expands immediately downstream of the bathtub 14, so that a vortex flow occurs in the downstream portion and goes from the channel box 2 to the fuel rod 3. Due to the velocity component, the liquid film 4 flowing along the inner wall of the channel box 2 that does not contribute to the cooling of the fuel rods is peeled off, and a part of the liquid film can be turned to the fuel rods 3 around the corners.

Further, the size of the bathtub 14 is not designed to greatly block the coolant flow path in consideration of interference between the bathtub 14 and the channel box 2, manufacturability, strength, and the like. The projected area of the fuel assembly can be slightly increased as compared with the conventional one, the increase in pressure loss can be minimized, and the limit output of the fuel assembly can be improved. Therefore, the thermal margin of the nuclear reactor is increased, the operation at a higher output is possible than before, the burnup of the fuel is made possible, the fuel cycle cost is reduced, and the economy is improved.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below using embodiments. 1 and 2 show one embodiment of the present invention.
As shown in FIGS. 1 and 2, a bathtub 14 having a longitudinal direction perpendicular to the coolant flow is formed on the side surface of the corner portion of the side band 13 constituting the fuel spacer.

FIG. 14 shows a coolant flow in the fuel spacer according to this embodiment. According to the present embodiment, the bathtub 14
In the further downstream area, the coolant flow path suddenly expands,
In the downstream part, a vortex flow 7 is generated, and the liquid film 4 flowing along the inside of the channel box 2 is separated by the velocity component from the channel box 2 toward the fuel rod 3, and a part of the liquid film is formed on the fuel rod 3. You can turn around.

With these structures, the liquid film 4 flowing on the inner wall of the channel box 2 which does not contribute to the cooling of the fuel rods 3
Water can be positively supplied to the fuel rods 3 around the corners where boiling transition is likely to occur, so that the thickness of the liquid film 5 flowing along the fuel rods 3 increases, and the critical output of the fuel assembly increases. Is improved.

In the present embodiment, the bathtub 14 is arranged so as to act on the fuel rods near the corners, which are particularly thermally severe, and the size of the bathtub 14 is determined by the interference between the bathtub 14 and the channel box 2, the manufacturability, and the strength. In consideration of the above, the structure is not configured so as to largely block the coolant flow path, so that the projected area of the fuel spacer of the present invention can be slightly increased as compared with the conventional one, and the pressure loss increases. And the critical power of the fuel assembly can be improved.

Furthermore, the same effect can be obtained by using all of the fuel spacers of the present invention in the axial direction of the fuel assembly or by using a plurality of fuel spacers above the fuel assembly where the coolant flows in the two-phase flow region. In order to reduce the pressure loss increased by the formation of the bathtub 14, the spacer located at the lower part of the fuel assembly, which is a single-phase water flow region, is inserted when the channel box 2 is inserted as shown in FIGS. The pressure loss in the fuel assembly can be further reduced by employing a structure in which only the flow tab 12 necessary for the guide is left.

FIGS. 3 and 4 show another embodiment of the present invention.
In the present embodiment, the bathtub 14 is formed in an inverted L shape. With this structure, water can be positively supplied to the fuel rods 3 around the corners as in the above-described embodiment, so that the thickness of the liquid film 5 flowing along the fuel rods 3 increases, and The limit output is improved. Further, since the bathtub 14 extended in the longitudinal direction of the assembly is formed like the bathtub 14 of the conventional spacer shown in FIG.
There is an advantage that the movement of the spacer 1 in the inside becomes smoother.

Also, by making the bathtub an inverted L-shape,
In particular, there is also a merit that more liquid film flowing on the side band 13 surface can be turned to the fuel rod 3 at the corner. Further, the same effect can be obtained by using all the fuel assemblies in the axial direction of the fuel assembly, or by using a plurality of coolants on the upper portion of the fuel assembly where the coolant flows in the two-phase flow region, as in the above embodiment.

FIGS. 5, 6, and 7 show another embodiment of the present invention. In the present embodiment, the bathtub 14 arranged at the corner is
The projection 15 is formed in the middle of the above. The projection height of the projection 15 is equal to or less than the projection height of the bathtub 14.

With this structure, similar to the above embodiment,
Since water can be positively supplied to the fuel rods 3 at the corners, the thickness of the liquid film 5 flowing along the fuel rods 3 increases, and the limit output of the fuel assembly improves. Further, there is an advantage that the protrusion 15 becomes an obstacle to the liquid film flowing on the surface of the side band 13 between the corner bathtubs 14 and the liquid film can be turned around by the fuel rods 3 around the corner.

The same effect can be obtained by using all of the fuel assemblies in the axial direction, or by using a plurality of coolants on the upper portion of the fuel assembly where the coolant flows in the two-phase flow region, as in the above embodiment.

In order to reduce the pressure loss increased by the formation of the flow tabs 14 and the projections 15, a spacer located at the lower part of the fuel assembly, which is a water single-phase flow region, is shown in FIG.
As shown in (1), the pressure loss in the fuel assembly can be further reduced by leaving only the flow tab 12 necessary for the guide when the channel box 2 is inserted.

[0024]

According to the present invention, a part of the liquid film 4 flowing on the inner wall of the channel box 2 can be supplied to the fuel rod 3 with a simple structure, and the liquid film 5 flowing on the fuel rod surface becomes thicker. As a result, the limit output of the assembly is improved, and the thermal margin is increased. The size of the bathtub 14 is also taken into consideration in consideration of interference between the bathtub 14 and the channel box 2, manufacturability, strength, and the like.
Since the structure does not have a structure in which the coolant flow path is largely closed, an increase in pressure loss can be minimized.

[Brief description of the drawings]

FIG. 1 is a perspective view of a spacer showing one embodiment of the present invention.

FIG. 2 is a side view of FIG. 1;

FIG. 3 is a perspective view of a spacer showing one embodiment of the present invention.

FIG. 4 is a side view of FIG. 2;

FIG. 5 is a side view of a spacer showing one embodiment of the present invention.

FIG. 6 is a side view of a spacer showing one embodiment of the present invention.

FIG. 7 is a side view of a spacer showing one embodiment of the present invention.

FIG. 8 is a side view of a spacer showing one embodiment of the present invention.

FIG. 9 is a side view of a spacer showing one embodiment of the present invention.

FIG. 10 is a side view of a spacer showing one embodiment of the present invention.

FIG. 11 is a perspective view of a conventional fuel spacer.

FIG. 12 is a side view of FIG. 11;

FIG. 13 is an explanatory diagram of a coolant flow in the upper part of the fuel assembly.

FIG. 14 is an explanatory diagram of a coolant flow in an assembly to which one embodiment of the present invention is applied.

[Explanation of symbols]

1 ... spacer, 2 ... channel box, 3 ... fuel rod,
4, 5: liquid film, 6: droplet, 11: cell, 12: flow tab, 13: side band, 14: bathtub, 15: protrusion.

Claims (4)

[Claims] 1. A round cell type fuel spacer which presses and supports a fuel rod, a longitudinal direction of a coolant flowing on a side surface of a corner portion of a side band forming the fuel spacer is defined as a longitudinal direction. A fuel bath, wherein the length of the bathtub in the longitudinal direction extends over a plurality of the fuel rods. 2. A round cell type fuel spacer which presses and supports a fuel rod, wherein a mainstream flow of coolant and a direction orthogonal to the mainstream direction are formed on side surfaces of the corners of side bands constituting the fuel spacer. A fuel spacer, wherein an inverted L-shaped bathtub having a longitudinal direction as a longitudinal direction is formed. 3. The fuel spacer according to claim 1, wherein a projection having a height lower than that of the bathtub is formed at an intermediate position of the bathtub. 4. The fuel spacer according to claim 1, 2 or 3,
A fuel assembly, comprising at least a plurality of fuel assemblies above a fuel assembly.