JPH07333371A - Fuel assembly - Google Patents

Abstract

PURPOSE:To enhance the thermal margin without increasing the pressure drop by disposing the gas plenum of fuel rod at the lower end of a spacer. CONSTITUTION:A fuel rod 1 is held, at the upper and lower ends thereof, by upper and lower tie plates 4, 3 while keeping an interval through a spacer member 8. A gas plenum 5 is disposed at the lower end of a plurality of members 8 arranged axially while sustaining the space thereof by means of a spring 6 and a tubular unheatable member 7 is disposed thereunder. Small holes are made through the upper and lower surfaces of the member 7 so that the member 7 also serves as a gas plenum. Consequently, pressure drop can be decreased. The liquid film becomes thinner through evaporation as the distance from the lower end of a heating member increases and becomes thinnest immediately before the member 8. When the member 7 is provided, the liquid film is protected against evaporation and a required film thickness can be ensured. This structure can ensure the flow rate of liquid film using a most stringent member 8 without increasing the pressure drop thus increasing the thermal margin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for improving the thermal margin of a pressure tube reactor and a boiling water reactor.

[0002]

2. Description of the Related Art Fuel assemblies for pressure tube reactors and boiling water reactors have a structure in which a gap between fuel rods is supported by spacers. Water, which is a coolant, generates steam in the assembly due to the heat generated by the fission reaction. As shown in FIG. 3, in the downstream of the assembly (see FIG. 2) where the ratio of steam in the coolant is large, as shown in FIG. Becomes an annular spray flow. When the spacer member 8 is arranged in the steam flow field of this flow mode,
The vortex generated in the spacer member 8 gives a velocity component to the droplet toward the surface of the fuel rod, and attaches to the liquid film on the surface of the fuel rod 1 which generates heat. Therefore, by disposing a large number of spacers in the axial direction, the disappearance of the liquid film on the surface of the fuel rod can be suppressed, and the limit output can be improved. For example, Japan Atomic Energy Society “1
992 Autumn Games ”p. 666-667 show the experimental results regarding the effect.

[0003]

In the above-mentioned conventional example, since a large number of spacers are arranged in the axial direction, the pressure loss of the assembly increases, and more energy is required to secure a predetermined coolant flow rate.

It is an object of the present invention to provide a nuclear fuel assembly which improves thermal margin without increasing pressure loss.

[0005]

The above problems can be solved by a fuel rod having a plurality of gas plenums arranged in the axial direction and a fuel assembly in which spacers are arranged so that the upper ends of the gas plenums are at the lower ends of the spacers.

[0006]

According to the fuel assembly of the present invention, it is possible to secure the liquid film flow rate upstream of the spacer, which is the most thermally severe, and increase the thermal margin without increasing the pressure loss.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to the drawings.

The upper and lower ends of the fuel rod 1 are held by the upper tie plate 4 and the lower tie plate 3, and the distance between the fuel rods 1 is maintained by the spacer member 8 (AA 'cross section is shown in the drawing). In order to reduce the pressure loss, a plurality of gas plenums (shown in FIG. 5), which were conventionally provided at the upper end of the fuel rod, are located at the lower end of the spacer member 8 in which a plurality of gas plenums are axially arranged. A space is maintained in the gas plenum 5 by a spring 6, and a cylindrical non-heating member 7 (BB 'sectional shape is shown in the drawing) is provided below them. A small hole is provided on the upper and lower surfaces of the cylindrical non-heating member, which also functions as a gas plenum.

FIG. 4 shows a comparison of changes in the liquid film thickness on the fuel rod when the non-heated portion is not provided at the lower end of the spacer member (conventional) and when it is provided (invention). The liquid film becomes thinner as the distance from the lower end of the heat generation length increases due to evaporation or the like, but it can be seen that the liquid film is slightly recovered downstream due to the promotion of droplet adhesion caused by the spacer. The position where the liquid film becomes the thinnest (thermally severest) is the position immediately before the spacer. By locally stopping the heating as in the present invention, the liquid film disappears immediately before the spacer, and the liquid film thickness You can see that you can secure the quality.

The length for providing the non-heated portion may be sufficiently smaller than the total heat generation length, and the present invention may be carried out only on the upper portion of the assembly which is the annular spray flow shown in FIG. Has a small effect. Further, if the present invention is applied only to the fuel rod having a large relative output, the influence on the pellet loading amount can be further reduced.

FIG. 6 shows another embodiment. In the present invention, since the cylindrical non-heating member 7 is provided in the fuel rod 1 and the spring 6 for holding the fuel pellet 2 is provided only in the lower gas plenum 5, the structure is simple and the same effect can be obtained. As the cylindrical non-heating member 7, a metal material such as a zirconium alloy having a small neutron absorption is suitable.

[0012]

According to the fuel assembly of the present invention, the thermal margin can be increased without increasing the pressure loss.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory view showing a position where an annular spray flow exists in the aggregate.

FIG. 3 is an explanatory view showing an influence of a spacer member in an annular spray flow.

FIG. 4 is an explanatory diagram showing a change in liquid film thickness due to a spacer.

FIG. 5 is a sectional view showing a conventional fuel rod structure.

FIG. 6 is a sectional view showing another embodiment of the present invention.

[Explanation of Codes] 1 ... Fuel rod, 2 ... Fuel pellet, 3 ... Lower tie plate, 4 ... Upper tie plate, 5 ... Gas plenum, 6 ... Spring, 7 ... Unheated member, 8 ... Spacer member.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location G21C 3/328 GDD

Claims (3)

[Claims] 1. A plurality of fuel rods in which a large number of pellets and gas plenums having different enrichments are arranged, upper and lower tie plates supporting the fuel rods, and a plurality of fuel rods for maintaining a constant gap between the fuel rods. A fuel assembly comprising the spacer, the fuel rod, and a channel containing the spacer, wherein the gas plenum of the fuel rod is provided at a lower end position of the spacer. 2. The fuel assembly according to claim 1, wherein the gas plenum of the fuel rod is provided at a lower end position of a plurality of spacers arranged above the assembly. 3. The fuel assembly according to claim 1, wherein the gas plenum of the fuel rod is provided at a lower end position of a spacer of a plurality of fuel rods having a large relative output.