JPS62145192A - nuclear fuel assembly - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a nuclear fuel assembly for a boiling water nuclear reactor, and in particular improves core stability by increasing the margin against fuel rod burnout and reducing pressure loss within the assembly. The present invention relates to a nuclear fuel assembly suitable for improving.

[Background of the invention]

In a nuclear fuel assembly for a boiling water reactor, approximately 64 fuel rods are regularly arranged in an 8x8 square grid, and coolant flows between them from the bottom to the top, and heat is removed by the heat transfer phenomenon that accompanies boiling. has been done. The spacers are provided to support the vertical axes of the fuel rods in parallel and to prevent the fuel rods from coming into contact with each other, and spacers are provided at approximately 0.5 m intervals in the effective heating section of the approximately 3.6 m fuel rods. Approximately 7 pieces are installed.

The maximum thermal output of a nuclear fuel assembly is determined by the heat removal capacity of the coolant. When a heat output exceeding this heat removal capacity is generated, the fuel rod surface is covered with a vapor film (boiling transition).
The output of the nuclear fuel assembly must be kept below a certain output (limit output) because the temperature will rise rapidly and burnout will occur, causing fission products with high radioactivity to flow out.

This limit output value is said to depend on the configuration shape of the spacer, and various shapes have been devised. Japanese Patent Publication No. 59-635
No. 89 proposes a structure in which an upward protrusion curved inward is provided at a part of the corner of the outermost structural member of the spacer.

On the other hand, in a boiling water reactor, from the standpoint of nuclear thermal and hydraulic stability, it is desirable that the pressure loss of the coolant in the reactor core be small. Therefore, when considering the configuration of the spacer and fuel assembly to improve the limit output, it is necessary to consider reducing pressure loss. Japanese Patent Publication No. 59-635
Providing an inwardly curved protrusion like No. 89,
Although the coolant flow path area is locally reduced and the critical output is improved, there is a problem in that the pressure loss increases.

[Purpose of the invention]

An object of the present invention is to provide a nuclear fuel assembly structure that can increase the margin against fuel rod burnout, that is, increase the critical output, reduce pressure loss within the assembly, and improve core stability. It is in.

[Summary of the invention]

In order to achieve the above object, in the present invention, a spacer having a configuration for improving the limit output is installed only at a location where the flow state of the coolant is an annular flow.

The reason for adopting the above structure in the present invention is as follows.

Generally, the pressure cuff is 0 atm, the power density is 50 kW/Q
In current boiling water reactors operated at
/S plays. As it flows through the fuel assembly, the coolant removes heat from the fuel rods, causes boiling, and flows out of the fuel assembly in an annular flow through bubble flow and slug flow. An annular flow is a state in which a liquid film flows on the surface of the fuel rod, vapor flows in other parts, and liquid droplets are mixed in the flow. The liquid film on the fuel rod surface becomes thinner toward the top of the assembly due to heat removal from the fuel rod. When this liquid film disappears, heat cannot be removed sufficiently from the fuel rod, so the surface temperature rises rapidly and burnout occurs. In other words, focusing on the fact that boiling transition occurs in the annular flow region, improvements in the spacer configuration shape to improve the critical output have the greatest effect on spacers installed in the annular flow region. become. It is desirable to consider the configuration of the spacer installed in the region upstream of the annular flow region with emphasis on reducing pressure loss rather than improving the limit output.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 1 shows a general view and a sectional view of a fuel assembly according to an embodiment of the present invention. Fuel rod 1 is 8X with spacer 2
They are arranged in an 8 square grid.

Seven spacers 2 are installed at equal intervals between the effective heat generating parts of the fuel rods 1. As the coolant flows from the lower orifice 4 with a slight subcool, etc., and flows inside the fuel focus surrounded by the channel box 3, it boils, passes through the states of bubble flow and slug flow, and becomes an annular flow. and flows out from the top.

The feature of the present invention is that among the seven spacers, the structure of the spacers at positions 2A and 2B on the downstream side in the flow direction of the coolant is
Upstream side 2C, 2D, 2E, 2F.

The structure of 2G is different. FIG. 2 is a sectional view of the spacer 2 provided at spacer positions 2A and 2B (
a) and a side view (b). This spacer is used in an assembly using a thick water rod 5 in its center. The central part of the spacer cross section is a circular cell 6 and a fixing holding member 7 for the fuel rod 1 attached thereto. Spring-like holding material 8 and fixed holding material 9 for water rod 5. It is composed of a spring-like holding material 10. The feature of this spacer is that an inwardly directed protrusion 12 is provided on the upper part of the outermost peripheral structural member 11, as shown in the dotted circle C in the side view (b). This protrusion 12
has the function of redirecting the coolant flowing from below toward the center of the fuel assembly, making it easier for droplets in the steam in the annular flow to adhere to the fuel rod surface.

As a result, the thickness of the liquid film on the fuel rod surface increases, making boiling transition less likely to occur. On the other hand, 2G on the upstream side.

For spacers attached to positions 2D, 2E, 2F, and 2G,
The structure is such that the protrusion 12 shown in FIG. 2 is not provided.

As a result, in the assembly of the present invention, all the spacers are the same as in the past, and the structure is as shown in FIG. 2. It can secure the same margin for boiling transition as the body.

In addition, the upstream spacer has the protrusion 12 removed,
Since local pressure loss is reduced, pressure loss throughout the fuel assembly is also reduced, and core stability can also be improved.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to secure margin against fuel rod burnout and improve core stability.

[Brief explanation of drawings]

FIG. 1 is a general view and a sectional view of a fuel assembly according to an embodiment of the present invention, and FIG. 2 is a sectional view and a side view of a spacer installed in the fuel assembly of the present invention. 1... Fuel rod, 2... Spacer, 11... Spacer outermost structural material.

Claims (1)

[Claims] 1. In a nuclear fuel assembly in which a large number of fuel rods are arranged regularly with spacers, and when loaded into a nuclear reactor, coolant flows from the bottom to the top and heat is removed by the boiling heat transfer phenomenon of the coolant. A nuclear fuel characterized in that, regarding a spacer installed at the top of a fuel assembly in which the flow state of the coolant becomes an annular flow during operation, the outermost structural member of the spacer is provided with a structure that directs the flow of the coolant toward the center of the fuel assembly. Aggregation.