JPH0519081A - Fuel assembly - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a fuel assembly that expands a mixed region of high-temperature and low-temperature coolant downwards to mitigate thermal shock. [Constitution] The fuel assembly 1 constitutes the core of a fast breeder reactor using liquid metal as a coolant, and the opening inside the handling head 4 expands as it goes upward. [Effect] Since the mixed region of the fuel assembly flowing out of the high temperature coolant and the coolant of the adjacent low temperature coolant flowing out is expanded, the temperature fluctuation of the coolant above the core can be suppressed and the heat The impact is relieved, and the fatigue to the upper mechanism can be relieved.

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 a fast breeder reactor.

[0002]

2. Description of the Related Art A fuel assembly 1 for a conventional fast breeder reactor is shown in FIG.
I will explain based on. An inlet nozzle 3 having a plurality of coolant inlets 2 is attached to the lower end of the fuel assembly 1, and a handling head 4 is attached to the upper end. In the fuel assembly 1, a fuel rod 5 and a wire 6 spirally wound.
The support plate 7 is horizontally mounted in the lower part of the fuel assembly 1 via the.

The coolant flows in through the coolant inlet port 2 of the inlet nozzle 3, passes through the passage 8 between the fuel rods 5 while removing heat, reaches the handling head 4, and flows out to the upper part of the fuel assembly 1. The coolant flowing out from the fuel assembly 1 is the fuel rod 5
It is hot because of the large amount of heat it generates. On the other hand, for example, the coolant flowing out from an assembly having a small heating value such as a control rod assembly or a blanket fuel assembly,
The temperature becomes lower than that of the coolant flowing out from the fuel assembly 1.
In a fast breeder reactor, blanket fuel assemblies are loaded in the reactor to produce more plutonium than it has consumed. Blanket fuel assembly is fuel assembly 1
It is an aggregate of almost the same structure as
Of the O ₂ ), uranium 238 absorbs neutrons and undergoes several stages of decay to become plutonium. Since the blanket fuel assembly uses depleted uranium whose fissionable uranium 235 is less than 0.3%, the temperature rise from the inlet nozzle 3 to the handling head 4 is small in the early stage of use when the amount of plutonium produced is small. , Out of the handling head 4 above the blanket fuel assembly at a temperature close to that at the inlet nozzle 3. This low temperature coolant lowers the outlet temperature of the coolant in the reactor loaded with the blanket fuel assemblies, degrading the thermal properties of the reactor.

In the upper part of the core, these high-temperature and low-temperature coolants are mixed and rise to the upper structure 9. The upper structure 9 has a function of surely inserting the control rod into the core, a function of supporting various instrumentation mechanisms for core monitoring, and a function of rectifying the coolant. The vicinity of the surface of the upper structure 9 exposed to the high-temperature and low-temperature coolants is subjected to thermal shock, which may cause fatigue cracks to develop. For this reason, there is a method of covering the surface of the upper structure 9 with a heat-resistant alloy such as Inconel, but there is no record of use in the nuclear reactor, and manufacturing and processing are difficult, resulting in high cost.

To solve this problem, Japanese Patent Application No. 63-
There is known a structure in which the holes are provided in the trumpet tube 10 of the fuel assembly 1 described in the specification of No. 109392 in the circumferential direction, and it is intended to mix high-temperature and low-temperature coolants near the holes. is there.

[0006]

In the above-mentioned prior art, the flow of the fuel assembly and the control rod assembly is branched from the main flow path and mixed in the middle, so that a part of the fluid of the fuel assembly having a large flow rate However, it may flow into a control rod assembly having a small flow rate, which is not preferable.

It is an object of the present invention to provide a fuel assembly which expands a mixed region of high-temperature and low-temperature coolant downwards to mitigate thermal shock.

[0008]

The fuel assembly of the present invention constitutes the core of a fast breeder reactor using liquid metal as a coolant, and the opening inside the handling head expands upward. Is.

[0009]

In the upper part of the core using the fuel assembly of the present invention having the above structure, the mixing region with the low temperature coolant flowing out adjacently expands downward, and the temperature fluctuation of the coolant in the upper part of the core is suppressed. . Therefore, the fatigue damage to the upper structure of the core is alleviated.

[0010]

1 is a vertical sectional view of a fuel assembly 1 constructed according to the present invention. Here, in the handling head 4, the inner opening is enlarged as it goes upward. The coolant flows into the fuel assembly 1 from the coolant inlet port 2 of the inlet nozzle 3, passes through the flow path 8 between the fuel rods 5, reaches the handling head 4, and flows out to the upper part of the fuel assembly 1. The coolant flowing out from the fuel assembly 1 has a high temperature because the calorific value of the fuel rods 5 is large. On the other hand, for example, the coolant that flows out from the control rod assembly or the blanket fuel assembly that generates a small amount of heat has a lower temperature than the coolant that flows out from the fuel assembly 1. At the upper part of the core, these high-temperature and low-temperature coolants are mixed and rise toward the upper structure 9. FIG. 2 is a diagram schematically showing a state in which a high-temperature coolant flowing out from a fuel assembly 1 that has been conventionally used and a low-temperature coolant flowing out from an adjacent assembly are mixed in the upper part of the core. FIG. 3 is a diagram showing a state in which the high temperature and low temperature coolants flowing out from the fuel assembly 1 according to the present invention are mixed. In both figures, a shaded area A to the lower right shows an area where the high temperature coolant flows out, and a shaded area B to the bottom left shows an area where the low temperature coolant flows out. C indicates a region where the high temperature and low temperature coolants are mixed.
In the lower part of the area C, the high-temperature and low-temperature coolants fluctuate in a turbulent state, but the temperature becomes uniform as they flow upward. As shown in FIG. 2, in the conventionally used fuel assembly 1, high-temperature and low-temperature coolants are not sufficiently mixed, and severe thermal shock occurs near the surface of the upper structure 9 exposed to the coolant. It will be. When the fuel assembly 1 according to the present invention is used, as shown in FIG. 3, the region of the area C is expanded to just above the assembly to sufficiently perform the mixing, and the thermal shock is mitigated near the surface of the upper structure 9. It

Since the fuel assembly according to the present invention has no holes on the side surface of the trumpet tube 10, there is no mutual interference of the coolant itself between the assemblies, and a highly accurate design can be achieved. Further, the manufacturing and processing of the handling head 4 can be performed by the conventional technique.

[0012]

EFFECTS OF THE INVENTION According to the present invention, the mixed region of the fuel assembly that flows out of the high temperature coolant and the coolant of the adjacent assembly that flows out of the low temperature coolant is expanded, so that the coolant in the upper core is Since the temperature fluctuation of the above can be suppressed and the thermal shock can be mitigated, the thermal fatigue of the upper mechanism can be mitigated.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a fuel assembly according to an embodiment of the present invention.

FIG. 2 is an explanatory view of a state where low-temperature coolant flowing out from the assembly is mixed in the upper part of the core.

FIG. 3 is an explanatory diagram showing a state where low-temperature coolant flowing out of the assembly is mixed in the upper part of the core.

FIG. 4 is a vertical sectional view of a conventional fuel assembly.

[Explanation of symbols]

1 ... Fuel assembly, 2 ... Coolant inlet, 3 ... Inlet nozzle,
4 ... Handling head, 5 ... Fuel rod, 6 ... Wire, 7
... Support plate, 8 ... Flow path, 9 ... Upper structure, 10 ... Trumpet tube.

Claims (1)

Claim: What is claimed is: 1. In a fuel assembly in which a coolant flows upward from below, the opening inside the handling head of the fuel assembly expands as it goes upward. Fuel assembly to do.