JPH07159570A - Fuel assembly - Google Patents

Abstract

PURPOSE:To expand the mixed region of high-temperature and low-temperature coolants downward and mitigate a thermal shock by providing a conical center rod increasing the cross sectional area toward the upper face from the lower face at the upper section of a handling head and at its center. CONSTITUTION:A coolant flows into a fuel assembly 1 from the coolant inflow port 2 of an inlet nozzle 3, it passes through passages among fuel rods 5, reaches a handing head 4, and flows out to the upper section of the fuel assembly 1. The high-temperature coolant flowing out from the fuel assembly 1 and the low-temperature coolant flowing out from adjacent control rods are mixed at the upper section of a core. Part of the coolant flows out along a conical center rod 14 provided at the upper end section of the handling head 4 of the fuel assembly 1, the mixed region of the high/ low temperature coolants is expanded to directly above the fuel assembly 1, and they are thoroughly mixed. The thermal shock is mitigated near the surface of the connection section of the guide pipes and lattice plates for a control rod driving mechanism. The thermal shock of the fuel assembly 1 at an optional portion of the upper structure of the core can be also mitigated.

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 conventional fuel assembly 1 for a 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 on the lower part of the fuel assembly 1 through the.

The coolant flows from 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. To do. The coolant flowing out of 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 flowing out of the control rod assembly 15 or the assembly having a relatively small heat generation amount such as the blanket fuel assembly has a lower temperature than the coolant flowing out of the fuel assembly 1. Since the blanket fuel assembly uses depleted uranium whose fissionable uranium 235 is 0.3% or less, the temperature rise from the inlet nozzle 3 to the handling head 4 is small in the early stage of use when the production of plutonium is small. It is small and flows 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 reduces the exit temperature of the coolant in the reactor loaded with blanket fuel assemblies.

At the upper part of the core, these high-temperature and low-temperature coolants are mixed and rise toward the upper structure 9. The upper structure 9 is a supporting function of various instrumentation mechanisms for core monitoring,
It also has the function of rectifying the coolant. The rectifying device includes a control rod guide tube 11 and a rectifying cylinder 12 supported by a lattice plate 13 arranged on the lattice around the control rod guide tube 11, and the like. It will be subject to thermal shock near the exposed surface. In particular, the thermal shock increases at the joint between the control rod guide tube 11 and the lattice plate 13.
For this reason, there is a method of covering the surface of the upper structure 9 with a heat-resistant alloy, but there is no record of use in the nuclear reactor, and manufacturing and processing are difficult, so the cost is high.

In order to solve this problem, Japanese Patent Application No. 63-
A fuel assembly 1 described in the specification of No. 109392 has a structure in which holes are provided in the trumpet tube 10 distributed in the circumferential direction, and mixing of high-temperature and low-temperature coolant is started near the holes. .

[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 is discharged. However, it may flow into the control rod assembly having a small flow rate, which is not preferable.

An object of the present invention is 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 is located above the handling head, at the center of the handling head, and from the bottom surface to the top surface. It is provided with a conical center rod whose cross-sectional area increases over time.

[0009]

In the specific portion of the core upper portion using the fuel assembly of the present invention having the above-described structure, the mixing region with the low temperature coolant flowing out adjacently expands downward, and the temperature fluctuation of the coolant in the core upper portion 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, and FIG. 2 is a transverse sectional view of a handling head 4 of the fuel assembly 1 constructed according to the present invention.
Here, in the upper part of the handling head 4, a conical center bar 14 whose cross-sectional area increases from the lower surface to the upper surface is provided at the center of the handling head. 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. Fuel assembly 1
The coolant flowing out from the fuel rod 5 has a high temperature because the calorific value of the fuel rod 5 is large. On the other hand, for example, the coolant flowing out from the control rod assembly 15 or the blanket fuel assembly that generates a small amount of heat has a lower temperature than the coolant flowing out of the fuel assembly 1. In the upper part of the core, the high temperature and low temperature coolants are mixed and rise toward the upper structure 9. FIG. 3 is an explanatory view of 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, and FIG. FIG. 6 is a diagram showing a state in which high-temperature and low-temperature coolants flowing out from the fuel assembly 1 according to the above are mixed.
In both figures, a shaded area A to the lower left shows an area where high-temperature coolant flows out, and a shaded area B to the lower right shows an area where low-temperature coolant flows out. C
Indicates a region where the hot and cold 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. 3, in the conventionally used fuel assembly 1, the 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. become. When the fuel assembly 1 according to the present invention is used, a part of the coolant flows out along the center rod 14 provided at the upper end portion of the handling head 4, so that the region of the area C is the assembly as shown in FIG. As a result, the heat shock is reduced near the surface of the joint between the control rod drive mechanism guide tube 11 and the lattice plate 13 by expanding to just above.

In the fuel assembly 1 according to the present invention, not only the connecting portion between the control rod drive mechanism guide tube 11 and the lattice plate 13,
The thermal shock at any part of the core upper structure 9 can be relaxed.

Further, since the fuel assembly 1 according to the present invention has no holes on the side surface of the trumpet tube 10, mutual interference of the coolant itself between the assemblies is small, and a more accurate design can be performed. Further, the manufacturing and processing of the handling head 4 can be performed by the conventional technique.

[0013]

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

Further, by preventing the upper structure from being damaged, it is possible to prevent the flow passage of the coolant from being blocked due to the structure falling into the fuel assembly.

[Brief description of drawings]

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

FIG. 2 is a cross-sectional view of a fuel assembly according to an embodiment of the present invention.

FIG. 3 is an explanatory view showing a state in which a high temperature coolant flowing out from a fuel assembly which has been conventionally used and a low temperature coolant flowing out from an adjacent control rod assembly are mixed in the upper part of the core.

FIG. 4 is an explanatory view showing a state in which a high temperature coolant flowing out from a fuel assembly according to the present invention and a low temperature coolant flowing out from an adjacent control rod assembly are mixed in an upper portion of the core.

FIG. 5 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, 11 ... Control rod guide tube.

Claims (1)

[Claims] 1. A conical member having a cross-sectional area increasing from the lower surface to the upper surface at the center of the handling head in the upper portion of the handling head of the fuel assembly in which the coolant flows from the lower side to the upper side. And the fuel assembly.