JPH063472A - Fuel assembly - Google Patents

Abstract

PURPOSE:To obtain a fuel assembly which allows the releaf of heat impact by expanding a mixed area of high and low temperature coolants downward in a specified area. CONSTITUTION:A fuel assembly 1 composes a core of a fast breeder employing a liquid metal as coolant. Inside a handling head 4, a rotatable cylinder 14 is provided having an opening which is decentered gradually in one direction upward with respect to the center axis of the fuel assembly 1.

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 via the.

The coolant flows in through the coolant inlet 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 due to the large amount of heat it generates. On the other hand, for example, the coolant flowing out of the control rod assembly 15 or the assembly having a small heat generation amount such as the blanket fuel assembly has a lower temperature than the coolant flowing out of the fuel assembly 1. In the fast breeder reactor, blanket fuel assemblies are loaded in the reactor to produce more plutonium than it has consumed. The blanket fuel assembly is an assembly having almost the same structure as the fuel assembly 1, but uranium 238 of the depleted uranium (UO ₂ ) absorbs neutrons and undergoes several stages of nuclear decay to become plutonium. 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 at the early stage of use when the production of plutonium is small. Small, inlet nozzle 3
At a temperature close to the temperature in the above, it flows out of the handling head 4 above the blanket fuel assembly. 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.

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 drive mechanism guide tube 11 and a rectifying cylinder 12 supported by a lattice plate 13 arranged on the lattice around the guide tube 11.
In the vicinity of the surface of the upper structure 9 which is exposed to the high temperature and low temperature coolants, thermal shock is generated, and fatigue damage increases. In particular, a fatigue crack due to thermal shock is likely to occur and propagate at the joint between the guide tube 11 for the control rod drive mechanism 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 such as Inconel, but there is no record of use in the nuclear reactor and it is difficult to manufacture and process, so the cost becomes 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 inside the handling head, the center of the fuel assembly increases as it goes upward. A rotatable cylinder having an opening eccentric to the axis in one direction was provided.

[0009]

In the specific region such as the joint between the control rod drive mechanism guide tube and the lattice plate in the upper part of the core using the fuel assembly of the present invention having the above-mentioned structure, the mixing region of the low temperature coolant flowing out adjacently Expands downward, suppressing temperature fluctuations in the coolant above the core. Therefore, fatigue damage to the core upper structure is mitigated.

[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, inside the handling head 4, there is provided a rotatable cylinder having an opening that is eccentric in one direction with respect to the central axis of the fuel assembly 1 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 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 from the control rod assembly 15 or the blanket fuel assembly having a small heat generation amount 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 a diagram schematically showing a state in which a high temperature coolant flowing out of a fuel assembly 1 that has been conventionally used and a low temperature coolant flowing out from an adjacent assembly are mixed in an upper portion of the core. 4 is a fuel assembly 1 according to the present invention
It is the figure which showed the state in which the high temperature and low temperature coolant which flowed out from are mixed. The cylinder 14 attached to the handling head 4 of the fuel assembly 1 in FIG. 4 is installed so that the upper part of the opening thereof faces the adjacent control rod assembly 15. 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 are fluctuating 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, the coolant flows out along the eccentric opening of the handling head 4, so that the region C extends to a position right above the assembly as shown in FIG. Sufficiently performed, the thermal shock is alleviated in the vicinity of the surface of the joint between the control rod drive mechanism guide tube 11 and the lattice plate 13.

In the fuel assembly 1 according to the present invention, the direction of the opening of the cylinder 14 mounted inside the handling head 4 can be set in any direction, and the guide rod 11 for the control rod drive mechanism and the lattice plate 13 can be set. The thermal shock can be mitigated not only in the joint portion of the above, but also in any portion of the core upper structure 9.

Further, since the fuel assembly 1 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.

[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 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 schematically 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 schematically showing a state in which a high temperature coolant flowing out of a fuel assembly according to the present invention and a low temperature coolant flowing out of an adjacent control rod assembly are mixed in an upper portion of a 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, 5 ... Fuel rod, 6 ... Wire, 8
... channel, 10 ... trumpet tube, 14 ... cylinder.

Claims (1)

[Claims] 1. Inside a handling head of a fuel assembly in which a coolant flows upward from below, the opening is eccentric in one direction with respect to the central axis of the fuel assembly as it goes upward. A fuel assembly having a rotatable cylinder.