JPH021586A - Gas purge mechanism for fuel aggregate of nuclear reactor - Google Patents

Abstract

PURPOSE:To eliminate a load in the axial direction for buckling a cooling pipe by constituting the cooling pipe of the inside of a guide pipe so that its upper end part becomes a double structure in which it has been turned back to the outside in a position where it has crossed over the upper end of the guide pipe and forming a bend path between said cooling pipe and the guide pipe. CONSTITUTION:A space to which a fuel body 4 is loaded is filled wit a liquid metal 5 for quickening a heat transfer at the time of transferring heat from the fuel body 4 to a coolant flowing through the inside of a cooling pipe. As for the upper supporting grid plate 8, its periphery is joined airtightly to the inside surface of a shell pipe 2, and this plate is provided with many cooling pipe through-holes 18 which have been pierced like a perforated plate in the same way as the lower supporting grid plate. The cooling pipe 3 passes through the inside of a guide pipe 10 and extends to the upper part, and its upper end becomes a double pipe structure in which it has been turned back to the outside in a position where it has crossed over the upper end of the guide pipe 10. That is, a bend path is formed by an outside pipe 3b and the guide pipe 10, and this part forms a gas purge part gas plenum 15.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a tube-in-shell type fuel assembly used in a liquid metal cooled nuclear reactor, and particularly relates to a tube-in-shell type fuel assembly used in a liquid metal cooled nuclear reactor. This invention relates to a gas purge mechanism that can absorb displacement without causing damage such as buckling of cooling pipes even if a difference in axial elongation occurs due to thermal expansion.

[Prior art] In a fuel assembly used in a liquid metal cooled nuclear reactor, in a bottle-shaped fuel in which fuel material is housed in a cladding tube, the structure for releasing FP gas (fission product gas) inside the fuel assembly is a - membrane Also known as vented fuel. For example,
9-109792 and Japanese Unexamined Patent Publication No. 50-5797, a typical example thereof is shown in FIG.

That is, in FIG. 3, reference numeral 30 indicates bottle-shaped fuel. A bottle-shaped fuel 30 is provided with a lower end plug 2 at the lower end of the fuel cladding tube 21.
An upper end plug 23 is welded to the upper end of 2. Inside the fuel tube 21, cylindrical nuclear fuel pellets 25 are stacked and loaded in the axial direction. In addition, this nuclear fuel pellet 25
is immersed in an internal filling liquid metal (e.g. sodium) 5.

There is a gas plenum 26 in the fuel bottle above the stacked nuclear fuel pellets, and a partition plug 40 is provided to separate the gas plenum 26 from the gas plenum 27 in the vent mechanism.

Further, the partition plug 40 is provided with a gas outlet pipe 29 that extends toward the gas plenum of the vent mechanism section, while an external gas outlet tube 28 extends from the upper end plug 23 side toward the gas plenum of the vent mechanism section. There is.

A plurality of bottle-shaped fuels 30 configured as described above are stored as a bundle in a wrapper tube (hexagonal cylindrical shell), not shown, and loaded into the reactor core. Therefore, when loaded in the reactor core, the area around the bottle-shaped fuel 30 is filled with reactor coolant (for example, sodium) 17.

Therefore, the coolant 17 is
enters the vent mechanism gas plenum 27, and a liquid level 17a is formed where the pressure is balanced.

In such a conventional gas barge structure for bottle-shaped fuel, the FP gas generated in the fuel part by neutron irradiation increases the internal pressure of the gas plenum 26 in the bottle, so the generated FP gas passes through the gas outlet pipe 29 to the vent mechanism gas plenum. 27, and eventually, when the liquid level 17a is pushed down to the lower end of the gas external discharge pipe 28, it is discharged into the reactor coolant 17 through the gas external discharge pipe 28.

[Problem to be solved by the invention] In contrast to conventional bottle-shaped fuel in which the fuel element is cooled from the outside,
In so-called tube-in-shell fuel assemblies, which are based on a new concept with a coolant flow path inside the fuel body, the cooling tubes and shell tubes extend in the axial direction by growth due to thermal expansion or neutron irradiation, but many cooling tubes do not interact with each other. There is a difference in the amount of elongation between the cooling pipe and the shell pipe, and unless a structure is adopted to absorb this difference, there is a risk that compressive force will be applied to the cooling pipe and cause the cooling pipe to buckle. On the other hand, in a structure in which the FP gas inside the shell tube is released to the outside of the assembly, it is necessary to prevent the internal liquid metal from being released to the outside.

In order to solve both of these problems, the conventional gas release Akebonobashi for bottled fuel was able to prevent the liquid metal inside the bottle from being released to the outside, but there was no need to absorb the elongation of the cooling pipe, so we devised a new method. became necessary.

This invention aims to solve the above-mentioned problems.

[Means for Solving the Problems l] Corresponding to this object, a gas purge mechanism for a reactor fuel assembly according to the present invention includes loading a fuel body having a large number of through holes in the axial direction into a shell pipe; A lower support grid plate is arranged at a distance in the axial direction so as to secure a lower support grid plate at the bottom and an FP gas plenum section at the upper part;
A dual-in-shell fuel assembly comprising a cooling pipe inserted into a through hole of the fuel body and forming a coolant flow path between the lower support grid plate and the upper support grid plate,
The upper support grid plate is provided with a guide tube extending upwardly in the through hole of the cooling tube, and the cooling tube disposed inside the guide tube has an upper end thereof extending outside at a position beyond the upper end of the guide tube. It has a double-tube structure that is folded back, and is characterized by forming a bending path between it and the guide tube.

[Operation] The gas stored in the gas plenum inside the shell tube passes through the bent path, the lower end of the outer tube that is folded back to the outside, and is discharged into the reactor coolant.

When the temperature of the gas plenum in the shell tube decreases or the external pressure increases, the liquid metal level formed in the bend passes upward, and in some cases, the reactor coolant enters the fuel region in the shell tube.

By appropriately setting the volume ratio of the gas plenum in the gas barge section (the space inside the bent path) and the gas plenum in the shell tube, it is possible to limit the amount of liquid metal that enters the inside of the shell tube to a certain amount or less. Further, by ensuring that the volume of the gas plenum in the shell pipe is larger than the expansion capacity of the fuel body and internal liquid metal, it is possible to prevent the liquid metal filled in the shell pipe from flowing backward through the gas purge part gas plenum and leaking to the outside.

On the other hand, the difference in axial elongation between the cooling tube and the shell tube or between the cooling tubes can be absorbed by the cooling tubes sliding freely since the cooling tubes are not fixed to the upper support grid plate.

[Embodiments] Next, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is an external view of a tube-in-shell type fuel assembly equipped with a gas purge mechanism according to the present invention, and FIG. 2 is a cross-sectional explanatory view of a main part for explaining details of the gas purge mechanism according to the present invention. For simplicity, only a portion corresponding to one cooling pipe is shown. In FIGS. 1 and 2, reference numeral 1 indicates a tube-in-shell type fuel assembly.
As clearly shown in FIG. 1, a hexagonal cylindrical shell tube 2 is provided with an entrance nozzle 11 at the bottom and a handling head 12 at the top, and the overall appearance is a hexagonal cylinder. It is a fuel assembly, and its appearance is almost the same as a conventional liquid metal cooled nuclear reactor fuel assembly that houses a bundle of bottle-shaped fuel inside. However, as mentioned above, the new concept tube-in-shell type fuel assembly has a structure in which the inside of the shell tube can be cooled from the inside by providing a coolant flow path within the fuel body. That is, the arrow in the figure indicates the flow direction of the coolant 17.

Specifically, what is indicated by the reference numeral 4 is a fuel body, and the fuel body 4 is formed into a hexagonal column shape with a size that can be loaded into the shell tube 2. This fuel body 4 is provided with a large number of through holes 9 that penetrate in the axial direction, and the cooling pipes 3 are inserted into the through holes 9. The lower ends of the cooling pipes 3 are hermetically welded to the holes, that is, the openings, of the perforated lower support grid plate 7 . The periphery of the lower support grid plate 7 is, of course, hermetically joined to the inner surface of the shell tube 2. Therefore, the fuel body 4 is loaded in the space formed by the cooling pipe, the shell pipe, and the lower support grid plate. The fuel body 4 has a hexagonal columnar shape with many vertical holes like a charcoal briquette, but it is not necessarily an integral type, and an faWJ type in which it is divided into several parts in the axial direction may be adopted.

On the other hand, the space in which the fuel body 4 is loaded is filled with liquid metal 5 for promoting heat transfer when transmitting the heat from the fuel body 4 to the coolant flowing in the cooling pipe 3.
A liquid level 5a is formed in the gas plenum 6 in the shell pipe above the fuel body 4. Reference numeral 8 denotes an upper support grid plate, and the upper support grid plate 8 has its periphery airtightly joined to the inner surface of the shell tube 2, and similarly to the lower support grid plate, has a large number of cooling pipes perforated in a perforated shape. A through hole 18 is provided. A cylindrical guide tube 10 is airtightly fixed in the through hole 18 upwardly. The cooling pipe 3 is connected to this guide pipe 1.
The guide tube 10 extends upwardly through the guide tube 10, and its upper end is folded back to the outside at a position beyond the upper end of the guide tube 10, forming a double-tube structure. That is, an outer tube 3b having a double-tube structure folded back to the outside (the cooling tube section on the inner side is referred to as an inner tube 3a for convenience) and the guide tube 10.
This section forms a curved path, and this section forms a gas plenum 15 in the gas purge section.

When the fuel assembly is manufactured, the outer tube 3b has its lower end fitted into an annular groove 16 cut into the upper surface of the upper support grid plate 8, and the opening is sealed with a low melting point alloy. When the fuel assembly is loaded into the reactor vessel and the reactor is in an output state, this seal portion melts into an open state, and the state of connection between the cooling pipe and the upper support grid plate is released.

The imaginary line of the double pipe section in FIG. 2 indicates the displacement (axial extension) of the cooling pipe 3 due to thermal expansion.

As is clear from this figure, the cooling pipe 3 is not fixed to the upper support grid plate 8 and can freely slide in the guide pipe 10 in the axial direction. In addition, the coolant liquid level 17a in the gas purge part gas plenum 15 in FIG. The surface is lowered to the frontage.

When the fuel output is stopped, the humidity in the fuel body 4 decreases, the pressure in the gas plenum 6 in the shell pipe decreases, the gas contracts due to a drop in temperature, etc.), and the coolant 17 flows into the gas plenum (bent passage space) in the gas barge. 15, the liquid level rises, and a liquid level is formed, for example, at a position indicated by reference numeral 178'.

In the figure, reference numeral 13 indicates a strainer 14, a coolant inlet, 19, an upper spacer pad, and 20, an intermediate spacer pad.

[Effects of the Invention] As is clear from the above description, according to the present invention, even if there is a difference in axial elongation between the cooling pipe and the shell pipe or between the cooling pipes, the displacement can be absorbed without restraining it. axial loads that would cause the cooling pipes to buckle are not applied.

In addition, by appropriately setting the volume ratio of the gas plenum in the shell pipe and the gas plenum in the gas purge section, the amount of reactor coolant flowing into the fuel part in the shell pipe can be kept within a certain limit, and the liquid metal filled inside can be It is possible to provide a gas barge mechanism for a reactor fuel assembly that can prevent leakage to (inside) the reactor fuel assembly.

[Brief explanation of the drawing]

Fig. 1 is an external view of a tube-in-shell type fuel assembly equipped with a gas barge mechanism of the present invention, Fig. 2 is a cross-sectional explanatory view of main parts for explaining details of the gas barge mechanism of the present invention, and Fig. 3 is a It is a figure which shows an example of the FP gas vent mechanism in the conventional bottle-shaped fuel. 1...Tube-in-shell type fuel assembly 2...Shell pipe 3...Cooling pipe 3a...Inner tube 3b of double tube section...Outer tube 4 of double tube section...Fuel Body 5...Internal filling liquid metal 6...Gas plenum in shell pipe 7...Lower support grid plate 8...Upper support grid plate 9...Different hole 10...Guide tube 15...Gas purge Gas plenum 16...Annular groove 17...Reactor coolant 17a...Liquid level 18...Cooling pipe d Excessive Figure 1

Claims (1)

[Claims] A fuel body having a large number of through holes in the axial direction is loaded into a shell pipe, and an upper support is arranged at a distance in the axial direction so as to secure a lower support grid plate at the bottom of the fuel body and an FP gas plenum section at the top. In a tube-in-shell type fuel assembly comprising a grid plate and a cooling pipe inserted into a through hole of the fuel body and forming a coolant flow path between the lower support grid plate and the upper support grid plate, the upper support The grid plate is provided with a guide tube extending upward in the through hole of the cooling tube, and the cooling tube arranged inside the guide tube is folded back to the outside at a position where the upper end thereof exceeds the upper end of the guide tube. A gas purge mechanism for a nuclear reactor fuel assembly, characterized in that the gas purge mechanism has a double pipe structure, and a bent path is formed between the guide pipe and the guide pipe.