JPS6239714B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel assembly in which the structure of an outer tube housing fuel rods is improved.

As is well known, the core of a nuclear reactor is constructed by regularly arranging a plurality of nuclear fuel assemblies.

By the way, the above-mentioned nuclear fuel assembly, for example, a nuclear fuel assembly used in a fast expansion reactor,
Usually, as shown in FIG. 1, a plurality of fuel rods 2 are arranged and housed in a triangular lattice in a fuel channel 1. The fuel channel 1 includes an outer cylindrical tube (hereinafter referred to as a wrapper tube) 3 formed in a hexagonal shape, for example, from a stainless steel plate, and this wrapper tube 3.
It consists of an entrance nozzle 4 connected to the lower end and a handling head 5 connected to the upper end of the wrapper tube 3. The entrance nozzle 4 is provided with a plurality of holes 6 for guiding the coolant into the fuel channel 1, and the radiation emitted from the fuel rods 2 is emitted into the handling head 5 through the opening of the handling head 5. Radiation resistant object 7 to prevent people from going straight from
is provided.

A plurality of nuclear fuel assemblies configured as described above are properly arranged in a honeycomb lattice shape adjacent to each other in the reactor core with the entrance nozzle 4 side facing downward.

Thus, the coolant flows into the fuel channel 1 from the hole 6 of the entrance nozzle 4, absorbs the heat generated in each fuel rod 2 while rising in the fuel channel, and then surrounds the radiation-resistant material 7. The liquid then flows out through the opening of the handling head 5.

Incidentally, the nuclear fuel assembly configured as described above has a function of separating the flow of coolant in the fuel channel from the flow of coolant in the fuel channel in an adjacent assembly by the wrapper tube 3. As a result, the flow rate of coolant flowing in from each assembly entrance nozzle hole 6 can be adjusted in order to equalize the heat removal capacity from the fuel rod bundles in the fuel assemblies having different calorific values throughout the reactor core.

However, in nuclear reactors, especially fast breeder reactors, the amount of heat generated from the fuel rods of the fuel assembly in the reactor core is large, and the cross-sectional area of the cooling flow path in the fuel channel is very narrow. Therefore, if foreign matter gets mixed into the coolant or if a spacer or the like that maintains the gap between the fuel rods is damaged, a local flow blockage occurs in the fuel channel. The flow of coolant around the blockage is therefore disrupted, reducing the ability to remove heat from the fuel rods at that location and potentially causing local overheating of the fuel element. In particular, if we assume that the cooling flow path within the fuel assembly is clogged to an excessive extent (60% or more of the cross section of the flow path within the fuel assembly) due to the accumulation of blockages over a long period of time, boiling of the coolant may occur, and further damage to the fuel elements may occur. There are concerns that the fuel rods could break and the fission product gases contained within the fuel rods could be released.

On the other hand, the entrance nozzle 4 of the fuel assembly has a multi-channel structure in order to prevent flow channel blockage caused by foreign matter mixed into the coolant as described above. If the fuel assembly 1 rises for some reason and the flow path connecting the inlet entrance nozzle hole 6 to the connecting pipe between the multiple layers of core support plates that support the fuel assembly 1 at the bottom becomes excessively blocked. , the cooling flow into the fuel assembly is shut off. As a result, the temperature inside the fuel channel will rapidly rise, which is expected to cause coolant boiling, fuel element damage, and even fuel melting.

If such an excessive blockage of the flow path is assumed, there is a possibility of fuel damage within the fuel assembly, but this is because the cooling flow rate within each fuel assembly is separated by the wrapper tube as described above. It also causes

The present invention has been made in view of these points, and even if a severe flow path blockage occurs within a certain fuel assembly, or even if a severe flow path blockage occurs at the inlet of a fuel assembly. The fuel assembly has sufficient heat removal capacity to prevent excessive temperature rise inside the fuel assembly, resulting in coolant boiling, fuel damage, and even fuel melting, and is designed to prevent accidents from propagating to surrounding fuel assemblies. The purpose is to provide a nuclear fuel assembly that has a suppressive effect.

Hereinafter, parts that are the same as those in Figure 1 are designated by the same reference numerals.
An embodiment of a nuclear fuel assembly according to the present invention will be described with reference to the drawings.

The present invention is directed to the wrapper tube 9 of the nuclear fuel assembly 8 shown in FIG.
Provided is a lapper tube structure having coolant flow holes 10 provided with a mesh structure having a plurality of narrow eyes regularly arranged in a range corresponding to the length of the heat generating region of the fuel rods 2 housed in the assembly. There is something to do.

By providing a plurality of mesh-structured flow holes 10 in the wrapper tube in this way, the coolant flowing within the fuel assembly 1 is not separated for each fuel assembly, which has been the case in the past, but is instead transferred between adjacent fuel assemblies. It can be easily mixed with other coolants. Particularly when the above-mentioned flow passage blockage accident occurs in a fuel assembly, or when the passage is blocked at the inlet of the assembly, the pressure loss due to the blockage increases. Therefore, the flow rate of coolant supplied from the fuel assembly inlet entrance nozzle 6 decreases compared to the normal state, but the coolant pressure within the fuel assembly also decreases correspondingly to the decrease in flow rate, so the coolant flow within the adjacent fuel assembly decreases. A pressure gradient occurs between the pressure and the pressure. As a result, the flow rate of coolant within the surrounding fuel assembly is naturally supplied to the fuel assembly on the accident side in the high heat flux area where the fuel rod heat generating part is located through the multiple mesh-shaped flow holes provided in the lapper tube. , the function of suppressing the inconsistency between the calorific value and the cooling flow rate in the accident-side assembly works, and it is possible to prevent the accident from spreading (coolant boiling, fuel damage, and fuel melting) within the assembly.

Furthermore, even if the flow path blockage becomes severe enough to cause localized fuel damage within the accident assembly, and broken pieces of the fuel cladding and fuel particles are released into the coolant flow path, such damage may occur. To prevent fragments, etc. from flowing into the cooling channels in adjacent aggregates, that is, from propagating flow path blockage from the accident side aggregate to the adjacent aggregates, by using the flow holes in the side parts of the wrapper pipes having a steel mesh structure. I can do it.

As explained above, in the present invention, in the case of a core local accident due to flow path blockage involving a single fuel assembly in the core of a nuclear reactor, the overheating phenomenon occurring within the accident assembly can be suppressed by cooling from surrounding fuel assemblies. It is expected that the heat removal effect due to the lateral flow of material will sufficiently suppress the heat, and at the same time, even if fuel failure or fuel release occurs within a fuel assembly, it will have a function that will not spread the flow path blockage to adjacent assemblies. It can be said that it has an inherent safety effect.

[Brief explanation of the drawing]

FIG. 1 is a side view showing a part of a conventional nuclear fuel assembly in cross section, and FIG. 2 is a side view showing a part of an embodiment of the nuclear fuel assembly of the present invention in cross section. 1...Fuel channel, 2...Fuel rod, 3...
Rupture pipe, 4... Entrance nozzle, 5... Handling head, 6... Entrance nozzle hole, 7... Radiation resistant material, 8... Fuel channel, 9... Rupture pipe, 10... Coolant distribution hole .

Claims (1)

[Claims] 1. In a nuclear fuel assembly in which a plurality of fuel rods are arranged in a matrix in an outer tube of a fuel channel and a coolant is allowed to flow through the fuel channel, the outer tube has a plurality of fuel rods arranged in a matrix along the flow direction of the coolant. What is claimed is: 1. A nuclear fuel assembly characterized in that a coolant flow hole formed by a plurality of regularly arranged meshes is provided in an axial region corresponding to a heat generating part of a fuel rod.