JPH0631754B2 - Reactor fuel assembly support grid - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a fuel assembly of a nuclear reactor, and more particularly to improvement of a support grid that constitutes the fuel assembly.

[Prior Art] For example, as shown in the transverse cross section in FIG. 4, in a pressurized water reactor 1, a reactor core 3 is disposed inside a cylindrical reactor melter 2, and the reactor core is A large number of constituent fuel assemblies 5 are surrounded by a partition plate 4, which is usually called a baffle plate, inside the reactor core 3. Because each fuel assembly 5 has a substantially square cross section, the baffle plates 4 adjacent to the fuel assemblies 5 located around the core have a stepped cross section as shown.

Therefore, as shown in FIG. 5 loaded between the lower core plate 6 and the upper core plate 7 of the core, a flat baffle is provided near at least one side of the fuel assembly 5 around the core. When the plates 4 are close to each other and the reactor is in operation, the coolant is provided not only inside the fuel assemblies and between the fuel assemblies but also the fuel assemblies around the core as shown by an arrow 8. The gap between the baffle plate 5 and the baffle plate 4 also flows upward. As a result, the flow 8 may cause vibration in the fuel assembly 5 (hereinafter referred to as grid vibration).

On the other hand, the fuel assembly 5 includes an upper nozzle 5a and a lower nozzle 5b.
And a rigid skeletal structure composed of a plurality of control rod guide tubes 5c having upper and lower ends attached to them and a plurality of support grids 5e spaced apart along the longitudinal direction of the fuel assembly 5. The fuel element or fuel rod 5d has a structure having a body and extending vertically in a cell (not shown) defined by the support grid 5e. Further, the support grid 5e is a dimple extending into each cell defined by the inner straps 9a, 9b and the outer strap 10 as shown in FIG. 6 by enlarging a region surrounded by a circle VI in FIG. Each fuel rod 5d is sandwiched and supported by 11 and the spring 12. Incidentally, 13
Is a guide vane, and 14 is a guide tab.

[Problems to be Solved by the Invention] Therefore, as is clear from FIG. 6, when grid vibration occurs in the fuel assembly 5 and the support grid 5e vibrates, the fuel rod
On the surface of the cladding tube of 5d, scratches due to abrasion with the dimples 11 and the spring 12 occur, which is rare, but this scratch grows into a hole penetrating the cladding tube, which may lead to fuel leakage. is there.

Therefore, it is an object of the present invention to provide a support grid for a reactor fuel assembly that can reduce vibration of the fuel assembly as much as possible.

[Means for Solving the Problems] To this end, according to the present invention, the support grids of a reactor fuel assembly loaded in a core having a baffle plate are assembled in a grid shape to receive fuel rods. An inner strap defining a cell, a support means extending into the cell for supporting the fuel rod, and an outer strap surrounding the inner strap, the outer strap having upper and lower edges. A flow hole is formed in at least one of the guide vane and the guide tab provided.

[Operation] During the operation of the reactor loaded with the fuel assembly including the support grid according to the present invention, the coolant flows upward between the baffle plate and the fuel assembly around the core. Since a flow hole is formed in at least one of the guide vanes and the guide tabs provided on the outer strap of the support grid of the fuel assembly, the coolant reaching the support grid passes through the flow hole, Therefore, the contraction or expansion of the coolant in the support grid is prevented from occurring, which suppresses the vibration of the fuel assembly.

[Embodiment] Next, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals indicate the same or corresponding portions.

Referring to FIG. 1, a first embodiment of a support grid according to the present invention is shown in perspective view. This support grid 5e comprises, as is conventional, an inner strap 9a, 9b interleaved in a known manner to define a number of cells and an outer strap 10 surrounding the inner strap 9a, 9b. The fuel rod 5d is inserted into the cell.
In the figure, typically two fuel rods 5d are shown.

Further, although only the spring 12 is shown on the outer strap 10 in the figure, the fuel rod 5d is sandwiched by the spring and dimple which are the supporting means, as described in connection with the conventional example of FIG. Although such dimples are formed on the strap, the illustration is omitted. Further, the inner strap may be provided with a protruding piece (not shown) called a mixing blade for mixing the coolant.

Further, when loading or unloading the fuel assembly into or out of the core, the outer strap 10 is provided with guide vanes 13 having a substantially triangular shape extending upward along the upper edge to guide the fuel assembly. Are formed at appropriate intervals, and the substantially trapezoidal guide tabs 14 extending downward are formed at appropriate intervals along the lower edge. Both the guide vanes 13 and the guide tabs 14 are curved inward of the lattice.

According to the knowledge of the inventors of the present application, in the grid vibration described above in relation to the conventional technique, the flow passage cross-sectional area of the flow 8 (see FIG. 6) is partially reduced at the position of the support grid 5e. The guide tabs 14 on the outer strap 10 cause a contraction phenomenon in which the coolant is collected, and the outer strap 10
It is considered that the guide vanes 13 cause a spreading phenomenon in which the coolant diffuses, and due to these phenomena, it is an automatic vibration based on the fluid force used for the support grid 5e.

Therefore, in a preferred embodiment according to the present invention, in order to reduce the occurrence of the above-mentioned contraction and expansion, as shown in FIGS. 1 and 1A, circular flow holes are formed in the guide vanes 13 and the guide tabs 14. 13a and 14a are formed by punching, for example. In the case of the embodiment, the diameter of the flow hole 13a is 3 mm,
The flow passage area is 7 mm ² , and the diameter of the flow hole 14a formed in the guide tab 14 smaller than the guide vane 13 is 1.5 m.
m, the flow channel area is 1.8 mm ² .

When the core is loaded with the fuel assembly having the support grid 5e described above and the reactor is operated, the coolant flows upward in the core as described with reference to FIG. In that case, in the core loaded with the fuel assembly having the support lattices 5e in which the flow holes 13a and 14a are formed according to the present invention, as shown in FIG.1A, between the baffle plate and the fuel assembly around the core, The coolant flow 8 passes partly between the baffle plate and the outer straps 10 of the support grid, but partly on the lower guide tabs 14
In the guide vane 13, it flows upward through the flow hole 14a as shown by the arrow 8 ', and also flows out from the flow hole 13a formed therein in the guide vane 13 as shown by the arrow 8 ". It is possible to prevent the contraction flow in the guide tabs 14 and the expansion flow in the guide vanes 13.

After loading one simulated fuel assembly having the support grid 5e in which the flow holes 13a and 14a having the above-described dimensions are formed in the experimental device with the baffle plate, the simulated fuel assembly in the experimental device is loaded. Water as a coolant was caused to flow from below to above at various speeds, and the swing of the support grid was measured. The results shown in FIG. 3 were obtained. As is clear from comparison of this result with the measured value without flow hole, in the case of the experimental example, when the flow velocity exceeds about 5.6 m / sec, a significant decrease in amplitude occurs due to the formation of flow hole. confirmed.

In the embodiment of FIGS. 2 and 2A, the flow hole 13
In order to compensate for the decrease in strength of the guide vane 13 and the guide tab 14 due to the formation of a and 14a, the guide vane 13 and the guide tab 14
At 14, raised portions 13b, 1 raised toward the inside of the support grid
4b is provided, and flow holes 13a and 14a are formed in the raised portion. The raised portions prevent the guide vanes and the guide tabs from being deteriorated in strength.

Although the preferred embodiment of the support grid according to the present invention has been described above, the shape of the flow hole does not necessarily have to be circular, and the strength of the guide vane and the guide tab and the vibration suppressing effect are taken into consideration. It is optimized with the size and quantity. Further, in the preferred embodiment, the flow holes are formed in both the guide vanes and the guide tabs, but it is obvious to those skilled in the art that both the guide vanes and the guide tabs can be used as long as the desired vibration suppressing effect can be obtained. It may be formed only on one side.

EFFECTS OF THE INVENTION According to the present invention, since the flow hole is formed in at least one of the guide zone and the guide tab of the outer strap of the support grid, the fuel assembly having the support grid is loaded into the core to operate the reactor. In this case, the generation of grid vibration due to the coolant flow between the baffle plate and the fuel assembly support grid is suppressed, and as a result, it is possible to greatly improve the integrity and reliability of the fuel assembly when it is used in the reactor. Can contribute.

[Brief description of drawings]

FIG. 1 is a perspective view showing a part of a first embodiment of a support grid according to the present invention, FIG. 1A is a sectional view taken along the line IA-IA in FIG. 1, and FIG. 2 is a support according to the present invention. FIG. 2A is a perspective view showing a part of a second embodiment of the lattice, FIG. 2A is a sectional view taken along the line IIA-IIA in FIG. 2, and FIG. 3 is a view showing a fuel assembly having a conventional supporting lattice and the present invention. FIG. 4 is a graph showing the experimental results of the fuel assembly having the supporting lattice of FIG. 1 according to the present invention; FIG. 4 is a cross-sectional view showing the positional relationship between the baffle plate in the core and the fuel assembly; [Fig. 6] is a side view of a fuel assembly around the core located near the baffle plate, and Fig. 6 is an enlarged cross-sectional view of a region surrounded by a circle VI in Fig. 5. 1 ... Reactor, 4 ... Baffle plate 5 ... Fuel assembly, 5d ... Fuel rod 5e ... Support grid, 9a ... Inner strap 9b ... Inner strap, 10 ... Outer strap 11 ... Dimple ( Supporting means) 12 …… Spring (supporting means) 13 …… Guide vane, 13a …… Flow hole 14 …… Guide tab, 14a …… Flow hole

Claims (1)

[Claims] 1. A support grid for a reactor fuel assembly loaded into a core having baffle plates, the inner straps being assembled in a grid pattern to define cells for receiving fuel rods, and the cells. At least one of a guide vane and a guide tab provided on the upper edge and the lower edge of the outer strap, the support means extending inside to support the fuel rod, and an outer strap surrounding the inner strap. A support grid for a reactor fuel assembly with flow holes formed in it.