JPH057679B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to a center fuel support fitting applied to a boiling water nuclear reactor, and in particular to a center fuel support fitting for preventing fuel assemblies from being damaged by foreign objects. Regarding fuel support fittings.

"Prior Art" In a boiling water nuclear reactor, a reactor core is constructed by loading a large number of fuel assemblies. As shown in FIG. 4, the fuel assembly has a large number of fuel rods 2 held at predetermined intervals by spacers 3 and housed in the inner tube of a channel box 1 having a rectangular cylindrical shape. Both upper and lower ends of each fuel rod 2 are restrained and fixed by an upper tie plate 4 and a lower tie plate 5, respectively.

Among these, the lower tie plate 5 is constructed by integrally molding a tie plate main body 6 and a fuel rod support portion 7. The lower end plug 8 of each fuel rod 2 is fitted or screwed into a hole 9 of the fuel rod support 7 and restrained. Furthermore, the fuel rod support section 7 includes a coolant flow path 1.
0 is formed. A coolant inlet 11 is opened at the lower end of the tie plate main body 6, and the coolant flowing from the coolant inlet 11 flows into the coolant flow path 1.
0 and is guided into channel box 1.

The fuel assembly is fitted into a fuel support fitting attached to the core support plate, the lower end is positioned and the weight is held, and the upper part of the fuel assembly is positioned by the upper grid plate.

There are two types of fuel support fittings: central fuel support fittings and peripheral fuel support fittings. As shown in FIG. 5, the center fuel support fitting 12 has a cross-shaped groove 13 in the center for passing the control rod, and the area around it is divided into four chambers for holding four fuel assemblies. At the top of the chamber is a fuel support hole 14 that mates with the fuel assembly. There is a coolant inlet hole 15 in the lower side of each chamber.

Although not shown, the peripheral fuel support fitting has a cylindrical shape, holds one fuel assembly at the upper part,
The structure is such that the coolant enters from the bottom. Peripheral fuel support fittings are installed on a part of the outermost periphery of the core. The other part is the center fuel support fitting.

Lower cylindrical portion 12a of central fuel support fitting 12
The upper part of the control rod guide tube is connected to the upper part of the control rod guide tube. The control rod guide tube has a hole through which coolant passes, and this hole corresponds to a coolant inlet hole 15 in the lower part of the central fuel support fitting 12.

``Problems to be Solved by the Invention'' By the way, during periodic inspections of nuclear reactors, the top cover of the reactor pressure vessel is opened to replace the fuel assembly. During this inspection work, metal pieces such as wire used in the work may fall into the reactor pressure vessel inner pipe. Additionally, parts of the reactor structure may become detached due to oxidation or other causes, and these detached pieces may similarly fall into the reactor pressure vessel. These foreign objects such as metal pieces and flakes are usually small in weight, so
During reactor operation, coolant is guided into the reactor core as it flows, and a portion of it flows together with the coolant through holes in the upper part of the control rod guide tubes. The incoming coolant and foreign matter enter the center fuel support fitting 12. In addition, in the peripheral fuel support fitting, by entering from the coolant inlet hole 15,
The foreign matter flows into the lower tie plate 5 of the fuel assembly and passes through the coolant passage 10 of the fuel rod support 7. Since the coolant flow path 10 has a wide flow path area, only large foreign objects will be caught, and there is a high possibility that small foreign objects will be caught in the spacer 3 portion having the narrowest flow path area.

On the other hand, the flow of the coolant flowing into the assembly is disturbed by the lower tie plate 5, spacer 3, etc.
The flow velocity changes locally due to voids occurring in the upper part of the fuel assembly. Due to the influence of this change in flow velocity, etc., the foreign matter and the fuel rod 2 are slightly vibrating.

If this vibrating foreign object were to come into contact with the fuel rod 2, there is a possibility that a fretting phenomenon would occur.

This fretting phenomenon occurs when a foreign object such as a metal piece comes into contact with the cladding tube of the vibrating fuel rod 2 and peels off the corrosion film attached to the cladding tube, and this peeled part becomes newly corroded, resulting in this peeling and corrosion. This is a phenomenon in which erosion of the cladding progresses due to the synergistic action of As this fretting phenomenon progresses, a hole opens in the cladding tube of the fuel rod 2, and the coolant erodes into the fuel rod 2 through this hole. This eroded coolant reacts with the inner surface of the cladding tube, and hydrides are generated, causing damage to the cladding tube. Such damage to the cladding tube may cause a large amount of radioactivity within the fuel rods 2 to diffuse into the coolant, which may impair the safety of the nuclear reactor.

The present invention has been made in consideration of the above facts, and an object of the present invention is to provide a central fuel support fitting that can maintain the integrity of fuel rods and improve the safety of a nuclear reactor.

"Means for Solving the Problems" In order to achieve the above object, the central fuel support fitting according to the present invention has an orifice at the entrance of the coolant flow path, and a net-like structure on the upstream side of the orifice. A foreign object capture member is attached. The orifice determines the flow rate, and the net-like foreign matter capturing member captures foreign matter to prevent foreign matter from flowing into the fuel assembly.

"Embodiment" An embodiment of the present invention will be described below based on the drawings.

As shown in FIG. 3, a reactor pressure vessel 21 of a boiling water reactor accommodates a reactor core 22 and is filled with coolant.

Core 22 is surrounded by a core shroud. The core shroud is cylindrical and separates the coolant into an upward flow within the core 22 and a recirculation flow downward through its outer annulus. The recirculation flow is pressurized by a recirculation pump outside the reactor pressure vessel and a jet pump inside the reactor pressure vessel, and is sent to the lower core plenum 23. The pressurized coolant rises between the control rod drive mechanism housing 24 and the control rod guide tube 25 in the lower part of the core, but is blocked by the core support plate 26, and the coolant rises between the control rod drive mechanism housing 24 and the control rod guide tube 25 in the lower part of the core, and is blocked by the core support plate 26. The fuel is guided to the central fuel support fitting 27 via a side hole in the upper part of the guide tube 25 . Some coolant goes directly into the peripheral fuel support fittings.

The configuration of the central fuel support fitting 27 is shown in FIGS. 1 and 2. An orifice 30 is attached to the coolant inlet hole 28 of the central fuel support fitting 27 to restrict the flow so that the amount of coolant flowing into the fuel assembly 29 is appropriate. There are several types of orifices 30 with different inner diameters, and by changing this, the flow rate of coolant to the fuel assembly 29 can be changed. The side hole 31 of the control rod guide tube, which is located in front of the orifice 30, has a larger hole diameter than the orifice 30, so that the orifice 30 can work effectively. Also, the coolant flow path 32 after the orifice 30
Similarly, the hole diameter has also increased.

A wire mesh 33 serving as a foreign matter capturing member is installed in the orifice 30 of the central fuel support fitting 27 . By disposing such a wire mesh 33, even if foreign objects such as metal pieces or peeled pieces in the coolant flow in together with the coolant, they will be caught by the wire mesh 33 and captured. As a result, foreign matter does not flow into the channel box where the fuel rods are located during the operation of the nuclear reactor, and damage to the fuel rods due to fretting phenomenon is prevented. Therefore, the integrity of the fuel rods can be maintained, and the safety of the nuclear reactor can be improved.

"Effects of the Invention" As described above, according to the central fuel support fitting according to the present invention, a net-like foreign matter capturing member is attached to the upstream side of the orifice, and this foreign matter capturing member prevents foreign matter from entering the fuel assembly. flow is prevented, the integrity of the fuel rods can be maintained, and as a result, the safety of the nuclear reactor can be improved.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of an embodiment of the central fuel support fitting according to the present invention, FIG. 2 is a sectional view of FIG. 1, and FIG.
The figure is a perspective view showing the core structure, FIG. 4 is a partial sectional view showing a fuel assembly, and FIG. 5 is a perspective view showing a conventional example. 27... Center fuel support fitting, 29... Fuel assembly, 30... Orifice, 33... Foreign matter capturing member.

Claims (1)

[Scope of Claims] 1. A central fuel support fitting that is disposed on the center side of a core support plate of a boiling water reactor and supports the lower end portions of a plurality of fuel assemblies, and that supports the fuel assemblies from below. A coolant flow path is formed in the coolant flow path, and an orifice is provided at the entrance of the coolant flow path;
A central fuel support fitting characterized in that a net-like foreign matter capturing member is attached to the upstream side of the orifice.