JPH06235784A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PURPOSE:To provide a BWR fuel assembly which deals with irradiation growth of a fuel rod due to high degree of burn-up while reducing difference of elongation between a water rod and the fuel rod and suppressing oscillation of the water rod and absorption of neutron. CONSTITUTION:In a fuel assembly where fuel rods 1 and water rods 6 bundled by means of spacers 8 are fit to upper tie plates 4 through rod springs 3 provided for upper terminal plugs 2 thus ensuring a predetermined gap between the tie plates 4 and the fuel rods 1, tubular part of the water rod is extended 11 close to the rear surface part of the upper tie plate 4 without altering the material thereof.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a boiling water reactor (hereinafter referred to as B
WR) fuel assembly, especially to cope with irradiation growth of fuel rods accompanying higher burnup, reduce the difference in elongation between water rods and fuel rods, suppress vibration of water rods, and It relates to a BWR fuel assembly that also reduces absorption.

[0002]

2. Description of the Related Art Conventionally, BWR fuel assemblies which have been conventionally used are, as shown in FIG. 5, mainly composed of an upper tie plate 4, a lower tie plate 7, a number of fuel rods 1, a central portion 1 and 2.
It is composed of a hollow water rod 12, a spacer 8 and a channel box 5 for accommodating them.

A plurality of spacers 8 are provided in the channel box 5 at intervals in the axial direction.
The fuel rods 1 and the water rods 12 including the above are aligned and supported at regular intervals. These focused fuel rods 1 and water rods 12
The lower end plugs 9 of the eight tie fuel rods 1a are screwed to the lower tie plate 7 and the upper end plugs 2 are engaged with the upper tie plate 4 by the nuts 10.
It is integrated with these upper and lower tie plates 4 and 7.

The upper end plugs 2 of the fuel rod 1 and the water rod 12 have a solid rod shape, and as shown in FIG. 4, these upper end plugs 2 have coiled rod springs 3 mounted thereon. It is slidably fitted and inserted into a plurality of bosses 4 a provided on the upper tie plate 4. That is, the fuel rod 1 and the water rod 12 are provided with the upper tie plate 4 and the channel box 5 attached thereto via the rod spring 3 and the lower surface of the upper tie plate with a predetermined interval. The structure is elastically supported, and this structure can absorb the difference in expansion in the axial direction due to irradiation of the fuel rod 1 or thermal expansion.

The above conventional boiling water reactor fuel assembly has fuel rods and water rods arranged in 8 rows and 8 columns, and both rods and water rods have rod springs as described above. Is installed.

[0006]

By the way, in recent years, the development of high burnup fuel has been advanced. As one of the methods, the number of fuel rods is increased by changing the fuel rod arrangement to 9 rows and 9 columns. A design has been proposed in which the fuel rod surface area is increased, the surface heat flux is lowered to improve the dryout performance, and the linear output density is lowered to suppress the FP gas release amount and PCI.

However, in general, in the fuel rod and the water rod, the growth due to irradiation growth is larger in the fuel rod due to the difference in the fast neutron flux density. Therefore, the difference in the expansion between the fuel rod and the water rod is high burnup as described above. It will grow as it becomes. For this reason, there is a risk that the distance between the upper and lower tie plates will expand beyond the extension of the water rod, and the end plugs of the water rod will fall off the upper tie plate.

The present invention has been made in consideration of the above situation, and corresponds to the difference in irradiation growth between the fuel rod and the water rod due to the increase in the burnup, and the elongation of the water rod and the fuel rod. The purpose is to reduce the difference, suppress the vibration of the water rod, and reduce the neutron absorption.

[0009]

That is, the feature of the BWR fuel assembly of the present invention which meets the above object is that the fuel rods and water rods arranged in a square matrix are converged by a plurality of spacers, and The upper end plugs provided on each upper part of the water rod are slidably inserted into the boss of the upper tie plate,
In addition, in the fuel assembly in which the upper tie plate is supported by interposing a rod spring in the upper end plug of the fuel rod so that a predetermined space is provided between the rod rod and the upper end plug, To near the bottom of the tie plate,
It is in the place where it was extended with the same material.

In this case, the rod spring is not inserted in the upper end plug of the water rod, but if the number of fuel rods increases in 9 rows and 9 columns as described above, the rod spring is attached to the water rod. Even if it is not installed, the weight of the upper tie plate and the channel box can be supported only by the rod spring of the fuel rod, so that there is no particular structural problem.

[0011]

[Function] When the zircaloy material forming the water rod is irradiated with fast neutrons and the atoms are knocked out from the crystal lattice,
Since the atoms are projected along the slip surface of the dense hexagonal crystal (Equation 1), the C axis, which is the central axis, contracts, and the a axis orthogonal to the C axis extends. That is, in the zircaloy pipe material according to the present invention, which is the extended portion of the water rod, the texture is controlled by the rolling method, the heat treatment, or the like at the time of production so that the zircaloy pipe material is elongated in the axial direction when irradiated with fast neutrons. Margin below

[0012]

[Equation 1]

On the other hand, in the conventional Zircaloy rod material constituting the upper end plug, the texture control which is generally carried out in the above-mentioned pipe material is not controlled, and the elongation in the axial direction is smaller than that of the pipe material. .

Therefore, in the present invention, the water rod
By using a Zircaloy pipe material in the portion where the Zircaloy rod material has been conventionally used, the elongation degree of the water rod is increased, thereby reducing the difference in elongation between the water rod and the fuel rod,
The upper end plug of the water rod can be prevented from falling out of the upper tie plate.

Further, when the rod material is a pipe material, the mass of zircaloy is reduced, so that the neutron absorption is reduced and the vibration of the water rod due to the flow of the coolant can be reduced. Since the amplitude of hydraulic oscillation is much larger in the case of two-phase flow than in the case of single-phase flow, the mass of the water rod is made smaller on the upper side of the fuel body in the two-phase flow state. That is very effective.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a partially omitted sectional view showing a BWR fuel assembly according to an embodiment of the present invention, and FIG. 2 is a partially enlarged view showing a main part of the embodiment.

In the fuel assembly of the embodiment, about 8 Thai fuel rods 1a and 1 or 2 water rods 6 are mixed to form a plurality of fuel rods 1 in 8 rows 8 columns or 9 rows 9 columns. It is focused by the spacer 8. Further, the upper end plugs 2 provided on the upper portions of the fuel rods 1 and 1a and the water rod 6, respectively, are slidably inserted into the bosses 4a of the upper tie plate 4, and further, the upper portions of the fuel rods 1 and 1a. By interposing the rod spring 3 on the end plug 2, a predetermined gap, that is, an absorption margin of irradiation growth is formed between the end plug 2 and the fuel rods 1 and 1a to support the upper tie plate 4.

In the bundled fuel bundle, the lower end plug 9 of the tie fuel rod 1a disposed near the outer peripheral portion is screwed to the lower tie plate 7, while the nut 10 of the upper end plug 2 is in the upper portion. By being tightened on the tie plate 4, the tie plate 4 and the lower tie plates 4 and 7 are integrated with the channel box 5 fixed to the upper tie plate 4.

Further, in the present invention, in the above fuel assembly, Zircaloy-2 or Zircaloy-
The tube portion of the water rod 6 consisting of the hollow pipe 4 of
As shown in FIG. 2, the pipe extending portion 11 is formed by extending the upper tie plate 4 to the vicinity of the lower surface thereof using the same zirconium alloy for pipe as it is.

FIG. 3 shows an embodiment in which a water rod 6'having a large diameter is used, and a water rod 6'having a large diameter is used.
At the upper part of the upper tie plate 4, a tubular extension portion 11 having a smaller diameter is extended to the lower surface of the upper tie plate 4. And
Also in this case, the material of the extended portion 11 is the same zirconium alloy for pipes as the large-diameter water rod 6 ', even if the diameter is different.

However, in each of the embodiments of the present invention having the above-mentioned structure, the extending portion 11 of the water rod 6 irradiates the zircaloy bar material, which is the main body of the upper end plug, in the axial direction in advance. It is composed of zircaloy tubing that is structured to extend, which is why the upper end plug 2 is provided at the position of this extension 11 of the water rod, as compared to the conventional fuel assembly as shown in FIG. The degree of extension of the water rod 6 can be increased.

That is, this makes it possible to reduce the difference in elongation between the water rod 6 and the fuel rods 1, 1a and prevent the upper end plug 2 of the water rod 6 from falling off the upper tie plate 4.

Further, as described above, when the rod material is a pipe material, the mass of zircaloy is reduced, so that the neutron absorption is reduced and the vibration of the water rod 6 due to the flow of the coolant can be reduced. Although the vibration has been explained in the section of action, the amplitude of hydraulic vibration is much larger in the two-phase flow than in the single-phase flow. From this, it can be said that it is very effective to reduce the mass of the water rod 6 on the upper side of the fuel assembly in the two-phase flow state.

[0025]

As described above, in the BWR nuclear fuel assembly of the present invention, the fuel rod and the water rod, which are converged by the spacer, are connected to the upper end plug of the fuel rod through the rod spring, and the upper type In the fuel assembly in which a predetermined interval is provided between the tie plate and the fuel rod by inserting it into the rate plate, the tubular portion of the water rod is extended to the vicinity of the lower surface portion of the upper tie plate with the same material. Since the extension part of the water rod is made up of a zircaloy tube material that is pre-structured so as to predominantly irradiate and extend in the axial direction with respect to the zircaloy rod material that is the main body of the upper end plug, As compared with the conventional fuel assembly in which the upper end plug is provided at the position of this extended portion, the degree of extension of the water rod can be increased, thereby reducing the difference in extension between the water rod and the fuel rod. Prevents the water rod upper end plug from falling out of the upper tie plate, and reduces the neutron absorption by reducing the mass of zircaloy, and also has the remarkable effect of reducing the vibration of the water rod due to the flow of the coolant. It is a thing.

[Brief description of drawings]

FIG. 1 is a partially omitted sectional view showing a BWR fuel assembly according to an embodiment of the present invention.

FIG. 2 is a partially enlarged view showing the vicinity of the upper portion of the water rod of the same embodiment.

FIG. 3 is an enlarged view showing the vicinity of an upper portion of a water rod according to a second embodiment of the present invention.

FIG. 4 is an enlarged view showing the vicinity of an upper portion of a water rod of a conventional fuel assembly.

FIG. 5 is a partially omitted sectional view showing a conventional BWR fuel assembly.

[Explanation of symbols]

 1 Fuel rod 1a Thai fuel rod 2 Upper end plug 3 Rod spring 4 Upper tie plate 4a Boss 5 Channel box 6 Water rod 7 Lower tie plate 8 Spacer 9 Lower end plug 10 Nut 11 Extended part (water rod)

Claims (1)

[Claims] 1. A fuel rod and a water rod arranged in a square matrix are converged by a plurality of spacers, and upper end plugs provided on the upper portions of the fuel rod and the water rod are slidable on a boss of an upper tie plate. In the fuel assembly in which the upper tie plate is supported by inserting a rod spring into the upper end plug of the fuel rod to provide a predetermined gap with the fuel rod, A fuel assembly for a boiling water reactor characterized in that the tubular portion is made of the same material and extended up to near the lower surface of the upper tie plate.