JPS62287186A - Nuclear fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention [Object of the Invention] (Field of Industrial Application) The present invention relates to a fuel assembly loaded in the core of a boiling water reactor, and particularly relates to a fuel assembly loaded in the core of a boiling water reactor. The present invention relates to a fuel assembly with improved end plugs fixed to openings at both ends of a rod.

(Prior Art) A plurality of fuel assemblies loaded in the core of a conventional boiling water nuclear reactor accommodate a plurality of fuel rods arranged in a lattice shape in a rectangular cylindrical channel box.

These fuel rods have a plurality of stacked fuel pellets inserted into a cladding tube, and a lower end plug is welded and sealed to the upper end of the cladding tube, and a lower end plug is welded and sealed to the upper end of the rod.

Then, when this fuel assembly is assembled into the reactor core,
The structure is such that the coolant flows upward in the channel box from the bottom to the top. During this upward flow, the coolant is heated by the reaction heat of the nuclear fuel, and is then discharged from the upper part of the fuel assembly. The steam generated by this heating is guided to a turbine outside the reactor through piping and is used to drive the turbine.

By the way, the power distribution in the horizontal direction within the nuclear fuel assembly is not uniform, and in the periphery where the cooling water has a large neutron moderating effect, the thermal neutron flux is large, so the power is high.
In the center, the neutron moderation effect is small and the thermal neutron flux is small, so the output tends to be low.

Therefore, in the past, one or two fuel rods in the center were missing, and instead, water rods with approximately the same external shape as the fuel rods were loaded, and cooling water was configured to flow through the water rods for cooling. Efforts are being made to enhance the neutron moderating effect of water. In addition, the enrichment of nuclear fuel is lowered for the fuel rods in the periphery, and the enrichment of nuclear fuel is increased for the fuel rods in the center, thereby making the power distribution in the horizontal direction more uniform.

However, because conventional water rods have a small diameter, it is not possible to achieve a sufficiently uniform power distribution, and it is complicated to set a difference in nuclear fuel enrichment between the periphery and the center. was there.

Therefore, as shown in Figures 7 and 8, we used a large-diameter water rod whose outer diameter was approximately twice that of the fuel rods and whose accommodation space was equivalent to approximately four fuel rods. It has been proposed to further improve the distribution evenness and neutron moderation effect.

As shown in the figure, a nuclear fuel assembly 1 has a large number of fuel rods 3 arranged in a rectangular cylindrical channel box 2 in a square lattice of, for example, 8 rows and 8 columns. In addition to disposing a plug 4 and a lower end plug 5, an upper tie plate 6 is attached to the upper part thereof, and a lower tie plate 7 is attached to the lower part thereof. Further, a large diameter water rod 8 having a large outer diameter and occupying a space for accommodating four fuel rods 3 is disposed at the center of the group of fuel rods 3 .

As shown in FIG. 7, this large-diameter water rod 8 has a coolant inlet 11 and a coolant outlet 12 bored in its side wall. These large numbers of fuel rods 3 and large-diameter water rods 8 are aligned and supported by a plurality of spacers 9 arranged in the axial direction, and are elastically supported by expansion springs 10.

With the above configuration, the nuclear fuel assembly 1 employing the conventional large-diameter water rod 8 can have extremely excellent nuclear properties.

(Problems to be Solved by the Invention) In the above configuration, when the fuel assembly 1 employing the conventional large-diameter water rod 8 is equipped with the channel box 2, the lower tie plate 7 is a plate having a strong spring force. It is firmly pressed against and supported by the channel box 2 via a spring.

However, the spring force of the plate p decreases due to the irradiation effect. In addition, when the channel box 2 is installed, it is expanded outward by the leaf spring, and while maintaining this state, the temperature inside the furnace increases and it is exposed to irradiation, so the creep rate increases and the channel box 2 deforms further outward than when it is installed. , the gap with the lower tie plate 7 increases.

Like this, lower tie plate 7 and channel box 2
In a fuel assembly where a gap is formed between the lower tie plate 7 and the lower tie plate 7, if a certain acceleration is applied in the horizontal direction, assuming an earthquake, for example, the lower tie plate 7 will tilt by a certain angle within the channel box 2. .

On the other hand, the conventional large-diameter water rod 8 is fitted and supported by an upper tie plate 6 and a lower tie plate 7 at its upper and lower parts via an upper end plug 4 and a lower end plug 5, similar to the fuel rod 3. Along with being there.

In order to prevent the spacer 9 from detaching due to rotation of the water rod 8, rotation is prevented by incorporating a prismatic portion at the lower end.

In the structure of such a large-diameter water rod 8, stress analysis during an earthquake was performed using the finite element method, taking into account the above-mentioned inclination of the lower tie plate 7 within the channel box 2. The water rod 8 is about twice as thick as the outer diameter of the fuel rod 3, so it has a much stronger sealing property than a fuel rod, and the amount of displacement when subjected to external force is also much smaller than that of a fuel rod. It was something.

For this purpose, the spacer (especially the first spacer at the bottom) supported by the large diameter water rod 8.

A very large load (reaction force) is applied. Further, the lower end plug of the large-diameter water rod is also subjected to extremely large bending stress due to the inclination of the lower tie plate.

If the large-diameter water rod 8 is used in this way, the integrity of the fuel during an earthquake will deteriorate.

Further, as described above, since the outer diameter of the large diameter water rod 8 is approximately twice that of the fuel rod 3, the flow of the coolant flowing from the lower part of the fuel assembly 1 is directed to the lower part of the large diameter water rod 8. This caused a large amount of disturbance, disrupting the flow of coolant within the fuel assembly and causing a large pressure loss.

An object of the present invention is to provide a highly stable nuclear fuel assembly that reduces the load and stress that occur during an earthquake, reduces disturbance of coolant at the bottom of a large-diameter water rod, and has excellent combustion efficiency. .

Means for Solving Problem C) The nuclear fuel assembly of the present invention includes a cylindrical fuel channel, an upper tie plate and a lower tie plate attached to the upper and lower parts of the fuel channel, respectively, and a plurality of spacers installed at intervals along the axial direction; fuel rods penetrating through these spacers and supported at both ends by the upper and lower tie plates and arranged in a square lattice; and the fuel rods; In a nuclear fuel assembly having a large-diameter water rod that is supported in the same manner as above and occupies an arrangement space for four or more fuel rods, the lower end plug of the large-diameter water rod and the upper end plug of the large-diameter water rod are At least the lower end plug should be fixed at a position inside the pipe end of the water rod, that is, in the case of a lower end plug, the upper position in the axial direction, and in the case of a lower end plug, in the lower position in the axial direction. .

The upper end plug has a long shaft portion and is fitted and supported by the lower tie plate and the upper tie plate, respectively.

(Function) In the fuel assembly of the present invention, the large-diameter portion of the large-diameter water rod can cover the function of the water rod in the fuel effective portion during reactor power output, and even during an earthquake, the lower portion with low rigidity and The long narrow diameter portion of the upper end plug easily deflects and absorbs external forces, making it mechanically sound.

Further, since the water rod tube is not removed even in the long narrow diameter portion of the end plug, the gap between the water rod and the fuel rod does not change in the axial direction, and the flow of the coolant is not disturbed.

(Example) The present invention will be described below with reference to an embodiment shown in FIG.

In FIG. 1 showing an embodiment of the fuel assembly 1 according to the present invention, a plurality of fuels 4ii121 are housed in a lattice shape in a rectangular cylindrical channel box 20. At the center of these fuel rods 21, a large-diameter water rod 22, which is formed to have a diameter larger than that of each of these fuel rods 21, is arranged. An upper end plug 23 is provided at the upper end opening of these fuel rods 21.
A lower end plug 24 is welded to each of the lower end openings to seal them.

These upper end plugs 23 are supported by an H-section tie plate 26 with an expansion spring 25 attached to their outer peripheries, and the lower end plugs 23 are
4 is supported by a lower tie plate 28 having a coolant inlet 27 opened at its lower part. The axially intermediate portions of these fuel rods 21 are restrained in a bundle by a plurality of spacers 29 supported by a large-diameter water rod 22 .

The large-diameter water rod 22 is constructed as shown in FIG. It is formed by

An upper end plug 32 is provided at the upper end of this large diameter water rod 22.
However, lower end plugs 31 are fixed to the lower ends of the water rods 22, respectively, at positions inside the pipe ends of the water rods 22. In the embodiment shown in FIG. 2, an upper end plug 32 and a lower end plug 3
Although both of the plugs 1 and 1 are fixed at positions inside the pipe end of the water rod 22, it is also effective even if, for example, only the lower end plug 31 is fixed inside.

This upper end plug 32. A plurality of through holes 33 are provided in the bottom of the lower end plug 31 to allow coolant to flow within the water rod. Note that although the through hole 33 is circular in the case of FIG. 3, it may have any shape. and - upper end plug 32. The shaft portion 34 of the lower end plug 31 is connected to the water rod 22
The tube is made long enough not to interfere with the upper/lower tie plate, and the shank at the end of the end plug fits into the upper/lower tie plate.

The shank portion of the lower end plug 31 of the water rod 22 that is fitted into the support fitting square hole of the lower tie plate 28 is shaped like a square column, and prevents the water rod 22 from rotating. Water rod 2 with end plug 31.32
There are the following methods for fixing to 2.

■ Insert the end plugs 31, 32 into the tube of the water rod 22. Weld from the outside of the tube using spot welding SW. In this case, the inner diameter of the pipe and the outer diameter of the end plug flange should be approximately the same, and the end plug should be fitted 6 (Fig. 4)■ Water rod 22
After inserting the end plugs 31 and 32 into the tube, the end plugs are fixed by caulking M from the outside of the tube. (Fig. 5) ■ Cut the water rod 22 pipe and attach the end plugs 31°32
between the two tube ends, and the end plug and TIG
Weld by welding, EB welding, etc. (Fig. 6) The shaft portion 34 of the upper end plug 32 or the lower end plug 3 of the large-diameter water rod 22 made by these methods has a small diameter and is long compared to the outer diameter of the water rod 22, so it cannot be held horizontally during an earthquake. Even when acceleration is applied in this direction, the external force can be absorbed without generating excessive stress. That is, the rigidity of the shaft portion 34 of the end plug 31, 32 is made smaller than the rigidity of the large diameter portion of the water rod.

It is formed to be flexible. As a result, the end plug 31°3
2. No large stress is applied to the spacer portion 29, and the mechanical integrity of the fuel can be maintained.

The outer surface of the water rod 22 is located near the lower tie plate 28, and the fuel rod 21 and water rod 22
The gap between the two is the same in the axial direction. Therefore, it is possible to prevent the flow of coolant in the horizontal direction (cross flow) that would occur if the gaps are different, and vibration of the fuel rods caused by the cross flow can be prevented, so that the integrity of the fuel can be maintained.

In addition, the bottom of the lower end plug 31 of the large diameter water rod 22 (
The horizontal portion) is provided with a through hole 33 through which cooling water flows, reducing pressure loss.

There is little disturbance in the flow of coolant. Furthermore, since the large diameter portion of the water rod 22 extends over the entire length of the fuel, it is possible to achieve uniform power distribution over the entire length of the fuel, which is the purpose of the water rod, and it is possible to have very excellent nuclear properties. Thereby, combustion efficiency can be improved.

〔Effect of the invention〕

As explained above, in the nuclear fuel assembly of the present invention, at least the lower end plug of the lower end plug and the upper end plug of the large-diameter water rod is fixed to a position inside the tube end of the water rod, and the end plug axis By increasing the length of the section so that it extends beyond the pipe end of the water rod, the load and stress generated during an earthquake are reduced, and the disturbance of the coolant at the bottom of the large-diameter water rod V! - can be reduced and fuel integrity can be maintained. Additionally, it provides a fuel assembly with excellent combustion efficiency6.

[Brief explanation of the drawing]

FIG. 1 shows the entire 4ff of an embodiment of the fuel assembly of the present invention.
The second section is a vertical cross-sectional view showing the large-diameter water rod used in the present invention with the axial position cut away, and FIG. 3 is a cross-sectional view taken along line 11-111 in FIG. , 4th
5 and 6 are cross-sectional views showing different embodiments of the end plug fixing structure for a large-diameter water rod of the present invention,
Figure 7 shows the longitudinal cross-section L of the combined fuel part incorporating a conventional large-diameter water rod! 21, FIG. 8 is a longitudinal sectional view of the large diameter waterlot shown in FIG. 7. 1...Fuel assembly 10...Channel box 2
1... Fuel rod 22... Large diameter water rod'
13... Upper end plug 24... Lower end plug 26.
... Upper tie plate 28 ... Lower tie plate 29 ... Spacer 31
...Large diameter lower end plug 32...Large diameter upper end plug 33...
・Coolant inlet 34...End plug small diameter shaft Agent Patent attorney Yoshiaki Inomata (one person) Figure I Figure 2 Figure 4M $5 Far-gloss i 1 [] []] ] Rollo 1 eye 1 i'' Figure Figure 8

Claims (3)

[Claims] (1) A cylindrical fuel channel, an upper tie plate and a lower tie plate fitted to the upper and lower parts of the fuel channel, respectively, and a plurality of tie plates installed at intervals along the axial direction inside the fuel channel. spacers, fuel rods that penetrate these spacers and are supported at both ends by the upper and lower tie plates and are arranged in a square lattice; and four or more fuel rods that are supported in the same way as the fuel rods and are In a nuclear fuel assembly having large-diameter water rods occupying an arrangement space, at least the lower end plug of the lower end plug and upper end plug of the large-diameter water rod is fixed to a position inside the pipe end of the water rod. With,
A nuclear fuel assembly characterized in that the length of the shaft of the end plug of the large-diameter water rod is made longer so that the portion of the end plug that fits with the tie plate protrudes beyond the pipe end of the water rod. . (2) The nuclear fuel assembly according to claim 1, wherein the upper end plug and the lower end plug of the large-diameter water rod are fixed inside the pipe end of the water rod. (3) The fuel assembly according to claim 1, wherein the tube of the large diameter water rod and the end plug are fixed by welding or caulking.