JPH0566553B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to a nuclear fuel assembly used in a boiling water nuclear reactor, and more particularly to a nuclear fuel assembly characterized by a water rod disposed in the center of the nuclear fuel assembly. Regarding.

[Technical background of the invention and its problems]

Conventionally, nuclear fuel assemblies used in boiling water reactors have a cladding tube filled with uranium dioxide (UO2) pellets, a lower end plug installed directly at the lower end, and a plenum spring plug installed above. It is constructed using a large number of fuel rods, each of which is welded and sealed. The interior of the nuclear fuel assembly configured as described above is configured such that cooling water flows from the bottom to the top. At this time, the cooling water is heated by the reaction heat of the nuclear fuel and is discharged from the upper part of the nuclear fuel assembly. The steam generated by this heating is sent to the outside of the pressure vessel through piping and is used to drive a turbine or the like.

By the way, the power distribution in the horizontal direction within the nuclear fuel assembly is not uniform; in the periphery, where the neutron moderating effect of the cooling water is large, the output is high due to the large thermal neutron flux, while in the center, the neutron moderating effect is large. Since the thermal neutron flux is small and the thermal neutron flux is small, the output tends to be low.

Therefore, in the past, one or two fuel rods in the center were missing, and in their place, water rods with approximately the same outer diameter as the fuel rods were loaded, and cooling water was configured to flow through the water rods. In addition to increasing the neutron moderation effect of the fuel rods, the enrichment of nuclear fuel is lowered for the fuel rods in the periphery, and the enrichment of nuclear fuel is increased for the central fuel rods in order to equalize the power distribution in the horizontal direction. I have to.

However, because conventional water rods had small diameters, the output distribution could not be made sufficiently uniform.
There was also the complexity of having to set a difference in the enrichment level of nuclear fuel between the periphery and the center.

Therefore, as shown in Figures 4 and 5, a large-diameter water rod, whose outer diameter is approximately twice that of the fuel rods and whose accommodation space is equivalent to approximately four fuel rods, is used to provide power distribution. It has been proposed to further improve the uniformity of neutrons and the neutron moderation effect.

As shown in the figure, a nuclear fuel assembly 1 includes a large number of fuel rods 3 in a rectangular cylindrical channel box 2.
For example, they are arranged in a square grid of 8 rows and 8 columns, and upper end plugs 4 and lower end plugs 5 are arranged at the upper and lower parts of the square grid, respectively. It is configured by attaching a rate 7. Further, a large diameter water rod 8 having a large outer diameter and occupying a space for accommodating four fuel rods 3 is arranged in the center of the group of fuel rods 3. 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.
10 is an expansion spring.

The lower part of the fuel assembly 1 is supported in pressure contact with the channel box 2 via leaf springs 13 attached to the four upper side surfaces of a lower tie plate 7 attached to the lower part. In addition, the upper part of the fuel assembly 1 is connected to one of the four posts 14 protruding from the four corners of the upper surface of the upper tie plate 6, and a triangular stop member 15 at the upper end of the channel box 2. It is fixed to the channel box 2 by tightening through the fasteners 16.

FIG. 5 shows a large-diameter water rod 8, in which a coolant inlet 11 and a coolant outlet 12 are bored in the side wall.

Although the nuclear fuel assembly 1 employing the conventional large-diameter water rod 8 described above has very excellent nuclear properties, it has problems in terms of mechanical performance and coolant pressure loss during earthquakes.

That is, as mentioned above, when the channel box 2 is installed, the lower tie plate 7
is firmly pressed against and supported by the channel box 2 via a leaf spring 13 having a strong spring force.

However, the spring force of the leaf spring 13 decreases due to the irradiation effect. Furthermore, since the axial elongation of the fuel assembly 1 and the fuel rods 3 due to irradiation growth is larger than the axial elongation due to irradiation growth of the channel box 2, the channel box 2 is pulled upward. In this case, the amount of engagement between the lower tie plate 7 and the channel box 2 is reduced.

Furthermore, when the channel box 2 is installed, it is expanded outward by the leaf spring 13, and while maintaining this state, the temperature inside the furnace increases and the channel box 2 is exposed to irradiation, so the creep rate increases and the box expands further outward than when it is installed. This results in deformation and the gap with the lower tie plate 7 increases.

In the fuel assembly 1 in which a gap is formed between the lower tie plate 7 and the channel box 2, for example, when a certain acceleration is applied in the horizontal direction assuming an earthquake, the sixth
As shown, the lower tie plate 7 is tilted within the channel box 2 by an angle θ.

On the other hand, like the fuel rod 3, the conventional large-diameter water rod 8 has its upper and lower portions fitted and supported by an upper tie plate 6 and a lower tie plate 7 via an upper end plug 4 and a lower end plug 5. At the same time, in order to prevent the spacer 9 from coming off due to the rotation of the water rod 8, rotation is prevented by fitting the 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 consideration the above-mentioned inclination of the lower tie plate 7 within the channel box 2. Since the water rod 8 is approximately twice as thick as the outer diameter of the fuel rod 3, it is much more rigid than the fuel rod 3, and its displacement when subjected to external force is also much smaller than that of the fuel rod 3. It was small.

For this purpose, the spacer 9 (especially the first spacer at the bottom) is supported by the large-diameter water rod 8.
As shown in FIG. 7, a very large load (reaction force) is applied. In addition, large diameter water rod 8
As shown in FIG. 8, the lower end plug 5 is also subjected to a very large bending stress due to the inclination of the lower tie plate 7.

In addition, as mentioned above, the large diameter water rod 8
Since the outer diameter of the large-diameter water rods 8 is approximately twice that of the fuel rods 3, the flow of coolant flowing from the lower part of the fuel assembly 1 is subjected to a large disturbance at the lower part of the large-diameter water rod 8, and the flow of coolant inside the fuel assembly 1 is greatly disturbed. This not only disturbed the flow of coolant but also caused a large pressure loss.

[Purpose of the invention]

The invention was made with these points in mind, and it reduces the load and stress that occur during an earthquake, reduces the disturbance of the coolant in the lower part of a large-diameter water rod, and further reduces the pressure loss in the upper part. The purpose is to provide a nuclear fuel assembly with excellent fuel efficiency and high guideability.

[Summary of the invention]

The nuclear fuel assembly of the present invention includes a cylindrical fuel channel, an upper tie plate and a lower tie plate that are fitted to the upper and lower parts of the fuel channel, respectively, and are installed at intervals along the axial direction inside the fuel channel. a plurality of spacers, fuel rods extending through these spacers and supported at both ends by the upper and lower tie plates and arranged in a square lattice pattern, supported in the same manner as the fuel rods, and
A nuclear fuel assembly having a large-diameter waterrod that occupies an arrangement space for four or more fuel rods is characterized in that the diameter of the large-diameter waterrod is narrower at its upper and lower portions than at its center.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIGS. 1 to 3.

1 and 2 are side sectional views showing one embodiment of a nuclear fuel assembly according to the present invention, and FIG. 2 is a side view of a large diameter water rod.

The main difference between the nuclear fuel assembly shown in FIG. 1 and the conventional nuclear fuel assembly shown in FIG. 8b and the lower narrow diameter portion 8c are formed with different outer diameters in the axial direction. The method of connecting these narrow diameter portions 8b, 8c with the upper and lower tie plates 6, 7 is the same as in the conventional method, with the upper end plug 4 having the shape of a round bar shank, and the lower part having the shape of a square bar shank. The lower end plugs 5 are fitted into respective tie plates 6 and 7.

Further, a coolant outlet 12 and a coolant inlet 11 are provided in the upper and lower parts of the large-diameter water rod 8a, respectively.

To explain further, this large diameter water rod 8a
The position and length of the large diameter portion are determined on the condition that the portion covers the fuel effective portion throughout the life of the fuel rod 3. However, the problem from the lower narrow diameter part 8c to the large diameter part is not fundamentally important because the coolant is not boiling in the first place, so this position is determined from the viewpoint of mechanical performance. . In other words, the lower tie plate 7 during an earthquake
The length is such that the slope can be absorbed without excessive stress/strain. In other words, the stiffness of the narrow diameter portion 8 is made smaller than that of the large diameter portion, so that it is easily bent.

On the other hand, the longer the length of the lower narrow diameter portion 8c, the smaller the stress/strain, but the diameter needs to be large above the boiling start point.

Further, the boundary position between the upper narrow diameter portion 8b and the large diameter portion 8a is determined on the condition that it must always be located above the fuel effective portion throughout the life. However, from the viewpoint of reducing pressure loss, the smaller the diameter, the smaller the pressure loss, so it is better to make the large diameter part as short as possible.

Next, the operation of this embodiment will be explained.

For example, when the effective fuel length is 3708 mm, in an example in which the length of the lower narrow diameter portion 8c is approximately 381 mm and the length of the large diameter portion is approximately 3378 mm, changes in the effective fuel length during irradiation are considered. However, the large diameter portion can cover the fuel effective portion throughout the life of the fuel. In addition, even in the event of an earthquake, the lower narrow diameter part 8 with low rigidity
c and the upper and lower narrow diameter portions 8b easily bend and absorb external forces, so it is mechanically sound. Furthermore, the coolant flow at the lower narrow diameter portion 8c is not disturbed. Furthermore, since a larger amount of steam can flow through the upper narrow diameter portion 8b compared to the conventional large diameter water rod, pressure loss can also be reduced.

Note that, as shown in FIG.
It is also possible to adjust the flow of the coolant in the large-diameter water rod 8a by drilling the hole in the large-diameter water rod 8a.

In this way, the nuclear fuel assembly according to this example is
Nuclear performance is improved by making the large diameter waterrod 8a large in the position corresponding to the fuel effective part, and by making the lower part small in diameter, it absorbs the strain caused by the tilt of the lower tie plate 7 during an earthquake, and the waterrod The load from 8a to spacer 9 can be significantly reduced. In other words, mechanical soundness can be significantly improved. Furthermore, by making the upper part smaller in diameter, the pressure loss of the coolant can be reduced compared to the conventional large-diameter waterrod 8, and core stability can be improved. Furthermore, the disturbance of the coolant at the bottom of the water rod that occurred in the conventional large diameter water rod 8 has been reduced by the fact that the diameter gradually increases from the small diameter part to the large diameter part, and the connection between the small diameter part and the large diameter part is tapered. By using a connector with 22, the cost can be significantly reduced.

〔Effect of the invention〕

As explained above, the nuclear fuel assembly of the present invention can reduce the load and stress generated during an earthquake, and can reduce the disturbance of the coolant in the lower part of the large diameter water rod. Furthermore, the pressure loss in the upper part is reduced, the combustion efficiency is excellent, and the stability is also high.

[Brief explanation of the drawing]

1 and 2 show one embodiment of the nuclear fuel assembly of the present invention, FIG. 1 is a partially cutaway side view of the entire assembly, FIG. 2 is a side view of a large diameter water rod, and FIG. 3 is a side view of a large diameter water rod.
The figure is a side view of a large-diameter water rod showing another embodiment, Figures 4 and 5 are similar views to Figures 1 and 2, showing a conventional example, and Figure 6 shows the angle of inclination θ of the lower tie plate. Fig. 7 is a characteristic diagram of the increase in the load acting on the first spacer depending on the inclination angle θ of the lower tie plate, and Fig. 8 is a characteristic diagram of the increase in stress acting on the lower part of the water rod depending on the inclination angle θ of the lower tie plate. It is a characteristic diagram. 1...Fuel assembly, 2...Channel box, 3...Fuel rod, 6...Upper tie plate, 7
...lower tie plate, 8a...large diameter water rod, 8b...upper narrow diameter section, 8c...lower narrow diameter section, 9...spacer.

Claims (1)

[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 fuel channel installed at intervals along the axial direction inside the fuel channel. a plurality of spacers, fuel rods extending through these spacers and supported at both ends by the upper and lower tie plates and arranged in a square lattice pattern; 1. A nuclear fuel assembly having a large-diameter water rod that occupies an arrangement space for fuel rods, wherein the large-diameter water rod is formed so that its upper and lower portions are narrower than its central portion. 2. The nuclear fuel assembly according to claim 1, wherein the large-diameter water rod has a large diameter at a portion corresponding to the fuel effective portion of the fuel rod, and a smaller diameter at other portions than the large diameter portion. body. 3. The nuclear fuel assembly according to claim 1, wherein the large-diameter water rod is formed such that the stiffness of the small-diameter portion thereof is smaller than that of the large-diameter portion.