JPS62134593A - 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 [Industrial Application Field] The present invention relates to a nuclear fuel assembly used in a boiling water reactor, and in particular to a nuclear fuel assembly characterized by a water rod disposed in the center of the nuclear fuel assembly. Concerning aggregates.

[Conventional technology]

Conventionally, nuclear fuel assemblies used in boiling water reactors have a cladding tube filled with uranium dioxide (UO3) pellets, a lower end plug provided directly at the lower end, and a blennium spring plug provided above. After that, a large number of fuel rods, each of which is welded and sealed, are inserted through the tube. At this time, the cooling water is heated by the heat of reaction 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, 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. In addition to increasing the neutron moderating effect of water, the enrichment of nuclear fuel is lowered in the peripheral fuel rods, and the enrichment of nuclear fuel is further increased in the central fuel rods in order to equalize the horizontal power distribution. That's what I do.

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 4 and 5, a large-diameter water rod whose outer diameter is approximately twice that of the fuel rods and whose housing space is equivalent to approximately four fuel rods is used to increase the output power. 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 . These large numbers of fuel rods 3 and large diameter water rods 8 are separated by a plurality of spacers 9 arranged in the axial direction.
It is aligned and supported by.

10 is an expansion spring.

The lower part of the fuel assembly 1 is supported in pressure contact with the channel box 2 via plate springs 13 attached to the four upper side surfaces of the 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 stopper member 15 at the upper end of the channel box 2 using a channel fastener. It is fixed to the channel box 2 by tightening via 16.

FIG. 5 shows a large-diameter water rod 8, in which a coolant inlet 1 and a coolant outlet 12 are bored in the side wall.

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

[Problem to be Solved by the Invention 1] In the above configuration, when the fuel assembly employing the conventional large-diameter water rod is equipped with a channel box, the lower tie plate does not move through the leaf spring with strong spring force. It is firmly press-fitted and supported with the channel box.

However, the spring force of the plate spring decreases due to the irradiation effect. In addition, since the axial elongation of the fuel assembly and fuel rods due to irradiation growth is larger than the axial elongation due to irradiation growth of the channel box, the channel box is pulled upwards, and this causes the lower part type when the channel box is installed. The amount of fitting between the rate and the channel box will decrease.

In addition, when the channel box is installed (it is expanded outward by the leaf spring, and while maintaining this state, it is exposed to an increase in the temperature inside the furnace and irradiation, so the creep rate increases and it deforms further outward than when it is installed). However, the gap with the lower tie plate increases.

In a fuel assembly in which a gap is formed between the lower tie plate and the channel box, for example, when a certain acceleration is applied in the horizontal direction, assuming an earthquake, as shown in Figure 6. , the lower tie plate 7 is tilted within the channel box 2 by an angle O.

On the other hand, conventional large diameter water rods, like fuel rods,
The upper and lower parts are fitted and supported by the upper tie plate and the lower tie plate via the upper end plug and the lower end plug, and the prismatic part at the lower end is fitted to prevent the spacer from coming off due to rotation of the water rod. This prevents rotation.

In the structure of such a large-diameter water rod, stress analysis due to an earthquake was performed using the finite element method, taking into account the above-mentioned inclination of the lower tie plate within the channel box.
As a result, the large-diameter water rod is approximately twice the outer diameter of the fuel rod, so it is extremely rigid compared to the fuel rod, and the amount of displacement when subjected to external force is also smaller than that of the fuel rod. It was very small.

Therefore, as shown in FIG. 7, a very large load (reaction force) is applied to the spacer (particularly the first spacer at the bottom) supported by the large-diameter water rod. Also,
As shown in FIG. 8, the lower end plug of a large-diameter water rod is also subjected to extremely large bending stress due to the inclination of the lower tie plate.

In addition, as mentioned above, the outer diameter of the large-diameter water rod is approximately twice that of the fuel rod, so the flow of coolant flowing from the bottom of the fuel assembly has a large disturbance effect at the bottom of the large-diameter water rod. This disrupted the flow of coolant within the fuel assembly and caused a large pressure loss.

Additionally, it is known that water rods elongate less in the axial direction than fuel rods when the power of a nuclear reactor increases, and if the length of the shank portion is insufficient, there is a risk of it separating from the lower tie plate support cavity. There is sex. For this reason, the prior art employs lower end plugs with sufficient shank length so that the shank portion of the lower end plug protrudes from the lower tie plate support cavity. In addition, friction or collision occurs between the lower tie plate support cavity and the shank due to the hydraulic vibration of the cooling water flowing into the water rod from below, which causes wear of the support cavity and the shank to progress and the play to increase. , the vibration of the water rod becomes even more intense,
In the end, there was a risk that the shank would break and the reactor would have to be shut down.

The present invention has been made in consideration of these points,
It simultaneously solves the problem of the water rod separating from the lower tie plate and the problem of vibration caused by cooling water.It also reduces the load and stress that occurs during an earthquake, and prevents the disturbance of the coolant at the bottom of the large diameter water rod. The purpose is to provide a highly stable nuclear fuel assembly with excellent combustion efficiency and reduced upper pressure loss.

[Means for solving interlayer points]

The nuclear fuel assembly of the present invention includes a cylindrical fuel channel, an upper tie plate and a lower tie plate that are respectively fitted to the upper and lower parts of the fuel channel, 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
In a nuclear fuel assembly having a large-diameter water rod that occupies an arrangement space for four or more fuel rods, the diameter of the large-diameter water rod is formed to vary depending on the axial position,
A threaded portion is provided in the lower end plug of the large diameter water rod,
It is characterized in that a rotation prevention mechanism is formed on the upper tie plate at a position facing the large diameter water rod and the upper end plug of the large diameter forter rod.

[For production]

With the fuel assembly configured as described above, the function of the water rod in the large-diameter part of the large-diameter part of the large-diameter part of the water rod during the reactor's power output can be controlled. Since the lower and upper narrow diameter portions, which have low rigidity, are easily bent and absorb external forces, they can be made mechanically sound. Furthermore, when the reactor power increases, even if the fuel rod extends in the axial direction beyond the large-diameter water rod, the threaded part of the lower end plug is connected to the lower tie plate.
Due to the anti-rotation mechanism of the upper end plug and the upper tie plate, there is no fear that the threaded portion of the lower end plug will loosen, and the water rod can be prevented from coming off from the lower tie plate.

〔Example〕

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. 4 is that they are formed as large-diameter warps with different outer diameters in the axial direction. Furthermore, upper and lower tie plates 6.7 with these narrow diameter portions 8b+8c
The method of connection is that the upper end plug 1 has a prismatic shank shape.
7, the lower part is fitted with the respective tie plate 6.7 by means of a threaded shank-shaped lower end plug 18.

To explain further, the position and length of the large-diameter portion of the large-diameter water rod 8a are determined on the condition that the portion covers the fuel effective portion throughout the life of the fuel rod 3. However, since the starting point from the lower narrow diameter portion 8C to the large diameter portion is originally a portion where the coolant is not boiling, it is not fundamentally important, so this position is determined from the viewpoint of mechanical performance.

That is, the length is set such that the inclination of the lower tie plate 7 during an earthquake can be absorbed without causing excessive stress/strain. In other words, the rigidity of the narrow diameter portion 8c is made smaller than the rigidity of the large diameter portion, making it easy to bend.

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 water rod 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, in the case where the effective fuel length is 3708 m+a, the length of the lower narrow diameter portion 8c is approximately 381 mm, and the length of the large diameter portion is approximately 33780111 mm.
Even after accounting for changes in fuel availability during irradiation, throughout its lifetime,
The large diameter portion can cover the fuel effective portion.

Furthermore, even in the event of an earthquake, the lower narrow diameter portion 8c and the upper narrow diameter portion 8b, which have low rigidity, are easily bent and absorb external forces, so that they are mechanically sound. Furthermore, the flow of coolant at the lower narrow diameter portion 80 is not disturbed. Furthermore, since a larger amount of steam can flow through the upper narrow diameter portion 8b compared to a conventional large diameter water rod, pressure loss can also be reduced.

Furthermore, even if the fuel rods 3 extend in the axial direction from the large-diameter water rods 8a due to irradiation growth when the reactor power increases, the threaded portion of the lower end plug 18 is firmly connected to the lower tie plate 7, so that separation will not occur. There's no fear. Furthermore, the upper end plug 1
7 and the fitting hole of the upper tie plate 6 have a rectangular cross section, so that the large diameter water rod 8a has a rotation prevention function.
This can prevent the screws from loosening at the bottom of the

In addition, as shown in FIG.
The flow of the coolant within the large-diameter water rod 8a may be adjusted by drilling in the large-diameter water rod 8a.

Further, in the present invention, the shank portion of the upper end stopper 17 is described as a square column, but it is sufficient that it has a function of preventing rotation. For example, a configuration in which a section of the upper end plug 17 is partially cut out may also have the same effect as the present invention. can be obtained.

In this way, the nuclear fuel assembly according to this embodiment improves nuclear performance by increasing the diameter of the large-diameter water rod 8a at the position corresponding to the effective fuel part, and by making the lower part smaller in diameter, it can be used in the event of an earthquake. The strain caused by the inclination of the lower tie breakers 1 to 7 can be absorbed, and the load from the water rod 8a to the spacer 9 can be significantly reduced. In other words, mechanical soundness can be significantly improved. Moreover, by making the upper part small in diameter, the pressure loss of the coolant can be reduced compared to the conventional large-diameter water rod 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 is caused by the gradual increase in diameter from the small-diameter part to the large-diameter part, and the coupling between the small-diameter part and the large-diameter part. can be significantly reduced by using the tapered connector 22.

〔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, reduce the disturbance of the coolant at the lower part of the large-diameter water rod, and further reduce the pressure loss at the upper part. In addition, it has excellent combustion efficiency and stability. Furthermore, it is possible to prevent the water rod from separating from the lower tie plate and reduce the vibration of the water rod.

[Brief explanation of drawings]

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 whole, FIG. 2 is a side view of a large-diameter water rod, and FIG. 3 is another example. Fig. 4 and Fig. 5 are similar views to Fig. 1 and Fig. 2 showing the conventional example, and Fig. 6 is a simulation showing an inclination angle of 0 for the lower tie plate. 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.
FIG. 2 is a characteristic diagram of the amount of increase in stress acting on the lower part of the water rod. 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 part, 8c... Lower narrow diameter part, 9... Spacer, 17... Soil end plug, 18... Lower end plug 9 Fig. 1 Fig. 2 Fig. 3 Fig. 4 Figure 6〕Bottom + n fidelate house! Self (deg) Figure 7

Claims (5)

[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, the diameter of the large-diameter water rods is formed to vary depending on the axial position, and a threaded portion is provided at a lower end plug of the large-diameter water rods, A nuclear fuel assembly characterized in that a rotation prevention mechanism is formed on an upper tie plate at a position facing the large-diameter water rod and an upper end plug of the large-diameter water rod. (2) The large diameter water rod is characterized in that the portion corresponding to the fuel effective portion of the fuel rod is formed to have a large diameter, and the other portions are formed to be thinner than the large diameter portion. nuclear fuel assembly. (3) The nuclear fuel assembly according to claim 1, wherein the large-diameter water rod is formed so that its upper and lower portions are thinner than its central portion. (4) The nuclear fuel assembly according to claim 1, wherein the large-diameter water rod has a small-diameter portion having a smaller rigidity than a large-diameter portion. (5) A patent claim characterized in that the rotation prevention mechanism consists of a notch formed in the upper end plug of the large diameter water rod and an upper tie plate that fits into the upper end plug of the large diameter water rod. The nuclear fuel assembly according to item 1.