JPS62291594A - 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 into the core of a boiling water reactor, and particularly relates to a fuel assembly loaded in the core of a boiling water reactor. This invention relates to a fuel assembly with an improved 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 the r1 tube, and a lower end plug is welded and sealed to the upper end of the rod, 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 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. In addition to increasing the neutron moderating effect of water, lowering the enrichment of nuclear fuel in the peripheral fuel rods and increasing the am content of the nuclear fuel in the central fuel rods 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 peripheral and central areas. was there.

Therefore, as shown in Figures 3 and 4, we used a large-diameter water rod whose outer diameter was approximately twice that of the fuel rods and whose accommodation space area was equivalent to approximately four fuel rods. ,
It has been proposed to further improve the equalization of the power distribution and the neutron moderation effect. In FIGS. 3 and 4, 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.

An upper end plug 4 and a lower end plug 5 are disposed at the upper and lower parts of the pipe, respectively, and an upper tie plate 6 and a lower tie plate 7 are mounted on the upper part and the lower part, respectively.

Further, in the center of the group of fuel rods 3, there is a large diameter water rod 8 with a large outer diameter that occupies a space for accommodating four fuel rods 3.
is located. 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 vertical direction, and are further elastically supported by expansion springs 10. Furthermore, the large-diameter water rod 8 has a coolant inlet 11 at the upper part of the side wall and a coolant outlet 1 at the lower part, as shown in FIG.
The nuclear fuel assembly 1 employing the conventional large-diameter water rod 8 with the above configuration in which the water rods 2 are perforated can have extremely excellent nuclear properties.

(Problems to be Solved by the Invention) Hereinafter, in the second configuration, the conventional large diameter water rod 8
In the fuel assembly 1 employing the above, when the channel box 2 is installed, the lower tie plate is firmly supported by pressure contact with the channel box 2 via a leaf spring having a strong spring force.

However, the spring force of the leaf spring 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 lower part is typed and the gap between -1 and 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, like the fuel rod 3, the conventional large diameter water rod 8 has its upper and lower parts 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. In order to prevent the spacer 9 from coming off due to rotation of the water rod, 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 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 is much more rigid than the fuel rod 3, and the amount of displacement when subjected to external force is also much smaller than that of the fuel rod. It was small.

Therefore, a very large load (reaction force) is applied to the spacer (especially the first spacer located at the bottom) supported by the large diameter water rod 8. Further, the lower end plug 8A of the large-diameter water rod is also subjected to extremely large bending stress due to the inclination of the lower tie plate 7.

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

An object of the present invention is to provide a highly stable nuclear fuel assembly that reduces the load and stress that occur during earthquakes and has excellent combustion efficiency.

[Structure of the invention]

(Means for Solving the Problems) The nuclear fuel assembly of the present invention includes 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 spacers installed at intervals along the axial direction, fuel rods penetrating these spacers and supported at both ends by the upper and lower tie plates and arranged in a square lattice shape; supported in the same way, and
In a nuclear fuel assembly having a large-diameter water rod that occupies an arrangement space for four or more fuel rods, on the upper and lower tube side portions of the large-diameter water rod.

A cooling water inlet and a cooling water outlet formed by arranging a plurality of long holes in parallel are provided.

(Function) In the fuel assembly according to 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 the reactor power output, and even during an earthquake, the lower portion with low rigidity and The upper section where a large number of through holes for coolant inflow and outflow are connected, or the spiral groove easily bends and absorbs external forces, making it mechanically sound.

(Embodiment) The present invention will be described below with reference to an embodiment shown in FIGS. 1 and 2. In FIG. 1 showing a fuel assembly 50 of the present invention, a plurality of fuel rods 21 are housed in a grid pattern. These fuel rods 2
A large-diameter water rod 22 having a diameter larger than that of each fuel rod 21 is disposed in the center of the fuel rod 1 . A lower end plug 23 is welded to the upper end opening of these fuel rods 21, and a lower end plug 24 is welded to the lower end opening, respectively, for sealing.

These upper end plugs 23 are supported by an upper tie plate 26 with expansion springs 25 attached to their outer peripheries, and the lower end plugs 23 are supported by an upper tie plate 26.
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 large-diameter water rods 22 .

The large-diameter water rod 22 is configured as shown in FIG. It is formed by A plurality of elongated holes are formed in parallel on the tube side surface near the fixed part of the upper end plug 32 at the upper end of the large-diameter water rod 22 and near the fixed part of the lower end plug 31 at the lower end. Inlet 33a and outlet 33
b is provided.

FIG. 2(a) shows an embodiment of an inlet 33a and an outlet 33b, in which a plurality of through holes are connected to each other. The inlet 33a and outlet 33b portions formed by connecting the through holes in this manner have a smaller cladding tube area in the cross section of the tube, and have lower rigidity than the portions without holes.

Since the rigidity is reduced, when a horizontal force is applied,
This part is flexible. Therefore, even if acceleration is applied in the horizontal direction during an earthquake, this portion flexes and can absorb the external force without generating excessive stress.

As a result, large stress is not applied to the end plugs and spacers, which would otherwise be subject to large stress and damage to the integrity of the large diameter water rod 22 when the deflection is small, thereby preventing damage and improving the mechanical integrity of the fuel. can be maintained.

FIG. 2(b) shows another embodiment of the inlet 33a and outlet 33b according to the present invention, in which one through hole is formed with parallel spiral grooves. The rigidity is further reduced than in the case of FIG. 2(a) in which a plurality of through holes are connected.

Due to this change in the shape of the through hole, there is little change in the flow of coolant inside the water rod, and since the large diameter part of the water rod spans the entire length of the fuel, it is possible to achieve a uniform power distribution, which is the purpose of the water rod. This can be achieved over the entire length of the fuel and can have very good nuclear properties. Thereby, fuel efficiency can be improved.

In addition, in the present invention, the soundness during an earthquake is improved by changing the rigidity of the water rod in the axial direction, so the gaps between the water rods and fuel rods are all the same in the axial direction, and one gap is different. There is no horizontal coolant flow (cross flow) that would otherwise occur, and vibration of the fuel rods can be prevented, so the integrity of the fuel rods can be maintained.

〔Effect of the invention〕

As explained above, the nuclear fuel assembly of the present invention.

By providing a cooling water inlet and a cooling water outlet with multiple long holes arranged in parallel on the upper and lower tube sides of the large diameter water rod, the load and stress generated during an earthquake can be reduced, and the large diameter water rod Disturbance of the coolant at the bottom of the fuel tank can be reduced, and fuel integrity can be maintained. Moreover, the fuel assembly has excellent combustion efficiency.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view showing the overall configuration of one embodiment of the fuel assembly according to the present invention, and FIGS. 2 a and b are configuration diagrams showing different embodiments of the large-diameter water rod shown in FIG. 1. , FIG. 3 is a vertical sectional view showing a fuel assembly incorporating a conventional large-diameter water rod, and FIG. 4 is a configuration diagram showing the large-diameter water rod shown in FIG. 3. 20...Channel box 21...Fuel rod 22
... Large diameter water rod 23 ... 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 33a...
・Coolant inlet 33b...Coolant outlet 50...
Fuel assembly agent Patent attorney Yoshiaki Inomata (and one other person) Marriage Iooo day [
][]L. 10 Rorororororo l Figure 1 (03 (b3 Figure 2 Figure 3 A certain figure 4

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, a cooling water inlet and a cooling water flow are formed by arranging a plurality of long holes in parallel on the upper and lower pipe side surfaces of the large-diameter water rods. A nuclear fuel assembly characterized by having an outlet. (2) The plurality of elongated holes forming the cooling water inlet and the cooling water outlet of the large-diameter water rod are formed by interconnecting a plurality of through holes. nuclear fuel assembly. (3) The nuclear fuel assembly according to claim 1, wherein the plurality of long holes forming the cooling water inlet and the cooling water outlet of the large diameter water rod are formed into a plurality of spiral grooves. body.