JPH0766069B2 - Reactor fuel assembly - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reactor fuel assembly, and more particularly to a reactor fuel assembly suitable for improving cooling efficiency of fuel rods.

[Conventional technology]

The core of a boiling water reactor is composed of control rods that are inserted between fuel assemblies arranged at regular intervals.

As shown in FIG. 4, a fuel assembly used in a boiling water reactor has a plurality of fuel rods 2, a spacer 3 for keeping a constant horizontal interval between the fuel rods 2, a fuel rod 2, and a water rod 1. Of the upper tie plate 7 and the lower tie plate 8 for supporting and maintaining a constant interval.

Water as a coolant flows in the fuel assembly having such a structure, and this water removes heat energy generated by the reaction of the fissile material such as uranium 235 existing in the fuel rod 4. Water also flows into the gaps between the fuel assemblies outside the channel box 9.

On the other hand, water also has a function as a moderator of neutrons,
High-energy fast neutrons generated by nuclear fission are decelerated by water, which is a moderator, to become low-energy thermal neutrons. When this thermal neutron is absorbed by the fissile material in the fuel rod 4, for example, uranium 235, it causes a fission reaction to generate energy.

In the core of a boiling water reactor with such a function,
Economical fuel development is needed. The fuel economy can be improved by increasing the burnup of the fuel. In order to increase the burnup of the fuel, the enrichment of uranium 235 existing in the fuel rod 2 may be increased, but if the enrichment is increased without increasing the water-uranium ratio, the neutron spectrum will be hardened. Therefore, the infinite multiplication factor of the fuel assembly does not become the maximum value of the infinite multiplication factor in the enrichment.

FIG. 5 shows how the relationship between the water to uranium ratio and the infinite multiplication factor changes as the fuel enrichment increases. As shown in FIG. 5, in order to obtain as large an infinite multiplication factor as possible at a constant enrichment, it is necessary to realize an optimum water-uranium ratio corresponding to the enrichment.

That is, in order to improve the economical efficiency of the fuel, it is necessary to increase the water rod because the optimum water-to-uranium ratio increases as the concentration increases.

In the conventional 8 × 8 type fuel, usually about two water rods are used, but in the case of highly enriched fuel, it is necessary to further increase the water rods.

FIG. 6 shows the uranium saving effect when the number of water rods is increased by the number corresponding to the number of fuel rods in the high burnup, high enrichment type fuel assembly of 9 rows × 9 columns arrangement.

From Fig. 6, the water rod is equivalent to the number of fuel rods, and the uranium saving effect is 6.5% or more when the number is 9 or more,
It can be seen that the uranium-saving effect does not increase much even if the number is increased to 9 or more.

FIG. 7 shows an example of a fuel assembly in which the water rod is equivalent to the number of fuel rods and has nine rods. Further, in order to suppress the amount of pressure loss and improve fuel stability, as shown in FIG. 8, a large diameter water rod 1 having a cross-sectional area of about nine fuel rods is used as a fuel assembly. A structure arranged in the central part or in the vicinity thereof is considered.

[Problems to be solved by the invention]

When the large diameter water rod 1 is arranged at or near the center of the fuel assembly, the large diameter water rod 1
A relatively large space is created in the outer diagonal direction of. In this space, the resistance against the flow of the coolant is smaller than the resistance between the fuel rods, so that the coolant tries to flow around the water rod 1 having a large diameter, so that the distribution of the flow rate in the assembly is biased. That is, the coolant flow rate is small between the fuel rods that require a sufficient coolant flow rate because it is a heat generating portion, and the coolant flow rate near the large diameter water rod 1 that does not require much coolant flow rate because it is a non-heat generating section When the flow rate increases, a phenomenon occurs and the cooling efficiency of the entire assembly deteriorates significantly.

In order to prevent the above phenomenon, it is necessary to uniformly disperse the coolant channels in the assembly.

As a means for this, it is conceivable to make the cross section of the large diameter water rod 1 polygonal so as to fill the space in the diagonal direction of the large diameter water rod 1 in order to improve the manufacturability of the large diameter water rod 1. Not only worsening, but also reducing the flow path area of the whole assembly and increasing the area in contact with the coolant will increase the pressure loss amount of the fuel assembly, thus stabilizing the channel. And core stability deteriorate.

An object of the present invention is to provide a reactor fuel assembly that has a uniform cooling rate distribution and a high cooling efficiency without increasing the pressure loss amount.

[Means for solving problems]

The above object is to make the coolant flow passage uniform in the assembly by changing the diameter and cross-sectional shape of the water rod, the fuel rod diameter without changing the flow passage area, and devising the arrangement of the fuel rods. It is achieved by dispersing.

[Action]

That is, by moving the fuel rods from the square lattice points, the space (coolant flow passage) near the large diameter water rod 1 is moved between the fuel rods, so that the coolant flow passages in the assembly are , Evenly distributed within the aggregate without reducing its cross-sectional area.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a cross-sectional view of a fuel assembly according to the present invention at a position of a spacer 3. A water rod 1 having a large diameter is arranged at the position of the third row and the third column at the center of the square grid point of the ninth row and the ninth column. Seventy-two fuel rods 2 are arranged around a large diameter water rod 1. The fuel rods 2 are each spaced by circular spacer cells 31. The side-by-side spacer cells 31 are welded at their contact points, and the outermost peripheral portion is attached to the spacer outer frame 32.
Are fixed together to form one spacer 3. Also,
Of the 72 fuel rods 2, forty-eight fuel rods 2 are arranged on a square lattice, but the fourth row, the sixth row, and the fourth and sixth rows of the fuel rods 2 are arranged. (24 pieces in total) are arranged in the direction of the large diameter water rod 1 and are displaced from the square lattice points once, and the large diameter water rod 1 is arranged.
The twelve fuel rods 2 adjacent to each other are arranged so that they are all equidistant from the water rod 1.

After producing the fuel assembly as described above, a two-phase flow with a void ratio of about 40% was made to flow through the fuel assembly, and points 5 and 6 in FIG.
Then, the flow rate was measured respectively. As a result, the ratio of the flow rates at the flow rate measuring points 5 and 6 obtained is shown in FIG.
FIG. 3 also shows the measurement results of the conventional example shown in FIG. 4, but it is easier to see that the flow rate distribution in the aggregate is made uniform according to this example.

FIG. 2 shows a second embodiment of the present invention. In the second embodiment, among the 72 fuel rods 2, the fuel rods 2 in the 4th to 6th rows and the 4th to 6th columns (total 36 rods) each have a large diameter water rod. Once moved to 1, the equidistant distance from the square lattice point, the remaining thirty-six fuel rods 2 are correctly positioned on the square lattice point. The ratio of the flow rates at the flow rate measuring points 5 and 6 in this embodiment is also shown in FIG. In this embodiment, the flow rate distribution cannot be made more uniform than in the first embodiment, but the improvement effect is remarkable as compared with the conventional example. In addition, in this embodiment, since eight spacer modules in which nine spacer cells 31 are assembled in a square lattice of three rows and three columns can be combined to form one spacer 3, there is an advantage that the manufacturing is easy. .

In the first and second embodiments, all the 72 rods are used as the fuel rods 2, but it is obvious that some of them may be replaced by the water rods 11 having a small diameter.

〔The invention's effect〕

According to the present invention, since the flow rate distribution in the aggregate can be made uniform without reducing the flow passage area, the cooling efficiency can be improved without increasing the pressure loss amount.

[Brief description of drawings]

1 is a cross-sectional view at a spacer position in a fuel assembly of a first embodiment of the present invention, FIG. 2 is a cross-sectional view at a spacer position of a fuel assembly of a second embodiment of the present invention, FIG. 3 is a diagram showing a flow rate ratio of two points in the fuel assembly showing the effect of the present invention, and FIG. 4 is a side view of a conventional fuel assembly.
Figure 5 shows the relationship between the water-uranium ratio and the infinite multiplication factor.
FIG. 6 is a diagram showing the relationship between the number of water rods and the uranium saving effect, and FIGS. 7 and 8 are transverse sectional views at the spacer position in the fuel assembly according to the prior art. 1 ... Large diameter water rod, 2 ... Fuel rod, 3 ... Spacer, 31 ... Spacer cell, 32 ... Spacer outer frame, 5
...... Flow rate measurement points (1), 6 ...... Flow rate measurement points (2), 7 ...
… Upper tie plate, 8 …… Lower tie plate, 9 ……
Channel box, 11 …… A thin water rod.

Claims (1)

[Claims] 1. A large-diameter water rod is arranged in the center, and 72 fuel rods or 72 fuel rods and small-diameter water rods are arranged around the large-diameter water rod in nine rows. In a reactor fuel assembly configured by arranging in 9 rows of lattices, a part of the fuel rods or the small diameter water rods is arranged at a position displaced from a square lattice point in the direction of the large diameter water rods. Reactor fuel assembly characterized by the following.