JPS6247115Y2 - - Google Patents

Description

[Detailed explanation of the invention] [Industrial application field] This invention relates to a fuel assembly for boiling water reactors, and is particularly aimed at flattening the power distribution in the axial direction of the reactor core and at the same time reducing the absolute value of the reaction void coefficient. FUEL ASSEMBLY WITH IMPROVED WATERROOD FOR TENDING.

[Prior art] As is well known, in a typical nuclear reactor, an increase in the core temperature causes a decrease in the density of the moderator around the nuclear fuel rods of the fuel assembly and the generation of bubbles, that is, the creation or increase of void spaces. If this occurs, an under-moderated core design with negative void coefficients and negative temperature coefficients is implemented to reduce the neutron deceleration rate and lower the thermal neutron level, thereby reducing the reactor's reactivity to core temperature increases. given the inherent safety that is reduced.

By the way, in a boiling water reactor, the coolant flowing from the lower part of the fuel assembly and flowing out to the upper part becomes void as it goes upwards, and as shown by curve A in Figure 1, the void ratio during operation is proportional to the height of the reactor core. It exceeds 0.5 at about 40% position and reaches 0.75 near the top of the furnace. For this reason, in conventional boiling water reactors, one or two of the nuclear fuel rods in the fuel assembly are replaced with hollow rods, in other words, water rods, through which neutron moderators flow, and the surrounding area is voided during operation. Even if the flow becomes a two-phase flow including It is being done. However, conventional water rods have the same size and shape as nuclear fuel rods and can be replaced with nuclear fuel rods due to structural limitations such as the upper and lower tie plates of fuel assemblies and spacers. The volume of water produced is not sufficient, and as a result,
Various problems remain. In other words, in transient conditions where the voids in the core are compressed, such as when the core internal pressure increases during reactor operation, deceleration occurs because the volume of coolant in the water rods that must compensate for this change in void ratio is small. It is less effective in suppressing an increase in the material-to-fuel volume ratio H/U, and therefore cannot be expected to be effective in reducing the absolute value of the void coefficient, making it impossible to prevent a rapid increase in the reactivity of the furnace. Furthermore, although the improvement effect of the water rod on the neutron moderation effect is originally not very large, the limitation of the coolant volume within the water rod makes this improvement effect smaller and smaller, and as a result, a particularly large number of voids are generated. The compensating effect for the power drop in the upper half of the core was not large enough, and as a result, the power distribution in the axial direction of the core had a peak in the lower part, as shown by curve B in FIG. 1, and its flattening was undesirable.

In order to increase the effect of the water rod mentioned above,
Conventionally, two water rods have been used in one fuel assembly, but this reduces the number of nuclear fuel rods and reduces the total power of the reactor core, or if the total power is kept the same, the remaining water rods are reduced. Either it is necessary to increase the output per nuclear fuel rod.

In addition, in order to flatten the axial power distribution, the fuel enrichment in the upper half of the fuel rod is made higher than that in the lower half to compensate for the decrease in reactivity in the upper half of the core, which has a high void ratio, and vice versa. In the lower half of the reactor core, where the output is relatively high, neutron-absorbing substances such as gadolinia are added to the fuel to absorb the reactivity in the lower half. Although flattening the power distribution has some effect, the concentration of fuel pellets loaded into a single fuel rod and the concentration of gadolinia added are different, so there is always a risk of incorrect loading, and the pellets after loading are Different types of fuel pellets are difficult to identify through non-destructive testing when there is a small difference in enrichment or gadolinia additive concentration, and many problems remain in the management of manufacturing and storage of fuel pellets and fuel rods.

Another method is to lower the output by partially inserting the control rod into the lower half, where the output is higher, but in this method, the output is lower than where the tip of the control rod was located when the control rod was removed. Because the increase in plutonium is progressing, it becomes too large and can easily damage the fuel. Therefore, restrictions on control rod operation, such as speed and method, must be increased, and operational margins must be increased. The disadvantage is that it becomes smaller.

[Problem that the invention attempts to solve]

This idea was devised in view of the above-mentioned problems, and it is possible to increase the effect of reducing the absolute value of the void coefficient per fuel assembly, and at the same time, it can be done without major design changes to the structural parts of the conventional fuel assembly. , and boiling with improved water rods that can effectively achieve an increase in the moderator-to-fuel volume ratio (H/U) and flatten the core axial power distribution without reducing the amount of nuclear fuel rods. The purpose is to provide fuel assemblies for water reactors.

[Means for solving problems]

In other words, in the fuel assembly of this invention, a plurality of nuclear fuel rods and one hollow rod through which a neutron moderator is circulated are arranged in a square manner with an interval between them, and the rods are arranged in a square manner with a space between them. The internal flow cross-sectional area of the lower part up to 20% of the core length is 70%
The cross-sectional area of the internal flow path of the upper portion is smaller than the cross-sectional area of the internal flow path of the upper portion, and the cross-sectional area of the internal flow path of the upper portion is larger than the area of a circle having a diameter corresponding to 90% of the arrangement pitch of the nuclear fuel rods, and the outer shape of the hollow rod is The dimensions are constant across the upper and lower parts. Here, the term "core length" corresponds to the effective length of the fuel assembly, and the bottom end of the core corresponds to the effective length of the fuel assembly.
%, is shown as a length percentage with the furnace top as 100%, and the range of the core length of the hollow rod from 20 to 70% is
Whether the large-diameter flow path or the small-diameter flow path is used can be changed depending on the core design.

[Effect]

In a boiling water reactor, the power distribution in the axial direction of the core peaks at the bottom, as shown by curve B in Figure 1, because, as mentioned above, there are many voids in the upper half, and the moderator density decreases, causing neutrons to This is because leakage and reduction in deceleration effect occur.

In this invention, in order to compensate for the decrease in moderator density in the upper half of the core where the void ratio is high, the inner diameter of the hollow tube serving as the water rod is changed at the top and bottom.
Since the moderator in the water rod can be concentratedly retained in the upper half, and the moderator is removed by the water rod volume in the lower half, the absolute value of the void coefficient can be reduced at the same time. We can expect great effects. Therefore, according to the present invention, first, the power reduction in the upper half of the core due to voids is effectively compensated for, and as a result, the axial power distribution is flattened.

〔Example〕

This idea will be explained in detail with reference to the drawings as follows.

Figure 2 is a cross-sectional view conceptually showing one fuel assembly as a general example. In this example, it has a square array of 8 x 8 fuel rods, where 1 is a nuclear fuel rod, 2 is a jacket, and 3 is a hollow. Rod (water rod), 4 indicates the control rod, 5 of the nuclear fuel rods 1 indicated by (G)
The book contains gadolinia, which is a poisonous substance. Now, FIGS. 3 and 4 are enlarged views of the hollow rod 3 at the spacer level, with FIG. 3 showing the conventional example and FIG. 4 showing the embodiment of this invention. In FIGS. 3 and 4, 5 is a spacer, and 30 is a hollow rod that is the main part of this invention.
FIG. 5 is a longitudinal cross-sectional view of the hollow rod 30 in FIG.
In the fuel assembly according to this invention, as is clear from a comparison between FIG. 3, FIG. 4, and FIG. 5, the hollow rod 30 as a water rod is thicker than the fuel rod 1, and the The cross-sectional area of the flow passage is larger than the cross-sectional area of the fuel rod 1, and the cross-sectional area of the lower flow passage is as small as a simple hole. Specifically, the upper part of the hollow rod 30 consists of a cylindrical pipe,
The outer diameter of this cylindrical pipe is larger than the inner diameter (distance between opposite sides) of the uniformly sized cells of the normal spacer 5, and the cell 6 for inserting the hollow rod is expanded to a larger diameter as shown in FIG. It may also be a large diameter cell portion. In the most common 8x8 fuel assembly, the typical value of the nuclear fuel rod arrangement pitch is 16.256.
mm, the wall thickness of the upper part of the hollow rod is 0.6 to 0.8 mm, and the wall thickness of the lattice strip of the spacer 5 is about 0.7 mm or less,
Based on these values, the upper portion of the water rod, which has a diameter so large that it cannot fit within a cell of normal uniform size, has an inner diameter that is 90% or more of the above-mentioned arrangement pitch size. The upper portion of the hollow rod 30 may be a rectangular pipe, as will be described later, and is not limited to the cylindrical pipe example shown in FIG. Expressing the limitation on the size of the internal flow passage cross-sectional area of the upper part of the hollow rod, including both cylindrical pipes and rectangular pipes, the internal flow cross-sectional area of the upper part of the hollow rod is 90% of the arrangement pitch of the nuclear fuel rods. This is larger than the area of a circle with the corresponding diameter. Therefore, in this design, hollow rod 3
0 is thick, and the shape of the hollow rod insertion portion cell 6 of the spacer 5 and the hollow rod 30 are adjusted accordingly.
The only difference is that the shape of the block spacer 7, which is elastically engaged in the cell 6, is different from the conventional one, and other structural members, such as the upper and lower end plugs of the hollow rod 30, the upper and lower tie plates, and the structure of their attachment parts, are the same as the conventional one. It may be the same as a fuel assembly.

As shown in Figure 3, the conventional hollow rod 3 has a large gap around the cell inside the cell, and there is quite a lot of space, but since the hollow rod itself does not generate heat during operation, the space inside the cell around the hollow rod is There is no need to flow a coolant, and therefore no problem will arise even if the cell 6 is fully inserted, as in the example of the present invention shown in FIG. Furthermore, in the present invention, the wall surface of the spacer 5 constituting the cell 6 is in contact with the adjacent fuel rod on the outside thereof, but the fixing protrusion 8 of the spacer 5 and the block spacer 7 are originally in engagement contact with the fuel rod 1. Therefore, there is no problem caused by the wall surface of the cell 6 coming into contact with the adjacent fuel rod.

In FIGS. 4 and 5, 31 is an upper end plug, and 32 is a lower end plug. The hollow rod 30 has a uniform outer diameter larger than the fuel rod outer diameter from the upper part to the lower part, but the inner diameter has a large diameter at the top and a small diameter at the bottom. The upper large diameter channel 33 is the fuel rod 1
The lower small-diameter flow path 34 is a narrow flow path similar to that of a simple through hole. In this example, the upper part 35 and the lower part 36 are joined to form a
It is a large diameter hollow rod of a book, and both parts are 35,3
6 is made of a material such as Zircaloy, for example, but the upper part 35 is made of a material such as beryllium, which has a large neutron reflecting effect, in order to compensate for the power drop in the upper part of the core, or to suppress the power peak in the lower part of the core. For this reason, the lower portion 36 may be formed of a material with a high neutron absorption effect, such as stainless steel, nickel-based alloy, or boron-added stainless steel.

[Effect of idea]

As described above, according to the present invention, the lower half of the hollow rod has an internal flow passage cross-sectional area as small as that of a simple hole, so it cannot hold much moderator (water).
In fact, since the water in the lower half of the reactor is eliminated compared to the conventional example, the moderation effect of neutrons is suppressed despite the low void ratio, and as a result, the power peak in the lower half of the reactor is suppressed by suppressing thermal neutronization. The effect of reducing the size can be expected.

Furthermore, the fact that a large volume of moderator is secured in a concentrated manner in the upper half of the fuel rod bundle section means that the ratio of the moderator volume in the fuel assembly to the moderator volume in the control rod gap between fuel assemblies is As a result, the radial power distribution in the upper half of the core is flattened, and at the same time, the nuclear life of the fuel assembly is extended because H/U increases in the upper half of the core. It is something that

[Brief explanation of drawings]

Figure 1 is a diagram showing an example of the relationship between the height position and void fraction change in the core of a boiling water reactor, and Figure 2 is a conceptual diagram of a general fuel assembly for a boiling water reactor. FIG. 3 is an enlarged cross-sectional view at the spacer level of a hollow rod portion according to a conventional example; FIG. 4 is an enlarged cross-sectional view at the spacer level of an upper portion of a hollow rod according to an embodiment of the present invention; FIG. 5 is a longitudinal sectional view of the hollow rod of FIG. 4. 1: fuel rod, 2: jacket, 30: hollow rod, 3
1: Upper end plug, 32: Lower end plug, 33: Large diameter channel, 34: Small diameter channel, 4: Control rod, 5: Spacer.

Claims (1)

[Claims for Utility Model Registration] (1) A boiling water reactor fuel comprising a plurality of nuclear fuel rods and at least one hollow rod through which a neutron moderator is circulated, spaced apart from each other in a square arrangement. In the assembly, an internal passage cross-sectional area of a lower part of the hollow rod up to 20% from the bottom of the core length is smaller than an internal passage cross-sectional area of an upper part of 70% or more of the core length, and the upper part The cross-sectional area of the internal flow path is 90% of the arrangement pitch of the nuclear fuel rods.
A fuel assembly for a boiling water reactor, characterized in that the area of the hollow rod is larger than the area of a circle with a diameter corresponding to . (2) The fuel assembly according to claim 1, wherein the upper portion of the hollow rod is comprised of a cylindrical pipe. (3) The fuel assembly according to claim 1, wherein the upper portion of the hollow rod is made of a rectangular pipe.