JPS6039589A - 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 [Technical Field of the Invention] The present invention relates to a fuel assembly used in a boiling water nuclear reactor.

[Technical dorsal fin of the invention] A large number of fuel assemblies are loaded as nuclear fuel into the core of a boiling water nuclear reactor.

FIGS. 1 and 2 show a conventional fuel assembly. The fuel assembly 11 or 12 includes fuel rods 1 in a hollow rectangular column-shaped channel box 13 or 14.
5 are arranged in a square grid. fuel rod 1
In the example shown in Fig. 1, 5 has a structure of 8 rows and 8 columns (even matrix), and in the case of the example shown in Fig. 2, it has a structure of 9 rows and 9 columns (
(odd matrix). Fuel assembly 11.12
are arranged around a control rod 16 having a cross-shaped cross section, forming a unit grid. A reactor core is made up of many such unit cells.

Now, while the reactor is operating, the channel box 13.1
A neutron moderator is placed in the so-called bypass area outside of the
Only light water of C exists, and channel box 13
．． Inside 14 there is a mixed flow of light water and steam as a coolant. For this reason, neutron deceleration is mainly performed in the bypass region. As a result, the distribution of thermal neutron flux within the fuel assembly 11.12 becomes smaller at the center and becomes larger toward the periphery. As a result,
Combustion is suppressed at a low level in the center of the combustion rod, and combustion becomes more intense toward the periphery. Such non-uniform combustion results in uneven local power output, which is undesirable from the standpoint of fuel integrity.

Therefore, conventionally, fuel assemblies have been generally used in which the concentration of uranium-235 as fuel is lowered in the fuel rods located in the peripheral part. 1 and 2 show examples of fuel assemblies provided with such a so-called sprint distribution for the concentration of uranium-235. That is, the number given to each fuel rod 15 of the fuel assembly 11,12 represents the degree of enrichment of uranium-235, and the number 1
The one represented by the number 5 has the highest concentration, and the one represented by the number 5 has the lowest concentration. Fuel assembly 11.12
In addition to the highly enriched fuel rod 15, there is a symbol W in the center of the
Two or three water rods (water rods)
17 is placed and C.

Further, some of the fuel rods 15 shown in these figures may contain burnable poison in the uranium fuel. However, these burnable poisons are not particularly relevant to the present invention, which will be explained below.

Therefore, detailed description of burnable poisons will be omitted in this specification.

[Problems with the background art] In fuel assemblies such as Kosasa, highly enriched fuel rods are arranged in a portion of the medium-heated part of the fuel assembly where the thermal neutron flux is small. Therefore, there is a large loss in terms of the reactivity (hereinafter referred to as infinite multiplication factor) of the -C fuel assembly. Furthermore, when the fuel rods are removed from the core after the prescribed combustion period, a larger amount of uranium-235 remains in the fuel rods placed in the medium-heated part than in those placed in the periphery, causing the temperature to drop to °C. .

In order to solve these disadvantages from the viewpoint of fuel economy and effective use of uranium, it is necessary to (i) increase the infinite multiplication factor of the fuel assembly throughout the combustion period to a greater degree than before; and ( ii) Uranium 2 at the end of combustion
It is necessary to reduce the remaining amount of 35 as much as possible. For this reason, the enrichment distribution of uranium-235 has traditionally been
Studies are being carried out to adjust the fuel rods and light water in various ways. However, the effect cannot be said to be sufficient.

Furthermore, in conventionally used fuel assemblies, uranium-238 absorbs neutrons and -C-generated plu-1 to nium-239 tend to accumulate in the central portion of the fuel assembly.

This is because the thermal neutron flux is small in this central portion, and in order to sufficiently burn plutonium-239, it is desirable that the thermal neutron flux be high in this central portion.

[Object of the Invention] In view of the above circumstances, an object of the present invention is to provide a fuel assembly that can utilize uranium resources more effectively.

[Summary of the invention] In the present invention, when U(i)N is an even number for fuel Y31 rods or fuel rods and water rods forming a square lattice of N×N, the fuel assembly is made up of (N/2) X(N /2) Divide into 4 small assemblies consisting of fuel rods or water rods, and use the area corresponding to the 4 fuel rods in the medium heat section as a non-boiling IIk water flow channel, and place these small assemblies in the middle of the fuel. Change the direction of your body so that the remaining fuel in the central fuel rods is burned at the periphery. Also (i
i) If N is an odd number, the fuel assembly is (N+1)/2
Book
The area corresponding to the void surrounded by the small assemblies is defined as a fuel assembly. Similarly, the direction of the small aggregates is changed during combustion to make more effective use of the fuel.

[Embodiments of the invention] The present invention will be described in detail with reference to Examples below.

[First Embodiment] FIG. 3 shows a fuel assembly forming a square lattice of 8 rows and 8 columns used in a boiling water nuclear reactor. The fuel rods 15 constituting this fuel assembly 21 are each housed in a subchannel box 22 of 15 pieces, and are grouped into four small assemblies 231 to 234. A fuel rod spacer, an upper tie plate 1-1 and a lower tie plate (both not shown) are provided for each of these small assemblies 231 to 234.

These small assemblies 231 to 234 are housed in a channel box 24 consisting of a lower tie plate provided for 8×8 fuel assemblies and a channel portion connected to the lower tie plate.

Each of the small assemblies 231 to 234 has one fuel rod at each corner.
It is equipped with a square lattice consisting of incompletely shaped fuel rods 15h1 in which each tree is missing. In the chipped portion C of the fuel rod, a subchannel box 22 made of, for example, a zirconium alloy is roundly recessed inside, and constitutes a non-boiling water channel wall 24A. Non-boiling water channel wall 2 of each small aggregate 231 to 234
When 4A is arranged toward the center of the fuel assembly 21 as shown, they form a circular wall surface as a whole.

With each of the small aggregates 231 to 234 arranged in this manner, combustion of uranium 235 is started. At this time, fuel rod 15
The enrichment degree of is distributed in five stages as represented by numbers 1 to 5. Many fuel rods 15 with high enrichment are arranged near the center i=J of the fuel assembly 21, and many fuel rods 15 with low enrichment are arranged around the tunnel box 24.

A circular portion surrounded by four non-boiling water channel walls 24A forms a non-boiling water channel 26.

Since the non-boiling water flow channel 26 is much larger than a conventional water rod, the presence of this channel increases the number of thermal neutrons that are moderated. This flattens the thermal neutron flux distribution within the fuel assembly 21 considerably.

However, the closer to the periphery of the channel box 24, the faster the uranium 235 burns, and the more unburned fuel remains in the center of the fuel assembly 21.

Therefore, in this fuel assembly 21, the small assemblies 231 to 234 are rearranged at a predetermined time when the infinite multiplication factor has exceeded its peak. The predetermined time is, for example, the time when one year has passed after the start of combustion in terms of rated output.

FIG. 4 shows the fuel assembly after reassembly. Each of the small aggregates 231 to 234 is rotated by 180 degrees and then stored in its original position. Due to this arrangement, the fuel rods that have undergone the most combustion are moved to the center of the fuel assembly 21. In addition, fuel rods with unburned fuel are placed near the four corners of the channel box 24 where combustion is most active. By rearranging the small aggregates 23+ to 234, the non-boiling water channel walls 24A are dispersed at the four corners of the channel box 24, forming a relatively small non-boiling water channel 27. These non-boiling water flow channels 27 further activate the combustion of unburned fuel. Moreover, the combustion of the plutonium 239, which has been generated and accumulated in the center of the fuel assembly 21 (FIG. 3) before the recombination, also progresses as a result of this recombination.

As a result of the above, when the fuel assembly 21 is rearranged, the infinite multiplication factor increases at this point, and the Nycle burnup can be extended. Furthermore, if the cycle burnup is kept constant, the enrichment of the fuel can be reduced by that amount.

A fuel assembly C1 in which four unit lattices each consisting of N xN square lattices are arranged around a control rod.When N is an even number as in this example, the fuel assembly is (N/2)
'(N/2) Divide into 4 small aggregates. An area corresponding to one fuel rod arranged at the corner of each small assembly is utilized as a non-boiling water flow channel through which non-boiling water flows.

[Second Embodiment] FIGS. 5 and 6 are for explaining an embodiment in which N is an odd number. Of these, Figure 5 shows the fuel assembly 31.
Figure 6 shows the arrangement before recombination and the arrangement after recombination. The fuel rods 15 constituting the fuel assembly 31 are housed in the leave channel box 32 in groups of 19, and are grouped into four small assemblies 331 to 334.

These small assemblies 33+ to 334 are housed in a C channel box 34 in the same manner as the fuel assembly 21 of the first embodiment.

Each of the small assemblies 331 to 334 has one fuel rod at each corner.
It is equipped with a square lattice consisting of incompletely shaped fuel rods 15b+, each of which is missing. At the chipped portion of the fuel rod, a subchannel box 32 made of, for example, a zirconium alloy is roundly recessed inward, and a non-boiling water flow path 134A is formed.
It consists of As shown in FIG. 5, in the state before the fuel is reassembled, these non-boiling water flow path walls 34Δ are oriented toward the center of the fuel assembly 21. A void partitioned into the shape of a non-boiling water flow channel 35 is formed.The numbers attached to the fuel rods 15 in the figure represent the stages of enrichment of uranium-235.

In the fuel assembly 31 (FIG. 5) before this reassembly, a relatively large non-boiling water flow channel 35 is formed in the center thereof, so that the thermal neutron flux distribution is similarly flattened. In addition, in the reassembled fuel assembly 31 (Fig. 6), as a result of rotating the arrangement of each of the small assemblies 331 to 334 by 180 degrees,
Two types of non-boiling water flow channels are formed: non-boiling water flow channels 36 distributed at the four corners and non-boiling water flow channels 37 in the medium heat section. The non-boiling water channel 36 contributes to the combustion of unburned fuel H, and the non-boiling water channel 37 has the effect of reducing the accumulation of plutonium 239 near the center.

When N is an odd number as in this embodiment explained above,
Divide into 4 small fuel assemblies: (N+1)/2X (N-1)/2. Then, the area corresponding to one fuel rod to be placed at the corner of each small assembly is utilized as a non-boiling water flow channel through which non-boiling water flows.

[Modification] Diagram F7 shows the shape of the non-boiling water flow channel when N is an even number. Of these, A in the figure is the non-boiling water flow channel 26 used in the first embodiment. In this embodiment, each corner of the rectangular sub-denel box 22 is recessed into a quarter circle, so that the overall UII
? A non-boiling water flow channel 26 whose i-plane is circular is formed.

On the other hand, in FIG. B, the subchannel box 22
The corners of the cross section are concave in a square shape, and the overall shape is 01.
A non-boiling water flow channel 41 with a square surface is formed. In addition, in FIG. C, the corners of the sub-channel box 220 are concave in the shape of a right isosceles triangle in cross section, and a water flow channel 42 with a square cross section is formed as a whole.

The shape of the non-boiling water channel can be further modified in various ways.

FIG. 8 shows the shape of the non-boiling water flow channel when N is an odd number. The figure shows a non-boiling water flow channel 35 used in the second embodiment.

In Figure B, each corner of the sub-channel box 32 is square in cross section, forming a non-boiling water flow channel 43 with a cross-shaped cross section as a whole. Same figure C
Now, the corners of the non-boiling water flow channel 32 are angular. A car-shaped non-boiling water flow channel 44 is formed. In this case as well, the shape of the non-boiling water channel can be modified in various ways.

In the LC fuel assembly described above, one corner of the glue butty channel box of each small assembly was recessed to form a non-boiling water flow channel. Alternatively, it is also possible to place a water rod in one corner of a rectangular four-knob channel box, and the purpose of this defense can be achieved by rearranging small aggregates.

FIGS. 9 and 10 show such a variation when N is an even number. 9 shows the fuel assembly 51 before being reassembled. Each small aggregate 5
The sub-density V tunnel boxes 53 21 to 524 have a substantially square cross section, and inside them, fuel rods 15 and water rods 17 are arranged in a 4 x 4 grid pattern. One of the water rods 17 is located near the center of the fuel assembly 51 . Four water rods in the center 17
has the same effect as the non-boiling water channel described above.
do. These water rods 17 may have a larger diameter than the fuel rods 15.

During combustion, each subchannel box 53 in the combustion chamber 24 is taken out and rotated 180 degrees.
”-U Return to the original position. This recombined model fuel assembly 5
1, as shown in FIG. 10, fuel rods with high enrichment levels are moved to the periphery, and the combustion of unburnt fuel is activated.

On the other hand, FIGS. 11 and 12 show such a modification when N is an odd number. Of these, FIG. 11 shows the fuel assembly 61 before being reassembled.

Subzi V tunnel box 6 for each small aggregate 62+ to 624
3 has a substantially rectangular cross section, and inside thereof fuel rods 75 and water rods 17 are arranged in a grid of 4×5 rods. Of these, one wood near the medium heat part of the fuel assembly 61 is the water rod 17. Small collection of 4 bodies 621
The void surrounded by 624 is used as a non-boiling water flow channel 64 and contributes to flattening the thermal neutron flux distribution together with the water rod 17. Also in this modification, the thickness of the water rods 17 does not need to be the same as that of the fuel rods 15.

In the middle of combustion, each subchannel box 63 in the channel box 34 is rotated 180 degrees LJ-' (if rearranged, it will become as shown in Fig. 12. In this case as well, the highly enriched fuel rods will move to the periphery). This activates the combustion of unburned fuel.

[Effects of the Invention] As explained above, according to the present invention, the four small aggregates are rotated 180 degrees during combustion and rearranged, so that the amount of uranium-235 remaining at the end of combustion is reduced and the combustion It is possible to obtain fuel efficiency gains and improve fuel cost economy.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a conventional fuel assembly without a square lattice of 8 rows and 8 columns, Figure 2 is a cross-sectional view of a conventional fuel assembly without a square lattice of 9 rows and 9 columns, and Figure 3 is a cross-sectional view of a conventional fuel assembly without a square lattice of 9 rows and 9 columns. The drawings to FIG. 12 are for explaining embodiments or modified examples of the present invention, and among these, FIG. 3 and FIG. 4 are! ′! As an example of Figure 1, 8 rows 8
5 and 6 are cross-sectional views showing before and after recombination of a fuel assembly forming a square lattice of columns (even matrix); FIGS.
The figure shows 'M' with 9 rows and 9 columns (odd matrix) as an example of Figure 2.
Cross-sectional views showing before and after recombination of fuel assemblies forming an E-force lattice, and FIGS. 7A to 7C are even-numbered matrices. Various schematic diagrams showing the possibility of deformation of the shape of the non-boiling water flow channel in the fuel assembly of , and FIGS. Figures 9 and 10 are 8-by-8 square grids using water rods; cross-sectional views showing the fuel assembly before and after reassembly; Figure 11; 83
and Fig. 12 shows the horizontal r1 before and after rearrangement of a fuel assembly forming a square lattice of 9 rows and 9 columns using water rods.
7i side view. 15・・・・・・・・・・・・Fuel rod 17・・・・・・・・・Water rod 21.31.51.61 ・・・・・・・・・・・・Fuel Aggregate 24.34...Channel box 22.32.53.63...Zub channel box 26
．． 27.35. 37.41-44. 64・・・・・・・・・Patent attorney for non-boiling water flow channels Suyama Sa - 7th mouth 8 Figure 11 Garden 12th mouth fi? 3

Claims (1)

[Claims] '(1) In a boiling water reactor in which four unit cells each consisting of N x N square lattices are arranged around a control rod, when N is an even number, the unit cell Ributi I
(surrounded by (N/2) wood
(N/2) fuel rods or more fuel rods than this, or divided into 4 small assemblies consisting of fuel rods and water rods, and 4 rods worth of fuel that goes into the center of the channel box. A fuel assembly characterized in that a region corresponding to a rod or a water rod is used as a flow path for non-boiling water. (2) Each of the four small assemblies has an upper tie plate, a lower tie plate, and a fuel rod spacer, and from the lower tie plate that supports the four small assemblies as one unit and the V tunnel part connected to this. Claim 1 characterized in that the channel box is removably housed in a channel box.
Fuel assembly as described in section. (3) The fuel assembly according to claim 1, wherein the cross section of the flow path through which the non-boiling water flows in each small assembly is approximately 1/4 circular. (4) The fuel assembly according to claim 1, wherein the cross section of the flow path through which the non-boiling water flows in each small assembly is approximately square. (5) The fuel assembly according to claim 1, wherein the cross section of the flow path through which the non-boiling water flows through each small assembly is approximately an isosceles triangle. (6) A patent characterized in that each subassembly is equipped with a 11-channel box with a rectangular cross section, and a water rod is arranged at each corner of the fuel assembly according to claim 1. body. (7) The fuel assembly according to claim 6, wherein the water rods are thicker than the fuel rods. (8) i! is formed by arranging four unit cases f consisting of N x N square lattices around the control rod! ll1III water type 1
In the Kyoshi reactor, when N is an odd number, each of the intermediate grids is surrounded by a subchannel box (N+1>
/2 X (N-1>/2 fuel rods or a total number of fuel rods less than 1 tree), or divided into 4 small aggregates consisting of fuel rods and water rods, and A region corresponding to one fuel rod or water rod in the center and a region corresponding to a void surrounded by these four small aggregates are characterized as flow paths for non-swelling water. Fuel assembly. (9) Each of the four small assemblies has an upper tie plate, a lower tie plate, and a fuel rod spacer, and is connected to a lower tie plate that supports the four small assemblies as one. The fuel assembly according to claim 8, characterized in that the fuel assembly comprises an IC channel portion and is detachably housed in a channel box. (10) A fuel assembly in which each small assembly has a square 17i surface. 1
9. A fuel assembly according to claim 8, characterized in that the fuel assembly comprises a 7-channel box, and water rods are arranged at each corner of the box. (11) The fuel assembly according to claim 10, wherein the water rods are thicker than the fuel rods.