JP2000081495A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel assembly for a D-grid, capable of increasing takeout combustibility by enhancing average enrichment, while securing thermal tolerance by reducing local output peaking to an appropriate value. SOLUTION: This fuel assembly comprises a plurality of first fuel rods 2 containing no combustible poison and a plurality of second fuel rods 3, 4 containing combustible poison, which are arranged in a square grid consisting of nine rows and nine lines. The first fuel rods, placed within two layers from the outermost periphery, on a cross section in which the enrichment of each fuel pellet used is 5 wt.% or less and average enrichment on the cross section is 4.5 wt.% or more, are all of maximum enrichment, and when the cross section is divided into two by a diagonal 7 connecting the centers of two of the outermost-periphery corner fuel rods located at diagonal positions, all the outermost-periphery fuel rods on one side are the first fuel rods of the maximum enrichment except those at the corners, and one or more first fuel rods of the maximum enrichment and three or more first fuel rods that are not of maximum enrichment are placed in positions on the other side at the outermost periphery except at the corners.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly loaded in a boiling water reactor, and more particularly to a fuel assembly aimed at increasing the burnup of a D lattice core.

[0002]

2. Description of the Related Art As shown in FIG. 2, in a fuel assembly loaded in a boiling water reactor, a coolant flows through a large number of fuel rods 2 in which fuel pellets containing fissile material are sealed. The water channel 6 and the like are supported by the upper tie plate 12, the lower tie plate 13, the spacers 14, and the like, and the channel box 1 is surrounded by these.
Is mounted and loaded into the core. In many cases, some fuel rods contain fissile materials and burnable poisons such as gadolinia, and are used for core reactivity control.

[0003] In order to reduce the cost of nuclear power generation, it is desired to extend the continuous operation period to increase the plant operation rate. It is also desired to increase the burnup to increase the energy generated per unit fuel weight. In order to extend the continuous operation period and increase the burnup, it is necessary to increase the initial uranium enrichment in the fuel assembly.

For example, JP-A-6-102384 discloses 45
It is described that the average enrichment of the fuel assembly should be 4 wt% or more in order to obtain an output burnup higher than GWd / t. On the other hand, the publication also states that the current fuel production facility has a maximum uranium enrichment limit of 5 wt%.

[0005] In a boiling water reactor, when a fuel assembly is loaded into the core, there is a type in which the distance between the fuel assemblies differs between the control rod insertion side and the non-insertion side, and is called a D lattice core. . The space between the fuel assemblies is filled with almost saturated water, and when viewed from the fuel assembly, the amount of the neutron moderator differs between the control rod insertion side and the non-insertion side. Is generated, and the power distribution in the cross section of the fuel assembly tends to be biased.

In order to make the power distribution flat in consideration of this, it is known to make the enrichment distribution in the fuel assembly asymmetric. FIG. 1 of JP-A-3-108690, and
FIG. 7 of JP-A-9-159777 discloses an example of an enrichment distribution of a fuel assembly for a D-lattice core. The fuel rods on the control rod insertion side have a relatively low enrichment and no control rods are inserted. The side fuel rods are set to a relatively high enrichment.

On the other hand, in JP-A-3-108690, JP-A-9-159777 and JP-A-9-236676, a water channel or a water rod is displaced from the center of the fuel assembly. Thus, a fuel assembly in which the asymmetry of the moderator distribution is reduced is described.

[0008]

According to the study of the present inventors, for example, when the continuous operation period is 18 months or more, the discharge burnup 50
To achieve about GWd / t, an average fuel assembly enrichment of about 4.2 wt% is required. When providing low enrichment regions at the upper and lower ends of a fuel assembly having poor combustion efficiency to enhance fuel economy, it is expected that the average enrichment in cross section will be 4.5 wt% or more. As described above, the currently available maximum enrichment is 5 wt%, and in this case, the cross-sectional average enrichment and the maximum enrichment are only 10%. In the above prior art, sufficient consideration has not been given to the enrichment distribution, the arrangement of the fuel rods containing burnable poisons, etc. under such restrictions on the enrichment.

In order to realize a fuel assembly for a D lattice having a cross-sectional average enrichment of 4.5 wt% or more under the constraint of a maximum enrichment of 5 wt%, the present invention aims at the following three points.

[0010] A first object of the present invention is to provide a fuel assembly capable of securing a thermal margin while keeping local output peaking in a cross section at an appropriate value.

A second object of the present invention is to maintain the fuel integrity of a fuel rod containing a burnable poison having a slightly poor thermal conductivity, so that the reactivity of the fuel assembly is relatively high during the first half of the life of the burnable poison. An object of the present invention is to provide a fuel assembly capable of keeping the output of a fuel rod containing fuel sufficiently lower than the output of a uranium fuel rod.

A third object of the present invention is to reduce the critical output of the fuel assembly in consideration of the uneven flow of the coolant when the water rod or the water channel is arranged off the center of the fuel assembly for the D grid. It is to provide a fuel assembly that can be kept.

[0013]

SUMMARY OF THE INVENTION A first object of the present invention is to provide a fuel cell system comprising a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison. In the fuel assemblies arranged in a square lattice as described above, the enrichment of the fuel pellet used has a cross section of 5 wt% or less, and the average enrichment has a cross section of 4.5 wt% or more. The first fuel rods located within the second layer from the outermost periphery of the rod arrangement are all the highest enrichment fuel rods, and a diagonal line connecting the centers of two outermost corner fuel rods at diagonal positions of the fuel rod arrangement. When the cross section is divided into two, the outermost fuel rods on one side are all the first fuel rods having the highest enrichment except for the corners, and the outermost outer circumferences on the other side include three fuel rods including the corners. This is achieved by arranging the first fuel rods not having the highest enrichment.

Particularly, in a fuel assembly in which a water rod or a water channel is arranged at the center of the fuel assembly, a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are included. In the fuel assemblies arranged in a square lattice of 9 rows and 9 columns or more, the enrichment of the fuel pellets used is 5 wt% or less, and the cross section has an average enrichment of 4.5 wt% or more. At least one large water rod having a cross section of one grid or more is arranged so as to be symmetrical with respect to a diagonal line connecting the centers of two outermost corner fuel rods at diagonal positions of the fuel rod array. All the second fuel rods included on one side with respect to the diagonal line include the burnable poison-containing pellets with the highest enrichment, and are not vertically and horizontally adjacent to the water rod at positions other than the outermost periphery. set on On the other side of the diagonal line, a second fuel rod having a lower enrichment is placed at a position other than a position adjacent to the outermost periphery and the water rod in the vertical and horizontal directions. Is achieved.

In a fuel assembly in which a water rod or a water channel is displaced from the center of the fuel assembly, a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are provided. Are arranged in a square lattice of 9 rows and 9 columns or more, the enrichment of the fuel pellets used is 5 wt% or less, and the average enrichment is 4.5 watts.
t% or more, and in the cross section, a cross section of one lattice or more so as to be asymmetric with respect to a diagonal line connecting the centers of two outermost corner fuel rods at diagonal positions of the fuel rod array. When a large water rod having at least one is disposed, the cross section is divided into two by the diagonal line, and a region including a large number of water rod regions is a region N, and the other is a region W.
The first object of the present invention can be achieved by preventing the second fuel rod from being adjacent to the water rod in the vertical and horizontal directions on the water rod region N side.

Further, in a fuel assembly in which a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are arranged in a square grid of 9 rows and 9 columns or more, The enrichment of the fuel pellets used is all 5 wt% or less, and the three enrichments with the highest enrichment are 10%
It has a cross section having the above enrichment intervals and an average enrichment of 4.5 wt% or more, and in this cross section, a diagonal line connecting the centers of two outermost corner fuel rods at diagonal positions of the fuel rod array. At least one large water rod having a cross section of one lattice or more is arranged so as to be asymmetric with respect to,
When the cross section is divided into two by the diagonal line, and a region including a large amount of water rod region is defined as a region N and the other is defined as a region W, a region N
The fuel pellets containing burnable poisons contained in the second fuel rods, which are adjacent to the water rod in the vertical and horizontal directions on the side, have the third enrichment in the fuel assembly from the highest in the fuel assembly. Can be achieved.

[0017] Further, the plurality of first burn-in-free first poisons.
In a fuel assembly in which a fuel rod and a plurality of second fuel rods containing burnable poisons are arranged in a square lattice of 9 rows and 9 columns or more, the enrichment of the fuel pellet used is 5 wt% or less, and the average The enrichment has a cross section of 4.5 wt% or more, and in the cross section, 1 is set so as to be asymmetric with respect to a diagonal line connecting the centers of two outermost corner fuel rods at diagonal positions of the fuel rod array. When at least one large water rod having a cross section equal to or greater than the lattice portion is arranged, and the fuel assembly cross section is divided into two by the diagonal line, a region containing a large number of water rod regions is referred to as a region N, and the other region is referred to as a region W. Of the smallest square area including the outermost corner fuel rods on the side of the area W and the water rods, the area L corresponding to the second and third layers from the outermost circumference of the fuel rod array of the fuel assembly includes 2 More than half of the fuel rods are placed in the area The third object of the present invention can be achieved by making all the first fuel rods of L contain fuel pellets with the highest enrichment.

[0018]

(Embodiment 1) Hereinafter, a first embodiment of a fuel assembly according to the present invention will be described with reference to FIG. FIG.
Indicates a cross section of the fuel assembly, 1 indicates a channel box, 2 indicates a fuel rod containing uranium dioxide pellets, 3 and 4 indicate fuel rods containing gadolinia, which is a burnable poison, and a burnable poison containing uranium dioxide, and 5 indicates a fuel rod. This is a large circular cross section water rod that functions as a neutron moderator rod. 9 fuel rods in 9 rows
The water rods are arranged in the form of a square lattice, and the water rods occupy an area corresponding to seven lattices of the fuel rod arrangement lattice. When the fuel assembly is loaded into the core, the control rod is inserted at the upper left.

The enrichment of the fuel rod is indicated by an alphabetic character attached to the fuel rod. In this embodiment, a = 4.9 wt%, b = 4.4 wt%,
c = 3.9 wt%, d = 3.6 wt%, e = 2.8 wt
%, F = 2.4 wt%. The fuel rods with G are burnable poison-containing fuel rods, and a to f indicate the uranium enrichment of the burnable poison-containing fuel pellets as described above. Therefore, the maximum enrichment is 4.9 wt% of 5 wt% or less, and the cross-sectional average enrichment is 4.64 wt%.

Within the second layer from the outermost periphery of the fuel rod array,
Six fuel rods containing burnable poison are placed, and all fuel rods containing no burnable poison are the highest enriched fuel rods. Also,
The fuel rods on the outermost two sides of the control rod non-insertion side area 8 at the lower right are all the fuel rods having the highest enrichment containing no burnable poison except for the corners. On the other hand, eight fuel rods with the highest enrichment and seven fuel rods with the low enrichment are arranged on the outermost two sides of the control rod insertion side region 9 at the upper left.

The water rod 5 of the present embodiment has a cross section of at least one grid of the fuel rod array, and the water rods 5 at diagonal positions of the fuel rod array are arranged.
The outermost corner fuel rods are arranged symmetrically with respect to a diagonal line 7 connecting the fuel rods. All the fuel rods containing burnable poison contained in the control rod non-insertion side area 8 at the lower right with respect to the diagonal 7 have a maximum enrichment of 4.9 wt%, and the burnable poison contained in the control rod insertion side area 9 at the upper left. The enrichment of the fuel rods containing the fuel rods is less than 4.4 wt%.

In order to determine the enrichment distribution of the D-lattice fuel, all the fuel rods are enriched at 4.9 wt% without burnable poisons.
FIG. 3 shows the result of evaluation by the inventors of the present invention on the fuel rod output distribution in the fuel assembly at the beginning of combustion. The number of the grid position indicates the magnitude of the fuel rod output, and when viewed as a relative output value, 1:
1.5 or more, 2: 1.5-1.4,3: 1.4-1.3
4: 1.3 to 1.2, 5: 1.2 to 1.15, 6: 1.15
~ 1.1,7: 1.1 ~ 1.05,8: 1.05 ~ 1.0,
9: 1.0 to 0.85, 10: less than 0.85.

Based on these results, considering the arrangement of fuel rods for increasing the cross-sectional average enrichment as much as possible under the enrichment constraint, the highest enrichment is set in ascending order of the position where the fuel rod output is unlikely to increase. It became clear that fuel rods should be arranged. Here, when comparing the control rod non-insertion side area 8 and the control rod insertion side area 9 for the outermost circumference, the number of the control rod non-insertion side is higher, and it can be seen that the fuel rod output is less likely to increase.

Therefore, when the highest enrichment fuel rod is placed in the control rod insertion side area 9, the control rod non-insertion side area 8 should have the highest enrichment except for the corners. In the control rod insertion side area 9, the fuel rod output is likely to be high at the corner and the three adjacent positions, so it is necessary to place a fuel rod with a lower enrichment. By observing the above rules regarding the enrichment distribution, the first object of the present invention is to increase the average enrichment of the cross-section while keeping the thermal margin by keeping the local output peaking at an appropriate value. It becomes possible.

Next, the enrichment of the fuel rod containing the burnable poison will be considered. Since fuel pellets containing burnable poisons have a slightly lower thermal conductivity, the fuel rod reactivity is relatively high, and the output of burnable poisoned fuel rods in the first half of life is sufficiently lower than that of fuel rods containing no burnable poison. It is necessary to control. Cross section average enrichment of 4.5 w
When it is set to t% or more, most of the control rod non-insertion side region 8 becomes the highest enrichment fuel rod as described above. At this time, as a result of examining the relative output of the burnable poison-containing fuel rod placed at a position other than the outermost periphery of the control rod non-insertion side region 8 and not adjacent to the water rod, about 25 GWd / t in the first half of the life was obtained. Up to 85% of the maximum output of fuel rods containing no burnable poisons.

Although the neutrons are effectively decelerated and the fuel rod output is likely to increase at the outermost periphery and the position adjacent to the large water rod, as shown by reference numeral 10 in FIG. It can be seen that the region 8 is the region where the fuel rod output is least likely to be high in the fuel assembly. Therefore,
It has been found that it is possible to place the highest enriched burnable poisoned fuel rod here. On the other hand, the control rod insertion side area 9
In this case, the fuel rod output is likely to be high, so that the fuel rod containing the burnable poison with the highest enrichment cannot be placed in all parts except the outermost periphery and the position adjacent to the water rod.

By determining the enrichment of the burnable poison-containing fuel rod as described above, the second object of the present invention, namely, to maintain the fuel integrity of the burnable poison-containing fuel rod having a slightly poor thermal conductivity. In the first half of the service life when the reactivity of the fuel assembly is relatively high, it is possible to keep the output of the burnable poison-containing fuel rod sufficiently lower than the output of the burnable poison-free fuel rod.

(Embodiment 2) Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 4 shows a cross section of the fuel assembly, and 6 is a water channel functioning as a neutron moderator rod. The fuel rods are arranged in a square grid with 9 rows and 9 columns.
The water rods occupy an area corresponding to nine grids of the fuel rod array grid.

The enrichment of the fuel rod is indicated by an alphabetic character attached to the fuel rod. In this embodiment, a = 4.9 wt%, b = 4.4 wt%,
c = 3.9 wt%, d = 3.6 wt%, e = 3.2 wt
%, F = 2.8 wt%. The fuel rods marked with G are burnable poisoned fuel rods, and the lower case letters indicate uranium enrichment of burnable poisoned fuel pellets as described above. Therefore, the maximum enrichment is 4.9 wt% and the cross-sectional average enrichment is 4.65 wt%. 3 from the highest concentration
Types a to c are set to 0.5 wt% difference,
It has 0% spacing.

Within the second layer from the outermost periphery of the fuel rod arrangement,
Six fuel rods containing burnable poison are placed, and all fuel rods containing no burnable poison are the highest enriched fuel rods. Also,
The fuel rods on the outermost two sides of the control rod non-insertion side region 8 are the fuel rods of the highest enrichment that do not contain any burnable poison except for the corners. On the other hand, on the outermost two sides of the control rod insertion side region 9, twelve fuel rods with the highest enrichment and three fuel rods with the low enrichment are arranged.

The water channel 6 of this embodiment occupies an area corresponding to nine grids of the fuel rod array, and is located at a diagonal position of the fuel rod array.
The outermost corner fuel rods are arranged asymmetrically with respect to a diagonal line 7 connecting the fuel rods. Control rod non-insertion side area 8
Contains many water channels. The enrichment of the burnable poison-containing fuel rod vertically and horizontally adjacent to the water channel 6 on the control rod non-insertion side is the third highest enrichment in the fuel assembly.

An outermost corner fuel rod on the control rod insertion side;
Of the smallest square area 10 including the water channel, in the area 11 corresponding to the outermost second and third layers, 10 or more fuel rods containing burnable poisons are placed. In addition, all burnable poison-free fuel rods located in region 11 are the highest enriched fuel rods.

FIG. 5 shows the output distribution in the fuel assembly when the water channel is located at a position shifted from the center of the fuel assembly. This is the result of the inventors' evaluation of the power distribution in the fuel assembly at the beginning of combustion, assuming that all the fuel rods are fuel rods containing no burnable poison and having a concentration of 4.9 wt%. The number of the grid position indicates the magnitude of the fuel rod output, and as a relative output value, 1: 1.5 or more, 2: 1.5 to 1.4, 3: 1.4 to
1.3, 4: 1.3 to 1.2, 5: 1.2 to 1.15, 6:
1.15-1.1,7: 1.1-1.0,8: 1.05-
1.0, 9: 1.0 to 0.85, 10: less than 0.85.

From these results, it is confirmed that, even when the water channels are arranged asymmetrically, the output of all fuel rods on the outermost periphery of the control rod non-insertion side is equal to or lower than that of the outermost periphery of the control rod insertion side except corners. Was done. Therefore, when the highest enrichment fuel rod is placed in the control rod insertion side region 9, all the control rod non-insertion side regions 8 except for the corners should have the highest enrichment.

In the control rod insertion side area 9, the fuel rod output is likely to be high at the corner and the three adjacent positions, so it is necessary to place a fuel rod with a lower enrichment. By observing the above rules regarding the enrichment distribution, the first object of the present invention is to increase the average enrichment of the cross-section while keeping the thermal margin by keeping the local output peaking at an appropriate value. It becomes possible.

Next, as a result of examining the enrichment of the fuel rods containing burnable poisons, the position of the lower right region adjacent to the water channel in the vertical and horizontal directions has a relatively high neutron moderating effect and the fuel rod output becomes high. I found it easy. Therefore,
Fuel rods with burnable poisons in these locations cannot be at maximum enrichment.

As a result of further quantitative evaluation, by setting the enrichment lower by about 20% than the maximum enrichment, the output of the burnable poison-containing fuel rod in the first half of the life is reduced to the maximum output of the fuel containing no burnable poison. On the other hand, it turned out that it can be suppressed to 85% or less. In general, the enrichment of the fuel pellets used in the fuel assembly is set so as to maintain an enrichment interval of 10% or more from each other for the convenience of manufacturing inspection. In this case, the enrichment lower by about 20% than the highest enrichment is the third highest. Therefore, the enrichment of the burnable poison-containing fuel rod adjacent to the water channel on the non-insertion side of the control rod including many water channels needs to be the third highest from the highest.

In the fuel assembly in which the water channels are arranged asymmetrically as in this embodiment, there is an effect that the asymmetry of the assembly interval is reduced and the power distribution is flattened. However, if attention is paid to the coolant flow in the fuel assembly, it is described in Japanese Unexamined Patent Application Publication No. 9-236676 that the limit output characteristic tends to be severe in the region 11 of FIG. Therefore, in the present invention, many fuel rods containing a burnable poison having a small heat output are arranged in the area 11, and all the fuel rods containing no burnable poison have the highest enrichment. With this measure,
The third object of the present invention, that is, when a water rod or a water channel is arranged at a position shifted from the center of the fuel assembly for the D grid, while maintaining the critical output of the fuel assembly in consideration of the bias of the coolant flow, It is possible to increase the cross-sectional average enrichment.

The fuel assembly of the present invention has a feature that the local power peaking of the cross section is small because the large water channel region is arranged asymmetrically as compared with the fuel assembly of the first embodiment.

(Embodiment 3) Next, a third embodiment of the present invention will be described with reference to FIG. The fuel rods are arranged in a square grid of 9 rows and 9 columns, and the water rods occupy an area corresponding to 9 grids of the fuel rod array grid. The enrichment of the fuel rods is indicated by the letters attached to the fuel rods and is the same as in FIG. Maximum concentration is 4.9
wt%, and the average cross-sectional enrichment is 4.68 wt%.

Within the second layer from the outermost periphery of the fuel rod array,
Six fuel rods containing burnable poison are placed, and all fuel rods containing no burnable poison are the highest enriched fuel rods. Also,
The fuel rods on the outermost two sides of the control rod non-insertion side region 8 are the fuel rods of the highest enrichment that do not contain any burnable poison except for the corners. On the other hand, on the outermost two sides of the control rod insertion side region 9, twelve fuel rods with the highest enrichment and three fuel rods with the low enrichment are arranged.

The water channel 6 of this embodiment occupies a region corresponding to nine grids of the fuel rod array, and is located at a diagonal position of the fuel rod array.
The outermost corner fuel rods are arranged asymmetrically with respect to a diagonal line 7 connecting the fuel rods. Control rod non-insertion side area 8
Is a region containing many water channels, and there is no burnable poison-containing fuel rod adjacent to the water channel 6 in the vertical and horizontal directions on the control rod non-insertion side.

An outermost corner fuel rod on the control rod insertion side;
Of the smallest square area 10 including the water channel, in the area 11 corresponding to the outermost second and third layers, 10 or more fuel rods containing burnable poisons are placed. Further, all the fuel rods containing no burnable poison placed in the area 11 are the highest enrichment fuel rods.

When the water channel is arranged at the center of the fuel assembly, the fuel rod output in the control rod non-insertion side region 8 is low as shown in FIG. On the other hand, by arranging the water channel asymmetrically, the neutron moderating effect is increased on the non-insertion side of the control rod of the water channel and the fuel rod output is increased, so that the output distribution in the fuel assembly is flattened. is there.
However, if a burnable poisoned fuel rod is placed there,
It has been found that the effect of flattening the power distribution is reduced because the decelerated thermal neutrons are absorbed.

Therefore, when the water channels are arranged asymmetrically, it is effective to place the burnable poison-containing fuel rods as little as possible on the side of the region containing many water channels. The fuel assembly of FIG. 6 in which the arrangement of the burnable poison-containing fuel rods is different from that of the fuel assembly of FIG. 3 has an effect of reducing the output peaking generated on the control rod insertion side by about 2%.

(Embodiment 4) Next, a fourth embodiment of the present invention will be described with reference to FIG. In the present embodiment, the position and the enrichment of the fuel rod containing the burnable poison are changed from the fuel assembly of the first embodiment. The maximum enrichment is 4.9 wt% and the cross-sectional average enrichment is 4.63 wt%.

In the second layer from the outside of the fuel rod arrangement, 16
The fuel rods containing burnable poison are placed, and all fuel rods containing no burnable poison are the highest enriched fuel rods. The fuel rods on the outermost two sides of the control rod non-insertion side region 8 are the fuel rods of the highest enrichment that do not contain any burnable poison except for the corners. On the other hand, on the outermost two sides of the control rod insertion side region 9, eight fuel rods with the highest enrichment and seven fuel rods with the low enrichment are arranged. The water rod 5 of the present embodiment has a cross section of at least one grid of the fuel rod array, and is symmetrical with respect to a diagonal line 7 connecting two outermost corner fuel rods at diagonal positions of the fuel rod array. Are located. All the fuel rods containing burnable poison contained in the control rod non-insertion side area 8 with respect to the diagonal line 7 have a maximum enrichment of 4.9 wt%. Concentration 4.
4 wt% or less is included.

In this embodiment, since the fuel rods containing burnable poisons that absorb neutrons are concentrated at the corners of the second layer, the output of the fuel rods near the corners is 2 compared to the fuel assembly of FIG. %. However, since the output of the fuel rod is likely to be higher at the position from the corner, it is necessary to keep the concentration of the fuel rod containing the burnable poison low. Therefore, the cross-sectional average enrichment is slightly lower than that of the fuel assembly of FIG.

[0049]

According to the present invention, it is possible to increase the average enrichment while flattening the power distribution in the cross section of the fuel assembly for the D lattice. As a result, it is possible to obtain a fuel assembly suitable for high burnup exceeding 45 GWd / t and continuous operation for 18 months or more without impairing the thermal margin.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a first embodiment of the present invention.

FIG. 2 is a longitudinal sectional view showing a schematic structure of a fuel assembly.

FIG. 3 is a diagram showing a power distribution in a transverse section of a fuel assembly in which a speed reduction rod is arranged at a symmetric position.

FIG. 4 is a transverse sectional view showing a second embodiment of the present invention.

FIG. 5 is a diagram showing a power distribution in a transverse section of a fuel assembly in which a speed reduction rod is arranged at an asymmetric position.

FIG. 6 is a transverse sectional view showing a third embodiment of the present invention.

FIG. 7 is a transverse sectional view showing a fourth embodiment of the present invention.

[Explanation of symbols]

1 ... channel box, 2 ... fuel rod, 3 ... fuel rod with burnable poison, 4 ... fuel rod with burnable poison of highest enrichment,
5 ... water rod, 6 ... water channel, 7 ... diagonal line, 8 ... control rod non-insertion side area, 9 ... control rod insertion side area, 10 ... control rod side corner square area including fuel rod and water channel, 11... The second and third layer areas of the smallest square area including the fuel rod and water channel on the control rod side, 12.
Upper tie plate, 13 ... Lower tie plate, 14 ... Spacer.

Claims (6)

[Claims] 1. A fuel assembly in which a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are arranged in a square lattice of 9 rows and 9 columns or more. The fuel pellets used have an enrichment of 5 wt% or less and a cross section having an average enrichment of 4.5 wt% or more. In the cross section, the fuel pellets are placed within the second layer from the outermost periphery of the fuel rod array. The first fuel rods are all the fuel rods having the highest enrichment. When the cross section is divided into two by a diagonal line connecting the centers of the two outermost corner fuel rods at diagonal positions of the fuel rod arrangement, The outermost fuel rods are all the first enriched fuel rods except for the corners, and the outermost outermost fuel rods on the other side are one or more first enriched fuel rods at positions other than the corners. The first fuel rods not having the highest enrichment of more than one are arranged. Fuel assembly. 2. A fuel assembly in which a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are arranged in a square grid of 9 rows and 9 columns or more. The enrichment of the fuel pellets used is 5 wt% or less, and the cross section has an average enrichment of 4.5 wt% or more. In the cross section, the two outermost corner fuels at diagonal positions of the fuel rod array are provided. At least one large water rod having a cross section of at least one grid is disposed so as to be symmetrical with respect to a diagonal line connecting the rod centers, and all second fuel rods included on one side with respect to the diagonal line The enrichment includes the largest burnable poison-containing pellet, and is arranged so as not to be adjacent to the water rod in the vertical and horizontal directions at positions other than the outermost periphery, and on the other side with respect to the diagonal, the outermost periphery and the outermost Next to water rod vertically and horizontally Besides position, the fuel assembly, wherein the second fuel rod lower enrichment is placed. 3. A fuel assembly in which a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are arranged in a square grid of 9 rows and 9 columns or more. The enrichment of the fuel pellets used is 5 wt% or less, and the cross section has an average enrichment of 4.5 wt% or more. In the cross section, the two outermost corner fuels at diagonal positions of the fuel rod array are provided. At least one large water rod having a cross section of one lattice or more is arranged so as to be asymmetric with respect to a diagonal line connecting the rod centers, and the cross section is bisected by the diagonal line, and a large amount of water rod area is included. A fuel assembly wherein the second fuel rod is not adjacent to the water rod in the vertical and horizontal directions on the side of the water rod in the area N when the other side is the area N and the other is the area W. 4. A fuel assembly in which a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are arranged in a square grid of 9 rows and 9 columns or more. The enrichment of the fuel pellets used is all 5 wt% or less, and the three enrichments with the highest enrichment are 10%
It has a cross section having the above enrichment intervals and an average enrichment of 4.5 wt% or more, and in this cross section, a diagonal line connecting the two outermost corner fuel rod centers at diagonal positions of the fuel rod arrangement. At least one large water rod having a cross section of one grid or more is arranged so as to be asymmetrical with respect to the area N. When the other is a region W, the second region adjacent to the water rod in the vertical and horizontal directions on the region N side
The fuel assembly, wherein the burnable poison-containing fuel pellets contained in the fuel rod have the third highest enrichment in the fuel assembly. 5. A fuel assembly in which a plurality of first fuel rods containing no burnable poison and a plurality of second fuel rods containing burnable poison are arranged in a square grid of 9 rows and 9 columns or more. The enrichment of the fuel pellets used is 5 wt% or less, and the cross section has an average enrichment of 4.5 wt% or more. In the cross section, the two outermost corner fuels at diagonal positions of the fuel rod array are provided. At least one large water rod having a cross section of at least one lattice is arranged so as to be asymmetric with respect to a diagonal line connecting the rod centers, and the diagonal line divides the fuel assembly cross section into two, and the water rod area is large. When the included region is the region N and the other region is the region W, two layers from the outermost periphery of the fuel rod array of the fuel assembly in the smallest square region including the outermost corner fuel rod on the side of the region W and the water rod. In the area L corresponding to the eyes and the third layer, More than half of the second fuel rod is placed in the plane, all of the first fuel rod region L, the fuel assembly, characterized in that it contains the highest enrichment of the fuel pellets. 6. The maximum concentration is 4.9 to 4.95 wt%.
The fuel assembly according to any one of claims 1 to 5, wherein