JP2001272489A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel assembly used in an initial loading core with raised average enrichment for higher burnup which is capable of ensuring thermal margin, properly suppressing excess reactivity and improving fuel economy. SOLUTION: In the middle of a fuel assembly of 9×9 fuel rod array, a water rod or a water box is arranged, fuel rods with Gd are arranged in positions except the outermost regions and the fuel assembly is separated with the diagonal into a channel fastener side region and an anti-channel fastener side region. In this case, more than 10 fuel rods with Gd are arranged in the anti-channel fastener side region and so the number of fuel rods with Gd existing in the anti-channel fastener side region is equal to or more than 3 compared with the number existing in the channel fastener side region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly to be loaded in an initially loaded core of a boiling water reactor (hereinafter, abbreviated as BWR).

[0002]

2. Description of the Related Art Reactors continue to emit energy through a chain reaction in which neutrons are absorbed by fissile material to cause fission, and neutrons released with energy cause the next fission. The state in which this chain reaction is in equilibrium is called critical, and a reactor operated at a constant output keeps this state. A state in which the chain reaction increases is called supercritical, and a state in which the chain reaction decreases decreases is called subcritical.

[0003] Since the nuclear reactor must continue to operate without refueling for a certain period of time, the core is loaded with more fissile material than is necessary for maintaining criticality. Thus, the reactor will be supercritical without control materials.
The excess reactivity is referred to as excess reactivity, and it is important to appropriately control the excess reactivity throughout the operation period. As a technique for controlling the excess reactivity throughout the operation period, a technique of mixing a burnable poison into fuel is well known.
The burnable poison is a neutron absorbing material that gradually burns during the operation period and its amount decreases, and gadolinia and the like used in combination with a nuclear fuel material are known.

Next, the manner of suppressing the reactivity of the burnable poison will be described with reference to the drawings. FIG. 22 shows an example of an infinite multiplication factor burnup change of a fuel assembly containing gadolinia, which is a kind of burnable poison. In general, if the number of fuel rods containing burnable poisons increases, the infinite multiplication factor at the beginning of combustion decreases. In addition, if the concentration of the burnable poison to be mixed is increased, the gadolinia burnout time can be delayed, and as a result, the maximum value of the infinite multiplication factor can be suppressed. By using this effect, the excess reactivity can be appropriately controlled by a combination of the concentration of the burnable poison and the number of fuel rods into which the burnable poison is mixed.

Next, the improvement in fuel economy of the initially loaded core of the nuclear reactor will be described. In the first loaded core, a part of the loaded fuel assembly is removed after the operation of the first cycle is completed, and is replaced with a new replacement fuel assembly. The fuel assemblies taken out in the first cycle have lower burnup and lower energy generation than other fuel assemblies. Therefore, in order to effectively utilize the fissile material, an initially loaded core using a plurality of fuel assemblies in which the uranium enrichment is changed according to the staying time in the reactor is known.

[0006] As a conventional initially loaded core, Japanese Patent Laid-Open Publication No.
No. 49270 discloses that the average enrichment of a fuel assembly is 3.4%.
High-enrichment fuel assembly (hereinafter referred to as high-enrichment fuel),
The core consists of three types: 2.3% medium enriched fuel assembly (hereinafter referred to as medium enriched fuel) and 1.1% low enriched fuel assembly (hereinafter referred to as low enriched fuel). Have been. Further, in order to effectively utilize fissile material, it is described that a fuel assembly with a low enrichment is taken out of the core earlier, and a fuel assembly with a high enrichment is loaded into the core for a longer time.

[0007] In order to further increase the discharge burnup of the initially loaded core, it is necessary to further increase the average enrichment of the core and prolong the residence time of the fuel assembly in the reactor. For this reason, as described above, when the initial loading core is configured by combining various types of fuel assemblies having different average enrichments, the enrichment difference between the fuel assemblies increases, and the high enrichment fuel and the low enrichment The difference in nuclear properties with fuel has increased. When fuel assemblies having such a large difference in nuclear properties are adjacent to each other, neutron exchange occurs because the neutron spectrum of each fuel assembly is different. As a result, the maximum linear power density and the minimum critical power ratio become severe in the early stage of combustion, and it has been a problem to improve the thermal margin.

Conventionally, gadolinia-containing fuel rods (hereinafter referred to as gadolinia rods) are arranged as symmetrically as possible in the cross section of the fuel assembly from the viewpoint of improving the thermal margin. Since the number of gadolinia rods is determined from the viewpoint of appropriately controlling the excess reactivity of the core as described above, it is not always a multiple of 2 or a multiple of 4. Therefore, the fuel assembly does not have a completely symmetrical arrangement in the lattice arrangement. FIG. 23 shows an example of a conventional arrangement of gadolinia rods in a fuel assembly. in this case,
Of the fuel rods arranged in a 9 × 9 (9 rows × 9 columns) square lattice, the number of the gadolinia rods 12 is thirteen, and they cannot be arranged completely symmetrically on the four sides of the square. Maintains symmetry.

[0009]

If the average enrichment of the core is increased with the aim of increasing the burnup of the initially loaded core, the excess reactivity will increase and it will be necessary to insert many control rods into the core. Therefore, the radial peaking is increased, and the thermal margin is reduced. Also, the thermal margin is reduced due to the difference in nuclear properties between the fuel assemblies.

[0010] It is an object of the present invention to provide an initially loaded core having an increased average enrichment for higher burnup, which can appropriately suppress excess reactivity while securing a thermal margin, thereby improving fuel economy. It is to provide a fuel assembly for use in a loaded core.

[0011]

In order to obtain the above-mentioned effects, an initially loaded core according to the present invention comprises a plurality of fuel assemblies having a substantially square cross section and different average enrichments, and a plurality of fuel assemblies having a different cross section. A plurality of control rods in the shape of a letter, and one low enrichment fuel assembly having the lowest average enrichment and three fuel assemblies having an average enrichment higher than the low enrichment fuel assembly are arranged in a square, The control rods are arranged one at each of the four corners of the square to form a unit loading pattern, a plurality of the unit loading patterns are provided in a central region of the core, and the plurality of unit loading patterns are divided into four units. Each of the low-enrichment fuel assemblies in the loading pattern is arranged adjacent to each other to form a square control cell, and a fuel assembly having an average enrichment higher than the low-enrichment fuel assembly is controlled by its diagonal. Rod side area and anti-control rod side area If you divided into regions, the number of gadolinia containing fuel rods present in the counter-control rod side area, is from three or more number the number of gadolinia containing fuel rods present in the control rod side area.

A fuel assembly according to the present invention used for the first loaded core includes a plurality of fuel rods arranged in a 9 × 9 square lattice, and an upper tie plate supporting the plurality of fuel rods at an upper portion and a lower portion. And a lower tie plate, a channel fastener attached to one corner of the upper tie plate, and a water rod or water box through which cooling water flows, wherein the water rod or the water box is the fuel assembly. Wherein the plurality of fuel rods include gadolinia-filled fuel rods,
The gadolinia-containing fuel rod is disposed at a position excluding the outermost periphery of the fuel assembly, and the fuel assembly is divided into a channel fastener side region and an anti-channel fastener side region by a diagonal line. Are arranged in the anti-channel fastener side area, and the number of gadolinia-containing fuel rods present in the anti-channel fastener side area is three more than the number of gadolinia-containing fuel rods present in the channel fastener side area. More.

According to the initially loaded core of the present invention, in the central region of the core, the plurality of unit loading patterns are such that the four unit loading patterns are rotationally symmetric with respect to each corner on the side of the low enrichment fuel assembly. , The loading pattern in the central region can be made substantially uniform, so that peaking in the radial direction can be reduced. In this case, the central region can be configured with one type of unit loading pattern, so that the core configuration can be extremely simplified.

In order to see the effect of reducing the peaking in the radial direction according to the present invention, an initial loading core according to the present invention shown in FIG. 1 to be described later and an initial loading core described in Japanese Patent Application Laid-Open No. 5-249270 (conventional example). FIG. 5 shows the results of comparing the combustion changes in the radial peaking coefficient. It can be seen from the figure that the present invention can significantly reduce the radial peaking and increase the thermal margin as compared with the conventional example.

Further, from the viewpoint of thermal margin, the fuel assemblies located in the high-power portion of the reactor core have a severe thermal margin in the initially loaded core. As shown in the comparative example of FIG. 6, when the low-enrichment fuel 8 and the high-enrichment fuel 9a are adjacent, neutrons enter and exit between these fuels. Therefore, of the fuel rods in the high-enrichment fuel 9a, the outermost fuel rods adjacent to the low-enrichment fuel 8 due to the influx of the thermal neutron flux from the low-enrichment fuel 8 as shown in FIG. Fuel rod output increases.

On the other hand, in the present invention, FIG.
As shown in (2), in the highly enriched fuel 9, the gadolinia rods 12 are arranged intensively on the non-control rod side of the fuel assembly, so that the output on the non-control rod side can be suppressed low. That is, the output of the outermost fuel rods adjacent to the low-enrichment fuel 8 among the fuel rods in the high-enrichment fuel 9 can be reduced. In this case as well, the fuel rod output increases due to the influx of the thermal neutron flux from the low enrichment fuel 8, but the fuel rod output of the entire fuel assembly can be flattened as compared with the comparative example of FIG. . FIG. 8 shows this flattening. Such flattening of the fuel rod output also contributes to increasing the thermal margin.

FIG. 9 shows the case where the position of the gadolinia rod in the fuel assembly is changed and the number n ₁ of the gadolinia rods in the non-control rod side region is changed.
N ₁ between the control rod side area and the number n ₂ of gadolinia rods in the control rod side area
−n ₂ (hereinafter referred to as the gadolinia bar number difference) is plotted on the horizontal axis,
The maximum value of the fuel rod output is shown on the vertical axis.
Here, the control rod side region and the non-control rod side region are defined as a region close to the control rod 7 when the fuel assembly is divided into two regions by a diagonal line 19 as shown in FIG. 13
The area far from the control rod 7 is referred to as an anti-control rod side area 14. However, if there is a gadolinia rod passing through the diagonal line 19, 0.5 is added to each of the gadolinia rods in the control rod side area and the non-control rod side area. The values on the vertical axis in FIG. 9 are normalized by the maximum value of the fuel rod output of the fuel assembly shown in FIG. For example, points A and B in FIG. 9 correspond to the maximum values of the fuel rod outputs of the fuel assemblies shown in FIGS. 11 and 12, respectively.

FIG. 9 shows that the maximum value of the fuel rod output can be effectively reduced by setting the difference in the number of gadolinia rods to two or more. As shown in the figure, when the difference in the number of gadolinia rods becomes larger than two, the decreasing rate of the maximum value of the fuel rod output tends to gradually become saturated. Therefore, in order to more effectively reduce the fuel rod output, it is preferable to make the difference in the number of gadolinia rods three or more in which the tendency of saturation appears. Since the fuel assembly has a channel fastener for fixing the fuel assembly to the control rod at a corner portion on the control rod side, the above-described control rod side area corresponds to the channel fastener side area.

FIG. 9 does not show the upper limit of the difference in the number of gadolinia bars. However, there is an upper limit to the difference in the number of gadolinia rods, which is about １ ／ of the total number of fuel rods in the fuel assembly. This is because the gadolinia rods are arranged at positions other than the outermost periphery of the fuel assembly, so that more gadolinia rods than the fuel assembly shown in FIG. is there. In the case of FIG. 13, the difference in the number of gadolinia rods is 19, which is about 1/4 of the total number of fuel rods 74.

Next, the surplus reactivity will be described. By arranging gadolinia rods intensively in the non-control rod side region of the fuel assembly, gadolinia, which is a strong thermal neutron absorber, is increased around the gadolinia rods, so that the thermal neutron flux is reduced. For this reason, the combustion of gadolinia is delayed, and the same effect as when the concentration of gadolinia is increased can be obtained.
FIG. 14 shows a comparison between the conventional example in which the gadolinia rods are uniformly arranged in the fuel assembly and the infinite multiplication factor change of the present invention in which the gadolinia rods are intensively arranged in the region opposite to the control rod in the fuel assembly. Shown in As shown in the figure, according to the present invention, since the combustion of gadolinia is delayed, the infinite multiplication factor can be suppressed smaller than in the conventional example at the time when the combustion of the fuel assembly has advanced. Thereby, even if the average enrichment of the fuel is increased, the surplus reactivity can be suppressed to be low, so that a high burnup can be achieved.

As described above, according to the present invention, since the excess reactivity can be appropriately suppressed while securing the thermal margin, the first loading core having high fuel economy and high fuel economy can be realized. it can.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the first loading core according to the present invention.
FIG. 2 shows a cross-sectional view of the embodiment. This core is 2
Forty low-enriched fuels, 8,296 high-enriched fuels 9 and 336 high-enriched fuels 10, constitute a total of 872 fuel assemblies. Here, the highly enriched fuels 9 and 1
0 indicates that the arrangement of gadolinia rods in the fuel assembly is different as described later.

A unit loading pattern as shown in FIG. 2 is arranged in the high power region 1 where the power in the central portion in the core becomes high. FIG. 2 is a cross-sectional view of a first embodiment of the unit loading pattern according to the present invention. The unit loading pattern includes one low-enrichment fuel 8 and two high-enrichment fuels 9.
And one high-enrichment fuel 10, and these fuel assemblies are configured so that four cross sections are surrounded by control rods 7 having a cross shape. In each of the fuel assemblies constituting the unit loading pattern, the fuel rods 6 are arranged in a 9 × 9 square lattice shape (the outer shape of the cross section is substantially square), and water flows through the center of the fuel assembly. Two water rods 5 having a diameter are arranged. Here, the two water rods 5 are installed in an area where seven fuel rods can be arranged.

In the high power region 1, the four unit loading patterns are arranged such that each low enrichment fuel 8 constitutes a control cell 11 adjacent to each other. That is, the four unit loading patterns are arranged rotationally symmetrically with respect to the center of the control cell 11. In the case of FIG. 1, 37 control cells 11 are configured. The control cell 11 controls the control rod 7 when the reactor is operated.
Is inserted into the core to suppress the excess reactivity of the entire core. In the core shown in FIG. 1, the loading patterns of the fuel assemblies in the high power region 1 are all uniform, so that radial peaking in the core can be reduced. This allows
Thermal margin can be increased.

The number of gadolinia rods 12 in the highly enriched fuel 9 constituting the unit loading pattern shown in FIG. 2 is 16, five in the control rod side area, and eleven in the non-control rod side area. The difference between the numbers is six. On the other hand, the number of gadolinia rods 12 in the high-enrichment fuel 10 is 16, six in the control rod side region and ten in the non-control rod side region, and the difference in the number is four. . in this way,
By arranging many gadolinia rods in the non-control rod side region in the highly enriched fuel constituting the unit loading pattern, the output of each fuel rod in the unit loading pattern is flattened. This effect can be obtained by increasing the number of gadolinia rods in the non-control rod side area by two or more than the control rod side area, and more effective by providing a difference of four or more as in the present embodiment. is there. This also contributes to an increase in thermal margin.

Further, by arranging a large number of gadolinia rods in the non-control rod side region as described above, the combustion of gadolinia can be delayed as described above, so that the infinite multiplication factor can be suppressed even if the fuel advances. it can. Therefore, even if the average enrichment of the fuel is increased, the surplus reactivity can be suppressed low, so that the burnup can be increased and the fuel economy can be improved.

As shown in a partially cutaway perspective view of FIG. 4, an actual fuel assembly includes an upper tie plate 4a, a lower tie plate 4d, a channel fastener 4b, a spacer 4c, a water rod 5 (not shown), a fuel It comprises a bar 6, a channel box 4, and the like. Of these, the channel fastener 4b attached to one corner of the upper tie plate 4a is for fixing the fuel assembly to the control rod, so that when viewed as a single fuel assembly, the control rod side region is the channel fastener. Corresponds to the area on the side where.

FIG. 3 shows the distribution of enrichment and gadolinia in the axial direction of the highly enriched fuel 9 in FIG. The highly enriched fuel 9 includes fuel rods a1 to a4 containing uranium fuel over the entire effective fuel length and not containing gadolinia, and gadolinia containing uranium fuel only in the range of 1/24 to 15/24 from the bottom of the active fuel length. (Hereinafter referred to as "part length fuel rod") a5, and 1/24 to 15 /
A fuel rod a6 containing uranium fuel only in the range of 24 and containing gadolinia in the range of 1/24 to 8/24 from the bottom of the active fuel length; And a fuel rod a7 containing gadolinia in the range of 1/24 to 22/24. The number of each fuel rod is as shown in FIG.

The fuel rods a1 to a4 and a7 have natural uranium (enrichment 0.711w) in the lower end region of 1/24 from the lower end of the active fuel length and in the upper end region of 2/24 from the upper end of the active fuel length.
t%). The fuel rod a7 is loaded with 4.4 wt% of uranium fuel and 7.5 wt% of gadolinia in the range of 1/24 to 22/24 from the bottom of the active fuel length. The fuel rod a6 contains 4.4 wt% of uranium fuel and 7.5 wt% of gadolinia in the range of 1/24 to 8/24 from the bottom of the active fuel length, and 8/24 to 15/24 from the bottom of the active fuel length. Is loaded with only 4.4 wt% uranium fuel.
The fuel rod a6 is provided at the corner of the second layer from the outside on the side opposite to the control rod of the fuel assembly. Also, the fuel rod a
In No. 5, only 4.9 wt% of uranium fuel is loaded on the entire length of the partial length fuel rod.

The highly enriched fuel 9 is formed by combining the fuel rods shown in FIG.
The average enrichment in the section perpendicular to the axial direction in the region of 4.
15/24 to 22/2 from 59th to 59wt%
The average enrichment in the cross section perpendicular to the axial direction in the region 4 is 4.56 wt%.

On the other hand, the low-enrichment fuel 8 shown in FIG. 2 does not include gadolinia, and is 1/24 to 8 / 24,8
The average enrichment in the cross section perpendicular to the axial direction in each region of / 24 to 15/24 and 15/24 to 22/24 is 1.49, 1.64, and 1.75 wt%, respectively. Natural uranium is loaded in the lower end region and the upper end region of the active fuel length, similarly to the high-enrichment fuel 9.

Next, a second embodiment of the unit loading pattern according to the present invention will be described with reference to FIG. FIG.
FIG. 2 shows a cross-sectional view of the embodiment. The number of gadolinia rods 12 in the highly enriched fuel 9 constituting this unit loading pattern is thirteen, three in the control rod side area and ten in the non-control rod side area. Is seven. On the other hand, the number of gadolinia rods 12 in the highly enriched fuel 10 is 12, six in the control rod side area and six in the non-control rod side area, and the difference between the numbers is zero. .

As described with reference to FIG. 7, the reason why the fuel rod output is increased by the influence of the thermal neutron flux from the low-enrichment fuel 8 is that the high-enrichment fuel 9 is higher than the high-enrichment fuel 10. Remarkable. Therefore, as in the present embodiment, the difference in the number of gadolinia rods of at least the highly enriched fuel 9 is made two or more to three, so that the fuel rod output can be flattened, the thermal margin can be increased, and the excess reactivity can be increased. And the fuel economy can be improved.

Next, a third embodiment of the unit loading pattern according to the present invention will be described with reference to FIG. FIG.
FIG. 2 shows a cross-sectional view of the embodiment. The number of gadolinia rods 12 in the highly enriched fuel 9 constituting this unit loading pattern is 11, 3.5 in the control rod side area, and 7.5 in the non-control rod side area. The difference in the number is 4
It is a book. On the other hand, gadolinia rod 1 in high-enrichment fuel 10
The number 2 is thirteen, six in the control rod side area and seven in the non-control rod side area, and the difference in the number is one. In this embodiment, as in the second embodiment of FIG. 15, the fuel rod output can be flattened to increase the thermal margin, and the excess reactivity can be suppressed to improve the fuel economy.

Next, a fourth embodiment of the unit loading pattern according to the present invention will be described with reference to FIG. FIG.
FIG. 2 shows a cross-sectional view of the embodiment. In this embodiment,
In addition to two high enrichment fuels 9 and 10 having different gadolinia rod arrangements, a low enrichment fuel 8 and a medium enrichment fuel 16 are used. The average enrichment of the medium enrichment fuel 16 is lower than the high enrichment fuels 9 and 10 and higher than the low enrichment fuel 8.

The number of gadolinia rods 12 in the highly enriched fuel 9 is 12, two in the control rod side area and ten in the non-control rod side area, and the difference in the number is eight. is there. The number of gadolinia rods 12 in the high-enrichment fuel 10 is 9
Four in the control rod side region and five in the non-control rod side region, and the difference in the number is one. The number of gadolinia rods 12 in the medium enrichment fuel 16 is five, two in the control rod side area and three in the non-control rod side area, and the difference between the numbers is one.

In this embodiment, as in the second embodiment shown in FIG. 15, the fuel rod output can be flattened to increase the thermal margin, and the excess reactivity can be suppressed to improve the fuel economy.
FIG. 18 shows a cross-sectional view of the second embodiment of the initially loaded core in which the unit loading pattern of this embodiment is loaded in the high power region of the core.

Next, a fifth embodiment of the unit loading pattern according to the present invention will be described with reference to FIG. FIG.
FIG. 2 shows a cross-sectional view of the embodiment. Of the four fuel assemblies that make up the unit loading pattern of this embodiment, three of the high enrichment fuels 9 and 10 are the same as in the second embodiment of FIG. different. In the low-enrichment fuel 8 of this embodiment, the fuel rods 6 are arranged in an 8 × 8 (8 rows × 8 columns) square lattice shape, and one water rod is provided in a central area where four fuel rods can be arranged. 5 are arranged.

The number of gadolinia rods 12 in the highly enriched fuel 9 constituting this unit loading pattern is thirteen, three in the control rod side area and ten in the non-control rod side area. The difference between the numbers is seven. On the other hand, high enrichment fuel 1
The number of gadolinia rods 12 in 0 is twelve, six in the control rod side area and six in the non-control rod side area.
The difference between the numbers is zero. Even when the unit loading pattern is formed using fuel assemblies having different shapes as in the present embodiment, the fuel rod output can be flattened and the thermal margin can be increased as in the second embodiment of FIG. In addition, surplus reactivity can be kept low to improve fuel economy.

Next, a sixth embodiment of the unit loading pattern according to the present invention will be described with reference to FIG. FIG.
FIG. 2 shows a cross-sectional view of the embodiment. The four fuel assemblies constituting the unit loading pattern of this embodiment are all shown in FIG.
It has the same shape as the low-enrichment fuel 8 shown in FIG.

In FIG. 20, the fuel assembly 18 is a MOX fuel containing plutonium from the time of core loading. The fuel rods of the fuel assembly 18 include gadolinia-free fuel rods 2.
There is a gadolinia rod 21 including 0 and gadolinia. At least a portion of the fuel rod 20 contains plutonium. The gadolinia rod 21 includes gadolinia,
At least a portion of this fuel rod also contains plutonium. Of course, the gadolinia rod 21 may be configured not to include plutonium.

The number of gadolinia rods 21 in the fuel assembly 18 is 12, four in the control rod side area and eight in the non-control rod side area, and the difference in the number is four. .
The number of gadolinia rods 12 in the highly enriched fuel 9 is eight.
Four in the control rod side area and four in the non-control rod side area, and the difference between the numbers is zero. Even if the shape of the fuel assembly is changed as in the present embodiment, at least the difference in the number of gadolinia rods of the fuel assembly 18 is made two or more to four, thereby flattening the fuel rod output and increasing the thermal margin. In addition to this, the excess reactivity can be kept low and the fuel economy can be improved.

In this embodiment, the fuel assembly 18 containing two plutoniums and the low-enrichment fuel 8 and the high-enrichment fuel 9 which are the two fuel assemblies not containing plutonium are diagonally separated from each other. However, for example, a configuration may be adopted in which the highly enriched fuel 9 is replaced with the fuel assembly 18.

Next, a seventh embodiment of the unit loading pattern according to the present invention will be described with reference to FIG. FIG.
FIG. 2 shows a cross-sectional view of the embodiment. The four fuel assemblies constituting the unit loading pattern of the present embodiment have fuel rods arranged in a 9 × 9 square lattice, as in the first embodiment of FIG. A water box 15 is provided. The water box 15 occupies an area where nine fuel rods can be arranged, and the number of fuel rods in the fuel assembly is 72.

The number of gadolinia rods 12 in the highly enriched fuel 9 of this embodiment is fifteen, five in the control rod side area and ten in the non-control rod side area. Are five. The number of gadolinia rods 12 in the high-enrichment fuel 10 is 16, six in the control rod side region and ten in the non-control rod side region, and the difference in the number is four.
It is a book. Also in this embodiment, by making the difference in the number of gadolinia rods of the high-enrichment fuel four or more, the fuel rod output can be flattened to increase the thermal margin, as in the first embodiment in FIG. Reactivity can be kept low to improve fuel economy.

[0046]

According to the present invention, it is possible to realize a fuel assembly to be used for an initially loaded core capable of achieving a high burnup, which can appropriately suppress excess reactivity while securing a thermal margin and improve fuel economy. .

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a first embodiment of an initially loaded core according to the present invention.

FIG. 2 is a cross-sectional view of the first embodiment of the unit loading pattern according to the present invention.

FIG. 3 is a diagram showing the enrichment and gadolinia distribution in the axial direction of the highly enriched fuel of FIG. 2;

FIG. 4 is a partially cutaway perspective view of a fuel assembly according to the present invention.

FIG. 5 is an explanatory diagram of a reduction effect of radial peaking according to the present invention.

FIG. 6 is a diagram showing a comparative example of the present invention.

FIG. 7 is a view showing a fuel rod output distribution of the comparative example of FIG. 6;

FIG. 8 is a diagram showing a fuel rod output distribution of the present invention.

FIG. 9 is a graph showing the relationship between the difference in the number of gadolinia rods in the non-control rod side region and the control rod side region and the maximum value of the fuel rod output.

FIG. 10 is an explanatory diagram of a non-control rod side area and a control rod side area.

FIG. 11 is a transverse sectional view of the fuel assembly corresponding to a point A in FIG. 9;

FIG. 12 is a cross-sectional view of the fuel assembly corresponding to a point B in FIG. 9;

FIG. 13 is an explanatory diagram of an upper limit of the difference in the number of gadolinia bars.

FIG. 14 is a diagram showing a change in burnup at an infinite multiplication factor according to the present invention.

FIG. 15 is a cross-sectional view of a second embodiment of the unit loading pattern according to the present invention.

FIG. 16 is a cross-sectional view of a third embodiment of the unit loading pattern according to the present invention.

FIG. 17 is a cross-sectional view of a fourth embodiment of the unit loading pattern according to the present invention.

FIG. 18 is a cross-sectional view of a second embodiment of the initially loaded core according to the present invention.

FIG. 19 is a cross-sectional view of a fifth embodiment of the unit loading pattern according to the present invention.

FIG. 20 is a cross-sectional view of a sixth embodiment of the unit loading pattern according to the present invention.

FIG. 21 is a cross-sectional view of a seventh embodiment of the unit loading pattern according to the present invention.

FIG. 22 is an explanatory diagram of the reactivity suppression effect of the burnable poison.

FIG. 23 is a view showing an example of arrangement of gadolinia rods in a conventional fuel assembly.

[Explanation of symbols]

1 High output area 4 Channel box 4a Upper tie plate 4b Channel fastener 4c Spacer 4d Lower tie plate 5 Water rod
6, 20: fuel rod, 7: control rod, 8: low-enrichment fuel,
9, 10: Highly enriched fuel, 11: Control cell, 1
2, 21 ... gadolinia rod, 13 ... control rod side area, 14 ...
Non-control rod side area, 15: water box, 16: medium enriched fuel, 18: fuel assembly.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akiko Kanda 3-1-1, Komachi, Hitachi-City, Ibaraki Pref. Inside Hitachi, Ltd. Hitachi Plant (72) Inventor Hajio Aoyama 7, Omika-cho, Hitachi City, Ibaraki Prefecture No. 2 1 Hitachi, Ltd. Power and Electricity Development Division (72) Inventor Yoko Yuchi 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Hitachi, Ltd. Power and Electricity Development Division (72) Invention Person Junichi Yamashita 3-1-1, Yukicho, Hitachi-shi, Ibaraki Pref. Hitachi, Ltd. Hitachi Plant

Claims (4)

[Claims] 1. A plurality of fuel rods arranged in a square grid of 9 rows and 9 columns, an upper tie plate and a lower tie plate for supporting the plurality of fuel rods at upper and lower portions, and one of the upper tie plates. In a fuel assembly including a channel fastener attached to two corners, and a water rod or a water box through which cooling water flows, the water rod or the water box is disposed at a central portion of the fuel assembly, The plurality of fuel rods include gadolinia-filled fuel rods, and the gadolinia-filled fuel rods are arranged at positions other than the outermost periphery of the fuel assembly, and the fuel assembly is divided into a channel fastener side region and an anti-channel fastener by a diagonal line. When divided into side regions,
Ten or more gadolinia-filled fuel rods are disposed in the channel fastener-side region, and the number of gadolinia-containing fuel rods present in the anti-channel fastener-side region is gadolinia-containing fuel rods in the channel fastener-side region. 3. A fuel assembly, wherein the number of fuel assemblies is three or more. 2. The fuel rod according to claim 1, wherein the number of gadolinia-containing fuel rods existing in the opposite region of the channel fastener is:
A fuel assembly wherein the number of gadolinia-containing fuel rods present in the channel fastener-side region is 3 to 19 more. 3. The fuel assembly according to claim 1, wherein two large-diameter water rods are arranged in a central area of the fuel assembly where seven fuel rods can be arranged. body. 4. The fuel assembly according to claim 1, wherein one water box is arranged in a central portion of the fuel assembly where nine fuel rods can be arranged.