JP3262723B2 - MOX fuel assembly and reactor core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fuel assembly for a boiling water reactor, and more particularly to a MOX fuel assembly loaded with a mixed oxide of plutonium and uranium and a reactor core provided with the same.

[0002]

2. Description of the Related Art A reactor core of a nuclear reactor is shut down after operation for a predetermined period (= 1 cycle), and a part of a loaded fuel assembly is taken out and replaced with a new fuel assembly. The fuel loading of the new fuel assembly during this exchange is set so that the amount of fissile material required to keep the reactor critical for one cycle is loaded, but at the end of the operation period, Is set in advance so that In other words, the reactor is in a state where the criticality is exceeded except at the end of operation. Therefore, in the core of the boiling water reactor, the control rods inserted between the fuel assemblies and the burnable poison added to the fuel rods absorb the extra neutrons generated, and as a result, the criticality is maintained throughout the operation period. The state is maintained. In a boiling water reactor,
Light water (hereinafter, appropriately referred to as cooling water) is used as a coolant for removing heat generated by fission. Here, a space for inserting the control rod described above is previously secured between the fuel assemblies, and cooling water also flows into the space between the fuel assemblies. The cooling water flowing in the fuel assembly removes most of the heat generated in the reactor, but the cooling water flowing in the space between the fuel assemblies does not remove much heat. The degree of the presence of bubbles due to boiling is different, resulting in a difference in density. Here, such cooling water also plays a role as a neutron moderator, and has a property that the neutron is further decelerated when the water density is higher. Therefore, the deceleration effect of the cooling water flowing through the space between the fuel assemblies is greater than the neutron deceleration effect of the cooling water in the fuel assembly from which heat is being removed. That is, the thermal neutron flux is larger near the cooling water flowing in the space between the fuel assemblies than near the cooling water in the fuel assemblies. And in general, fissile material
Since the larger the thermal neutron flux is, the more easily the reaction occurs, the output of the fuel rods is relatively high on the channel box side near the cooling water flowing through the space between the fuel assemblies, and the output of the fuel rods is far inside the channel box. A relatively low power distribution occurs within the fuel assembly.

On the other hand, an important quantity related to the core of a nuclear reactor is a linear power density representing the power per unit length of a fuel rod. The linear power density is defined as “fuel assembly output” which is an absolute output value of the entire fuel assembly, and “fuel assembly axial relative value” representing the relative distribution of power at each axial position in the fuel assembly. The output is represented by the product of three quantities of "power output" and "fuel rod relative output" representing the relative power distribution of each fuel rod, and the maximum value of the quantity in the reactor is the maximum linear power density. If the maximum linear output density is excessive and exceeds a predetermined value, the fuel rod center temperature becomes too high and the fuel rod pellets are melted. It is in a state where there is room. In general, in fuel assembly design, a plurality of types of fuel rod pellets are prepared, an enrichment distribution is appropriately provided, and the maximum value of “fuel rod relative output” which is one of three factors is suppressed.
The maximum linear power density has been reduced, and a thermal margin has been secured for the core.

In the core design of a nuclear reactor, both maintaining the above criticality and securing a thermal margin are both important, and cannot be considered independently. For example, if a large number of burnable poison-containing fuel rods are used to maintain the criticality, the output of the burnable poison-containing fuel rods is low, so the output of the other fuel rods rises and the thermal power of the core increases. Margin is reduced. Alternatively, if a large number of control rods are inserted into the core in order to maintain the criticality, the control cell core concept (= fuel assembly adjacent to the control rod inserted during operation), which is the mainstream in the current core design, In this case, a fuel assembly having a relatively high burnup is arranged in the core, thereby reducing the influence on the core average axial power distribution after the control rod is pulled out. . Therefore, as described above, the control rod and the burnable poison are usually used in combination to maintain the criticality.

[0005] Here, in the conventional boiling water reactor, uranium has been mainly used as fuel. But,
In recent years, from the viewpoint of effective utilization of fuel resources, a nuclear reactor using a mixed oxide fuel of uranium and plutonium (hereinafter, appropriately referred to as MOX fuel) has been proposed.

Here, since the absorption cross section of plutonium is larger than that of uranium, the difference in absorption cross section between plutonium and the burnable poison is smaller than the difference in absorption cross section between uranium and the burnable poison. . That is, the proportion of neutrons absorbed by the burnable poison when plutonium and the burnable poison coexist is smaller than the proportion of neutrons absorbed by the burnable poison when uranium and the burnable poison coexist. Therefore, in the reactor using the MOX fuel, the effect of suppressing the reactivity of the burnable poison becomes small, so that more fuel rods containing the burnable poison must be used than in the reactor using the uranium fuel. This results in an increase in the output of the other fuel rods and reduces the thermal margin.

[0007] To solve such problems, examples of known techniques in which the effect of burnable poisons are increased include, for example, JP-A-61-178693 and JP-A-7-301688. JP-A-61-178693 FIG. 2 and FIG. 3 of this prior art show that a combustible poison is placed on the outermost peripheral portion on the side not facing the control rods in the square lattice arrangement and on the inner part of the square lattice arrangement. A structure for disposing fuel rods is disclosed. JP-A-7-301688 discloses a fuel rod containing a burnable poison at positions adjacent to four corners of a square lattice arrangement and at intermediate positions away from the four corners in the outermost peripheral portion. A configuration to dispose is disclosed. In the above-described known technique, as described above, the amount of thermal neutrons absorbed by the burnable poison is increased by arranging the burnable poison-containing fuel rods on the outermost peripheral portion of the square lattice array having a large thermal neutron flux.

[0008]

The above-mentioned known techniques have the following problems. That is, in a fuel assembly using MOX fuel (hereinafter referred to as MOX fuel assembly as appropriate)
From the viewpoint of simplification of fuel assembly design, it is desired to reduce the number of types of fuel rod pellets. However, in the above-mentioned known art, the fuel rods containing burnable poisons are arranged at both the four corners and the inner part of the square lattice array in which the thermal neutron flux differs greatly, so that the uranium enrichment or burnable poison concentration can be reduced. Different types of burnable poisoned fuel rods are used. That is, the number of types of pellets of burnable poison-containing fuel rods cannot be reduced to one.

[0009] On the other hand, in the above-mentioned known art, it is not particularly indicated whether or not the burnable poison concentration in the fuel rod containing burnable poison is one type. However, in order to prevent burnable poisons from burning out in a short time in the fuel rods containing burnable poisons (hereinafter appropriately referred to as corner fuel rods) at positions adjacent to the four corners of the square lattice arrangement, the burnable poison concentration must be reduced to some extent. Must be higher. Then, if the concentration and the burnable poison-containing fuel rod at the intermediate position where the thermal neutron flux is small from the four corners in the outermost peripheral portion (hereinafter, as appropriate,
If the burnable poison concentration of the intermediate fuel rod is the same, the burnable poison remains unburned in the intermediate fuel rod until the end of combustion, and wasteful neutron absorption occurs. As described above, reducing the number of pellet types of burnable poison-containing fuel rods to one by setting the burnable poison concentration of the corner side fuel rod and the intermediate fuel rod to be the same causes a problem that neutrons are effectively used. . Further, at this time, in this known technique, three burnable poison-containing fuel rods per side are formed on the four outer peripheral sides formed by the arrangement of fuel rods at the outermost peripheral portion of the 9 × 9 square lattice array. Is arranged. However, since two sides (upper side and left side in FIG. 2) of the four outer peripheral sides are close to the control rod, if the number of burnable poison-containing fuel rods is large, the neutron absorption when the control rod is inserted is increased. There is also a problem that the effect is reduced and the control rod value is reduced.

As described above, simply decreasing the number of types of fuel rod pellets and decreasing the concentration distribution increases the maximum value of the relative output of the fuel rods, thereby increasing the maximum linear power density, and thus the thermal margin. Therefore, care must be taken to prevent this.

A first object of the present invention is to provide a MOX fuel assembly and a nuclear reactor core capable of reducing the number of types of pellets of burnable poison-containing fuel rods to one while securing a thermal margin. It is in. A second object of the present invention is to provide a MOX fuel assembly and a nuclear reactor core capable of reducing the number of types of pellets of burnable poison-containing fuel rods to one without hindering effective utilization of neutrons. It is in. A third object of the present invention is to reduce the control rod value without reducing
An object of the present invention is to provide a MOX fuel assembly and a reactor core in which the number of types of pellets of burnable poison-containing fuel rods can be one.

[0012]

According to the present invention, a MOX fuel rod filled with a mixed oxide of uranium and plutonium and a uranium filled with uranium oxide are provided. At least M of the fuel rods
A plurality of fuel rods, including OX fuel rods, and at least one water rod are arranged in a square grid arrangement of N rows and N columns,
In the MOX fuel assembly, a part of the plurality of fuel rods is a burnable poison-containing fuel rod to which a burnable poison is added, and the water rod is arranged near a central portion in the square lattice array. Of the four outer peripheral sides formed by the arrangement of the fuel rods in the outermost peripheral portion of the square lattice arrangement, all of the lattice positions occupied by the water rods and at least one of the rows and columns are the same. While arranging the burnable poison-containing fuel rods, at the grid positions other than the outer four sides,
A MOX fuel assembly is provided, wherein fuel rods to which the burnable poison is not added are arranged . According to the present invention, in order to achieve the first and third objects,
Of the MOX fuel rod filled with the mixed oxide of uranium and plutonium and the uranium fuel rod filled with the uranium oxide,
A plurality of fuel rods including at least the MOX fuel rod;
At least one water rod is arranged in a square grid arrangement of N rows and N columns, and a part of the plurality of fuel rods is a burnable poison-containing fuel rod to which burnable poison is added, In the MOX fuel assembly in which the water rods are arranged near the center in the square lattice arrangement, the inside of the fuel assembly is controlled.
When the rod side area and the counter control rod side area are divided into two,
In the rod side area, the outermost peripheral portion of the square lattice
Of the four sides on the outer peripheral side formed by the row of fuel rods,
Grid position occupies the same row and column
The burnable poisoned fuel rods are placed at all grid positions
And at the grid positions other than the four sides on the outer circumference
Arrange a fuel rod to which the burnable poison is not added,
Serial The control rod side area, at least one of said burnable poison containing per side one of the grid positions and row and column where the outer peripheral side four sides No Chi before Kisui rod occupied lattice positions is the same with arranging the fuel rods, MOX fuel assembly, characterized in that a said burnable poison containing fuel rod of the one even without least in the region of the inner peripheral side than the outer peripheral side four sides is provided. That is, the reactivity suppression effect of the burnable poison becomes higher at the lattice position where the thermal neutron flux is larger, so that the outermost part of the square lattice arrangement is more effective than the inner part, and the outermost part is closer to the four corners among the outermost parts. Get higher. At this time, the distribution of the thermal neutron flux at each lattice position in the outermost peripheral portion does not change much at the lattice positions near the central portion away from the four corners, but increases sharply as it approaches the four corners. Therefore, although the reactivity suppression effect can be maximized in the vicinity of the four corners, the following inconvenience is considered when arranging a plurality of fuel rods made of the same type of pellet having the same burnable poison concentration. Occurs. That is, for example, 4
The thermal neutron flux greatly differs between the corner and the adjacent position, causing a large difference in the burning period of the burnable poison. However, if the concentration of the burnable poison is unified to the concentration suitable for the position adjacent to the four corners, the poison burns at the four corners early. And the suppression of the reactivity becomes insufficient. Therefore, the concentration of the burnable poison must be adjusted to a concentration suitable for the four corners. In this case, however, the poison at the adjacent position remains unburned until the end of combustion, causing unnecessary absorption,
On the contrary, it hinders the effective use of neutrons. Therefore, in the present invention, first, as one means, among the four sides of the outermost peripheral portion, the fuel rods containing burnable poisons are arranged only at all the grid positions in which the rows or columns are the same as the water rods, that is, only at a plurality of positions near the center. I do. As described above, the effect of suppressing the reactivity by the poison is relatively high in the outermost part, and the thermal neutron flux distribution does not change much at the lattice position near the center, so that the inconvenience caused by the unburned poison as described above almost occurs. do not do. Therefore, it is possible to realize a configuration in which the fuel rod containing the burnable poison has only one kind of pellets while effectively utilizing the neutrons. On the other hand, in the above means, 4
Since the burnable poison-containing fuel rods are arranged in all the grid positions where the rows or columns are the same as the water rods, the control rods are inserted in two of the outermost peripheral sides on the control rod side. There is a concern that the neutron absorption effect of the gas will weaken and the control rod value will decrease. Therefore, in the present invention, as another means , the outside in the region opposite to the control rod inside the fuel assembly is provided.
The grid position occupied by the water rod and the row and column
Burnable poison at all grid positions where either one is the same
In addition to placing fuel rods containing
At least one burnable poison-containing fuel rod is arranged at the same grid position as the water rod or row in the four outer peripheral sides, and the burnable poison-containing fuel rod is also disposed on the inner peripheral side from the four outer peripheral sides. Deploy. That is, the lattice position in the vicinity of the central part, which is distant from the four corners, of the four outer peripheral sides has the lowest thermal neutron flux among the four outer peripheral sides, and the thermal neutron flux in the area on the inner peripheral side of the four outer peripheral sides. The difference is not so large. Therefore, by arranging the burnable poison-containing fuel rods in both the vicinity of the central portion of the four outer peripheral sides and the inner peripheral region, the occurrence of inconvenience due to unburned poison due to the difference in thermal neutron flux is minimized. At the same time, it is possible to reduce the number of burnable poison-containing fuel rods on the four sides on the outer peripheral side to prevent a reduction in the value of control rods, and to realize a configuration in which one kind of burnable poison-containing fuel rod pellets is used. . In any of the above means, fuel rods with a lower enrichment or a lower enrichment than the burnable poison-containing fuel rods are appropriately disposed at the four corners of the fuel rods containing the burnable poison on the outer four sides. By doing so, it is possible to reduce the maximum linear output density and secure a thermal margin. Further, in general, in the fuel assembly, the combustion of the outermost fuel rods relatively progresses in the early stage of combustion, so that the output of the inner fuel rods increases in the latter stage of combustion. Here, in the present embodiment, by adding burnable poisons to the fuel rods on the four sides on the outer peripheral side, the output in the initial stage of combustion can be suppressed, and the enrichment can be increased by that amount. That is, around the four corners of the four sides on the outer periphery,
It is possible to reduce the degree of enrichment in a portion inside the side. Therefore, it is possible to reduce the maximum linear output density in the initial stage and the later stage of the combustion, and to further improve the thermal margin.

The first and second or third objects are achieved.
According to the present invention, uranium and plutonium
Fuel rods and uranium oxide filled with mixed oxides of uranium
Of the uranium fuel rods filled with
A plurality of fuel rods, including fuel rods, and at least one water rod;
And N are arranged in a square lattice arrangement of N rows and N columns,
Burnable poison was added to some of the fuel rods
A fuel rod containing a burnable poison, wherein the water rod is
MOX fuel assemblies located near the center in a grid array
The fuel in the outermost peripheral portion of the square lattice arrangement
Of the four outer peripheral sides formed by the row of rods, the water rod is
The grid position occupied and either row or column are the same
At least one burnable poison per side at the grid position
A fuel rod with a material is arranged, and the area on the inner side from the four sides on the outer side is arranged.
Area, at least one fuel rod containing the burnable poison is provided.
The MOX fuel assembly is provided , wherein the four outer peripheral sides include at least one pair of two adjacent sides each having a different number of the burnable poison-containing fuel rods. .

Preferably, in the above MOX fuel assembly, the fuel rods include a plurality of the uranium fuel rods, and all of the burnable poison-containing fuel rods are the uranium fuel rods. A fuel assembly is provided. In other words, adding burnable poisons to uranium fuel rods is a technique that is currently used in light water reactors, and is therefore easier than adding burnable poisons to MOX fuel rods.

Preferably, in the above MOX fuel assembly, all of the burnable poison-containing fuel rods are formed of the MOX fuel rod.
An MOX fuel assembly is provided which is an X fuel rod. As a result, the amount of uranium oxide fuel can be reduced, so that the amount of plutonium loaded in one fuel assembly can be increased. That is, the number of MOX fuel assemblies manufactured from a certain amount of plutonium can be reduced. Thereby, the number of MOX fuel assemblies to be transported can be reduced.

Preferably, in the MOX fuel assembly, all the burnable poison-containing fuel rods have the same burnable poison concentration and the same plutonium enrichment. Is provided.

According to the present invention, in order to achieve the first to third objects, a plurality of fuel assemblies each loaded with a plurality of fuel rods filled with fissile material; At least one control rod inserted between;
In a reactor core having a plurality of in-core instrumentation tubes each provided with a measuring means, the plurality of fuel assemblies include the MOX fuel assembly, and the reactor core is provided. .

[0018]

Embodiments of the present invention will be described below with reference to the drawings. Each of the embodiments described below has a general core whose cross section is shown in FIG. 2, that is, a plurality of fuel assemblies each loaded with a plurality of fuel rods 503 and water rods 504 filled with fissile material. A core of a boiling water reactor having a body 500, at least one control rod 501 inserted between the fuel assemblies 500, and a plurality of in-core instrumentation tubes (not shown) each provided with measuring means. It is applied to. A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a cross-sectional view illustrating the structure of the MOX fuel assembly according to the present embodiment.
In FIG. 1, the MOX fuel assembly includes 74 MOX fuel rods 3 filled with a mixed oxide of uranium and plutonium, and two water rods 4 arranged in a space for seven MOX fuel rods 3. Are arranged in a square grid of 9 rows and 9 columns (I rows to IX rows and columns to columns), and are surrounded by a channel box 2.

The two water rods 4, 4 are arranged in the vicinity of the center of the square grid arrangement of 9 rows and 9 columns.

The MOX fuel rods 3 are arranged in three types in which the weight ratio of the fissile material contained in the pellets (= the ratio of the weight of the fissile material to the total fuel weight, hereinafter referred to as “enrichment” as appropriate) is different. And are represented by fuel rod numbers A1, B1, and C1, respectively. These three fuel rod numbers are
It shows that the enrichment of the MOX fuel rod 3 is higher in the order of A1>C1>B1>. At this time, the enrichment of the MOX fuel rods 3 of the fuel rod numbers A1 and C1 is higher than the fissile material weight ratio of the average of the cross section of the fuel assembly (that is, the average of all the fuel rods). MOX fuel rod 3 of B1
Has a lower enrichment. The enrichment degree is 3
A burnable poison (for example, gadolinia) is added to the MOX fuel rod 3 (fuel rod number C1) which is an intermediate between the two.

The MOX fuel rods 3 have 42 A1, 20 B1, and 12 C1 for each fuel rod number.
Books are arranged as shown. That is,
Four sides on the outer peripheral side formed by the arrangement of fuel rods at the outermost peripheral portion of the 9 × 9 grid array (I matrix to column, IX matrix to
Of the columns, columns I to IX, columns I to IX, all of the grid positions where the rows or columns are the same as the grid position occupied by the water rod (I matrix to column, IX matrix to column, column IV) line~
In row VI, columns IV to VI), 12 MOX fuel rods 3 (fuel rod number C1) containing burnable poison are placed, and the outer peripheral side 4
MOX fuel rods 3 (fuel rod number B1) having the lowest enrichment are arranged at other grid positions on the side.

Next, the operation of the present embodiment configured as described above will be described with reference to FIG. As described above, the thermal neutron flux is larger near the cooling water flowing in the space between the fuel assemblies than near the cooling water in the fuel assemblies. Therefore, the thermal neutron flux is particularly large on the four sides on the outer periphery of the lattice array, and among the four outer sides, a larger water gap region is present nearer to the corner of the fuel assembly. The four corners of the grid are the largest,
The thermal neutron flux becomes smaller as it approaches the center further away. This is shown in FIG.

FIG. 3 shows the distribution of the thermal neutron flux in the I matrix to the I column as an example of the four outer peripheral sides which are the outermost peripheral parts of 9 rows and 9 columns. The horizontal axis indicates the grid position in the row direction from column to column, and the vertical axis indicates the relative value when the thermal neutron flux in the I matrix corresponding to the center is set to 1.
In FIG. 3, the thermal neutron flux becomes higher as it moves away from the I matrix and toward the four corners (I matrix and column).
The rate of increase becomes sharper as the distance from the matrix I increases. That is, the I matrix and columns near the I matrix have a thermal neutron flux of about 1.02,
The number does not increase much to 7, but increases to about 1.24 at the four corners of the I matrix and column, and reaches about 1.74 at a stretch to the four corners of the I matrix and column. The neutron flux distribution of each fuel rod on the inner side of the fuel assembly from the four sides on the outer peripheral side is almost flat, and its thermal neutron flux is about 0.5 (I
About 50% of the matrix).

As described above, the effect of suppressing the reactivity of the burnable poison becomes higher at the lattice position where the thermal neutron flux is larger.
According to FIG. 3, when the burnable poison is arranged on the four sides on the outer periphery of the square lattice arrangement, the reactivity suppression effect is higher than when the burnable poison is arranged on the inner side of the fuel assembly. The closer to the position, the higher the reactivity suppression effect, and the four corners can maximize the reactivity suppression effect. However, in the case of arranging a plurality of fuel rods made of the same kind of pellets having the same burnable poison concentration in order to simplify the design of the fuel assembly, the following inconvenience occurs in this configuration. That is, in FIG. 3, for example, at the four corners (I matrix and column) and adjacent positions (I matrix and column), the difference in thermal neutron flux is as large as 0.5. There is a big difference in the period. Therefore, if the burnable poison concentration is unified to a concentration suitable for the four corner adjacent positions (I matrix and column), the poison burns at the four corners (I matrix and column) at an early stage, and the reactivity is insufficiently suppressed. Therefore, the concentration of the burnable poison must be adjusted to a high concentration suitable for the four corners (I matrix and column). In this case, on the contrary, the poison at the four corner adjacent positions (I matrix and column) is at the end of combustion. Unnecessary absorption occurs as a result of burning, which hinders the effective use of neutrons. This corresponds to not only the I matrix to the column but also other portions (IX matrix to column,
The same applies to columns I to IX and columns I to IX.

Therefore, in the present embodiment, the outermost four sides (I matrix to column, IX matrix to
All the grid positions (I matrix to column, I matrix to column,
The burnable poison-containing fuel rods are arranged in the IX matrix to column, column IV row to VI row, and column IV row to VI row (the I matrix to column correspond in the example of FIG. 3). As described above, the four sides on the outer peripheral side have a relatively high effect of suppressing the reactivity due to the poison, and the lattice positions of these lattice positions are not so different from each other in thermal neutron flux distribution (from FIG. However, the above-mentioned inconvenience due to unburned toxic substances hardly occurs. Therefore, it is possible to use only one kind of pellets of the burnable poison-containing fuel rod while effectively using the neutrons, so that the design of the fuel assembly can be simplified. In addition, on the four sides on the outer peripheral side, grid positions (I matrix-column and column-column, IX matrix-column and column-column, column I row) at four corners from the MOX fuel rod 3 (fuel rod number C1) containing burnable poison. -Lines III and VII-lines IX,
In columns I to III and VII to IX), the burnable poison M
MO with lower enrichment than OX fuel rod 3 (fuel rod number C1)
By arranging the X fuel rods 3 (fuel rod number B1), by appropriately adjusting the difference between these two enrichment levels, the maximum linear output density can be reduced and the thermal margin can be secured.
Further, at this time, in the fuel assembly, in general, the combustion of the outermost fuel rods relatively progresses in the early stage of combustion, so that the output of the inner fuel rods increases in the latter stage of combustion. Here, in the present embodiment, the I matrices on the outer four sides
By adding a burnable poison to the MOX fuel rods 3 in columns, IX matrix to column, column IV row to VI row, and column IV row to VI row, the output in the early stage of combustion can be suppressed, and the enrichment of these can be reduced in advance. Since it is possible to increase the enrichment, it is possible to reduce the enrichment degree at other lattice positions, that is, around the four corners on the outer peripheral side and inside the four outer peripheral sides. Therefore, it is possible to reduce the maximum linear output density in the initial stage and the later stage of the combustion, and to further improve the thermal margin. Further, the grid arrangement of 9 rows and 9 columns is constituted only by the MOX fuel rods 3 and the water rods 4, and the burnable poison is added to the MOX fuel rods 3 (fuel rod number C1), and no uranium fuel rods are arranged. Therefore, the loading amount of plutonium can be increased. As a result, the amount of uranium oxide fuel can be reduced.
The body's plutonium loading can be increased. That is, the number of MOX fuel assemblies manufactured from a certain amount of plutonium can be reduced. Thereby, the number of MOX fuel assemblies to be transported can be reduced.

In the first embodiment, all the fuel rods are MOX fuel rods 3. However, the present invention is not limited to this. That is, the fuel rod number C to which the burnable poison is added
One MOX fuel rod 3 may be replaced with a uranium fuel rod appropriately filled with uranium oxide fuel having a predetermined enrichment. In this case, the same effect is obtained while securing substantially the same thermal margin. Further, in this case, adding a burnable poison to a uranium fuel rod is a technique that is currently performed in a light water reactor, and therefore is an easier technique than adding a burnable poison to a MOX fuel rod.

A second embodiment of the present invention will be described with reference to FIG. This embodiment is an embodiment in which water rods having different shapes are used in the configuration of the first embodiment. Members equivalent to those in the first embodiment are denoted by the same reference numerals. FIG.
FIG. 3 is a cross-sectional view illustrating the structure of the MOX fuel assembly according to the embodiment. In FIG. 4, the MOX of the present embodiment is shown.
The fuel assembly is particularly different from the first embodiment in that a water rod 204 having a square cross section and a larger cross section is used. The fuel rod numbers A2, B2,
C2 is the fuel rod number A1, B1, C1 of the first embodiment.
Is equivalent to These numbers correspond almost in the same manner. However, as the water rod 204 has a large cross-sectional area and requires space for nine fuel rods, the MOX fuel rod 3 having the fuel rod number A2 has the fuel rod number A2. The number is reduced by two from the MOX fuel rod 3 having the rod number A1 to be 40 rods. Other structures are almost the same as those of the first embodiment. According to this embodiment, the same effect as that of the first embodiment is obtained.

A third embodiment of the present invention will be described with reference to FIG. This embodiment is an embodiment in which burnable poison-containing fuel rods are arranged on both the outer peripheral side and the inner peripheral side of the lattice-shaped array. Members equivalent to those of the first and second embodiments are denoted by the same reference numerals. FIG. 5 is a cross-sectional view illustrating the structure of the MOX fuel assembly according to the present embodiment.
In FIG. 5, the MOX fuel assemblies are disposed in 74 MOX fuel rods 3 filled with a mixed oxide of uranium and plutonium, and in a space for seven MOX fuel rods 3 as in the first embodiment. And two water rods 4 are arranged in a square grid of 9 rows and 9 columns, and the periphery thereof is surrounded by a channel box 2.
Are arranged in the vicinity of the center of a square lattice arrangement of 9 rows and 9 columns (I rows to IX rows and columns to columns).

The MOX fuel rod 3 has a fuel rod number A3> C
Three different types having higher enrichment in the order of 3> B3 are arranged. At this time, as in the first embodiment, the fuel rod number A3,
The enrichment of the MOX fuel rod 3 of C3 is higher, and the enrichment of the MOX fuel rod 3 of fuel rod number B3 is lower. The burnable poison (for example, gadolinia) is added to the MOX fuel rod 3 having the fuel rod number C3.

The MOX fuel rods 3 have 38 A3, 24 B3, and 12 C3 for each fuel rod number.
Each is arranged as shown. That is, the outer peripheral four sides (the I matrix to the column, the IX matrix to the column,
Some of the grid positions (I matrix, IX matrix to column, column V row, column IV row) in columns I to IX and column I to IX
8), eight MOX fuel rods 3 (fuel rod number C1) containing burnable poisons are arranged. Each of these is a grid position where the row or column is the same as the grid position occupied by the water rod, but the two sides (I
The matrix to column and column I to IX rows have only one MOX fuel rod 3 containing burnable poison (fuel rod number C1) on each side, whereas two sides (IX matrix to column) that are not on the control rod side And columns I to IX) have three sides. As a result, the four sides on the outer peripheral side include two sets of two sides adjacent to each other in which the number of the burnable poison-containing fuel rods is different from each other. In addition, four burnable poison-containing poisons are also located at lattice positions (II matrix and column, column III row and VII row) in an area on the inner side of the four outer sides (hereinafter appropriately referred to as an inner side area). The MOX fuel rod 3 (fuel rod number C1) is arranged.

As described above, the configuration of the present embodiment is particularly different from the first embodiment in that the flammability on the outer two sides (I matrix to column and column I row to IX row) on the control rod 1 side is different. The structure is such that the number of MOX fuel rods 3 (fuel rod number C1) containing poison is reduced and the number is moved to the inner peripheral area.
This difference has the following effect. That is, in the configuration of the first embodiment shown in FIG. 1, four sides on the outer peripheral side (I matrix to column, IX matrix to column, column I row to IX row,
All the grid positions (I matrix to column, IX matrix to column,
MO with burnable poison in columns IV to VI, columns IV to VI)
Since the X fuel rod 3 (fuel rod number C1) is arranged, the control rod 1 is inserted on two sides (I matrix to column and column I row to IX row) on the control rod side among the four outer peripheral sides. There is a concern that the neutron absorption effect will diminish and the control rod value will decrease.
On the other hand, in the present embodiment, the number of MOX fuel rods 3 (fuel rod number C1) containing burnable poisons on two outer peripheral sides (I matrix to column and column I row to IX row) on the control rod 1 side. Is reduced and the portion is moved to the inner peripheral region from those two sides. This is for the following reason. That is, the outer four sides (I matrix to column, IX matrix to column, column I row to
Lattice positions (for example, I matrix to column, IX matrix to) near the center of IX rows and columns I to IX
In columns, columns IV to VI, columns IV to VI), the thermal neutron flux is relatively low in the four outer sides (see FIG. 3 described in the first embodiment), and the inner peripheral region Is not so large (for example, about 50% as described in the first embodiment). Therefore, the lattice position (I matrix and column V row) near the central portion of the outer two sides (I matrix to column and column I row to IX row) on the control rod 1 side and the inner peripheral area (II matrix in this case) And columns, columns III rows and V
2), the burnable poison-containing MOX fuel rod 3 (fuel rod number C3) is arranged to minimize the occurrence of inconvenience due to the unburned poison due to the difference in thermal neutron flux, and to reduce the possibility of the control rod 1 side. The number of MOX fuel rods 3 containing burnable poisons (fuel rod number C3) is reduced while reducing the number of control rod values by reducing the number of burnable poison-containing MOX fuel rods 3 (fuel rod number C3) on the two outer sides. One type of pellet can be used. In this case, as in the first embodiment, the four corners of the burnable poison-containing MOX fuel rod 3 (fuel rod number C1) are located at the four corners (I matrix to column and column to column, IX) on the four outer peripheral sides. Matrix-column and column-column, column I row-
Rows IV and VI to IX, Columns I to III and VII to IX
Row), MOX fuel rod 3 containing burnable poison (fuel rod number C)
MOX fuel rod 3 (fuel rod number B) with lower enrichment than 1)
By arranging 1), by appropriately adjusting the difference between these two enrichment levels, it is possible to reduce the maximum linear output density and secure a thermal margin. Further, the grid arrangement of 9 rows and 9 columns is constituted only by the MOX fuel rods 3 and the water rods 4, and the burnable poison is MOX fuel rod 3 (fuel rod number C3).
And no uranium fuel rods are placed,
The loading of plutonium can be increased. As a result, the amount of uranium oxide fuel can be reduced, so that the amount of plutonium loaded in one fuel assembly can be increased. That is, the number of MOX fuel assemblies manufactured from a certain amount of plutonium can be reduced. Thereby, the number of MOX fuel assemblies to be transported can be reduced.

In the third embodiment, all the fuel rods are MOX fuel rods 3. However, the present invention is not limited to this. That is, the fuel rod number C to which the burnable poison is added
The third MOX fuel rod 3 may be replaced with a uranium fuel rod appropriately filled with uranium oxide fuel having a predetermined enrichment. In this case, the same effect is obtained while securing substantially the same thermal margin. Adding burnable poisons to uranium fuel rods is an easier technique than adding burnable poisons to MOX fuel rods, as it is a technique currently practiced in light water reactors. In the third embodiment, the grid positions (I matrix, IX matrix to column, column V
MOX with 8 burnable poisons in rows, columns IV-VI)
The fuel rods 3 (fuel rod number C1) are arranged, and some of the grid positions (II matrix and column, column III row and V
Although two burnable poison-containing MOX fuel rods 3 (fuel rod number C1) are arranged in row II), the present invention is not limited to this. In other words, at least one burnable poison-containing MOX fuel rod 3 per side is located at a grid position where one of the row and column is the same as the grid position occupied by the water rod 4 among the four outer peripheral sides.
(Fuel rod number C3) and at least one burnable poison-containing MOX fuel rod 3 (fuel rod number C3)
What is necessary is just to arrange 3). Then, the number of the burnable poison-containing MOX fuel rods 3 (fuel rod number C3) on the four sides on the outer peripheral side is appropriately adjusted, thereby preventing the fuel rod containing the burnable poison while preventing the control rod value from decreasing. Can be one kind of pellet. At this time, for example,
Four sides on the outer peripheral side are each provided with a burnable poison M
The number of the OX fuel rods 3 (fuel rod number C3) on the control rod 1 side is determined by including at least one pair of adjacent two sides having different numbers of OX fuel rods 3 (fuel rod number C3). It is preferable to reduce the number of the burnable poison-containing MOX fuel rods 3 (fuel rod number C3) on the side other than the control rod 1 side. Thereby, an effect is obtained that the control rod value can be more reliably prevented from decreasing.

A fourth embodiment of the present invention will be described with reference to FIG. This embodiment is an embodiment in the case of using water rods having different shapes in the configuration of the third embodiment. Members equivalent to those in the first to third embodiments are denoted by the same reference numerals. FIG. 6 is a cross-sectional view illustrating the structure of the MOX fuel assembly according to the present embodiment. In FIG. 6, the MOX fuel assembly of the present embodiment is particularly different from the third embodiment in that a water rod 204 having a large cross-sectional area similar to that of the second embodiment is used.
That is, was used. Note that the fuel rod numbers A4, B4, and C4 of the MOX fuel rod 3 are the fuel rod numbers A of the third embodiment.
3, B3, C3. Although these numbers correspond almost in the same way, the MOX fuel rod 3 having the fuel rod number A4 has the fuel rod number A4 because the water rod 204 has a large sectional area and a space for nine fuel rods is required. MOX of stick number A3
There are 36 fuel rods, two less than the fuel rods 3. Other structures are almost the same as the third embodiment. According to this embodiment, the same effect as that of the third embodiment is obtained.

In the third and fourth embodiments, in the inner peripheral area, the inner peripheral four sides (II matrix to column, VIII matrix to column, column II row) adjacent to the outer peripheral four sides. ~
MO with burnable poison in row VIII, columns II to VIII)
Although the X fuel rod 3 (fuel rod number C1) is arranged, it is not limited to this. That is, as described above, the neutron flux distribution of each fuel rod on the inner side of the fuel assembly from the four sides on the outer peripheral side is almost flat, so that the MOX fuel containing burnable poison is located in the area inside the four sides on the inner peripheral side. The rod 3 (fuel rod number C1) may be arranged. In other words, it is sufficient to dispose it somewhere in the inner peripheral area.

[0035]

[0036]

According to the present invention, the burnable poison-containing fuel rods are arranged at all the grid positions in the same row and column as the water rods among the four outermost peripheral sides, so that the neutrons can be effectively used. In addition, it is possible to realize a configuration in which the fuel rod containing the burnable poison has one type of pellet. Also, on the anti-control rod side
In the area, the grid position occupied by the water rod among the four sides on the outer peripheral side
And all grid positions where either row or column are the same
A fuel rod containing a burnable poison in the control rod side area.
Indicates the grid position occupied by the water rod among the four outer sides and
Per side at the grid position where either one of
At least one burnable poisoned fuel rod should be placed
At least one line is located in the region on the inner side of the four sides on the outer side.
Since the fuel rods containing the burnable poison are arranged, it is possible to realize a structure in which the fuel rods containing the burnable poison have only one kind of pellets while preventing the reduction of the control rod value. At this time, if the fuel rods having a lower enrichment or a lower enrichment than the burnable poison-containing fuel rods are appropriately arranged at the four corner grid positions of the burnable poison-containing fuel rods on the four outer peripheral sides, the maximum line is obtained. The power density can be reduced to secure a thermal margin.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a structure of a MOX fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating an example of a core to which a fuel assembly according to each embodiment of the present invention is applied.

FIG. 3 shows an example of an I matrix to an outermost peripheral portion of 9 rows and 9 columns.
FIG. 3 is a diagram showing a distribution of thermal neutron flux in a row.

FIG. 4 is a cross-sectional view illustrating a structure of a MOX fuel assembly according to a second embodiment of the present invention.

FIG. 5 is a cross-sectional view illustrating a structure of a MOX fuel assembly according to a third embodiment of the present invention.

FIG. 6 is a cross-sectional view illustrating a structure of a MOX fuel assembly according to a fourth embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 control rod 2 channel box 3 MOX fuel rod 4 water rod 204 water rod 501 control rod 503 MOX fuel rod 504 water rod

Continuation of the front page (72) Inventor Hajio Aoyama 7-2-1, Omika-cho, Hitachi City, Ibaraki Pref. Hitachi, Ltd. Electric Power & Electric Equipment Development Division (56) References JP-A-10-90460 (JP, A) JP-A-10-26682 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 3/328

Claims (7)

(57) [Claims] 1. An MOX fuel rod filled with a mixed oxide of uranium and plutonium, and a uranium fuel rod filled with uranium oxide, a plurality of fuel rods including at least the MOX fuel rod, and at least one water rod. Rods are arranged in a square lattice arrangement of N rows and N columns, and a part of the plurality of fuel rods is a burnable poison-containing fuel rod to which burnable poison is added, and the water rod is In the MOX fuel assembly arranged in the vicinity of the central portion in the square lattice array, the grid occupied by the water rods among four outer peripheral sides formed by an arrangement of fuel rods at the outermost periphery of the square lattice array and lines and arranged Then together the burnable poison-containing fuel rods in all one lattice position one is the same as column
In addition, the combustible poison is located at a lattice position other than the four sides on the outer peripheral side.
MOX characterized by arranging fuel rods that are not added
Fuel assembly. 2. A plurality of fuel rods including at least the MOX fuel rod, among MOX fuel rods filled with a mixed oxide of uranium and plutonium and uranium fuel rods filled with uranium oxide, and at least one water rod. Rods are arranged in a square lattice arrangement of N rows and N columns, and a part of the plurality of fuel rods is a burnable poison-containing fuel rod to which burnable poison is added, and the water rod is In the MOX fuel assembly arranged near the center in the square lattice arrangement, the inside of the fuel assembly is divided into a control rod side region and a non-control rod side region.
When divided, in the area opposite to the control rod side, the square lattice
Outer side formed by the arrangement of fuel rods at the outermost part of the array
Grid position, row and column occupied by the water rod among the four sides
All of the grid positions where one of
A fuel rod containing a burnable poison is arranged, and
The burnable poison is not added to grid positions other than the sides
The fuel rods are arranged, wherein the control rod side area, at least one per side in a grid position either is the same lattice position and row and column where the outer peripheral side four sides No Chi before Kisui rod occupies wherein with placing the burnable poison-containing fuel rods, one of the MOX fuel assembly, characterized in that a burnable poison containing fuel rods even without least on the inner peripheral side region than the peripheral side four sides of the . 3. Filling a mixed oxide of uranium and plutonium
MOX fuel rods and uranium fuel filled with uranium oxide
A plurality of rods including at least the MOX fuel rod
Fuel rods and at least one water rod are arranged in N rows and N columns
And a part of the plurality of fuel rods is provided with a burnable poison.
Fuel rods containing burnable poisons, and the water rods are positioned near the center in the square lattice arrangement.
In the arranged MOX fuel assembly, the arrangement of the fuel rods on the outermost peripheral portion of the square lattice arrangement is
The grid occupied by the water rod among the four outer peripheral sides to be formed
A grid position whose position is the same as either the row or column
And at least one burnable poisoned fuel per side
A material rod is arranged , and at least one piece is provided in an area on the inner peripheral side from the four outer peripheral sides.
The burnable poison-containing fuel rods are disposed, and the outer peripheral side four sides include at least one pair of adjacent two sides in which the number of the burnable poison-containing fuel rods is different from each other. MOX fuel assembly. 4. The MOX according to claim 1, wherein
In the fuel assembly, the fuel rod includes a plurality of the uranium fuel rods, and all of the burnable poison-containing fuel rods are the uranium fuel rods. 5. The MOX according to claim 1, wherein
In the fuel assembly, all of the burnable poison-containing fuel rods are the MOX fuel rods. 6. The MOX fuel assembly according to claim 5 , wherein all of the burnable poison-containing fuel rods have the same burnable poison concentration and the same plutonium enrichment. Aggregation. 7. A fuel assembly having a plurality of fuel rods loaded with a plurality of fuel rods filled with fissile material, at least one control rod inserted between the fuel assemblies, and measuring means, respectively. A reactor core having a plurality of in-core instrumentation tubes, wherein the plurality of fuel assemblies include the MOX fuel assembly according to any one of claims 1 to 3. Core.