JPH11174178A - Fuel assembly and reactor core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure uniformness of H/U ratio in diameter direction in a fuel assembly attempting high burnup, suppress non-uniformity in burnup of burnable poison and unify the power distribution in axial and radial directions. SOLUTION: Fuel rods 101 are constituted of first fuel rods 101a having normal fuel effective length not containing burnable poison, second fuel rods 101b having normal fuel effective length containing gadolinia and 14 third fuel rods 101c having short fuel effective length. In the case inside of a fuel assembly is separated in two regions of control rod side region 111 and counter control rod side region 112, a water box 102 is eccentrically arranged in the counter control rod side region 112. Also the number of the second and the third fuel rods 101b and 101c arranged in the control rod side region 111 is made larger than the number of the second and the third fuel rods 101b and arranged in the counter control rod side region 112.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used for a boiling water reactor and a reactor core using the same.

[0002]

2. Description of the Related Art FIG. 10 is a conceptual cross-sectional view showing a partial structure of a general boiling water reactor core. In FIG. 10, a reactor core 1 is configured by arranging a large number of fuel assemblies 2, and these fuel assemblies 2 are formed as a set of four fuel rods, and a control rod 3 having a cross-shaped cross section is inserted therebetween. It has become so.

In each fuel assembly 2, a large number of fuel rods 4 and one or more (two in the figure) water rods 5 are arranged in a square lattice to form a fuel bundle. A channel box 8 having a rectangular cross-section is arranged at the center of 6 (shown by a broken line) and surrounds the fuel bundle. The upper and lower ends of this fuel bundle are supported by an upper tie plate (not shown) and a lower tie plate (same) inserted into the channel box. In this example, 74 fuel rods 4 are provided, and fuel pellets filled with nuclear fuel substances are arranged in a cladding tube. Also those 74
Burnable poisons (for example, gadolinia) for suppressing the reaction at the initial stage of combustion are further added to the fuel pellets of some of the fuel rods 4 in the book. The water rod 5 is a fuel rod 4
Two are arranged in the space of seven. Note that a rectangular water box may be used instead of the water rod 5.

In the fuel assembly 2, during operation, slightly unsaturated cooling water flows between the fuel rods 4 from the holes of the lower tie plate and flows from the lower part to the upper part between the fuel rods 4. And boil, and flow out of the holes of the upper tie plate as a two-phase flow. As a result, the void ratio of the coolant is 0% at the lower portion of the fuel assembly 2 but reaches about 70% at the upper portion, and the moderator-to-fuel ratio, which is a factor that determines the nuclear characteristics of the fuel assembly 2, that is, The hydrogen to uranium ratio (H / U ratio) will differ significantly at the axial position.
The principle that the H / U ratio determines the nuclear characteristics of the fuel assembly 2 is as follows. In other words, since boiling water reactors use light water as a moderator, energy is generated by a chain reaction in which fast neutrons generated by nuclear fission are scattered and decelerated by the moderator water to become thermal neutrons, causing the next nuclear fission. I do. That is, water plays an important role in promoting the nuclear fission reaction. The fission reaction is promoted in a region where the amount of water is relatively large, and the fission reaction is suppressed in the region where the amount of water is relatively small.

Returning to FIG. 10, a gap is provided outside the channel box 8 of the fuel assembly 2 for disposing the control rod 3 and a neutron detector instrumentation tube (not shown). . This gap is filled with saturated water, and a gap water region 9, which serves as a flow path for light water of the coolant to flow without boiling.
10 are formed. In these gap water regions 9 and 10, the gap water region 9 into which the control rod 3 is inserted and the control rod 3
There are two types of gap water regions 10 that do not take in and out.
Due to the existence of such gap water regions 9 and 10, the effect of the saturated water on the fuel rods 4 in the periphery of the fuel assembly 2 (region close to the gap) and the fuel rods 4 in the center of the fuel assembly 2 Are different. That is, the periphery of the fuel assembly 2 near the gap water regions 9 and 10 is a region having a larger H / U ratio than the center. Therefore, the nuclear fission reaction is promoted at the peripheral portion of the fuel assembly 2 as compared with the central portion, and the local output peaking increases. In particular, in the core 1 of FIG. 10, the gap width of the gap water region 9 on the control rod side is larger than the gap width of the gap water region 10 on the non-control rod side.
It is a lattice core, and is of a type different from the C lattice core in which the gap widths of the gap water region 9 and the gap water region 10 are equal. Therefore, the H / U ratio greatly differs between the side facing the control rod 3 and the side opposite thereto, and the local output peaking also differs greatly.

With respect to the radial distribution of the H / U ratio as described above, it is very important to improve and optimize the H / U ratio from the viewpoint of improving the characteristics of the fuel assembly. Therefore, conventionally, the radial distribution of the H / U ratio has been improved by various methods. Hereinafter, these will be sequentially described.

(1) Radial H / U by arrangement of water rod
Improving the ratio As a measure for improving the H / U ratio in the radial direction of the fuel assembly, an increase in the number of water rods (or water boxes) arranged in the central portion of the fuel assembly, in particular, where the amount of water is low, is considered. There is a configuration to increase the size. Since the water inside the water rod or water box does not boil, water can be effectively taken into the central region of the fuel assembly where the neutron moderating effect is not sufficient. Thereby, the H / U ratio distribution in the radial direction can be improved, and the characteristics of the fuel assembly can be improved.

In particular, in the fuel assembly loaded in the D lattice core, as described above with reference to FIG. 10, the gap water region 9 on the side where the control rod 3 is located and the gap water region 10 on the opposite side.
Are not equal, the H / U ratio in the radial direction becomes uneven, and local output peaking tends to increase. On the other hand, the uranium 235
A method of adjusting the degree of concentration of is used. That is,
The fuel rod 4 on the side facing the narrow gap water region 10 where the thermal neutron flux is relatively small has a relatively high enrichment, and the fuel rod 4 on the side facing the gap water region 9 where the thermal neutron flux is relatively large and wide. By making 4 a relatively low enrichment, the output difference between the two is reduced, and local output peaking in the radial direction is suppressed.

(2) Improvement of H / U Ratio in High Burnup In recent years, in a boiling water reactor, as a method of effectively utilizing uranium resources while improving the plant utilization rate, a high burnup of fuel has been proposed. And a long-term operation cycle has been proposed. At this time, it is necessary to increase the enrichment in order to increase the removal burn-up of the fuel assembly.
Although the / U ratio is affected, it is necessary to consider a deceleration state in which an appropriate reactor shutdown margin and void reactivity coefficient are obtained. Further, the extension of the stay period in the furnace due to the long operation cycle means that the fuel is subjected to the effect that the H / U ratio differs in the radial direction for a long time in the core, and the influence of the H / U ratio is affected by this. It will be further expanded.

As a known technique for improving the radial H / U ratio in a fuel assembly with such a high burnup, there is, for example, JP-A-3-296689. In this prior art, a water rod is brought close to a narrow gap water region as a method of reducing a difference in a radial H / U ratio in a fuel assembly disposed in a D lattice core. A short fuel rod having a function of increasing the saturated water area is provided, and the short fuel rod is brought closer to a narrow gap water area. Are disclosed. All of these increase the H / U ratio near the narrow gap water region,
The H / U ratio is reduced near the wide gap water region. FIG. 1 shows an example of the structure of the fuel assembly in which the above is implemented.
It is shown in FIG. Portions common to FIG. 10 are denoted by the same reference numerals. The fuel assembly shown in FIG. 11 includes a square water box 11 that functions as a water rod in a 9 × 9 square lattice array,
It is eccentric by one line from the center of the fuel assembly to the narrow gap water region 10 side. Further, in the above-mentioned conventional technology, it is also shown that, when the operation is performed, it is not so effective to further perform the operation.

[0011]

However, the above-mentioned prior art has the following problems. That is,
In order to increase the burnup of the fuel assembly to increase the burnup, generally, the average enrichment of the fuel assembly is, for example, 3.8% by weight.
Although the amount is increased as described above, the amount of burnable poison mixed into the fuel pellets of some fuel rods in order to suppress the reactivity increases. At this time, the above is performed to
When the difference in the / U ratio is reduced, the water box 11 is biased toward the counter control rod side as shown in FIG. 11, for example. Inevitably. This is for the following reason. That is, first of all, the fuel rods containing burnable poisons burn out quickly if they are arranged at the outermost periphery of the square lattice array where the thermal neutron flux is the largest. Is generally arranged in the second and subsequent columns. Further, from the viewpoint of stable performance of the fuel assembly and prevention of a decrease in the value of the burnable poison, the fuel rods containing the burnable poison are arranged adjacent to each other (in the same row adjacent row or the same row adjacent row in the nxn row arrangement). Position) is generally avoided. For this reason, for example, in the example of FIG. 11, the fuel rod containing the burnable poison is difficult to be disposed in the vicinity of the non-control rod side corner (the lower right corner in the figure), so that the fuel rod containing the burnable poison is difficult. Are more arranged on the control rod side.

Here, the burning of the burnable poison strongly depends on the neutron spectrum, and the burning progresses as the neutron average energy becomes lower (the neutron spectrum becomes softer). In the control rod side region, the water box 11 is biased toward the control rod side, so that water is relatively reduced in the inner region other than the outermost periphery, and the neutron spectrum is hardened. When the poisoned fuel rods are arranged, the burning of these burnable poisons tends to be delayed, and the radial burning of the burnable poison becomes uneven. At this time, in particular, in the fuel assembly of the boiling water reactor, the cooling water boils in the axial direction, that is, flows while changing the phase, so that the void ratio is higher in the upper part in the axial direction (= the moderator density is higher). And the neutron spectrum becomes harder. Therefore, particularly in the upper part of the reactor core, the burning of the burnable poison is delayed, and the burning of the burnable poison in the axial direction becomes non-uniform. Due to the non-uniform combustion in the axial and radial directions of the burnable poison as described above, there has been a problem that the output distribution in the axial and radial directions becomes non-uniform and greatly distorted.

Here, in the above prior art, if the above (= the water rod region is made closer to the narrow gap water region) is carried out, the amount of the moderator near the narrow gap water region is almost sufficient. = Short fuel rods approach the narrow gap water area)
It is said that there is not much meaning in making the U ratio uniform. Therefore, by bringing the short fuel rods that do not contribute much to this uniformity closer to the wide gap water area on the control rod side, on the contrary, the effect of the radial H / U ratio uniformity is almost the same as when both are performed. If the neutron spectrum in the control rod side area, especially in the upper part in the axial direction, is softened to promote the burning of burnable poisons and achieve a uniform power distribution in the axial and radial directions while securing the effect of Seems much more informative. The above prior art does not consider such a point.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fuel assembly with a high burn-up, in which the H / U ratio is made uniform in the radial direction, and the non-uniform combustion of burnable poisons is suppressed, and the axial and radial directions are reduced. It is an object of the present invention to provide a configuration capable of achieving a uniform power distribution and to provide a reactor core using the fuel assembly.

[0015]

(1) In order to achieve the above object, the present invention provides a fuel cell system comprising: a plurality of fuel rods arranged in a square lattice; and an area in which one or more fuel rods can be arranged. At least one water rod or water box disposed, and a guide post provided on a control rod side for fixing a channel fastener, and the plurality of fuel rods are provided.
A plurality of first fuel rods containing nuclear fuel material and no burnable poison; a plurality of second fuel rods containing nuclear fuel material and burnable poison; more fuel-efficient than the first and second fuel rods; In a fuel assembly including a plurality of third fuel rods each having a short length and containing a nuclear fuel substance and not containing a burnable poison, the inside of the fuel assembly is divided into a control rod side region and a non-control rod side region. When divided, the water rod or the water box is biased toward the non-control rod side area side, and the second fuel rod and the third fuel rod are respectively located in the control rod side area. The number arranged is larger than the number arranged in the non-control rod side region. When a fuel assembly for a boiling water reactor is loaded in a D-lattice core, the amount of water as a moderator becomes relatively large on the control rod side because the interval between the fuel assemblies is wide. On the other hand, on the anti-control rod side, the space between the fuel assemblies is narrow and the amount of water is relatively small. In the present invention, first, by disposing the water rod or the water box in the non-control rod side region, the amount of water in the non-control rod side region is controlled by the action of the saturated water region in the water rod or water box. The H / U ratio is increased to increase the amount of fuel in the control rod side region to increase the H / U ratio.
Suppress U ratio. As a result, the radial arrangement of the water and the fuel is made more uniform, so that the radial direction H between the non-control rod side and the control rod side is increased.
/ U ratio is reduced and the radial H /
A uniform U ratio distribution is ensured. Here, the places where the plurality of second fuel rods including the burnable poison can be arranged usually have restrictions such as excluding the outermost periphery of the square lattice arrangement. Therefore, if the water rods or water boxes are arranged so as to be biased toward the non-control rod side as described above, the empty space in the non-control rod side area is reduced, so that the second fuel rod is not much disposed on the non-control rod side. become unable. Therefore, if the number of the second fuel rods is made equal on the control rod side and the counter-control rod side, the total number of the fuel rods is limited, and the predetermined amount of burnable poison necessary for achieving high burnup is required. It may not be possible to secure enough fuel rods. However, in the present invention, by arranging these second fuel rods more on the control rod side than on the non-control rod side, a fuel rod containing a predetermined amount of burnable poison required for achieving a high burnup can be obtained. A sufficient number can be secured. By the way, in general, the burning of burnable poison strongly depends on the neutron spectrum, and the burning progresses as the neutron spectrum becomes softer. As described above, if the water rod or the water box is unevenly distributed on the side opposite to the control rod, the water becomes relatively small in the control rod side region only in the inner region other than the outermost periphery, and the neutron spectrum tends to be hard. Therefore, if the second fuel rod containing the burnable poison is arranged in the control rod side region as described above, the combustion of the burnable poison is delayed, and particularly, the combustion is most delayed in the upper part in the axial direction. The directional output distribution becomes non-uniform. However, in the present invention, the second fuel rods containing burnable poisons are arranged more in the control rod side area, and the third fuel rods having a short active fuel length are also arranged more in this control rod side area. Thus, by the action of the saturated water region generated above the third fuel rod, the amount of water in the control rod side region, particularly in the upper portion in the axial direction, can be increased to soften the neutron spectrum. Therefore, the combustion of the burnable poison contained in the second fuel rod, particularly at the upper part in the axial direction, can be promoted to achieve uniform axial combustion. This also promotes combustion in the control rod side region as compared to the non-control rod side region, so that radial combustion can be made uniform. Therefore, the output distribution in the axial and radial directions can be made uniform.

(2) In the above (1), preferably,
All of the second fuel rods and the third fuel rods are arranged in the control rod side region.

(3) In the above (1), preferably, in the second fuel rod, the concentration of the burnable poison in the upper part in the axial direction is lower than the concentration of the burnable poison in the lower part in the axial direction. As a result, the remaining unburnable burnable poison in the upper part of the second fuel rod in the axial direction can be more effectively prevented, so that the axial power distribution can be further uniformed.

(4) In order to achieve the above object, the present invention provides a nuclear reactor core loaded with the above fuel assembly.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention.
This will be described with reference to FIG. FIG. 2 is a longitudinal sectional view showing the structure of a fuel assembly for a boiling water reactor according to the present embodiment, and FIG. 1 is a transverse sectional view taken along line II in FIG. 1 and 2, a fuel assembly 10 according to the present embodiment is shown.
Numeral 0 denotes a unit loaded in the unit cell 107 of the boiling water reactor D lattice core similarly to the core shown in FIG. 10, and is arranged in a square lattice of 9 rows and 9 columns and has uranium as a nuclear fuel material therein. 72 fuel rods 101 filled with -235,
One water box 102 in which nine fuel rods 101 are arranged in an area where nine fuel rods 101 can be arranged for the purpose of increasing the amount of water in the central region of the square lattice arrangement, and formed by these fuel rods 101 and water box 102 And a channel box 103 surrounding the periphery of the fuel bundle. The upper and lower parts of the fuel bundle are each an upper tie plate 10
4 and supported by the lower tie plate 105,
The fuel rods 101 are provided at a plurality of positions in the axial direction of the fuel bundle.
Further, a spacer 106 for keeping the interval between the water boxes 102 is provided. Also, the upper tie plate 10
4 control rod 108 side (hereinafter, simply referred to as control rod side as appropriate)
Guide posts 104a and 104b are formed integrally with each other at a corner portion on the opposite side (hereinafter, appropriately referred to as a non-control rod side). The connected channel fastener 109 is fixed.
This channel fastener 109 has one control rod 10
In addition to connecting the four fuel assemblies 100 around the fuel assembly 8 to each other and securing a space for inserting and pulling out the control rods 108 between these fuel assemblies 100, the control rod side of each fuel assembly 100 is controlled. Guide post 10
4a. The guide post 104b on the opposite side of the control rod is a dummy for balancing the weight with the guide post 104a on the side of the control rod.

The fuel rods 101 are composed of 52 first fuel rods 1 having a normal fuel effective length and containing no burnable poison.
01a and six second fuel rods 101b having a normal fuel effective length and containing gadolinia as a burnable poison.
And the active fuel length of the first and second fuel rods 101a, 101
14 third fuel rods, so-called short fuel rods shorter than 1b
And a fuel rod 101c. At this time, the mixed concentration of gadolinia in the second fuel rod 101b is uniform in the axial direction, and the active fuel length of the third fuel rod 101c is equal to the first and second fuel rods 101a, 101a.
The effective length of b is 15/24 (= 5/8). These fuel rods are not particularly shown or described in detail, but similar to those known as this type of fuel assembly,
It consists of a plurality of types of fuel rods with different enrichment distributions. The axial output peaking is flattened by appropriately providing the enrichment distribution in the axial direction for each type, and the radial output peaking is flattened by appropriately devising the arrangement of various fuel rods. . At this time, in such an arrangement of the fuel rods 101, the average enrichment in the fuel assembly 100 is 3.8% by weight.

The main part of the fuel assembly 100 according to the present embodiment is the method of arranging the water box 102 and the second embodiment.
And the third fuel rods 101b and 101c. In other words, the inside of the fuel assembly is
By 0, the control rod side area 111 and the non-control rod side area 112
When considered in two parts, the water box 102 is disposed so as to be biased toward the non-control rod side area 112 and the second and third fuel rods 101b, 1 disposed in the control rod side area 111.
01c is larger than the number of the second and third fuel rods 101b and 101c arranged in the non-control rod side region 112. Specifically, while ten second fuel rods 101b and five third fuel rods 101c are arranged in the control rod side area 111, the control rod side area 1
12, only four second fuel rods 101b and one third fuel rod 101c are arranged.

With such an arrangement of the water box 102, the second fuel rod 101b, and the third fuel rod 101c,
The present embodiment has the following operations.

(1) Improvement of H / U Ratio Due to Uneven Distribution of Water Boxes That is, when loaded in a D lattice core, the area of the gap water region 113 formed outside the channel box 103 on the control rod side increases. Therefore, the amount of water is relatively large, while the amount of water is relatively small because the area of the gap water region 114 formed outside the channel box 103 on the side opposite to the control rod is small.
However, in the present embodiment, the water box 10
2 is biased toward the non-control rod side, the amount of water in the non-control rod side area 112 is increased by the action of the saturated water area in the water box 102, and the H / U ratio is increased. The H / U ratio is suppressed by increasing the amount of fuel in the side region 111. Thereby, the radial arrangement of water and fuel is made more uniform, and the difference in the radial H / U ratio between the non-control rod side and the control rod side is reduced.
The H / U ratio distribution in the radial direction in the inside can be secured uniformly.

(2) Ensuring High Burnup Normally, the location of the fuel rod including gadolinia has the following restrictions. In other words, the fuel rod containing gadolinia burns out quickly if it is placed on the outermost periphery of the square lattice array where the thermal neutron flux is the largest, so it is usually not placed on the outermost periphery, but is placed on the second and subsequent rows from the outermost periphery. It is common to do. In addition, from the viewpoint of stable performance of the fuel assembly and prevention of a reduction in gadolinia value, fuel rods containing gadolinia are arranged adjacently (positions in the same adjacent row or the same adjacent row in the n × n row arrangement). It is common to avoid Therefore, if the water box 102 is displaced toward the non-control rod side area 112 according to the above (1), the empty space in the non-control rod side area 112 is reduced, and the second fuel rod 101b is moved to the non-control rod side area. It cannot be arranged in the area 112 much. Therefore, the second fuel rod 1
The control rod side area 111 and the non-control rod side area 1
If the number is equal to 12, the total number of arrangements is limited, and the number of predetermined gadolinia-containing second fuel rods 101b (14 in the example of FIG. 1) required for achieving high burnup is reduced. It may not be possible to secure. However,
In the present embodiment, by arranging these second fuel rods 101b more in the control rod side area 111 than in the non-control rod side area 112, a predetermined amount of burnable poison required for high burnup is achieved. Number of the second fuel rods 101b (1
4) can be secured as a whole.

(3) Uniform combustion in axial and radial directions In general, gadolinia combustion strongly depends on the neutron spectrum, and the more the neutron spectrum is softened, the more the combustion proceeds. When the water box 102 is unevenly distributed to the non-control rod side area 112 according to the above (1), the control rod side area 11
In the case of 1, water is relatively reduced only in the inner region other than the outermost periphery, and the neutron spectrum tends to be hard. Therefore, when a large number of gadolinia-containing second fuel rods 101b are arranged in the control rod side region 111 as in the above (2), combustion of gadolinia is delayed, and particularly, combustion is most delayed in the upper part in the axial direction. The directional and axial power distribution becomes non-uniform. However, in this embodiment,
The second fuel rod 101b containing gadolinia is moved to the control rod side region 1
11 and the third fuel rod 101 c having a short effective fuel length is also disposed in the control rod side region 111. Thus, by the action of the saturated water region generated above the third fuel rod 101c, it is possible to increase the amount of water in the control rod side region 111, particularly in the upper part in the axial direction, and to soften the neutron spectrum. Therefore, the second fuel rod 1
The combustion in the upper part of the gadolinia included in the gadolinia 01b, particularly in the axial direction, can be promoted to make the axial combustion uniform. This also promotes combustion in the control rod side region 111 as a whole, as compared to the non-control rod side region 112, so that uniform radial combustion can be achieved. Therefore, the output distribution in the axial and radial directions can be made uniform.

As described above, according to the fuel assembly 100 of the present embodiment, in the structure with high burn-up, the gadolinia combustion non-uniformity is maintained while the H / U ratio is made uniform in the radial direction. And the output distribution in the axial and radial directions can be made uniform.

Among the above-mentioned effects, in particular, the uniformization of the axial output distribution will be described using a comparative example. As a comparative example of the fuel assembly 100 of the present embodiment, all the third fuel rods 101c in the fuel assembly 100 are replaced with the first fuel rods 10c.
FIG. 3 shows the structure replaced with 1a. As shown in FIG. 3, the fuel assembly 150 according to the comparative example has the first fuel rods 101 a whose number is reduced from 52 to 58 by the above replacement.
It is increasing in books. Other structures are substantially the same as the fuel assembly 100 according to the present embodiment. And FIG.
The analysis results (curve a) of the axial power distribution of the boiling water reactor core loaded with the fuel assembly 100 according to the present embodiment in the same manner as in FIG. 10 and the fuel assembly 150 according to the comparative example
11 is a comparison between the analysis result (curve a) of the axial power distribution of the boiling water reactor core loaded in the same manner as in FIG. As shown in FIG. 4, in the core to which the fuel assembly 150 of the comparative example is applied, although the moderator density is small, the gadolinia combustion is delayed in the upper part in the axial direction where the neutron spectrum is hard, but the power distribution is swelled downward. In the core to which the fuel assembly 100 of the present embodiment is applied, the effect of the saturated water on the upper part of the third fuel rod 101c reduces the neutron spectrum hardening, thereby reducing the difference in the axial burning of the burnable poison, It can be seen that the axial power distribution is flatter.

In the above embodiment, in order to increase the burnup, the average enrichment of the aggregate is set to 3.8% by weight in a square grid-like arrangement of 9 rows and 9 columns. Although 14 fuel rods 101b are arranged, the invention is not limited to this. For example, the second fuel rod 10 containing gadolinia may be made slightly smaller than the average enrichment of the assembly.
The ratio of the number 1b may be reduced. An embodiment in such a case will be described below with reference to the second to fourth embodiments. In the following embodiments, the control rod side region 111 and the non-control rod side region 112 are appropriately omitted in order to avoid complication.

A second embodiment of the present invention will be described with reference to FIGS. FIG. 5 is a cross-sectional view showing the structure of the fuel assembly for a boiling water reactor according to the present embodiment. FIG. 5 is a view substantially corresponding to FIG. 1 of the first embodiment, in which common members are denoted by the same reference numerals and description thereof is omitted.

In FIG. 5, the fuel assembly 200 according to this embodiment is the same as the fuel assembly 1 of the first embodiment shown in FIG.
The difference from 00 is that the number of second fuel rods 101b is slightly reduced. That is, the second with gadolinia
The number of fuel rods 101b is reduced by six to eight in total, six in the control rod side area 111 and two in the non-control rod side area 112. Correspondingly, the number of first fuel rods 101a is increased by six to 58. The number and arrangement position of the third fuel rods 101c do not change. The average concentration of the aggregate is x [% by weight] (where x
> 3.8). Other structures are substantially the same as the fuel assembly 100 of the first embodiment.

According to this embodiment, the same effects as those of the first embodiment can be obtained.

In the first and second embodiments, the concentration of gadolinia mixed in the second fuel rod 101b is uniform in the axial direction. However, the present invention is not limited to this. It may be provided. That is, as shown in FIG. 6, the gadolinia concentration of the second fuel rod 101b is set to β [% by weight] at the upper part and to α
[% By weight] (where α> β). In this case, since the upper gadolinia concentration in the second fuel rod 101b can be more effectively prevented by the relatively lower upper gadolinia concentration, the axial power distribution can be further uniformed. . In the first and second embodiments, the water box 102 is provided in a space where nine fuel rods 101 can be arranged. However, the present invention is not limited to this, and a water rod having a circular cross section may be provided. FIG. 7 shows a structure of a fuel assembly 200A in which two water rods 102A, 102A are provided instead of the water box 102 in the fuel assembly 200 of the second embodiment. Again, the second
The same effect as that of the embodiment is obtained.

A third embodiment of the present invention will be described with reference to FIG. FIG. 8 is a cross-sectional view illustrating a structure of a fuel assembly for a boiling water reactor according to the present embodiment. FIG. 8 shows the first
FIG. 6 is a view substantially corresponding to FIG. 1 of the second embodiment and FIG. 5 of the second embodiment.

In FIG. 8, the fuel assembly 300 according to this embodiment is the same as the fuel assembly 2 of the second embodiment shown in FIG.
The difference from 00 is that all the second fuel rods 101b and the third fuel rods 101c are arranged in the control rod side region 111. Other structures are almost the same as those of the fuel assembly 200 of the second embodiment. According to this embodiment, the same effect as that of the second embodiment is obtained.

A fourth embodiment of the present invention will be described with reference to FIG. The present embodiment is an embodiment in which the present invention is applied to a fuel assembly in which fuel rods are arranged in a 10 × 10 square grid. FIG. 9 is a cross-sectional view illustrating a structure of a fuel assembly 400 for a boiling water reactor according to the present embodiment, and FIG.
FIG. 9 is a diagram substantially corresponding to FIG. 1 of the embodiment, FIG. 5 of the second embodiment, and FIG. 8 of the third embodiment.

In FIG. 9, the fuel assembly 400 of the present embodiment has a structure basically similar to that of the second fuel assembly 200 except that the number of fuel rods arranged is increased.
That is, the fuel assemblies 400 are loaded in the unit cells 407 of the boiling water reactor D lattice core, are arranged in a square lattice of 10 rows and 10 columns, and are filled with uranium-235 as a nuclear fuel material. 91 fuel rods 401
A water box 402 in which nine fuel rods 401 are arranged in an area where nine fuel rods 401 can be arranged for the purpose of increasing the amount of water near the center of the square lattice arrangement; And a channel box 403 surrounding the periphery of the fuel bundle formed by the above.

The fuel rods 401 have 75 fuel rods 4 having a normal fuel effective length and containing no burnable poison.
01a and nine second fuel rods 401b having a normal fuel effective length and containing gadolinia as a burnable poison.
And the active fuel lengths of the first and second fuel rods 401a, 401
7b, which are so-called short fuel rods shorter than 1b. Further, these fuel rods 401 are composed of a plurality of types of fuel rods having different enrichment distributions, and the enrichment distribution in the axial direction is appropriately provided for each type, and the arrangement of the various fuel rods is appropriately devised so that the fuel rods 401 are disposed in the axial direction. -Flattening of radial output peaking is achieved. At this time, the average enrichment in the fuel assembly 400 is x [% by weight]. When the inside of the fuel assembly is divided into a control rod region 411 on the control rod 408 side and an anti-control rod side region 412 on the opposite side by the vertical boundary surface 410, the water box 402 is The second and third fuel rods 40 which are arranged in the non-control rod side area 412 and are arranged in the control rod side area 411
The number of 1b and 401c is made larger than the number of the second and third fuel rods 401b and 401c arranged in the non-control rod side region 412, and the difference between the numbers is five.

According to the present embodiment, the gap water regions 313 and 31 are based on substantially the same principle as in the second embodiment.
H / U ratio unevenness based on the area difference of 4
2 and the number of the second fuel rods 401b is arranged in the control rod side area 111 to secure the number required for high burnup, and the third fuel rods 401c are moved to the control rod side area 111. By arranging many, the neutron spectrum is softened, and gadolinia combustion is promoted, so that axial and radial combustion can be made uniform.

In the first to fourth embodiments, each of the water boxes 102 and 402 is arranged in a region where the nine second fuel rods 101b and 401b can be arranged. The present invention is not limited to this, and it suffices that the second fuel rods 101b and 401b are arranged in a region where they can be arranged. In these cases, a similar effect is obtained.

[0040]

According to the present invention, in a fuel assembly with high burn-up, the H / U ratio can be kept uniform in the radial direction, and the non-uniform combustion of burnable poison can be suppressed in the axial direction. Uniform radial power distribution can be achieved, and good core characteristics can be realized.

[Brief description of the drawings]

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

FIG. 2 is a longitudinal sectional view of the fuel assembly shown in FIG.

FIG. 3 is a cross-sectional view illustrating a structure of a fuel assembly according to a comparative example.

FIG. 4 is a diagram showing a comparison of analysis results of an axial power distribution of a boiling water reactor core loaded with the fuel assemblies of FIGS. 1 and 4, respectively.

FIG. 5 is a cross-sectional view illustrating a structure of a fuel assembly for a boiling water reactor according to a second embodiment of the present invention.

FIG. 6 is a diagram for explaining a modified example regarding the concentration of gadolinia mixed in the second fuel rod.

FIG. 7 is a cross-sectional view illustrating a structure of a modification in which two water rods are provided instead of a water box.

FIG. 8 is a cross-sectional view illustrating a structure of a fuel assembly for a boiling water reactor according to a third embodiment of the present invention.

FIG. 9 is a cross-sectional view illustrating a structure of a fuel assembly for a boiling water reactor according to a fourth embodiment of the present invention.

FIG. 10 is a conceptual cross-sectional view showing a partial structure of a general boiling water reactor core.

FIG. 11 is a diagram showing a structural example of a conventional fuel assembly in which a water rod is brought closer to a narrow gap water region in order to reduce a difference in a radial H / U ratio.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 fuel assembly 101 fuel rod 101 a first fuel rod 101 b second fuel rod 101 c third fuel rod 102 water box 102 A water rod 104 a guide post 108 control rod 109 channel fastener 110 boundary surface 111 control rod side region 112 counter Control rod side region 200 Fuel assembly 200A Fuel assembly 300 Fuel assembly 400 Fuel assembly 401 Fuel rod 401a First fuel rod 401b Second fuel rod 401c Third fuel rod 402 Water box 408 Control rod 410 Boundary surface 411 Control rod side area 412 Anti-control rod side area

Claims (4)

[Claims] A channel fastener is fixed to a plurality of fuel rods arranged in a square lattice, at least one water rod or water box arranged in a region where one or more fuel rods can be arranged. And a plurality of first fuel rods including a nuclear fuel substance and no burnable poison, and a plurality of first fuel rods including a nuclear fuel substance and no burnable poison. And a plurality of third fuel rods having a shorter effective fuel length than the first and second fuel rods and containing a nuclear fuel substance and containing no burnable poison. In the fuel assembly, the inside of the fuel assembly is divided into a control rod side region and a non-control rod side region.
When divided, the water rod or the water box is disposed so as to be biased toward the non-control rod side region side, and
A fuel assembly, wherein the number of fuel rods and the number of third fuel rods arranged in the control rod side area are larger than the number of fuel rods arranged in the non-control rod side area. 2. The fuel assembly according to claim 1, wherein all of said second fuel rod and said third fuel rod are arranged in said control rod side region. 3. The fuel assembly according to claim 1, wherein the concentration of the burnable poison in the upper portion in the axial direction is lower than the concentration of the burnable poison in the lower portion in the axial direction. Characteristic fuel assembly. 4. A reactor core loaded with the fuel assembly according to claim 1.