JP2809626B2 - Fuel assembly - Google Patents

Description

The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for a boiling water reactor having a long operation cycle and a high shutdown margin. About. (Prior Art) A fuel assembly of a boiling water reactor is configured by regularly arraying a large number of fuel rods filled with nuclear fuel material inside a metal cladding tube and housed in a rectangular channel box. ing. In the core of a boiling water reactor, cells each usually composed of one cross control rod and four fuel assemblies surrounding the control rod are regularly arranged. That is, the fuel assemblies and control rods of the core of the boiling water reactor are arranged so that their axes are vertical and parallel to each other, and the coolant having a function as a moderator is applied from below to above the core. It is configured to flow toward. Although no air bubbles are generated near the lower end of the core effective portion, that is, near the lower end of the heat generating portion, a large amount of air bubbles are generated from the center to the upper end of the core, and the generated air bubbles flow above the core. If the volume ratio of the bubbles, that is, the void ratio, increases, the neutron moderating characteristics deteriorate, so that the thermal neutron flux decreases and the output decreases. To avoid this, the concentration of fission nuclides, that is, enrichment, should be increased at sites with a high void fraction,
Alternatively, combustible poisons have been added to prevent the output from increasing in the portion having a low void ratio. Therefore, in the boiling water reactor, the combustion in the upper part of the core tends to be delayed, which causes the U-235 concentration to be relatively higher than the other parts, and also generates fissionable nuclides such as Pu-239 by voids. It is well known that the reactor shutdown margin tends to be severe in the upper part of the reactor core. Further, efforts have been made to extend the operation cycle and improve the burnup of fuel, mainly for the purpose of improving economy. In this case also, the enrichment of the fuel is inevitably increased, so that the shutdown margin of the nuclear reactor is further increased. Next, typical examples of a fuel assembly used for a boiling water reactor and a fuel assembly expected to be used in the near future will be described with reference to the drawings. FIGS. 22 (a) and 22 (b) are a perspective view of a conventional fuel assembly and a schematic vertical sectional view of a fuel rod constituting the fuel assembly, respectively. In FIG. 2A, a fuel assembly is a water rod (not shown).
And the fuel rods 2 are fixed by an upper tie plate 4, a spacer 5, and a lower tie plate 6, and the outside thereof is surrounded by a channel box 1. As shown in FIG. 1B, the fuel rods 2 are provided with fuel pellets 8 disposed in a cladding tube 7 and a spring 9 provided in a gas plenum above the fuel pellets.
An upper end plug 10 is provided at an upper end and a lower end plug 11 is provided at a lower end. FIG. 23 is a cross-sectional view of the conventional fuel assembly shown in FIG. In the channel box 1, 62 fuel rods 2 and 2
The water rods 3 are arranged to form a fuel assembly. The water rod 3 suppresses the shortage of water as a moderator inside the assembly. However, since the water rod 3 is uniform in the axial direction, there is a problem that water is excessive below the core and water is insufficient above the core. There is a point. The fuel assembly shown in FIG. 24 has been developed to improve the characteristics of the fuel assembly. One large-diameter water rod 12 is disposed inside the assembly to introduce non-boiling water. I have. However, even in this example, there is a problem that water is excessive below the core and water is insufficient above the core. The fuel assembly shown in FIG. 25 is also an improvement of the fuel assembly shown in FIG. 23, in which four small channel boxes 13 are provided, boiling cooling water is provided in the small channel boxes 13, and a space between the small channel boxes 13 is provided. The cross-shaped gap 14 has a non-boiling cooling water area to flatten the power distribution in the horizontal direction, but this type of fuel assembly also suffers from excess water below the core and insufficient water above it. There is a problem. The fuel assembly shown in FIG. 26 has been developed as an improved version of the fuel rod shown in FIG. This fuel assembly is composed of nine sub-assemblies 15 and each sub-assembly 15
Reference numeral 15 denotes nine fuel rods 2 each. A slightly wider gap 16 is provided between the subassemblies 15.
In this fuel assembly as well, the problem of excess and deficiency of water in the upper and lower portions of the core has not been solved. (Problems to be Solved by the Invention) As described above, air bubbles are not generated at the lowermost end of the fuel assembly, which is the heating section of a boiling water reactor (BWR), but air bubbles are generated anywhere in other parts. The generated air bubbles flow upward (downstream). Therefore, BW
The bubble ratio (void ratio) of R becomes higher toward the upper part of the core.
As a result, the neutron moderation characteristics are reduced, so that the fission rate is reduced. That is, combustion proceeds below the core and is delayed above the core. Therefore, it has been proposed to increase the fission nuclide concentration above the core in order to suppress a decrease in power above the core. However, increasing the void fraction above the core and increasing the fission nuclide concentration reduces the subcriticality above the core when the reactor is shut down. On the other hand, fuel enrichment must be further increased in order to prolong the operation cycle and improve economics, but this will make the subcriticality in the upper part of the core increasingly shallow, and eventually It is possible that the reactor could not be shut down. That is, this point is a bottleneck, and there has been a problem that the operation cycle cannot be lengthened in the conventional reactor core. The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a boiling water reactor having an axial power distribution improved while enabling a reactor shutdown even when the fuel enrichment is increased. An object of the present invention is to provide a fuel assembly constituting a core. [Constitution of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly in which a large number of fuel rods filled with nuclear fuel pellets are regularly arranged inside a metal cladding tube. In the body, some fuel rods are inserted with a certain length of intervening member having a greatly reduced fissile nuclide concentration so that nuclear fuel pellets are present in the upper and lower portions in the metal cladding tube, and the length of the intervening member is reduced. Is the thermal neutron diffusion distance or longer during reactor power operation, and the subcriticality is reduced when the axial insertion position of the interposed members in the fuel rods becomes critical during reactor operation. A position including a shallow portion, wherein the interposed member-inserted fuel rods are arranged inside the fuel assembly in a linear or intersecting linear or nodular shape. (Operation) As described above, according to the fuel assembly of the present invention, the neutron interaction (coupling effect) on both sides of the intervening region is weakened in the cold state, with the particularly low portion of the fissile material concentration, that is, the intervening region interposed therebetween. During high temperature operation, especially when voids are generated, a phenomenon occurs. This phenomenon can be mainly explained by the action of thermal neutrons with a short diffusion distance. That is, since the density of water (about 1.0) is large in the cold state, the diffusion distance of thermal neutrons is short, and the interaction of neutrons on both sides of the intervening region is reduced. As a result, the neutron multiplication characteristics are reduced. During high temperature operation, the boiling water reactor has a water temperature (standard value) of about 286 ° C and a water density of about 0.74 g even when no voids are generated.
/ a cm ^3. Thermal neutron migration distance in water is 1 / 0.74 of that in cold
(= 1.35) times. Furthermore, the density of the air-water mixture at the time of void generation decreases to about 0.3, and as a result, the thermal neutron diffusion distance in the air-water mixture increases by 1 / 0.3 (1/3) times. As a result, the neutron interaction on both sides of the intervening region increases, and the neutron multiplication characteristics increase. By utilizing the above-mentioned effects, the introduction of the intervening region reduces the multiplication factor in the cold state, that is, increases the reactor shutdown margin (subcriticality), and reduces the amount of fuel during the high temperature operation by introducing the intervening region. Even in this case, it is possible to prevent a decrease in the multiplication factor or to increase the multiplication factor more than in the case where there is no intervening region, if a suitable design is performed. Next, the operation of the present invention will be described with reference to FIG. As shown in FIG. 3A, it is assumed that there are two fuel regions I and II having a rectangular cross section, and a water gap having a width W exists between them. The width Wf of the fuel regions I and II in the same direction as the water gap width W is sufficiently wider than the water gap width W. The relationship between the water gap width W and the change in the neutron multiplication factor at this time is as shown in FIG.
The part is enlarged and shown in FIG. Here, “change in neutron multiplication factor” indicates that the change is from the neutron multiplication factor when the water gap width is 0 at both a high temperature (broken line) and a cold state (solid line). In the direction perpendicular to the axial direction (normally the horizontal direction in a light water reactor) in the fuel assembly, it is difficult to obtain a wide water gap region. In other words, widening the water gap in the range given the outer shape means that the fuel region becomes narrower and the heat generation region becomes narrower. In the present invention, since the interposed region is inserted in a direction perpendicular to the axis of the fuel assembly, it is necessary to clarify the characteristics of the interposed region having a small width. FIG. 3 (c) is an enlarged view of part c of FIG. 3 (b) with this in mind. The theoretical calculation value for a case where a water gap of at most about 2 cm is provided also gives a curve substantially similar to FIG. That is, during high temperature operation (when voids are generated), the change in the multiplication factor increases with the water gap width in the positive direction (effective multiplication factor keff increases), and when cold, when the water gap width exceeds about 1 cm, it becomes remarkable. It can be seen from this figure that keff decreases with increasing water gap width and helps to increase the subcriticality when the furnace shuts down. In the above description of the operation, the viewpoint of the change in the neutron interaction between the two fuel regions sandwiching the water gap is considered. However, the infinite multiplication factor k _の of the fuel assembly is divided into four factors that have been known for a long time. It can also be described in a scheme.
In this system, the curve in FIG. 3 (c) is also explained mainly by changes in the properties of thermal neutron utilization and the probability of escaping resonance. If the water gap is increased without reducing the number of fuel rods inside the fuel assembly, the gap between the fuel rods must be reduced, which increases the shielding effect of the resonance neutrons between the fuel rods in the resonance absorption. As a result, the effect of increasing the probability of escaping resonance is increased, while the effect of reducing the thermal neutron flux ratio between the fuel region and the water gap is reduced, and as a result, the effect of reducing the thermal neutron utilization is produced. FIG. 3 (c) is substantially determined by the offset effect of the above two effects on the water density and the water gap width. In order to fix the gap between fuel rods and widen the water gap, fuel material must be removed from within the fuel rods.
In that case, the change in the probability of escaping the above resonance absorption is
The probability of escape from resonance increases due to an increase in the deceleration effect, not the shielding effect of the resonance neutrons. In other words, when the reactor is operating at a high temperature and voids are generated, the moderator is in a shortage state, and the introduction of a water gap mitigates the moderator. As a result, the probability of escape from resonance also increases. I do. The change in the thermal neutron utilization is almost the same as in the above example. In the present invention, the latter is mainly used, and the former is also used as needed. An object of the present invention is to provide a fuel assembly which effectively produces the operation of the present invention without reducing the amount of fuel loaded in the assembly as much as possible (in order not to reduce the absolute amount of output). (Example) An example of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view of an embodiment of the present invention. FIG. 1A is a longitudinal sectional view taken along the line II of FIG. 1B, and FIG. 1B is a plan view. In the fuel assembly of this embodiment, a square water rod 32 is arranged in the center, and fuel rods 30 and 31 are regularly arranged in 9 rows and 9 columns except for the central part.
, And the upper and lower ends thereof are fixed by an upper tie plate 34 and a lower tie plate 35, respectively. The fuel rod 31 is an interposed member inserted fuel rod p. Intervening member 36
The preferred length l is 1 / 3H or less, in this example 30-60 cm
It is about. The height of the center of the interposed member is from the lower end of the active fuel length
3 / 4H. This is because the subcriticality is particularly shallow near this 3 / 4H and within about 1 / 4H. Although details of the intervening member 36 will be described later, insertion of a hollow tube, insertion of graphite, insertion of ZrH ₂ pellets, introduction of water, and the like are typical examples. Further, as described later, an output spike suppressing member is inserted adjacent to the intervening member to prevent the output spike. Further, the fuel rods p inserted into the interposition member are arranged in a cross shape (a straight line perpendicular to the center) with the center water rod 32 as a center. 3
Since the neutron interaction between the main group is small in this example,
It is not always necessary to limit to the same height. FIG. 2 (a) is a schematic sectional view in which the fuel assembly of the present invention is applied to a boiling water reactor, and FIG. 2 (b) is a diagram showing a void ratio and a subcriticality distribution in a core axis direction. The hatched portion in FIG. 2 (a) is the position of the interposed member.
It is effective to make the heights of the intervening members in the fuel rods effective, but it is not always necessary to make the height uniform between the assemblies. This is because the outer periphery of the core does not contribute much to the increase in the stop margin. Further, the width of the intervening region is approximately equal to the width of the water gap at the outer periphery of the aggregate, and the operation of the coupling effect inside the aggregate. In a narrow intervening layer, the effect is largely masked by the outer water gap, and the effect on adjacent aggregates is small. The axial length of the intervening member varies depending on the number and arrangement of the fuel rods in which the intervening member is placed in the assembly, but is generally in the range of about 15 to 90 cm. The effect is small when it is 15 cm or less, and the effect is relatively large when the length is longer than 90 cm (a relatively large decrease in the heating substance (fuel) loading).
Become smaller. And since an interposition member is arrange | positioned in the site | part where subcriticality becomes the shallowest, this invention functions effectively. FIG. 4 is a plan view of a second embodiment of the present invention. The same parts as those in the first embodiment described above are denoted by the same reference numerals and described. The same applies to the following embodiments. In the figure, since the corner portions also have the effect of improving the margin for stopping, the fuel rods p inserted into one of the three intervening members of each set of three crosses are replaced with the fuel rods 30 of each corner portion. In this fuel rod, by inserting a burnable poison (Gd or the like) to such an extent that the poisoning action disappears only at the center at the end of the cycle, it is possible to contribute to the achievement of the above object without generating an output spike. FIG. 5 is a plan view of a third embodiment of the present invention, in which fuel rods p for inserting intervening members are arranged diagonally in a cross shape with a center water rod 32 as a center. Therefore, in this assembly, the number of fuel rods p inserted into the intervening member is 16 and the number of fuel rods 30 is 60. FIG. 6 is a plan view of a fourth embodiment of the present invention. In this embodiment, a conventional assembly (two thin water rods 37 instead of a large water rod) is used.
This is the case where the present invention is applied. Therefore, in the present assembly, 13 fuel rods p with interposed members and 49 fuel rods 30 are inserted.
It is composed of books. In this embodiment, since there is no large-diameter water rod, the interaction between the p-groups (3 to 4 each) of the "intervening member-inserted fuel rods" in the cruciform arrangement is slightly larger than in the first embodiment. It is slightly more advantageous to arrange the heights. FIG. 7 is a plan view of a fifth embodiment of the present invention. This embodiment is a case where it is applied to a system using two thin water rods 37 as in the embodiment of FIG. In this embodiment, the fuel rods p are inserted diagonally, and the number of fuel rods p is 14 and the number of fuel rods 30 is 48. The operation is almost the same as that of the embodiment shown in FIG. FIG. 8 is a plan view of a sixth embodiment of the present invention. This embodiment is similar to the embodiment of FIG. 6 and FIG.
This is a case where the method is applied to a method using two 37s. In the present embodiment, the fuel rods p inserted with intervening members are arranged in a nodular shape at the center of the assembly. The number of fuel rods p inserted with intervening members is 14 and the number of fuel rods 30 is 48. The operation is substantially the same as that of the embodiment shown in FIGS. 6 and 7. FIG. 9 is a plan view of a seventh embodiment of the present invention. This embodiment is a case where it is applied to a system using two small diameter water rods as in the embodiment of FIGS. 6 and 7. In this embodiment, the fuel rods p inserted by the intervening members are arranged in two rows inside the assembly so as to be parallel to the diagonal line, and have substantially the same operation as the embodiment shown in FIGS. 6 and 7. . In this embodiment, the number of the fuel rods p inserted into the interposed member is eight, and the number of the fuel rods 30 is 54. The number of the fuel rods p inserted into the interposed member is small. May be divided. Therefore, it can be utilized for adjusting the axial power distribution together with the furnace stop margin. In other words, the water shortage during the high-temperature operation in the upper part of the core is mitigated by the interposed member of the fuel rod p inserted into the interposed member. FIG. 10 is a plan view of an eighth embodiment of the present invention. This embodiment is a case where the present invention is applied to a system using two thin water rods. In this embodiment, the arrangement of the interposed member inserted fuel rods p is a double cruciform arrangement, with 26 interposed member inserted fuel rods p and 36 fuel rods 30. At the insertion height of the interposed member, the assembly is substantially composed of four 3 × 3 sub-bundles, and is greatly separated by the interposed member of the fuel rod p inserted into the interposed member. It has the property to create a furnace shutdown margin. Therefore, the degree is adjusted by adjusting the axial length. The length of the interposed member is
A large effect can be expected even with about 15 to 30 cm (normal). If necessary, the height of the interposed member inserted fuel rod p may be changed between two opposing rows. As a result, the output distribution can be changed effectively. FIG. 11 is a plan view of a ninth embodiment of the present invention. This embodiment is a case where a large-diameter water rod 38 for four fuel cells at the center of the embodiment shown in FIG. 10 is used.
The number of fuel rods and fuel rods 30 is 44. The operation and effect are slightly smaller than the embodiment of FIG. FIG. 12 is a plan view of a tenth embodiment of the present invention. This embodiment is a case where a square water rod 39 for four fuel cells at the center of the embodiment of FIG. 7 is used.
The number of fuel rods and fuel rods 30 is 48. The operation and effect are slightly smaller than the embodiment of FIG. FIG. 13 is a plan view of an eleventh embodiment of the present invention. In this embodiment, the fuel cells are composed of 11 rows and 11 columns, and a large-diameter water rod 40 for nine fuel cells is used at the center. The number of fuel rods 30 is 16 and the number of fuel rods 30 is 96. FIG. 14 is a plan view of a twelfth embodiment of the present invention. This embodiment is a modification of the embodiment of FIG. 13 in which the fuel cells are constituted by 11 rows and 11 columns. In the center, a square water rod 41 for 9 fuel cells
And 36 fuel rods inserted into the intervening member and 76 fuel rods 30 are used. The fuel area is 9 at the interposed member insertion height.
It is divided into two sub-regions, which can effectively increase the furnace stop margin and increase the effective multiplication factor keff during high-temperature operation. FIG. 15 is a plan view of a thirteenth embodiment of the present invention. In this embodiment, a square water rod 42 for five fuel cells is arranged at 45 degrees with respect to the fuel bundle at the center, and further arranged in a cross shape by using twelve fuel rods p inserted with intervening members. 4
There are three sub-regions (sub-bundles), and the fuel rods 30
Consists of 64 pieces. Therefore, the distance between the sub-regions can be increased at the height of the interposed member insertion fuel rod p of the central cross-shaped interposed member, so that the effect of increasing the high-temperature effective multiplication factor hot keff and the low-temperature effective multiplication factor cold keff are increased. It has the effect of reducing the width (furnace stop margin is large). Further, even if the length of the interposed member of the interposed member inserted fuel rod p is made relatively short, a great effect is obtained. FIG. 16 is a plan view of a fourteenth embodiment of the present invention. In this embodiment, a square water rod 32 for five fuel cells is arranged at 45 degrees in the center with respect to the fuel bundle, and the whole is composed of nine sub-regions. A gap is provided. Twelve interposed member inserted fuel rods p are used at the center of the fuel bundle and arranged in a cross shape, and one fuel rod p is arranged at each corner of the fuel bundle. The fuel rod 30 is composed of 60 rods. In this embodiment, the effective multiplication factor hot
It has the effect of increasing the keff and the effect of lowering the cold multiplication factor cold keff (large furnace stop margin), but is slightly smaller than in the previous embodiment. FIG. 17 is a plan view of a fifteenth embodiment of the present invention. This embodiment is a modification of the embodiment of FIG. 16, in which a square water rod 43 larger than that of the above embodiment is arranged at the center, and one fuel rod is additionally arranged at a position facing this square water rod 43. It was done. Therefore, the fuel rod p inserted into the interposed member is 16
The number of the fuel rods 30 is 60. Central square water stick
Since 43 is made larger, the furnace stop margin is larger than in the above embodiment. FIG. 18 is a plan view of a sixteenth embodiment of the present invention. This embodiment is a modification of the embodiment of FIG. 15, in which a large diameter water rod 44 and a cruciform wide water gap are offset with respect to the fuel bundle, and a core (BWR) having a different water gap width on the outer periphery of the assembly. The present embodiment can be effectively applied to (called -D grating). In such a core, the cruciform control rod preferably has a configuration in which a central tie rod is disposed on the upper left side of the figure. That is, many fuel rods are arranged on the side where the water gap is wide. Therefore, the number of fuel rods p inserted into the intervening member is 14 and the number of fuel rods 30 is 63. FIG. 19 is a plan view of a seventeenth embodiment of the present invention. In this embodiment, four sub-bundles 45 are provided, a cross-shaped gap 46 between the respective sub-bundles is used as a non-boiling cooling water region, and the fuel rod p inserted into the corner portion located at the center of the assembly in each sub-bundle 45. Are arranged in a nodular manner, and the number of fuel rods p inserted by the intervening members is 12 and the number of fuel rods 30 is 52. This embodiment has the effect of increasing the high-temperature effective multiplication factor and the effect of reducing the low-temperature effective multiplication factor (large furnace stop margin). FIG. 20 is a plan view of an eighteenth embodiment of the present invention. This embodiment is composed of nine sub-bundles 47 as a whole. Each sub-bundle 47 is composed of nine fuel rods 30, and a slightly wide gap 48 is provided between each sub-bundle 47. The sub-bundles at the center of the fuel assembly are all interposed member inserted fuel rods p. Therefore, the number of interposed member inserted fuel rods p is nine and the number of fuel rods 30 is 72. This embodiment has the effect of increasing the high-temperature effective multiplication factor and the effect of reducing the low-temperature effective multiplication factor (large furnace stop margin). 21 (a) to (d) are longitudinal sectional views of different fuel rods according to the present invention. That is, the fuel rod shown in FIG. 3A has a region in the cladding tube 20 that does not contain a fuel substance, which is approximately 15 to 90 cm, and in which graphite 21 is inserted. Graphite
21 is one of the most suitable examples having excellent high-temperature characteristics, low absorption of thermal neutrons, and also having a function as a moderator. Low-density (porous) Al ₂ O ₃ , ZrO _2, etc., although not excellent in deceleration characteristics, have good heat resistance characteristics, and it is possible to use such a substance having a small neutron absorption. Instead of solid graphite, hollow graphite, hollow Al ₂ O ₃ , Zr
O ₂ , hollow natural uranium, hollow depleted uranium, or the like may be used, and the hollow portion may be used as a gas plenum. It is natural that solid depleted uranium, natural uranium, or the like can be used in place of the hollow if the use as a gas plenum is not considered. The most important point in the characteristics required in this region is that the thermal neutron absorption at the end of the cycle is smaller than the fuel region sandwiching this region. In the fuel substance adjacent to the graphite 21, an output peak (spike) occurs in a range of about 2 cm (at most 5 cm), which is disadvantageous in terms of fuel integrity,
Only two pellets (approximately 2 cm) each containing a burnable poison are arranged near the axis. These pellets 2
In No. 2, the output does not fluctuate relatively throughout the entire operation cycle because no poison is contained in the outer periphery. The design is such that the absorption characteristics of the poison disappear as the end of the cycle is approached, and the output of this portion gradually increases. Region with low fissile nuclide concentration (hereinafter referred to as intervening region)
The neutron interaction (coupling effect) in the fuel region in the horizontal direction between the two is reduced, and as a result, the subcriticality of the stopped reactor can be increased. The difference between the fuel rod shown in FIG. 21 (b) and the fuel rod shown in FIG. 21 (a) is that a tube 24 made of zircaloy having a small thermal neutron absorption cross section is inserted instead of the graphite 21. In this example, many variations are possible. (1) When used as a gas plenum, use an unsealed pipe. (2) When ZrH ₂ (called zirconium hydride, zirconium hydride, etc.) is densely packed, ZrH ₂ should be accurately written as ZrH _x (0 <x2). However, it is generally desirable to seal the tube because x becomes brittle when x becomes large. The relatively small gap in the tube, H ₂ is slightly released from ZrH ₂
Provided for use as a gas plenum. (3) Since Be and BeO are toxic, it is preferable to put them in a tube. Since Be also generates He gas by reaction with neutrons, a small He gas plenum (gap) is provided. Between the zircaloy tube 24 and the fuel pellets 23, small heat insulating material pellets 25, Al ₂ O ₃ , ZrO ₂ , depleted uranium, etc. are interposed to improve the fuel integrity. Insulation pellets 25
It is desirable that the thermal neutron absorption characteristics be small at the end of the operation cycle. Therefore, Al ₂ O ₃ −G with burnable poison
d ₂ O _3, like impaired uranium UO ₂ -Gd ₂ O ₃ pellets are preferred. In the fuel pellets adjacent to the zircaloy tube 24 in the axial direction, it is preferable to arrange the pellets 22 containing flammable toxicity up to about 2 cm (about 5 cm at longest) from the end face. FIG. 21 (b) shows the fuel pellet 22 in which a small-diameter Gd pellet is inserted. However, Gd may be mixed into the entire pellet, and the fuel rod shown in FIG. 21 (a) and FIG. Similarly, Gd may be mixed into the entire pellet. The difference between the fuel rod shown in FIG. 21 (c) and the fuel rod shown in FIG. 21 (b) is that water is introduced. That is, water passage holes 26 are provided up and down in the cladding tube 20 of the portion where the Zircaloy tube of the fuel rod is located as shown in FIG.
An intermediate plug 27 and a heat insulating material pellet 25 are arranged above and below 26, respectively, and a pellet 22 containing a burnable poison is provided above and below, and then a fuel pellet 23 is arranged above and below. FIG. 21 (d) in the fuel rod and drawing differences between the fuel rods (a) graphite showing _{(Al 2 O 3, ZrO 2} , Al 2 O 3 -ZrO or the like ₂₎ in addition to the burnable poison This is the point that the interposed layer 28 is provided. According to this embodiment, no burnable poison is contained in the fuel, so that there is a manufacturing advantage. [Effects of the Invention] As described above, according to the present invention, the following effects can be obtained. (1) Since the water temperature is low and the water density is high when the reactor is shut down, the diffusion distance of thermal neutrons is small. However, according to the fuel assembly of the present invention, the region where the fissile material concentration is low (intervening region)
The neutron interaction (coupling effect) in the fuel region in the horizontal direction is reduced, so that the subcriticality of the stopped reactor can be increased. (2) During high-temperature operation, the average density of water is significantly reduced, so that the thermal neutron diffusion distance is significantly (2 to 3 times) extended. As a result, the coupling effect across the intervening region is improved, and the effective multiplication factor can be increased even if a little, despite the fact that there is a region where the fissile material concentration is significantly reduced. There is no disadvantage by introducing an intervening region. (3) In the present invention, no local output peaks (output spikes) occur because the flammable toxicity is effectively located in the intervening region or in a limited portion of the fuel axially adjacent to it. Fuel integrity is maintained.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 (a) and (b) are schematic longitudinal sectional views and plan views of one embodiment of the present invention, and FIGS. 2 (a) and (b) are fuel assemblies of the present invention. FIG. 3 is a schematic cross-sectional view in which is applied to a boiling water reactor and a diagram showing a void ratio and a subcriticality distribution in a core axis direction. FIG. 3 is a diagram for explaining the operation of the present invention, and FIGS. All figures are plan views of different embodiments of the present invention,
FIGS. 21 (a) to (d) are longitudinal sectional views of different fuel rods according to the present invention, and FIGS. 22 (a) and (b) are perspective views of a conventional fuel assembly and a fuel assembly, respectively. 23 is a cross-sectional view of the fuel assembly shown in FIG. 22, and FIGS. 24 to 26 are cross-sectional views of a conventional fuel assembly. 30 fuel rod 31 fuel rod p 32,37,38,39,40,41,42,43,44 interposed member water rod 33 channel box 34 upper tie plate 35 lower Tie plate 36 ... intervening member 45,47 ... sub-bundle 46 ... cross-shaped gap 48 ... gap

Claims (1)

(57) [Claims] In a fuel assembly composed of a large number of fuel rods filled with nuclear fuel pellets inside a metal cladding tube arranged regularly, some of the fuel rods have intervening members of a certain length that have significantly reduced the fissile nuclide concentration. Nuclear fuel pellets are inserted so as to be present at the upper and lower portions in the metal cladding tube, and the length of the interposed member is set to be equal to or longer than the thermal neutron diffusion distance during the reactor power operation, and The axial insertion position is a position including a portion where the subcriticality becomes shallow at the time when the reactor shutdown margin becomes severe within the reactor operation period, and the interposed member inserted fuel rod is linearly or intersectingly linear or nodular. A fuel assembly, wherein the fuel assembly is disposed inside the fuel assembly. 2. 2. The fuel assembly according to claim 1, wherein the insertion length of the interposed member is equal to or less than 1/3 of the active fuel length. 3. 2. The fuel assembly according to claim 1, wherein the interposed member-inserted fuel rods that are close to each other in a linear or intersecting linear arrangement are arranged adjacent to each other or every other fuel rod. 4. 2. The fuel assembly according to claim 1, wherein in the fuel assembly including the water rod, the nodular arrangement is such that an interposed member-inserted fuel rod is disposed adjacent to or surrounding the water rod. Aggregation. 5. 2. The fuel assembly according to claim 1, wherein in the fuel assembly including no water rod, the nodular arrangement is such that a plurality of interposed member inserted fuel rods are arranged adjacent to each other. 6. 2. The fuel assembly according to claim 1, wherein the insertion position of the interposition member is a position where the center position of the interposition member is 3/4 from the lower end of the active fuel length. 7. 2. The fuel assembly according to claim 1, wherein the intervening member is a uranium pellet having a lower fissile nuclide concentration than an adjacent fuel pellet containing a burnable poison in the entire pellet or an axial center of the pellet. 8. 2. The fuel assembly according to claim 1, wherein the interposed member of the interposed member inserted fuel rod is configured such that neutron absorption characteristics become as small as possible at the end of a reactor operation cycle.