JP2635694B2 - Fuel assembly - Google Patents

Description

The present invention relates to a fuel assembly, and more particularly to a light water reactor such as a boiling water reactor (BWR) having a long operation cycle and a high margin of shutdown. The present invention relates to a fuel assembly suitable for (LWR).

(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 moderation 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 fuel in the upper part of the core is easily 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 to improve the burn-out of fuel with the main 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. 19 (A) and (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. 1A, 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 8.
An upper end plug 10 is provided at an upper end and a lower end plug 11 is provided at a lower end.

FIG. 20 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 regularly arranged in a grid 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. 21 has been developed to improve the characteristics of the fuel assembly. One large-diameter water rod 12 is arranged 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. 22 is also an improvement of the fuel assembly shown in FIG. 20, 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 moderator water area to flatten the horizontal power distribution, but this type of fuel assembly also has excess water below the core and insufficient water above it. There is a problem.

The fuel assembly shown in FIG. 23 has been developed as an improved version of the fuel rod shown in FIG. This fuel assembly consists of nine subassemblies 15 and a nuclear subassembly.
Reference numeral 15 denotes nine fuel rods 2 each. A slightly wider space 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 or increase the extraction burnup and improve economics, which further reduces the subcriticality in the upper part of the core Eventually, it may be impossible to safely shut down the reactor at low temperatures. In other words, this point becomes a problem, and there has been a problem that the operation cycle cannot be lengthened or the take-out burnup cannot be increased 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 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 multiplicity of fuel rods filled with nuclear fuel material inside a metal cladding tube and a cross-sectional area larger than the fuel rods. In a fuel assembly constituted by regularly arranging a large diameter water rod having a cross-sectional area, some of the fuel rods have an intervening member of a fixed length in which the fissile nuclide concentration is significantly reduced is inserted in the metal cladding tube. And the interposed member fuel rod is disposed so as to surround the large-diameter water rod, and the length of the interposed member is longer than the thermal neutron diffusion distance when the reactor is in a cold state. Characterized by being less than 1/3 of the length of the axial heating portion of the assembly and the interposed member being located at or near a position where the neutron importance distribution peaks when the reactor is shut down. It is.

(Operation) As described above, according to the fuel assembly of the present invention, the portion having a particularly low fissile material concentration, that is, the intervening region is arranged so as to surround the large-diameter water rod. The diameter water rod and its surroundings become excessive water, that is, a fuel shortage, the neutron doubling rate of the fuel decreases, the subcriticality increases at the time of cold shutdown, and the reactor shutdown margin increases. On the other hand, during operation at high temperature, particularly when voids are generated on the outer periphery of the large diameter water rod, excess water is eliminated, and the neutron multiplication factor of the fuel is recovered. This phenomenon can be mainly explained by the action of thermal neutrons with a short diffusion distance. That is,
In the cold state, the water density (about 1.0) is large, so the diffusion distance of thermal neutrons is short, and the interaction of neutrons on both sides of the intervening region is reduced, and as a result, the neutron image magnification characteristic is reduced.
During high temperature operation, the boiling water reactor has a water temperature (reference value) of about 286 ° C and a density of about
0.74 g / cm ³ Therefore, the thermal neutron migration distance in water increases to 1 / 0.74 (= 1.35) times that of cold. further,
The density of the air-water mixture at the time of void generation is reduced to about 0.3, and as a result, the thermal neutron diffusion distance in the air-water mixture is 1 /
It increases by 0.3 (≒ 3) times. For thick water rod in is a design usually voids does not occur, the density of water is about 0.74 g / cm ^3, reduced to about 0.3 g / cm ³ in the outer periphery, the mean value of both 0.5g / cm ³ . As a result, the neutron interaction on the outer periphery surrounding the intervening region around the large diameter water rod increases, and the neutron multiplication characteristics increase.

Utilizing the above-mentioned effects, it is possible to design a design that reduces the multiplication factor in a cold state, that is, increases the reactor shutdown margin (subcriticality), and does not decrease the multiplication factor in a high-temperature operation even when the fuel is reduced. Become.

 Next, the operation of the present invention will be described with reference to FIG.

The present inventor has studied through experiments and analysis how the core characteristics change when a water region is introduced into the fuel region. A part of the study is included in the proceedings of the 3rd International Conference on Peaceful Use of Nuclear Energy held in 1964 (Proc.3
rd. Geneva Conf., P846, No. 3 Page 79 (1964). Fig.1
5).

FIGS. 3 (A) and 3 (B) are obtained by rewriting and partially adding the drawings described in the above-mentioned paper in order to clearly show the basic principle of the present invention.

Now, as shown in FIG. 2 (A), a case is considered where an annular core is formed in water and the size (radius r) of the central water region is changed. The outer periphery of the core region is a water reflector. Criticality experiments were performed with a fuel enrichment of 2.6% and a water to fuel volume ratio of about 1.8 in the core region. FIG. 2 (B) shows the critical mass at a constant core height as a function of the radius r as a relative value. The vertical axis on the right side of the figure assumes that the core radius is constant (20 cm).
This is a scale when the reactivity of the core is changed according to the radius of the central water region, that is, converted into a water region reactivity effect. By using the scale of the figure, it was possible to show the critical mass and the reactivity effect on the same curve (in general, it is not possible to show the same curve). When the radius r is between 0 and 2 cm, the critical mass decreases rather because the core is slightly decelerated (unde
r moderate). Such a phenomenon is a real machine
This is the same example that often occurs in LWR cores. That is,
In the same way, extracting some fuel from the upper half of the BWR core with a high void fraction and introducing a moderator such as a water rod improves the reactivity of the core. The critical mass increases when the radius of the central water region is larger than 2 cm. That is, the reactivity becomes negative. If the core outer diameter is fixed at r = 0 and only r is made larger than 2 cm, the core becomes subcritical, and the subcriticality increases in a quadratic curve with r.

Now, in FIG. 2 (B), the radius of the central water area is 2.5c.
Assuming a change from m to 3.8 cm, from the curve b, the reactivity of the core changes 0.8% △ K / K toward the side where the subcriticality becomes deeper (b ₁ ).
Occurs. Here, the density of water in the central water region is 1.0 g / cm ³ . Suppose now that the water density in the central water area is 0.7 g / cm ³ . In this case, considering the reaction probability of neutrons with water, the radius of the central water region is approximately 0.7 × 2.5 cm (1.8 cm) to 0.7 × 3.
Equivalent to a change of 8 cm (2.7 cm). At this time, the reactivity change of the core is not 0.8% △ K / K, but 0.3% △ K / K (b ₂ ).
Such a characteristic is a rudimentary principle explanation of the characteristic that the reactivity loss due to a water rod at a high temperature of the LWR is small, but is large in a cold state (large stoppage margin). In the actual LWR, since the water density changes even in the core region, the curve in FIG. 2 (B) shows the curve (b) widened to the right.

Figures 3 (A) and (B) show the current core length of 3m or more.
FIG. 3 illustrates the concept of BWR axial characteristics and characteristics when the fuel of the present invention is loaded in a reactor core. In other words, as shown in Fig. 2 (B), in the BWR, the void ratio is higher in the upper part of the reactor core, so that the rate of generation and accumulation of plutonium is high, and the rate of consumption of uranium 235 and plutonium 239 due to fission is low. When the furnace is cold-stopped, assuming that the total effective length of the core effective heat generating portion is L, the subcriticality becomes the shallowest around 1 / 6L to 1 / 4L from the upper end. This part (A)
When the present invention is carried out as described above, the subcriticality distribution (importance distribution) in a shallow part is improved (deepened) as shown in FIG.

FIG. 4 is an example in which the characteristics of FIG. 3 are actually calculated. In this case, the fuel assembly has an axially uniform configuration having a cross section shown in FIG. 6 described later. This assembly is a Japanese patent application
Proposed in part of 82124. That is, the present invention is applied to a system improved in many points such as a high reactivity, a flat power distribution, a large stop margin, a low coolant pressure loss, and an improvement in thermal characteristics (indicated by a circle) to further improve the stop margin. This is a calculation system, and the average fuel enrichment of the aggregate is 4.4 wt%.
Burnup was assumed to be 30 GWd / st (33 GWd / t) on average, and a typical axial burnup distribution was assumed, and the axial power distribution was determined taking into account the difference in fuel composition due to the axial void distribution. 4th
FIG. 7A shows an axial output distribution during a cold stop. The “output distribution during stoppage” is not a very real phenomenon, and it may be better to use a calculated output distribution. But,
It is well known that the shape of this curve itself is not proportional to the local subcriticality, but corresponds very well. In other words, the power distribution form well represents a local subcritical area, and a position having a high reactivity substantially corresponds to a power peak position.

For example, in the assembly as shown in FIG. 6, the twelve fuel rods marked with ● have the maximum relative output in the conventional assembly near 1/4 from the upper end of the core (see FIG. 4 (A)). (Dashed line)
When inserted over a length of 15 cm, the peak portion of the relative output significantly decreases (solid line in FIG. 4 (A)). This indicates that the subcriticality of the locally shallow subcriticality is effectively improved.

FIG. 4 (B) is an axial output distribution during rated output operation. Changes due to the application of the present invention are minute. Since the power distribution is improved in the intervening region, the axial distribution is slightly flattened.

FIG. 5 shows the core reactivity during rated output operation and cold shutdown when the position of the intervening area (fuel elimination part) was fixed and the length of the intervening area was changed in the system obtained in FIG. The change is shown. The intervening region was set to a vacuum in this calculation example.
When the length of the fuel exclusion section (intervening area) changes, the core reactivity during the rated power operation hardly changes, and the curve of the core reactivity change during the cold shutdown is convex downward. It can be seen that the rapid saturation phenomenon does not occur and the subcriticality can be deepened with the length. The relative output distribution in FIG. 4 is obtained when the length of the fuel elimination section in FIG. 5 is set to 15 cm. Note that a similar core reactivity change occurs when depleted uranium is introduced without evacuating the intervening region, but the spread of both curves is slightly reduced. In addition, the effect appears even if natural uranium is added.
Although the amount of fissile nuclides in the aggregate decreases slightly due to the arrangement of the intervening region,
If the enrichment is increased by about%, the recovery can be achieved, and the risk of impairing fuel economy can be eliminated.

In addition, in the case of a core with a short effective core length, for example, about 2 m, the peak position of power analysis (inboardance distribution) in the cold state shifts below the current core, so the intervening amount should be set near the peak position accordingly. Is natural.

In addition, when the axial length of the intervening layer is increased, the reactivity at low temperature decreases. The degree of this decrease gradually becomes saturated,
If it exceeds one-third of the total fuel length, there is almost no further decrease in reactivity. For this reason, it is appropriate that the axial length of the intervening layer be 1/3 or less of the entire fuel length.

(Example) An example of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view of one embodiment of the present invention, and FIG.
Is a schematic longitudinal sectional view taken along the line AA in FIGS.
(B), (C), (D), (E), and (F) of the same figure show the BB line, CC line, DD line, EE line, and F-line in FIG. It is a schematic transverse sectional view which follows the F line.

As shown in FIGS. 1 (A) to 1 (F), a fuel assembly 20 according to the present embodiment includes a water rod (hereinafter, referred to as an ultra-large diameter rod) 25 having an extremely large diameter at the upper part and a small diameter at the lower part. The fuel rods are arranged in the center of the body 20 and the fuel rods are arranged in three rows and three columns around the super-large diameter water rod 25.
Sub-bundles 29 are arranged via a gap 31 (dense / dense grid configuration), the outside of which is surrounded by a channel box 30, and the upper and lower ends thereof are respectively formed by an upper tie plate and a lower tie plate (not shown). Fixed. In addition, the intake 27
However, a drain port 28 is provided at the top.

Normal fuel rods are used as fuel rods constituting the sub-bundle 29
21, an interposed member-containing fuel rod 22 (represented by p in the figure), a long part long fuel rod 23 (represented by LR in the figure), and a short part long fuel rod 24 (represented by SR in the figure) are used.

In the present embodiment, as described above, the super-large diameter water rod 25 is arranged at the center of the fuel assembly 20, and the fuel rod of the sub-bundle 29 facing the super-large diameter water rod 25 is used as the interposed member containing fuel rod 22. Yes,
In addition, a long part-length fuel rod 23 is arranged at the center of the sub-bundle 29 located at each corner. Eight short part-length fuel rods 24 are arranged around the small diameter part below the super-large diameter water rod 25. Therefore, in this embodiment, the normal fuel rods 21 are 56, the intervening member-containing fuel rods 22 are 12, the long part long fuel rods 23 are 4,
The short and long fuel rods 24 are composed of eight rods. By the way, the long part length fuel rod 23 and the short part length fuel rod 24 are provided with a plenum on the lower side in principle to prevent deterioration of fuel economy. This is because the lower plenum has a low neutron flux and the fuel does not advance much. Further, a plenum may be provided on the upper side of the long part-length fuel rod 23 if necessary.

In the fuel rod 22 having the interposed member, the height of the center of the interposed member 26 is 3 / 4H from the lower end of the effective fuel length. This is this 3 / 4H
This is because the subcriticality becomes shallow especially within about 1 / 4H in the vicinity. Although details of the interposition member 26 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.

Normally, during operation, the pressure loss increases in the high void portion in the upper part of the fuel, but in this embodiment, a burnishing region is formed in the upper part of the fuel by adopting the long partial length fuel rod 23. Since the coolant pressure loss is reduced by providing the burnishing region and enlarging the flow path in this manner, the axial water-to-fuel ratio is optimized, and high-power operation becomes possible. Also, the fuel top 15
In the case of ~ 30cm, natural uranium blanket is often used, so it is not necessary to use a super-large diameter water rod, and a small diameter water rod is used as shown in the figure. In the case where depleted uranium is used as the intervening member, it is preferable in terms of fuel economy to use depleted uranium also for the blanket.

Further, during the shutdown of the furnace, especially at the portion where the subcriticality becomes the least, that is, in the vicinity of DD in FIG.
Since the interposed member 26 is disposed, the central non-fuel region is effectively enlarged.

FIG. 6 is a cross-sectional view of a second embodiment of the present invention, and corresponds to FIG. 1 (D). 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.

As shown in the figure, the fuel assembly 32 of the present embodiment has a configuration in which the long and partial fuel rods 23 are arranged at the center of each sub-bundle 29 of the first embodiment. First
The number of burnishing portions is larger than that of the embodiment. There is no change in the arrangement of the interposed member fuel rods 22. Therefore, in this embodiment, the normal fuel rods 21 are 52, the interposed member-containing fuel rods 12 are 12,
The long part long fuel rod 23 is composed of eight pieces, and the short part length fuel rod 24 is composed of eight pieces.

FIG. 7 is a cross sectional view of a third embodiment of the present invention. As shown in the figure, the fuel assembly 33 of the present embodiment has a square tubular water rod 34 corresponding to four fuel rods arranged at a 45-degree inclination in the center of a channel box 37. The water rod 34 is further provided with a cross-shaped channel 36, and a sub-bundle 35 of 5 rows and 5 columns (one innermost fuel bundle of the sub-bundle) is provided in a space separated by the rectangular cylindrical water rod 34 and the cross-shaped channel 36. The rods are removed for the rectangular cylindrical water rod 34), and the fuel rod facing the rectangular cylindrical water rod 34 is defined as the interposed member-containing fuel rod 22. Square cylindrical water rod 34 and cross-shaped channel 36
Non-boiling moderator water flows in the inside, and boiling cooling water flows in the sub-bundle 35. Each corner of the rectangular cylindrical water rod 34 is provided with a water passage hole. Therefore, since the flow path is expanded, it is possible to reduce the furnace stop margin and pressure loss, improve the void coefficient, and improve the axial power distribution. In this embodiment, the normal fuel rod 21 is
84 rods, 8 fuel rods 22 with intervening members, long partial length fuel rods 23
Is composed of four lines.

It is more effective if the lower part of the rectangular cylindrical water rod 34 is made thinner than the upper part, and the short and long fuel rods 24 are arranged around the small diameter part. The same applies to each of the following embodiments, and a description thereof will not be given.

FIG. 8 is a cross-sectional view of a fourth embodiment of the present invention, and corresponds to FIG. 1 (D).

The fuel assembly 38 of the present embodiment is different from the first embodiment (FIG. 1) in that two large-diameter water rods 29 are used instead of one super-large-diameter water rod disposed at the center of the fuel assembly. The fuel rods 22 are disposed diagonally adjacent to each other, and the fuel rods 22 are disposed so as to surround the two large-diameter water rods 39, and have the same operation as the first embodiment. In this embodiment, the normal fuel rod 21 is composed of 56 rods, the interposed member-containing fuel rod 22 is composed of 14 rods, and the long part-length fuel rod 23 is composed of 4 rods.

FIG. 9 is a cross-sectional view of a fifth embodiment of the present invention, and corresponds to FIG. 1 (D).

In the fuel assembly 40 of the present embodiment, an ultra-large diameter water rod 41 corresponding to nine fuel rods is arranged at the center of a channel box 42,
The fuel rods are arranged in 9 rows and 9 columns in other spaces. The fuel rod surrounding the super-large diameter water rod 41 is inserted into the fuel rod
22. Thus, the shortage of water during high-temperature operation in the upper part of the core is alleviated by the intervening members of the interposed member fuel rods 22. In this embodiment, there are usually 56 fuel rods 21 and 16 fuel rods 22 with interposed members.

FIG. 10 is a cross sectional view of a sixth embodiment of the present invention, corresponding to FIG. 1 (D).

In the fuel assembly 43 of the present embodiment, a square tubular water rod 44 corresponding to nine fuel rods is arranged at the center of the channel box 42,
The fuel rods are arranged in 9 rows and 9 columns in other spaces. The fuel rod surrounding the rectangular cylindrical water rod 44 and the fuel rod in the middle of the second row are used as the interposed member-containing fuel rods 22. Thus, the shortage of water during high-temperature operation in the upper part of the core is alleviated by the intervening members of the interposed member fuel rods 22. In this embodiment, the normal fuel rod
The number 21 is 56, and the number of the fuel rods 22 containing the intervening members is 16.

FIG. 11 is a cross-sectional view of the seventh embodiment of the present invention, and corresponds to FIG. 1 (D).

The assembly 45 of the present embodiment has a 4-1-4 dense and dense lattice structure, and a super-large diameter water rod 25 for nine fuel cells is arranged at the center, and the fuel rod 22 with the interposed member is super-large diameter water rod. Twelve fuel rods are arranged around the periphery of 25, and the fuel rods 21 are usually composed of 60 rods. Therefore, since the water region can be made large at the interposed member insertion height of the interposed member inserted fuel rod 22, the effect of increasing the effective multiplication factor hot keff at high temperature and the effect of greatly reducing the effective multiplication factor cold keff at low temperature (furnace shutdown) Room).

FIG. 12 is a cross-sectional view of the eighth embodiment of the present invention, and corresponds to FIG. 1 (D).

The assembly 25 of this embodiment has a 4-2-3 dense / dense grid structure, and a super-large diameter water rod 25 for nine fuel cells is disposed at the center, and the fuel rod 22 with the interposed member is connected to the super-large diameter water rod. Twelve fuel rods are arranged around the periphery of 25, and the fuel rods 21 are usually composed of 60 rods. This fuel assembly is effective when the water gap around the assembly is not uniform in the circumferential direction. It has the same function as the first embodiment with a large furnace stop margin.

FIG. 13 is a cross sectional view of a ninth embodiment of the present invention, corresponding to FIG. 1 (D).

The assembly 47 of this embodiment has a 4-2-4 dense / dense lattice structure, in which a large-diameter water rod 48 for four fuel cells is arranged at the center, and the fuel rod 22 with the interposed member is connected to the large-diameter water rod 48. Twenty fuel rods are arranged around the periphery, and the fuel rods 21 are usually composed of 76 rods.

FIG. 14 is a cross sectional view of a tenth embodiment of the present invention, corresponding to FIG. 1 (D).

The assembly 50 of this embodiment has a 4-2-4 dense / dense lattice structure, in which a super-large water rod 51 for 12 fuel cells is disposed at the center, and the super-large water rod 22 with the interposed member-containing fuel rod 22 is provided. Twelve fuel rods 21 are arranged around 51, and the fuel rods 21 are usually composed of 76 rods.

 FIG. 15 is a plan view of an eleventh embodiment of the present invention.

This embodiment is an example applied to a core for a pressurized water reactor (hereinafter, referred to as PWR). The figure is a schematic diagram of PWR 17 × 17,
The assembly 53 of the present embodiment includes a normal fuel rod 54 and a fuel rod containing an intervening member.
It comprises a control rod guide tube 56 having a larger diameter than the fuel rod 55 and an in-core instrumentation tube 57 arranged at the center of the assembly.
An interposed member fuel rod 56 is arranged around the control rod guide tube 55. The control rod guide tube 55 is a water rod during operation. Therefore, a large water area can be obtained at the height of the interposed member of the interposed member fuel rod 56, so that the furnace stop margin can be effectively increased. In addition, the fuel rod 56 with an interposed member may be arranged around the in-furnace instrumentation guide tube 57. However, in order to prevent the instrumentation error from being enlarged, the fuel rod 54 is usually used in this embodiment.

 FIG. 16 is a plan view of a twelfth embodiment of the present invention.

This embodiment is an example applied to a PWR 14 × 14 core. The assembly 58 of this embodiment has a control rod guide tube 5 having a diameter larger than that of the normal fuel rod 54, the fuel rod 56 containing the interposed member, and the control rod guide tube of the PWR 17 × 17.
It consists of nine. An interposed member fuel rod 56 is arranged around the control rod guide tube 59. This control rod guide tube
Since 59 becomes a water rod during operation, it has the same effect as the above embodiment.

 FIG. 17 is a plan view of a thirteenth embodiment of the present invention.

The assembly 60 of the present embodiment is composed of ordinary fuel rods 61, fuel rods 62 with intervening members, and spectral shift rods (SSR) 63. 64 is a channel box and 65 is a cross control rod.

As a future BWR core, research is being conducted on a system (K lattice) in which the bundle is enlarged and the control rods are arranged as shown in the figure. In this embodiment, nine water rods of the K lattice are arranged for four fuel cells. If the water rod is used as a void tube in the first half of the operation cycle and as a water rod in the second half, plutonium generation is improved and fuel economy is improved. Since such a water rod is intended for spectrum shift operation, it is called a spectrum shift rod (SSR). The structure is such that the intermediate SSR completely surrounds the fuel containing the intervening member and the outer SSR also surrounds more than half, so that a very large reactor shutdown margin can be obtained.

18 (A) to 18 (D) are longitudinal sectional views of different fuel rods according to the present invention.

That is, the fuel rod shown in FIG. 5A has a region in the cladding tube 66 that does not contain a fuel substance, this region is approximately 15 to 90 cm, and an intervening member 67, for example, graphite is inserted. Graphite 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, and the} like do not have excellent deceleration characteristics, but have good heat resistance characteristics, and such a substance having low neutron absorption can be used as an intervening member. Instead of solid graphite, hollow graphite, hollow Al ₂ O ₃ , ZrO ₂ , hollow or solid natural uranium, depleted uranium, etc. are used. If a hollow part is provided, the hollow part can be used as a gas plenum. Good.

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 67, an output peak (spike) occurs in a range of about 2 cm (at most 5 cm), which is disadvantageous for fuel soundness.
Two fuel pellets 68 (approximately 2 cm) each containing a burnable poison are arranged only near the axis. Since the outer periphery of these fuel pellets 68 does not contain toxic substances, the output has relatively little fluctuation over the entire operation cycle. As the end of the cycle is approached, the design is such that the absorption characteristics of the toxic substance disappear, and the output and the gradual rise of this part.

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. 18 (B) and the fuel rod shown in FIG. 18 (A) is that a zircaloy tube 70 having a small thermal neutron absorption cross section is inserted instead of the graphite 67. In this example, many variations are possible. (1) When used as a gas plenum, use an unsealed pipe.

(2) When ZrH ₂ (referred to as zirconium hydrite, zirconium hydroxide, 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.

A small heat insulating material pellet 71, Al ₂ O ₃ , ZrO ₂ , depleted uranium, etc. are interposed between the zircaloy tube 70 and the fuel pellet 69 to improve the fuel integrity. It is desirable that the thermal insulator pellets 71 have small thermal neutron absorption characteristics at the end of the operation cycle. Therefore, Al ₂ O ₃ −Gd ₂ added with burnable poisons
O ₃ , depleted uranium UO ₂ —Gd ₂ O ₃ pellets and the like are preferred. In the fuel pellets adjacent to the Zircaloy tube 24 in the axial direction, it is preferable to arrange the pellets 68 containing the burnable poison up to about 2 cm (about 5 cm at longest) from the end face. FIG. 18 (B) shows a fuel pellet 68 into which a small-diameter Gd pellet is inserted. However, Gd may be mixed into the entire pellet, and the fuel rod shown in FIG. 18 (A) and FIG. Similarly, Gd may be mixed into the entire pellet.

The intervening members 67 and 70 each have both ends 0.5 to
The part of about 3cm is hafnia (HfO ₂ ) and yttria (Yb ₂ O ₃
The same effect can be obtained by mixing and burning pellets.

The difference between the fuel rod shown in FIG. 18 (C) and the fuel rod shown in FIG. 18 (B) is that water is introduced. That is, water passage holes 72 are provided up and down in the cladding tube 60 in the portion where the Zircaloy tube of the fuel rod is located, as shown in FIG.
That is, an intermediate plug 73 and a heat insulating material pellet 71 are arranged above and below 72, respectively, and a pellet 68 containing a burnable poison is provided above and below, and then a fuel pellet 69 is arranged above and below.

FIG. 18 (D) in the fuel rod and drawing differences between the fuel rods (A) are 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 74 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 fuel assembly of the present invention,
When the reactor is stopped, the low temperature is low and the water density is high, so the diffusion distance of thermal neutrons is small. Since the interposed member area is arranged, there is almost no effect during the power operation, and during the shutdown of the furnace, the subcriticality is effectively improved by the action of the excess water and the intervening member, so that a longer cycle and higher enrichment are achieved. Becomes possible. Further, the fact that the subcriticality can be made deeper during the shutdown of the furnace corresponds to the fact that the amount of the burnable poison can be reduced, and there is an effect that the fuel economy can be greatly improved at the time of high burnup.

[Brief description of the drawings]

FIG. 1 is a schematic view of one embodiment of the present invention, and FIG.
Is a schematic longitudinal sectional view taken along the line AA in FIGS.
(B), (C), (D), (E), and (F) of the same figure show the BB line, CC line, DD line, EE line, and F-line in FIG. FIGS. 2 (A) and 2 (B) are schematic cross-sectional views taken along line F, and FIGS. 3 (A) and 3 (B) are views for explaining the operation of the present invention.
FIG. 4 is a schematic cross-sectional view in which the fuel assembly of the present invention is applied to a boiling water reactor and a diagram showing a void ratio and a subcriticality distribution in a core axial direction, and FIGS. FIG. 5 is a diagram in which the characteristics of the figure are divided into those during cold stop and during rated output operation and compared with the conventional example. FIG. 5 shows the rated output operation when the position of the interposed area is fixed and the length of the interposed area is changed. FIGS. 6 to 17 are cross-sectional views of different embodiments of the present invention, and FIGS. 18 (A) to 18 (D) are diagrams showing a change in core reactivity during cold shutdown. FIGS. 19 (A) and (B) are a perspective view of a conventional fuel assembly and a schematic longitudinal sectional view of a fuel rod constituting the fuel assembly, respectively, and FIGS. Is a cross sectional view of the fuel assembly of FIG. 19, and FIG.
FIG. 23 to FIG. 23 are cross-sectional views of a conventional fuel assembly. 20,33,38,40,43,45,46,47,50 ... Assembly 21,54,61 ... Fuel rod 22,56,62 ... Fuel rod with interposed member 23 ... Long part length fuel Rod 24 …… Short part length fuel rod 25,34,39,41,44,48,51 …… Water rod 26 …… Interposed member 29,35 …… Sub-bundle 30,37,42,49,52 …… Channel Box 31 ... gap

Continuation of the front page (56) References JP-A-61-226685 (JP, A) JP-A-61-278788 (JP, A) JP-A 1-223194 (JP, A) JP-A 1-189591 (JP U.S. Pat. No. 4,968,479 (US, A)

Claims (7)

(57) [Claims] 1. A fuel assembly comprising a plurality of fuel rods filled with nuclear fuel material inside a metal cladding tube and a large-diameter water rod having a cross-sectional area larger than a cross-sectional area of the fuel rods. Some fuel rods are provided with a certain length of interposed member having a significantly reduced fissile nuclide concentration in the metal cladding tube as the interposed member-containing fuel rod, and the fuel rod is surrounded by the large diameter water rod. The length of the interposed member is longer than the thermal neutron diffusion distance when the reactor is in a cold state, and the length of the interposed member is not more than 1/3 of the length of the axial heating portion of the fuel assembly, and A fuel assembly, wherein the disposition position is at or near a position where the neutron importance distribution peaks when the reactor is shut down. 2. The fuel rod comprises a full-length fuel rod and at least one kind of short fuel rod. The large-diameter water rod has a small diameter at a lower part and a large diameter at an upper part, and a small-diameter part of the water rod. A patent wherein a short fuel rod is disposed on the outer periphery, and the interposed member arrangement portion of the interposed member-containing fuel rod is disposed so as to surround the entire outer periphery of the large diameter portion of the water rod or most of the outer periphery. The fuel assembly according to claim 1. 3. Both ends of the interposed member in the axial direction are composed of fuel pellets containing a burnable poison in the entire pellet, in the axial center of the pellet or in the outer periphery of the pellet, and the fissile nuclide concentration is determined by the axial distance from the interposed member. 2. A uranium pellet having a concentration lower than the fissile nuclide concentration of a fuel pellet following the direction further away from the fuel pellet.
Item 3. The fuel assembly according to Item 2 or 2. 4. The intervening member has a region of about 0.5 to 3 cm at both ends in the axial direction, and is made of hafnia (HfO ₂ ).
_2. The fuel assembly according to claim 1, wherein the fuel assembly is a mixed sintered pellet of and yttria (Yb ₂ O ₃ ). 5. A fuel rod adjacent to the fuel rod containing the interposed member, wherein the fuel enrichment at the side surface of the interposed member is not higher than the fuel enrichment at a portion other than the side surface of the interposed member in the adjacent fuel rod. Claims 1, 2, 3, 4
Item 13. The fuel assembly according to Item. 6. The patent wherein the interposed member of the interposed member-filled fuel rod has a neutron absorption characteristic as small as possible at the end of the reactor operation cycle, except at both ends in the axial direction of the fuel rod. The fuel assembly according to claim 1, 2, 3, 4, or 5. 7. The interposition member, except for both ends in the axial direction,
A hollow or solid graphite, a non-sealing zircaloy tubular body, a hollow or solid beryllium, alumina, zirconia, a hollow or solid depleted uranium or natural uranium pellet, or a combination thereof; Are those containing burnable poisons or those containing no burnable poisons, and those containing burnable poisons are characterized in that their concentrations are such that they burn almost completely at the end of the driving cycle The fuel assembly according to claim 6.