JPH01105193A - Fuel assembly - Google Patents

Abstract

PURPOSE:To reduce the pressure drop of a coolant so as to improve power generation efficiency and to increase the allowance of a thermal limit value to fuel by chipping the inside surfaces of a channel box to the thickness smaller nearer the down stream side, thereby widening a flow passage area. CONSTITUTION:A large-sized square cylindrical water rod 3 corresponding to 9 pieces of fuel rods 4 is disposed to the central part in the channel box 1 having a uniform outside shape. The fuel rods 4 are disposed in correct order in 9 lines and 9 rows in the other space part of the channel box 1. The water rod 3 is formed to a thin wall 3b by chipping the wall thickness 3a thereof in such a manner that the sectional area of the flow passage on the down stream side of the cooling water is widened on the outside thereof. The inside surfaces of the channel box 1 are reduced in thickness to 1a, 1b, 1c toward the down stream to widen the inside area. The bundles are not horizontally retained except in the lower part by the mere reduction in the thickness and are, therefore, unstable. Spacer contact parts 2 are locally mounted to the inside surfaces of the channel box 1 and the horizontal positions of the bundles are fixed at these thin-walled parts in order to avoid the above-mentioned instability.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly, and in particular to a boiling water nuclear reactor that can simultaneously achieve improved thermal margin and fuel economy. The present invention relates to a fuel assembly suitable for.

(Prior art) The fuel assembly of a conventional boiling water reactor (BWR) is
As shown in FIGS. 7 to 19, a large number of fuel rods 30 filled with nuclear fuel material are regularly arranged inside a metal cladding tube (not shown) and fixed by a plurality of spacers 31 to form a fuel bundle), Upper tie plate 3 on top
3. It is constructed such that a lower tie plate 34 is attached to the lower part and housed inside a rectangular channel box 32. 35 is a handle.

In the core of a boiling water reactor, cells each consisting of one cross-shaped control rod and four fuel assemblies surrounding it are regularly arranged. That is, BWR
The fuel assemblies and control rods in the core are arranged so that their axes are perpendicular and parallel to each other, and the cooling water, which functions as a moderator, flows from the bottom of the core to the top. has been done. The fuel rods generate heat due to the nuclear fission reaction, and the cooling water flows upward (downstream) while removing the heat from the fuel rods. A portion of the cooling water evaporates and generates bubbles (voids). Bubbles do not occur near the effective lower end of the core, i.e., near the lower end of the heat generating part, but bubbles are generated everywhere in the core inside the tunnel box, except near the lower end, and from the axial center of the core to the upper end. The percentage of bubbles in the cooling water passages, that is, the void ratio, increases significantly towards the end of the core, reaching 70% near the top of the core.
exceed. As the void ratio increases, the cross-sectional area of the cooling water passage in the direction perpendicular to the axis remains constant in the height direction within the channel box, so the flow rate of the cooling water inevitably increases. Since the frictional resistance of the flow path increases approximately in proportion to the square of the flow velocity, the pressure loss of the cooling water increases from the center to the top of the core where the flow velocity is high (the void ratio is high).

The pressure loss ΔPT/ΔZ per unit length is expressed as the sum of the following four elements.

Here, △Ph is position loss and △Pa is acceleration loss.

ΔPf is friction loss, and ΔPL is local loss due to spacers and the like. The largest factor in the section without spacers is friction loss, which is expressed by the following equation.

Here, M is the coolant (water) quality flow L, A is the coolant flow path area, P is the coolant density, 7 is the gravitational constant, f is the fuel bundle friction loss coefficient, and △Pf is the fuel bundle friction loss. .

The force required to flow the coolant is mainly provided by the discharge pressure at the outlet of the recirculation pump, so a large pressure loss means that a large amount of power must be applied to the pump, which increases the size of the equipment and reduces power generation efficiency. This causes a decrease in Therefore, if this pressure loss can be reduced, the pump power can be reduced.

(Problems to be Solved by the Invention) The present invention was made in view of the above circumstances, and its purpose is to improve power generation efficiency by reducing the pressure loss of the coolant and to provide a margin for the thermal limit value for the fuel. The object of the present invention is to provide a fuel assembly that improves the soundness of the fuel by improving the neutron efficiency of the reactor core, and further improves the neutron economy of the reactor core to effectively utilize nuclear fuel.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention arranges an upper tie plate at the upper end and a lower tie plate at the lower end, and connects the upper tie plate and the lower tie plate. A plurality of spacers are arranged at predetermined intervals between the upper tie plate, the lower tie plate, and the plurality of spacers, and a large number of fuel rods are regularly arranged to form a fuel bundle, and the outer periphery of the fuel bundle is Place a metal channel box in the
In a fuel assembly configured such that cooling water flows around the fuel rods in the axial direction of the fuel rods and removes heat generated in the fuel rods, the channel box has an outer shape that is uniform in the axial direction of the channel box. The wall thickness of the channel box material is thicker upstream of the flow of cooling water, and except for the downstream end where the upper tie plate comes into contact, the wall thickness of the channel box material becomes gradually thinner toward the downstream, and the internal cross-sectional area of the channel box is It is characterized by being configured to increase in size.

(Function) In the fuel assembly of the present invention, the inside of the channel box is cut to expand the flow path area downstream where the pressure drop of cooling water becomes large, so that the following functions and effects are achieved. play.

■Pressure loss can be effectively reduced. This means that the pump power can be reduced, which immediately means an improvement in the economic efficiency of power generation. Conversely, by increasing the cooling water flow m above the normal level at the end of the cycle and lowering the void ratio, it is possible to increase the core reactivity and extend the cycle.
This also leads to improved economic efficiency of power generation.

(2) As the wall thickness of the tunnel box decreases, the volume occupied by the cooling water, which functions as a moderator, increases, which improves the neutron moderating effect and improves the neutron multiplication factor.

(2) The flow rate of cooling water decreases due to the expansion of the flow path within the fuel bundle. As a result, the pressure loss described in (1) above is reduced, and the flow rate of cooling water on the fuel surface is reduced, making it difficult for the liquid film on the surface to peel off and improving the cooling efficiency.
Improves health.

■As a result of the flow velocity decreasing downstream of the cooling water flow,
The difference in moving speed between the cooling water and the voids, that is, the slip phenomenon increases, and the void ratio decreases.

■As a result, the output in the downstream region increases, and the characteristics of BWR, which tends to decrease in the upper half of the core, are improved, and the power distribution in the vertical direction of the core is flattened, increasing the margin for fuel integrity. do.

■As mentioned above, the void rate in the upper part of the core decreases,
Decrease in core reactivity due to voids is suppressed. Although this is a small amount, it contributes to lengthening the cycle and thus helps improve economic efficiency. In addition, suppressing the decrease in core reactivity due to voids helps to improve the safety of the core, since an abnormal increase in core reactivity when the void rate decreases in an abnormal situation is suppressed to some extent.

Next, the basic idea of the present invention will be explained.

When looking at the assembly in the axial direction, the pressure loss of the cooling water mainly occurs in the axially downstream part where the flow is a two-phase flow.

Therefore, by enlarging the flow path of the cooling water in the axially downstream portion, the speed of the two-phase flow can be reduced and an increase in pressure loss can be suppressed.

Therefore, we examined the excessively thick part of the channel box from both the stress and creep issues.
We came up with the idea that the excessively thick portion corresponds to the portion (downstream) that requires expansion of the flow path, and completed the present invention.

In general, by examining the axial distribution of stress in the channel box and the axial distribution of creep phenomena associated with neutron irradiation and channel internal pressure, various effects can be obtained by adjusting the wall thickness of the channel box material.

It has become clear that the effect can be effectively demonstrated.

That is, the channel box needs to be thicker at the upstream portion (lower end) of the coolant flow where the internal pressure is high. especially,
In areas near the upstream where the neutron irradiation dose is high, the channel box swells outward due to the continuous action of internal pressure, causing a channel creep phenomenon, so it is necessary to avoid making the wall thin in this area.

The area near the axial center of the channel box can be made thinner than the lower end in order to cope with the not-so-large channel internal pressure and stress generated during earthquakes, but it should be avoided to make it significantly thinner. The stress distribution during an earthquake is maximum near the axial center, but is usually smaller than the stress generated by internal pressure. Note that if there is a step-like change in the thickness of the channel box near the center in the axial direction, stress will be concentrated during an earthquake, so the step should be avoided near the center.

Above the center of the tunnel box, stress due to internal pressure and stress during earthquakes are small, and since the internal pressure is also small, the channel creep phenomenon hardly occurs, so it can be made thin. However, near the upper end that contacts the upper grid, that is, the portion that contacts the upper tie plate, stress (although not very large or rather small) is generated during an earthquake. Therefore, in areas where large earthquakes are expected, it would be better to make the wall near the top of the channel box a little thicker.

In any case, the areas where thickness can be reduced from the axial distribution are as follows:
Since this area almost coincides with the area where the area inside the channel is particularly desired to be expanded as a countermeasure against pressure loss, the present invention produces many resonant and synergistic actions and effects as described above.

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

FIG. 1(A) is a vertical cross-sectional view of a channel box which is a main component of an embodiment of the present invention, and FIG. The figure ([) is a front view of the spacer abutting part seen from the E-E direction in Figure 1 (^), and the figure (F) is a cross-sectional view taken along the line F-F in the figure (G). FIG.
) is a vertical cross-sectional view taken along line GG (however, fuel rods are omitted).

The fuel assembly of this example, as shown in FIG. 1(F),
A thick rectangular cylindrical water rod 3 corresponding to 9 fuel rods 4 is arranged in the center of the channel box 1 having a similar external shape, and fuel rods 4 are arranged in 9 rows and 9 in the other space of the channel box 1.
It has a structure in which columns are arranged regularly.

The outer surface of the rectangular tubular water rod 3 has a thinner wall 3b by cutting its wall thickness 3a on the downstream side so that the downstream side of the cooling water expands.
As shown in Figures (8) to (D), toward the downstream la,
The walls have been made thinner to lb and lc, and the inner area has been expanded. If the bundle is only made thinner, the bundle becomes unstable because there is no horizontal support except for the lower part. To avoid this, spacer abutting portions 2 are locally attached to the inner surface of the channel box 1 at thin-walled locations, and the horizontal position of the bundle is fixed at these thin-walled locations.

In addition, a thick rectangular cylindrical water rod 3 (within which a moderator of non-boiling water flows gently downstream) is placed in the center of the bundle. The outer surface is carved and thinned toward the downstream so as to enlarge the channel.

Next, the effects of the present invention will be explained from the aspect of stress acting on the channel box.

As shown in Figure 2 (^), the cooling water flows inside and outside the channel box, but the stress △P generated by the water pressure difference inside and outside the channel box is much greater upstream, as shown in Figure 2 (B). In addition, the stress σ generated during an earthquake is considered to be a horizontal vibration, but
The bottom end of the tunnel box is almost a fixed end,
Since the upper end is somewhat variable, it becomes as shown in the same figure (C). . Therefore, the maximum stress σ is shifted slightly above the center.

Therefore, when the resultant stress acting on the channel box is determined, the resultant stress ΔP+σ is as shown in FIG. 2 (0). In the analysis of the resultant stress up to this range, it is not a very important issue whether to cut the inner surface of the channel box or the outer surface, and it is also possible to cut the gong.

FIG. 3 is a diagram for explaining the stress acting on the channel box of the present invention by roughly adding the channel creep phenomenon to the stress in FIG. 2.

FIG. 3(A) is a schematic configuration diagram of a normal fuel assembly,
In the illustrated fuel assembly, a fuel bundle in which fuel rods 4 are regularly arranged with spacers 5 is housed in a channel box 1, and a lower tie plate 6 is attached to the lower end of the fuel bundle, and an upper tie plate 7 is attached to the upper end of the fuel bundle.

By the way, the channel creep phenomenon occurs due to pressure difference between the inside and outside and neutron irradiation, but the range where this phenomenon is likely to occur is within the tertiary range.
As shown in Figure (B), since this is the range (A) on the upstream side, the wall thickness of the channel box 1 on the upstream side is made thicker than this area.

After this, the composite stress on the downstream side becomes almost flat (1),
Further, on the downstream side, the composite stress becomes even smaller after about 173 days, so the wall thickness of the channel box is reduced stepwise as the composite stress decreases.

FIG. 4 is a diagram for explaining the channel box of the present invention from a thermal-hydraulic perspective. Figure 4 (^) shows a spacer arrangement diagram of the fuel assembly, Figure CB) is a sectional view of a conventional channel box, and Figure 4 (C) is a sectional view of the channel box of the present invention. be. Roughly, Figure 4 (
0) is a comparison of the void ratio distribution curve (broken line) when the channel box of the present invention is applied and the void ratio distribution curve (solid line) when the conventional channel box is applied. The void ratio has decreased slightly due to slip from the aqueous phase. In addition, the inch-jannel pressure loss curves of both cooling water are as shown in FIG.
The pressure loss is smaller than that of

FIG. 5 explains the effect of cooling water in a fuel assembly using the channel box of the present invention. In other words, as shown in FIG. 3A, according to the present invention, since the inner surface of the channel box and the outer surface of the square tubular water rod 3 located at the center of the fuel assembly are shaved, the fuel rod The local flow of boiling cooling water within the gap flows in the direction marked with *. These flows reduce the axial flow velocity inside the bundle, making it difficult for the liquid film to peel off from the surface of the fuel rods 4. Therefore, as shown in FIG. 5(8), the thermal limit output of the present invention (solid line) is improved compared to the conventional one (dotted line).

Also, the thickness of conventional channel boxes is 2.0 to 3.
The thickness is 0mm, and depending on the axial location, it is 1mm or 1mm thick.
It may be less than thick. On the other hand, water sticks were originally 1.0 to 1
, 5 mm, and even in thin areas, reducing the thickness to 0.5 mm or less poses a problem in terms of mechanical strength.

Therefore, it is not possible to expand the flow path area by thinning the water rod to the right of point A1 in FIG. The thickness of the channel box can be reduced more roughly than point A2. therefore,
The composite effect curves from straight line 2 to straight line C at point A3, parallel to straight line A, and rises to the right. That is, as shown in FIG. 6, in the present invention, a straight line C is obtained as a combination of the straight line A, which is the effect of scraping the inner surface of the channel box, and the straight line opening, which is the effect of scraping the outer surface of the water rod.

Therefore, since the increase in ω in the infinite multiplication factor is approximately proportional to the flow passage enlargement area (combined value), the fuel assembly of the present invention has a longer cycle time and improves economic efficiency. In addition, since the amount of cooling water as a moderator in the upper half of the core increases, the output in the upper half of the core increases, and the power distribution becomes flat as shown by the dotted line in Figure 7, compared to the conventional one. The fuel integrity is improved compared to the solid line).

FIG. 8 is a longitudinal sectional view of a second embodiment of the invention. In this embodiment, as shown in the figure, the channel box 1 and the lower nozzle 10 are integrally connected, and the lower tie plate 6 and the lower nozzle 10 are configured to be separable. Therefore, in such a fuel assembly, the bundle attached to the upper tie plate 7 is connected to the channel box 1.
The structure is such that it can be inserted and removed inside the device. For this reason, the abutting portion 9 of the channel box 1 is attached to the spacer 8 at the portion where the inner surface of the channel box 1 is shaved to make it thinner. As shown in FIG. 10, this spacer 8 has channel contact portions 9 formed on both sides of its outer periphery. and,
The contact portion 9 on the outer periphery of the spacer 8 is configured to bulge outward significantly at the upper portion of the channel box, and to bulge slightly at the lower portion of the channel box as in the conventional case.

Note that 13 is a 70-tab.

FIG. 9(^) is a vertical cross-sectional view of a channel box which is a main component of the third embodiment of the present invention, and FIG.
)) is a cross-sectional view taken along line G8 to line D in FIG. 9(A), and FIG. 9(E) is an enlarged view of portion E in FIG. 9(B).

In this embodiment, the wall thickness of the channel box 11 from the upstream side to the downstream side is set as shown in FIG.
As shown in D), it is configured to become thinner stepwise from 11a to llb to llc. The characteristic feature of this embodiment is that, as shown in FIG. This simultaneously achieves attachment/detachment of the channel box 11 and prevention of the bundle from moving laterally within the channel. This groove 12
is provided slightly below the lowest spacer (spi > position in Figure 4 (^)). However, this groove 12 is provided to the bottom of the channel box 11 in order to prevent reta flow to the out channel and to cope with large internal pressure. Not provided.The corners inside the channel box generate a large outward stress due to the internal pressure of the coolant in the inch channel, so the thick wall 11a is used.In other words, the part where the groove 12 is provided has the outer stress at the corner and the area near the corner. Since it becomes a balance part for the internal stress of the side surface, the stress is small, and therefore it can be made thin.This allows the formation of the groove 12 which becomes the passage part of the channel contact part (spacer fixation).(Please note the details of the reason for this) (This will be explained in FIG. 13, which will be described later.) Inward stress is generated in the subsequent part in response to internal pressure, so it cannot be made as thin as the upper part.

FIG. 11 is a longitudinal sectional view of a fourth embodiment of the present invention. The thickness of the channel box 14 of this embodiment is structured in two steps (14a, 14b) in the axial direction.

This channel box 14 is effective in plants where earthquake resistance is strongly required. That is, although the effect of reducing pressure loss is slightly reduced, the wall thickness is relatively thick even at the upper part of the channel box, so it is strong against lateral stress. Further, the depth of the vertical groove 15 in this embodiment is the same as the thickness 14b of the upper part of the channel box, and the channel abutting portion of the spacer is inserted into this vertical groove 15.

FIG. 12 shows a fifth embodiment of the present invention, and FIGS. ) No. 8-
Longitudinal cross-sectional views along line A or line B-B, (C) to (E
) is a cross-sectional view taken along line C-C to line E-E in FIGS. 2A and 2B, and FIGS. FIG. 2 is a longitudinal cross-sectional view of the core, a horizontal plan view of the upper part of the core, and a horizontal plan view of the lower part of the core.

When the channel corner is chamfered or the radius of curvature of the corner is increased, the maximum value of stress based on internal pressure generated at the corner is reduced. In this embodiment, this idea is added to make the wall thickness of the channel box 16 even thinner.

In this case, the fuel rods at the bundle corners are removed as shown in (G) and ([1) of the same figure. Since stress is small in the upper part of the core, the channel corners are not chamfered as in the conventional type, as shown in Figure CB). As shown in the same figure (^) and (B), the wall thickness of the channel box 16 is 16a, 16b, from the upstream side to the downstream side.
It is said to be thin at 16c. As a result, the cooling water flow path at the corner portion is expanded, the flow velocity of the cooling water is reduced, pressure loss is reduced, fuel health is improved, and the void ratio is also reduced to some extent due to the reduction in void slip. In addition, 17 is a fuel bundle, 18 is a spacer, 19 is a channel contact part, and 20 is a water rod.

FIG. 13 is a stress distribution diagram based on the internal pressure of the cooling water in the channel box. That is, in the figure, the solid line A is the conventional channel box, the dotted line opening is the channel box of the present invention (one with a large corner radius), and the dashed line C is the channel box of the present invention (one with a corner surface). It is an internal pressure stress distribution diagram. As can be seen from the figure, the stress is reduced at all locations where the grooves for passing the channel abutting portion of the spacer are provided, so forming vertical grooves does not pose any problem.

FIG. 14 is a plan view of the sixth embodiment of the present invention, in which a cross-shaped channel 22 is provided in the channel box 1, and a space separated by the cross-shaped channel 22 and the channel box 1 is divided into four rows and four columns. One sub-bundle 21 is arranged. In the cross-shaped channel 22 there is non-boiling cooling water, and also in the sub-bundle 2.
Boiling cooling water is flowing inside 1. This sub-bundle 2
The inside of the channel box 1 is thinned so that the flow path of boiling cooling water flowing through the channel box 1 expands toward the downstream.

In other words, the wall thickness of the channel box 1 is structured in a stepped manner so that it is thicker on the upstream side and thinner on the downstream side.
It is omitted in the figure.

By the way, the sub-bundles are not limited to 4 rows and 4 columns, but sub-bundles in other formats can also be applied, and the non-boiling water area in the channel box can be modified in various ways, such as placing thick water rods roughly in the center. Conceivable.

FIG. 15 is a plan view of a seventh embodiment of the present invention, in which a large-diameter water rod 23 corresponding to nine fuel rods 4 is arranged in the center of a channel box 1 having a similar external shape. In the other space 1, fuel rods 4 are regularly arranged in 9 rows and 9 columns.

With this configuration of the fuel assembly, the flow of the cooling water 28a between the channel box 1 and the corner fuel rods 4a tends to deteriorate, and the flow of the cooling water 28b around the large-diameter water rods 23 tends to become excessive. By employing the channel box 1 whose inner surface is thinned and the large diameter water rod 23 whose outer surface is thinned, the non-uniformity of the cooling water flow described above is improved.

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

As shown in the figure, there are four
A rectangular cylindrical water rod 24 corresponding to a real fuel rod 4 is arranged at an angle of 45 degrees, and a roughly cross-shaped channel 25 is provided in this rectangular cylindrical water rod 24. Four rows and four columns of sub-bundles 26 (however, the innermost fuel rod of the long bundle is removed for the rectangular cylindrical water rod) are arranged in the space separated by the channel 25 and the channel box 1, respectively. The configuration is such that one piece is placed at a time. Non-boiling cooling water flows in the rectangular cylindrical water rod 24 and cross-shaped channel 25, and boiling cooling water flows in the sub-bundle 26. The inside of the channel box 1 is cut and thinned so that the flow path of the boiling cooling water flowing within the sub-bundle 26 expands toward the downstream. Similarly, the cross-shaped channel 25 and the rectangular cylindrical water rod 24 are gradually reduced in thickness. A water passage hole 2 is provided at each corner of the roughly rectangular tubular water rod 24.
7 is provided. If the flow path is enlarged, it will be possible to improve the reactor shutdown margin, reduce pressure loss, improve the void coefficient, and improve the power distribution (in the axial direction).

[Effects of the Invention] As explained above, in the fuel assembly of the present invention, the inner surface of the channel box is shaved and the downstream side is thinly combed to expand the flow passage area, which reduces the flow rate of cooling water and reduces the cooling water pressure. It has remarkable effects on loss reduction, improvement of thermal limit output, and improvement of reactivity at high output.

[Brief explanation of the drawing]

FIG. 1(A) is a vertical cross-sectional view of a channel box which is a main component of an embodiment of the present invention, and FIG. 1(8) to (D) are lines BB to [ - A cross-sectional view taken along line [), (E) is a front view of the spacer abutting part seen from the E-E direction in Figure 1 (^), and (F) is F in Figure 1 (G). -F
A plan view of one embodiment of the present invention along the line, (G) is the first
Vertical cross-sectional view along line GG in Figure (F), Figure 2 (^)~(
D) is a diagram for detailed explanation of the present invention, FIG. 3(A)
~(C) are diagrams for explaining the composite stress acting on the channel box according to the present invention, - Figures 4(A) to (E) are diagrams for explaining the channel box of the present invention from a thermo-hydraulic perspective. , Fig. 5(A) and Fig. 5(B) are diagrams for explaining the action of the fuel assembly of the present invention, and Fig. 6 shows the relationship between the infinite multiplication factor and the boiling cooling water flow passage expansion area during high-temperature output operation. 7 is a relative power distribution diagram in the axial direction of the core, FIG. 8 is a vertical sectional view of the second embodiment of the present invention, and FIG. 9(A) is a main view of the third embodiment of the present invention. A vertical cross-sectional view of the channel box which is a constituent element. Figures (B) to (0) are cross-sectional views taken along line B-B to D-[) in Figure 9 (^), and figure (E) is a cross-sectional view along line B-B to D-[) in Figure 9 (^). FIG. 10 is a schematic diagram of a spacer used in FIG. 9, FIG. 11 is a vertical cross-sectional view of the fourth embodiment of the present invention, and FIG. This is the fifth embodiment of the present invention, and (A) and (B) in the same figure are the channel boxes that constitute the main components of this embodiment, that is, (C) to (E) in the same figure.
Longitudinal cross-sectional view along line A- or line G8, the same figure (C) ~
(E) is a cross-sectional view taken along line C-C to line E-E in FIGS. 5A and 3B, and FIGS. A partial vertical sectional view of the embodiment, a horizontal plan view of the upper part of the core, and a horizontal plan view of the lower part of the core, FIG. 13 is a stress distribution diagram based on the internal pressure of cooling water in the channel box, and FIGS. 14 to 16 The figures are all plan views of the sixth to eighth embodiments of the present invention, FIG. 17 is a perspective view of a conventional fuel assembly, and FIG. 18 is a vertical sectional view of the fuel assembly shown in FIG. FIG. 19 is a cross-sectional view of the fuel assembly of FIG. 18. 1, 11.14.16. ...Channel Box 2,
9.19... Contact portion 3, 20.23.24... Water rod 4... Fuel rod 5, 8.18... Spacer 6... Lower tie plate 7... Upper tie plate 10...Lower nozzle 12, 15...Groove 13...Flow tab 17...Fuel bundle 21, 26...Sub bundle 22, 25...Cross-shaped channel 27...Water hole (8733 ) Agent Patent Attorney Yoshiaki Inomata (Baka
1 person) (Don Kōki 0 Io (A) Jōryokomi 1 CB) Fig. 5 Fig. 4 CB) Fig. 1O Fig. 11 (A) (B) Fig. 12 Fig. 13 Man 14ifi ! 151m 16th cause 17th figure 19 figure 18

Claims (7)

[Claims] (1) An upper tie plate is arranged at the upper end, a lower tie plate is arranged at the lower end, and a plurality of spacers are arranged at a predetermined interval between the upper tie plate and the lower tie plate, and the upper tie plate and the lower tie plate are arranged. A large number of fuel rods are arranged regularly using multiple spacers to form a fuel bundle, a metal channel box is placed around the outer periphery of the fuel bundle, and cooling water flows around the fuel rods in the axial direction of the fuel rods. In a fuel assembly configured to remove heat generated in the fuel rods, the channel box has a uniform outer shape in the axial direction of the channel box, and the wall thickness of the channel box material is equal to the thickness upstream of the flow of cooling water. A fuel assembly characterized in that the channel box has a thick wall, and the wall is gradually thinned toward the downstream except for the downstream end where the upper tie plate abuts, so that the internal cross-sectional area of the channel box increases. (2) The wall thickness of the channel box is thick from 1/4L to 1/3L from the upstream end of the cooling water, and the upper tie plate is thick from the downstream end to at least 1/4L. 2. The fuel assembly according to claim 1, wherein the fuel assembly is made thin except for the downstream end where the channel box comes in contact with the fuel assembly, and is configured such that the cross-sectional area inside the channel box increases. (3) In a fuel assembly in which the lower nozzle for introducing cooling water is constructed inseparably from the lower tie plate, and the fuel bundle and channel box are assembled by attaching and detaching the channel box, the wall thickness of the channel box material is Except for the largest part, a spacer abutting part is fixed to the inner surface of the channel box at the position where the spacer abuts on the inner surface of the channel box, and the configuration is such that the position in the direction perpendicular to the axis of the fuel bundle inside the channel box is maintained correctly. A fuel assembly according to claim 1, characterized in that: (4) The lower nozzle for introducing cooling water and the lower end of the channel box are fixed to each other, and the lower nozzle and the lower tie plate are configured to be separable, and the combination of the fuel bundle and the channel box can be performed by inserting and removing the fuel bundle. In the fuel assembly to be carried out, except for the thickest part of the channel box material, a channel contact part for contacting the inner surface of the channel box is attached to the outer periphery of the spacer.
2. The fuel assembly according to claim 1, wherein the fuel bundle is configured to correctly maintain a position perpendicular to the axis of the fuel bundle within the channel box. (5) In thick-walled parts of the channel box material, excluding the part where the wall thickness is the minimum, pressure is generated by the cooling water inside the channel box between the corner of the channel box and the center of the adjacent corner of the flat plate part. 2. The fuel assembly according to claim 1, wherein the channel material is cut from the inside in a region where stress is reduced to form a passage for a channel abutting portion of the spacer. (6) All the fuel rods at each corner of the square-shaped fuel bundle are removed, and correspondingly, the corners of the square-shaped channel box are chamfered or the curvature of the corner is changed to the corner fuel rods. A patent characterized in that the wall thickness of the channel box material is reduced to the extent that the stress generated by the cooling water pressure inside the channel is reduced to the extent that it becomes impossible to insert the channel box material. A fuel assembly according to claim 1. (7) The water rod disposed inside the fuel bundle is thinned by cutting the outer surface of the water rod so that the downstream flow path of the cooling water is enlarged. fuel assembly.