JPH05256970A - Fuel assembly - Google Patents

Abstract

PURPOSE:To enlarge a cooling water area formed on the circumference of outermost fuel rods on the upper part of a fuel bundle without thinning the thickness of a channel box. CONSTITUTION:A fuel bundle 21 is arranged and a fuel assembly 20 is assembled within an angular barrel-like channel box 27. The fuel bundle 21 consists of a fuel rods 22, water rods 23, spacers 24a, 24b, an upper tie plate 25 and a lower tie plate 26. The interval between the neighboring fuel rods 22 is narrowed on the upper part of the fuel bundle 21 and widened on the lower part thereof. Thereby a cooling water area formed on the circumference of the outermost fuel rods is enlarged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly loaded in a boiling water reactor, and more particularly to improvement of the structure of a channel box and a fuel bundle.

[0002]

2. Description of the Related Art FIG. 16 shows a conventional boiling water reactor (BW).
R) for a fuel assembly, which is
A plurality of fuel rods 2 containing a nuclear fuel material, one or a plurality of water rods 3, spacers 4 for holding the fuel rods 2 and the water rods 3 at regular intervals, an upper tie plate 5, and a lower tie plate 6 It is provided with a fuel bundle 1 composed of and, and the fuel bundle 1 is surrounded by a channel box 7. The wall thickness of the channel box 7 is uniform in the vertical and circumferential directions, and the channel box 7 divides the fuel assembly into the inside and the outside.

Inside the fuel assembly, steam voids are generated due to heat transfer from the fuel rods 2, but outside the fuel assembly, there is little heat transfer and heat generation, so steam voids are not generated. Particularly in the BWR, the cooling water outside the fuel assembly has a high water density so as to be effective in the moderating action of neutrons, and has characteristics for the flow inside the channel box 7 and the flow outside so that there are no vapor voids. It is provided. Also, FIG.
Although not shown in FIG. 6, a cross-shaped control rod is arranged outside the channel box 7 so that the control rod can be inserted into the core by using the smooth outer wall of the channel box 7 as a guide.

Further, in a BWR in which steam voids are directly generated in the core, the amount of voids generated in the fuel assembly differs depending on the output of the fuel assembly, so that a channel box 7 is attached to each fuel assembly to make the coolant flow. If the path is not restricted in the lateral direction, a cross flow between the two-phase flow fuel bundles 1 (cross flow) occurs, and the coolant escapes from the periphery of the fuel bundle 1 having a large heat output to the fuel bundle 1 having a small heat output, Fuel cooling capacity is reduced.

There is a pressure difference between the inside and the outside of the channel box 7 provided for such a purpose,
The internal pressure is higher. As a result, the channel box 7 expands to the outside in addition to the thermal effect, the neutron irradiation effect, and the like. In order to suppress the expansion amount and smooth the insertion of the control rod and to secure the rigidity of the fuel assembly, the channel box 7 is made of a thick plate material made of a zirconium alloy having a small neutron absorption. Conventionally, such a channel box 7 is manufactured by bending two plate materials to form a square groove material having a U-shaped cross section, and combining two of them to integrally weld them.

FIG. 17 shows a position in the vicinity of the lower end of the channel box 7, where reference numerals 8 and 9 are water films and reference numeral 1 is shown.
Reference numeral 0 is a water drop, and reference numeral 11 is a vapor void.

By the way, this portion is a region in which the fuel assembly coolant is heated from the saturation temperature or lower to the saturation temperature, and there is no steam inside the channel box 7 and water is filled inside the fuel assembly. There is. Channel box 7
In most of the internal axial area, the coolant flow consists of a mixture of steam and water, and the inner surface of the channel box 7 has
One layer of water film 8 is in contact, and one layer of water film 9 is in contact with the surface of each fuel rod 2. Then, as it goes upward in the axial direction, the volume occupied by the steam generated on the surface of the fuel rod 2 in the coolant flow increases, and the thickness of the water film 9 in contact with the surface of the fuel rod 2 decreases.

If the thickness of the water film 9 in contact with the fuel rods 2 is too small and the heat generation of the fuel rods 2 there is large, heating and thermal instability occur (transition boiling, film boiling and insufficient cooling). Will be).

Water also flows upward by forming a water film 8 in contact with the inner surface of the channel box 7 surrounding the fuel bundle 1. However, unlike the fuel rod 2, the channel box 7
Is heated by neutrons and gamma rays, so
The water film 8 on the inner surface of the channel box 7 flows upward while remaining thick, and is relatively independent of the output of the fuel bundle 1, the flow rate of cooling water into the fuel bundle 1, and the axial height.

By the way, in the recent fuel assembly design, the maximum thermal neutron flux and the maximum local output are determined by the channel box 7
Occurring in the fuel rod 2 adjacent to the inner wall of the
In a WR, the upper part of the fuel bundle 1 usually has a smaller amount of water and a larger amount of steam than the lower part. Due to this difference in water density, the reactor power becomes maximum toward the lower part of the fuel bundle 1. The reactor in the cold shutdown state has the highest reactivity (infinite neutron multiplication factor) above the fuel bundle 1. This is because during the output operation, the fuel burnup is relatively low and the amount of plutonium produced is relatively large in the region where the volume of the vapor in the upper part of the fuel bundle 1 is large.

Conventionally, as a method for solving such a problem, as shown in, for example, Japanese Patent Laid-Open No. 63-261191, the average thickness of the upper portion of the channel box 7 is reduced to reduce the fuel bundle 1 Increase the amount of water in the upper part to increase neutron moderation, reduce void reactivity coefficient, flatten axial power distribution, increase reactor shutdown margin, and decrease two-phase flow pressure loss due to increase in inner area of channel box 7. On the inner surface of the channel box 7,
As shown in FIG. 18, slanted grooves are provided to form the flow tripper 13, and the water film 8 flowing upward on the inner wall of the channel box 7 is separated from the fuel adjacent to the channel box 7 by the edge 13 a of the flow tripper 13. The rod 2 is deflected as a water drop 10 to be effectively used for cooling the fuel bundle 1,
The occurrence of transition boiling of the fuel rods 2 adjacent to the channel box 7 is suppressed, and as a result, the limit output of the entire fuel bundle 1 (maximum output of the fuel bundle 1 at which transition boiling begins to occur somewhere in the fuel bundle 1) is improved. The method of making it do is proposed.

[0012]

By the way, in the conventional fuel assembly in which the flow tripper 13 is formed in the channel box 7, the flow tripper 13 is provided with the slanted groove on the inner surface of the upper portion of the channel box 7 as described above. The channel box 7 is formed by
When the fuel bundle 1 is covered from above, the protrusion of the outermost peripheral band of the spacer 4 is caught by the edge 13a of the flow tripper 13, which causes a problem in assembling the fuel assembly.

Further, in the process of assembling the channel box 7 by welding two square groove members, if the welded portion has the structure of the flow tripper 13, the cross-sectional thickness of the welded portion changes in the axial direction. Another problem is that it is not easy to control the welding heat input.

On the other hand, in the conventional fuel assembly in which the average thickness of the upper portion of the channel box 7 is reduced, reducing the average thickness of the channel box 7 is relatively easy by using a recent NC machine tool. Although it can be automated and is not a serious problem in manufacturing, there is a problem in that the rigidity of the fuel assembly is reduced and the lateral deflection during an earthquake is increased.

The present invention has been made in consideration of the above points, and when the channel box is manufactured by welding, the welding operation is easy and the spacer is the flow tripper when the fuel assembly is assembled. An object of the present invention is to provide a fuel assembly that does not have a problem that a spacer is damaged due to being caught in the fuel assembly.

Another object of the present invention is to provide a fuel assembly capable of flattening the axial power distribution and reducing the absolute value of the void reactivity coefficient without reducing the rigidity of the fuel assembly. It is in.

[0017]

The first invention of the present invention is as follows:
As means for achieving the above object, an upper tie plate,
A lower tie plate, a plurality of fuel rods whose upper and lower end portions are held by both of these tie plates, a plurality of spacers which hold each of these fuel rods at predetermined intervals to form a fuel bundle, and surround this fuel bundle. In a fuel assembly including a rectangular channel box that constitutes a coolant flow path, a water flow is provided on an inner wall of the channel box from a flow path adjacent to the inner wall of the channel box to a fuel rod at an outer peripheral portion of the fuel bundle. A plurality of lateral grooves functioning as a flow tripper for diverting are provided, and the welding portion of the inner wall of the channel box and the sliding portion with the spacer are flat surfaces without the lateral grooves. ..

The second aspect of the present invention is, as means for achieving the above object, an upper tie plate, a lower tie plate, a plurality of fuel rods whose upper and lower ends are held by both tie plates, and these. A fuel assembly comprising: a plurality of spacers that hold fuel rods at predetermined intervals to form a fuel bundle; and a prismatic channel box that surrounds the fuel bundle and forms a coolant flow path. The fuel rod spacing in the upper part of the fuel bundle is made narrower than the fuel rod spacing in the lower part of the fuel bundle, and the cooling water region around the outermost peripheral fuel rods in the upper part of the fuel bundle is made larger.

[0019]

In the fuel assembly according to the first aspect of the present invention, the inner wall of the channel box is provided with a plurality of lateral grooves that act as flow trippers, but the welding portion of the channel box and the sliding portion with the spacer are provided. Is not provided with a lateral groove, and the inner wall of the channel box is left as a flat surface. For this reason, the welding work is facilitated, and when the fuel assembly is assembled by covering the channel box from above the fuel bundle, the spacer slips well and the spacer can be prevented from being damaged.

Further, in the fuel assembly according to the second aspect of the present invention, the fuel rod interval at the upper part of the fuel bundle is made narrower than the fuel rod interval at the lower part of the fuel bundle, whereby the upper part of the fuel bundle is The cooling water area around the outermost fuel rod is made large. Therefore, it is not necessary to reduce the average thickness of the upper portion of the channel box, and it is possible to prevent the rigidity of the fuel assembly from decreasing.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 shows a fuel assembly according to a first embodiment of the present invention. The fuel assembly 20 includes a plurality of elongated fuel rods 22, a predetermined number of water rods 23, spacers 24a and 24b. The fuel bundle 21 includes an upper tie plate 25, a lower tie plate 25, and the like, and the fuel bundle 21 is housed in a channel box 27 having a rectangular tube shape.

Each fuel rod 22 is supported between an upper tie plate 25 and a lower tie plate 26, and penetrates a plurality of spacers 24a and 24b. These spacers 24a and 24b are intermediately supported. As a means, the fuel rods 22 are held at predetermined intervals and their horizontal vibrations are suppressed.

Each fuel rod 22 has pellets of fissionable fuel in a slender tube.
9 is hermetically sealed, and the lower end plug 28 is tapered so as to be fitted and supported in a hole of a fuel support frame 30 formed in the lower tie plate 26. In addition, an extension portion is formed on the upper end plug 29, and the upper tie plate 2
It is fitted and locked in the support hole provided in 5.

Also, some of the holes in the fuel support frame 30 of the lower tie plate 26 (for example, holes at specific positions on the periphery) are formed with threads, and these threads have a threaded lower end. A fuel rod 22 having a plug is connected. The extension portion of the upper end plug 29 of the fuel rod 22 penetrates through the support hole of the upper tie plate 25 and protrudes upward, and a retaining nut with a female screw (not shown) is coupled to the protruding portion. There is.

On the other hand, the channel box 27, as shown in FIGS. 1 to 4, is in the shape of a thin plate quadrangular cylinder whose horizontal cross section is substantially square.
Both tie plates 25, 26 and spacers 24a, 24
It has a slip-fit structure with respect to b. The channel box 7 is fixed to the fuel bundle 21 with bolts 31.

A tip piece is formed on the lower tie plate 26, and the tip piece is inserted into and supported by an opening of a fuel support fitting of a core support plate (not shown).

The spacers 24b are installed in the upper 1/4 to 1/2 section with respect to the fuel effective portion of the fuel bundle 21 (the portion in the fuel rods 22 filled with the fuel pellets), and its lattice. The pitch B is set to a value slightly smaller than the lattice pitch A of the lower spacer 24a.

2 and 3 show spacers 24a, 24
An example using a ferrule spacer is shown as b. In these spacers 24a and 24b, the same number of cylindrical sleeves 32a and 32b as the fuel rods 22 are arranged in a lattice shape, and the outer periphery thereof is a band-shaped side band 33a. 33b
It has a structure surrounded by.

Cylindrical sleeves 32a arranged in a grid pattern,
The adjacent parts of 32b are joined by spot welding, and two projections 34a and 34b are provided at the upper end thereof. These protrusions 34a and 34b are
Part of the cylindrical sleeves 32a and 32b is configured to project inward.

The cylindrical sleeves 32a and 32b are also provided with continuous loop springs 35a and 35b across two adjacent cylindrical sleeves 32a and 32b. The continuous loop springs 35a and 35b are arranged in the height direction. The shape is such that the central part projects outward.

Outside the side bands 33a and 33b,
Protrusions 36a and 36b, which maintain a distance from the inner wall of the channel box 27 and serve as a sliding portion between the spacers 24a and 24b and the channel box 27, are provided on the respective sides near the corners of the outer circumference of the spacers 24a and 24b. Two are provided. In addition, a plurality of flow tabs 37a, 3b are formed on the upper ends of the side bands 33a, 33b at regular intervals.
7b is provided.

The cylindrical sleeve 32b is a cylindrical sleeve 32.
An outer diameter (or thin wall) smaller than a is used, and the protrusion amount of the protrusion 34b and the protrusion amount of the central portion of the continuous loop spring 35b are reduced as the lattice pitch is reduced. The side band 3 of the spacer 24b
The outer size of 3b is smaller than that of the side band 33a, or the protruding amount of the protruding portion 36b is larger than that of the side band 33a, and the protruding amount of the protruding portion 36b is larger than that of the protruding portion 36a.
The distance between the inner wall and the surface of the fuel rod 22 at the outermost periphery of the fuel bundle 21 is
It is designed to be kept larger than the spacer 24a.

If there is a possibility that the fuel rod 22 is bent between the two spacers 24a and 24b and an unreasonable force is applied between the two spacers 24a and 24b, a third spacer is provided between the two spacers 24a and 24b. You may make it arrange | position a spacer. At this time, the third spacer may be manufactured separately,
In the dimension inspection of the components of the spacers 24a and 24b, in the case of the spacer 24a, a component having a smaller size is
Further, in the case of the spacer 24b, parts having larger dimensions may be collected, and the third spacer may be manufactured by this part. Thereby, the cost increase can be suppressed.

On the other hand, on the inner surface of the channel box 27, as shown in FIGS. 4 and 5, in the upper 1/4 to 1/2 section with respect to the fuel effective portion of the fuel bundle 21 (see FIG. 1), A flow tripper 38 is provided. As shown in FIG. 5, the flow tripper 38 is composed of sloped lateral grooves formed by cutting or corrosion etching, and a water film flow can smoothly flow into the groove bottom of the flow tripper 38. The taper is tapered and is at a shallow angle to prevent flow separation.

Further, as shown in FIG. 4, the flow tripper 38 includes the welded portion 39 of the channel box 27 and the side bands 33a and 33 of the spacers 24a and 24b.
It is not provided on the inner surface portion 40 on which the protrusions 36a, 36b of b slide, and this portion is a flat surface.

Next, the operation of this embodiment will be described.

The cooling water for the reactor is the lower tie plate 26.
Through the opening of the fuel support frame 30 into the cooling channel in the channel box 27. The cooling water receives the heat from the fuel rods 22 and reaches the saturation temperature, further vapor voids are generated, and the amount of vapor voids increases in accordance with the upward flow in the axial direction.

Such two-phase flow is generated by the surface of the fuel rod 22, the inner wall of the channel box 27 and the water rod 23.
A thin layer of cooling water is formed on the surface, and the thickness of the layer becomes smaller in the fuel rod 22 that is generating heat as it goes upward in the axial direction. In the space between the fuel rods 22, the flow is such that water drops are mixed in the steam flow, and the amount of water drops in the steam flow decreases as it goes upward.

By the way, in the fuel bundle 21 according to the present embodiment, the interval between the fuel rods 22 in the upper 1/4 to 1/2 section of the fuel effective portion is the interval between the fuel rods 22 in the lower position. Since the distance between the inner wall of the channel box 27 and the surface of the fuel rod 22 at the outermost periphery of the fuel bundle 21 is increased accordingly, the fuel bundle 21
The Dankov coefficient (1-C) in the upper portion is reduced, the effect of resonance absorption is reduced, and the reactivity (infinite neutron multiplication factor) in the upper portion of the fuel bundle 21 is increased as compared with the conventional case. As a result, the axial power distribution is flattened and the void reactivity coefficient also has a small absolute value.

The conventional distribution of the cooling water flow passage area around the fuel rod is based on a design in which the flow passage area which is almost the same as the central portion of the fuel bundle is also arranged at the outermost fuel rods of the fuel bundle. However, the fuel assembly of the BWR has a water gap outside the channel box, where neutrons are efficiently heated, so that thermal neutrons are large in the vicinity of the outermost periphery of the fuel bundle and the heat generation of the fuel rods is also Larger than the fuel rods inside the fuel bundle.

On the other hand, in this embodiment, the fuel rod 2
The coolant flow path for each of the two fuel rods 2 is the outermost fuel rod at the uppermost portion.
2, the coolant is supplied to more outermost fuel rods 22. The coolant flow in the fuel bundle 21 is
As the volume occupied by the vapor increases as it goes upward in the axial direction, the water film on the surface of the fuel rods 22 becomes thinner, and more coolant is supplied to the fuel rods 22 at the outermost periphery of the fuel bundle 21, which has a strong tendency. The limit output of the entire fuel bundle 21 is improved as compared with the conventional one.

Here, the fuel rod pitch of the fuel bundle 21 is made large at the bottom and reduced only at the top, and the reason is that the fuel rods 22 and the channels at the outermost periphery of the fuel bundle 21 are formed from the region where the vapor voids at the bottom are small. When the distance between the inner wall of the box 27 and the inner wall of the box 27 is increased, the flow of the cooling water is excessively offset to the outer periphery of the fuel bundle 21,
On the contrary, this is to prevent insufficient cooling of the fuel rods 22 inside the fuel bundle 21.

On the other hand, the water film on the inner wall of the channel box 27 does not change much even at the upper part and does not contribute to the cooling of the fuel rod 22. Therefore, the flow tripper 38 is provided in a 1/4 to 1/2 section above the fuel rods 22 where cooling of the fuel rods 22 at the outermost periphery of the fuel bundle 21 deteriorates. The membranous water flow that rises along the inner wall of the channel box 27 that is not heated flows into the flow tripper 38, turns to the fuel bundle 21 side at the upper end portion of the flow tripper 38, and heads to the adjacent exothermic fuel rods 22. Then, it adheres to the surface of the fuel rod 22 and prevents transition from nucleate boiling and overheating of the fuel rod 22.

The spacer 2 that bundles the fuel bundle 21
At the upper edges of the side bands 33a and 33b of the 4a and 24b, the flow tab 3 that attracts the cooling water flow into the fuel bundle 21 is provided.
7a and 37b are provided. This flow tab 37
By a and 37b, the cooling water flow between the inner wall of the channel box 27 and the fuel rod 22 at the outermost periphery of the fuel bundle 21 is directed to the fuel rod 22 at the outermost periphery and the fuel rod 22 of the second layer from the outside. Therefore, the water droplets in the cooling water flow that have passed through the spacers 24a and 24b adhere to the surface of the fuel rod 22, and the transition from nucleate boiling and the overheating of the fuel rod 22 are prevented.

In particular, in this embodiment, the fuel rod pitch of the upper spacer 24b is made smaller than that of the lower spacer 24a so that the cooling water is supplied to the outermost peripheral portion of the fuel bundle 21 at the upper portion. I made this a lot, this is Channel Box 2
7 Inner wall flow tripper 38 and spacers 24a, 24b
Since the flow tabs 37a and 37b are used to effectively attach water droplets to the outermost fuel rod 22 and the second-layer fuel rod 22 of the fuel bundle 21, an extremely large combination effect can be obtained.

The flow tripper 38 has a welded portion 39 of the channel box 27 and projections 36a and 36b.
Is not provided on the inner surface portion 40 on which the slides. As described above, the two channel materials are welded together to form the channel box 27.
If the flow tripper 38 is provided in the welded portion 39 in the step of assembling the above, the cross-sectional thickness of the welded portion changes in the axial direction, making it difficult to control the welding heat input.
That is, in the thin portion, the welded portion 39 is excessively melted, pits are formed, the welded portion strength is lowered, and the size and phase of the metal crystal grains of the welded portion 39 are changed, resulting in a decrease in corrosion resistance. is there. Further, when the fuel assembly 20 is assembled by covering the channel box 27 from above the fuel bundle 21,
The protrusions 36a and 36b may be caught by the flow tripper 38, and the protrusions 36a and 36b may be damaged.

However, in this embodiment, the welded portion 3
9 does not have the flow tripper 38, and the inner surface portion 40 on which the protrusions 36a and 36b slide is also not provided with the flow tripper 38, so that there is no problem as in the prior art.

As described above, since the distance between the inner wall of the channel box 27 and the surface of the fuel rod 22 at the outermost periphery of the fuel bundle 21 is larger in the upper portion of the fuel bundle 21 than in the lower portion, the Dankov coefficient (in the upper portion of the fuel bundle 21 ( 1-C) is reduced, the effect of resonance absorption is reduced, and the reactivity (infinite neutron multiplication factor) in the upper portion of the fuel bundle 21 can be increased more than in the conventional case.
Therefore, the output distribution in the axial direction is flattened, and the void reactivity coefficient can also have a small absolute value. Further, since the combustion of the upper part of the fuel bundle 21 progresses further, the reactor shutdown margin can be improved. Further, the absolute value of the void coefficient can be reduced to mitigate the pressure rise transient characteristic of the reactor.

Further, since the flow tripper 38 removes a part of the channel box 27,
The neutron absorption by the channel box 27 is reduced, the water-fuel ratio in the upper core is increased, the combustion of fuel in the upper core is promoted, the reactor shutdown margin is improved, and the void coefficient is reduced to make the reactor pressure rise transient. The characteristics can be relaxed.

Further, the outermost fuel rod 2 above the fuel bundle 21.
(2) Since the cooling flow path around the circumference is expanded, the flow of cooling water is attracted, and at the same time, the water film rising along the inner wall of the channel box 27 is attracted by the flow tripper 38 in the form of water droplets. The start of transitional boiling of the fuel bundle and overheating are prevented, and the limit output of the fuel assembly 20 can be improved more than before.

Further, in the conventional method in which the inner wall of the channel box 27 is shaved to expand the cooling water flow path in the upper part of the fuel bundle 21, the channel box 27 is made of a zirconium alloy, so that the cutting process is difficult, and the cutting is performed while considering oxidation prevention. Although a large area is required to be cut in spite of the necessity of processing, the manufacturing cost is increased. However, in the case of this embodiment, it is only necessary to prepare a plurality of types of spacers 24a and 24b. Easy to make.

A sufficient effect can be obtained by making the distance between the inner wall of the channel box 27 and the fuel bundle 21 slightly larger in the upper portion of the fuel bundle 21 than in the lower portion.

6 and 7 show a second embodiment of the present invention, in which a side band 43b is used instead of the side band 33b in the above embodiment.

That is, as shown in FIG. 7, in the side band 43b, the welded portion 44 with the cylindrical sleeve 32b is projected toward the cylindrical sleeve 32b side, which makes it possible to reduce the size of the protrusion 46b. The flow tab 47b can be brought close to the inner wall of the channel box 27. FIGS. 8A and 8B show this state.

That is, FIG. 8A shows the side band 43.
FIG. 8B shows the side band 33b of the first embodiment. In this way, by bringing the flow tab 47b closer to the inner wall of the channel box 27, the water film on the inner wall of the channel box 27 can be effectively attracted to the surface of the fuel rod 22 at the outer peripheral portion of the fuel bundle 21.

FIG. 9 shows a third embodiment of the present invention, in which a spacer 54b is used instead of the spacer 24b in the first embodiment.

That is, in this spacer 54b, the lattice pitch B on the inner side becomes smaller and the lattice pitch A on the outer side becomes larger from the second layer counted from the outer periphery of the fuel bundle 21. And therefore, the inner side is the cylindrical sleeve 32b.
Is used, and a cylindrical sleeve 32a is used on the outer side. In addition, two types of cylindrical sleeves 32a having different sizes,
Since 32b is used, an auxiliary band 55 is arranged on the outer peripheral portion of the cylindrical sleeve 32b.

As described above, in the case of this spacer 54b, since the auxiliary band 55 is required and the number of welding points is increased, the cost is somewhat high, but the spacer 24 of the first embodiment is used.
Performance can be improved more than b.

10 and 11 show a fourth embodiment of the present invention, in which a flow tripper 68 is provided instead of the flow tripper 38 in the first embodiment.

That is, the flow tripper 68 is composed of inclined lateral grooves, and the left and right ends thereof are bent obliquely upward and the angle thereof is set to 45 degrees or more.

As described above, the welded portion 39 of the channel box 27 and the protrusions 36a and 36b (see FIGS. 2 and 3).
The inner surface portion 40 on which the reference slip) slides is attached to the flow tripper 6
Since the fuel cell 22 is not provided, the water film from the inner wall of the channel box 27 is less likely to be peeled off and attached to the fuel rod 22 adjacent to the fuel rod 22. However, by bending both ends of the flow tripper 68 obliquely upward, as shown in FIG. 12, the water film in the flow tripper 68 is positively guided to both ends, and peeling adhesion can be increased. ..

FIG. 13 shows a fifth embodiment of the present invention, in which, in addition to the flow tripper 38 in the first embodiment, a flow tripper 78 is provided at the corner of the inner wall of the channel box 27. Is.

By additionally installing the flow tripper 78 in this manner, transition boiling or overheating of the fuel rods 22 at the outermost peripheral corners of the fuel bundle 21 can be prevented.

FIG. 14 shows a sixth embodiment of the present invention. In addition to the flow tripper 68 in the fourth embodiment, a V-shaped flow tripper 88 is provided at the corner of the inner wall of the channel box 27. It was done.

As described above, by additionally providing the flow tripper 88, the same effect as that of the fifth embodiment can be obtained, and the amount of the water film peeled off and attached can be further increased.

FIG. 15 shows a seventh embodiment of the present invention, which is applied to a fuel assembly having a cruciform water gap inside the fuel assembly.

That is, in the channel box 127 of this fuel assembly, the cooling channel partition plate 1 is attached to the outer cylinder 128.
29, and a water cloth 130 as a cross-shaped water gap is provided inside the fuel assembly. The four sections divided by the water cloth 130 are composed of a plurality of fuel rods 131. The small fuel bundles 132 are arranged respectively.

Flow trippers 138 are provided on the inner wall of the outer cylinder 128 and on the surface of the cooling channel partition plate 129 facing the small fuel bundles 132, and the distance between the fuel rods 131 of each small fuel bundle 132 is set to A spacer (not shown) is set so that the upper portion is smaller and the lower portion is larger. In FIG. 15, reference numeral 133 is a control rod.

Even with this structure, the same effect as that of the first embodiment can be expected.

Although the ferrule type spacers are used as the spacers in each of the above-described embodiments, spacers of other structures such as an egg plate type can be applied in the same manner, and the same effect can be expected.

[0072]

As described above, according to the first aspect of the present invention, the flow tripper is provided on the inner wall of the channel box, and the flow tripper is provided at the welding portion of the channel box and the sliding portion with the spacer. Since it is left as a flat surface without welding, the welding work is easy, and when the fuel assembly is assembled by covering the channel box from above the fuel bundle, the spacer slips well and the spacer is not damaged. Can be prevented.

In the second aspect of the present invention, the distance between the fuel rods in the upper portion of the fuel bundle is made narrower than the distance between the fuel rods in the lower portion of the fuel bundle. Since the cooling water region is made large, it is not necessary to reduce the average thickness of the upper portion of the channel box, and it is possible to prevent the rigidity of the fuel assembly from being lowered.

[Brief description of drawings]

FIG. 1 is a sectional view showing a fuel assembly according to a first embodiment of the present invention.

2 is an enlarged sectional view taken along line II-II of FIG.

FIG. 3 is an enlarged sectional view taken along line III-III of FIG.

FIG. 4 is a perspective view of the channel box of FIG.

5 is an enlarged view of a main part of FIG.

FIG. 6 is a view corresponding to FIG. 3 showing a second embodiment of the present invention.

FIG. 7 is an enlarged perspective view showing a main part of FIG.

8A is an explanatory diagram showing a flow tab of FIG. 7,
(B) is an explanatory view showing the flow tab of FIG.

FIG. 9 is a view corresponding to FIG. 3 showing a third embodiment of the present invention.

FIG. 10 is a view corresponding to FIG. 4 showing a fourth embodiment of the present invention.

11 is an enlarged view of a main part of FIG.

FIG. 12 is an explanatory view showing the effect of the flow tripper shown in FIG. 11.

FIG. 13 is a view corresponding to FIG. 5 showing a fifth embodiment of the present invention.

FIG. 14 is a view corresponding to FIG. 11 showing a sixth embodiment of the present invention.

FIG. 15 is a horizontal sectional view of a fuel assembly showing a seventh embodiment of the present invention.

FIG. 16 is a partially cutaway perspective view showing a conventional fuel assembly.

FIG. 17 is a schematic view showing a state of cooling water on the surface of the fuel rod and the inner wall of the channel box.

FIG. 18 is an explanatory view showing the effect of the flow tripper.

[Explanation of symbols]

 20 Fuel Assembly 21 Fuel Bundle 22,131 Fuel Rod 24a, 24b, 54 Spacer 25 Upper Tie Plate 26 Lower Tie Plate 27,127 Channel Box 38, 68, 78, 88, 138 Flow Tripper 39 Weld 40 40 Inner Surface A, B lattice pitch

Claims (2)

[Claims] 1. An upper tie plate, a lower tie plate, a plurality of fuel rods whose upper and lower ends are held by both of these tie plates, and a plurality of fuel rods which hold these fuel rods at predetermined intervals. A spacer and a rectangular tubular channel box that surrounds the fuel bundle to form a coolant channel, in a channel assembly in which an inner wall of the channel box is provided with a water flow adjacent to the inner wall of the channel box. A plurality of lateral grooves that act as flow trippers for diverting from the fuel bundle to the fuel rods on the outer peripheral portion of the fuel bundle, and the welded portion of the inner wall of the channel box and the sliding portion with the spacer are flat surfaces without the lateral grooves. The fuel assembly is characterized in that 2. An upper tie plate, a lower tie plate, a plurality of fuel rods whose upper and lower end portions are held by both of these tie plates, and a plurality of fuel rods which hold these fuel rods at predetermined intervals. Of the spacer and a rectangular tubular channel box that surrounds the fuel bundle to form a coolant flow path, in a fuel assembly, the fuel rod spacing at the upper portion of the fuel bundle is set to the fuel rod spacing at the lower portion of the fuel bundle. A fuel assembly characterized in that it is narrower than the rod interval and the cooling water region around the outermost peripheral fuel rod above the fuel bundle is enlarged.