JPH0377477B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel assembly, particularly suitable for use in a boiling water nuclear reactor, in which a large number of elongated fuel rods are supported in a bundle with their axes parallel to each other. The present invention relates to a fuel assembly having a fuel spacer for

Figure 1 shows the schematic structure of a fuel assembly conventionally used in boiling water reactors (for example,
Structure shown in Figures 1 to 4 of Publication No. 56-106183)
It shows. 1 is a fuel rod; 2 is a fuel spacer that maintains a horizontal interval between a plurality of fuel rods 1 to a predetermined width; and 3 is a fuel spacer that maintains a vertical interval of a plurality of fuel spacers 2 arranged in an axial direction to a predetermined width. The holding spacer support rods 4 and 5 are an upper tie plate and a lower tie plate that support both ends of each fuel rod 1 and spacer support rod 3.

The fuel spacer 2 is held at a predetermined position in the axial direction of the fuel assembly by a spacer support projection 6 provided on a spacer support rod 3.

A schematic structure of the fuel spacer 2 is shown in FIG. 2. The fuel spacer 2 includes dividers 7 and 8, bars 19 and 20, and bars 19 and 2 assembled in a grid pattern.
from the lantern box 9 provided at the intersection with 0, the lantern spring 10 attached to the lantern box 9, the S-bend 11 provided to the dividers 7 and 8, and the band 12 provided around the outer periphery of the fuel spacer 2. It's summery.

However, such a fuel spacer 2 has a considerably large projected area in the axial direction of the fuel rod due to the presence of the lantern box 9 and the S-bend 11, resulting in a large pressure loss. Furthermore, the fuel spacer 2 is a fuel assembly that is susceptible to output fluctuations due to control rod operation, since the intersections of the divider 7, divider 8, bar 19, and bar 20 cause stirring of the coolant flowing inside the fuel assembly. Film boiling occurs on the fuel rod surface around the body. Therefore, it has not been possible to improve the minimum value of the ratio of the critical power to the fuel assembly power during reactor operation (minimum critical power ratio, hereinafter referred to as MCPR).

With the development of improved boiling water reactors (ABWR) using high burnup fuel assemblies, improvements are desired from the viewpoints of core safety and thermal-hydraulic characteristics.

SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel assembly that can reduce pressure loss and improve the minimum output ratio, and can also easily support a fuel spacer.

The present invention is characterized by: (a) a plurality of cylindrical cells arranged in a square lattice with holes formed between them in the diagonal direction of the lattice;
(b) a spacer support rod having a spacer support protrusion on a side wall; (c) the fuel spacer is configured to cause the fuel rod inserted into the cylindrical cell to be inserted into the cylindrical cell; (d) the spacer support rod is inserted into a space surrounded by the plurality of cylindrical cells; e) a plurality of rod holding portions for holding the spacer support rods in the space in a direction perpendicular to the axis of the fuel assembly are provided in separate cylindrical cells forming the space; (f) the spacer support; The protrusion supports the rod holder, which is attached to a plurality of cylindrical cells among the rod holders, in the axial direction of the fuel assembly.

In the present invention having such characteristics, the fuel spacer has a plurality of cylindrical cells, and holes are formed between the cylindrical cells diagonally adjacent to each other in the lattice arrangement of these cylindrical cells. The projected area of the fuel spacer is reduced, and the pressure loss of the fuel spacer can be reduced.

Further, according to the above feature, the thermal margin of the fuel assembly can be improved, that is, the minimum critical output ratio thereof can be increased. The first reason why this effect is obtained is that the divider 7, divider 8, such as the conventional fuel spacer,
Intersections between the bars 19 and 20 are not formed, and holes are formed between diagonally adjacent cylindrical cells in the lattice arrangement of cylindrical cells in the fuel spacer. As a result, agitation of the coolant flowing into the cylindrical cell is significantly suppressed, and the minimum output ratio of the fuel assembly can be increased. The second reason is that the spacer support protrusion supports the rod holders attached to a plurality of cylindrical cells among the plurality of rod holders provided in the axial direction of the fuel assembly. . As a result, the spacer support protrusion is located below the coolant passage formed between the fuel rod and the cylindrical cell and does not disturb the coolant flowing into the coolant passage.
The minimum critical power ratio of the fuel assembly can be further increased.

Further, since the spacer support protrusion of the spacer support rod disposed within the space supports the rod holding portions attached to the plurality of cylindrical cells in the fuel spacer, the work of supporting the fuel spacer can be easily performed. That is, since the spacer support rod is inserted into the space surrounded by the plurality of cylindrical cells, the spacer support protrusion is inserted into the space formed in the space perpendicular to the axis of the fuel assembly. (a space formed between adjacent cylindrical cells) through which the fuel assembly can be easily moved in the axial direction of the fuel assembly. After this movement, the fuel spacer can be supported by the spacer support rod very easily by rotating the spacer support rod until the spacer support protrusion comes to the position of the rod holding portion. The fuel spacer support work, which is one of the fuel assembly assembly steps, can be easily performed.

A fuel assembly that is an embodiment of the present invention applied to a boiling water reactor will be described.

The fuel assembly of this embodiment has the configuration of the fuel assembly shown in FIG. 1, in which the conventional fuel spacer 2 shown in detail in FIG. 2 is replaced with a fuel spacer 14 shown in FIGS. 3 and 4. It is. Additionally, the fuel assembly of this embodiment includes spacer support rods 17. The fuel spacer 14 includes a large number of mutually adjacent circular cells 13 and a circular elastic body 18 for holding a spacer rod (hereinafter referred to as circular elastic holding section).

The fuel spacer 14 has a large number of independent cylindrical cells 13 arranged adjacent to each other in a square lattice shape. Adjacent cylindrical cells 13 are welded together. Each cylindrical cell 13 has fuel supporting rigid protrusions 15 provided at two locations in the circumferential direction. In two adjacent cylindrical cells 13,
One fuel support elastic member 16 is provided. The fuel rod 1 inserted into the cylindrical cell 13 has two fuel supporting rigid protrusions 15 and one fuel supporting elastic member 1.
6, it is supported in its radial direction. The fuel supporting rigid protrusion 15 is formed by making a notch in the side wall of the cylindrical cell 13 and extruding a part of the side wall inward, and protrudes into the cylindrical cell 13. For this reason, as shown in FIGS. 3, 5, and 11, an opening 26 in the fuel rod axial direction is provided between the side wall of the cylindrical cell 13 and the fuel support rigid protrusion 15, that is, the cooling water is allowed to flow in the axial direction. A passageway is formed. The fuel spacer 14 has two types of cylindrical cells 13a and 13 shown in FIGS. 6 and 7 as the cylindrical cells 13.
b is used in combination.

The cylindrical cell 13a has a U-shaped slit 16a in the axial center of the side wall, as shown in FIG. 6, and the cylindrical cell 13b has a rectangular slit 16b in the axial center of the side wall, as shown in FIG. has. The fuel support elastic member 16 is composed of a circular elastic body having a cylindrical shape, as shown in FIG. As shown in FIG. 5, the cylindrical cell 13a is inserted into the U-shaped slit 16a and the rectangular slit 16b located opposite to each other in the adjacent cylindrical cells 13a and 13b.
and 13b. A portion of the fuel support elastic member 16 is located within the cylindrical cell 13a, and the remaining portion is located within the cylindrical cell 13b.

That is, as shown in FIG.
The height of the rectangular hole portion of the U-shaped slit 16a provided at a and the vertical hole portion _16a1 is L, which is the same as that of the fuel supporting elastic member 16. The fuel support elastic member 16 is first inserted into the rectangular hole of the U-shaped slit 16a (FIG. 8). After half of the fuel support elastic member 16 is inserted into the rectangular hole of the U-shaped slit 16a, it is moved into the vertical hole _16a1 (the left state in FIG. 9). A portion of the fuel support elastic member 16 that protrudes outside the cylindrical cell 13a is inserted into the rectangular slit 16b of the cylindrical cell 13b (FIG. 9). The height of the rectangular slit 16b is also L, similar to that of the fuel supporting elastic member 16. The state in which the insertion of the cylindrical cell 13b into the rectangular slit 16b is completed, that is, the fuel supporting elastic member 1 as shown in FIG.
6 is assembled into each slit of adjacent cylindrical cells 13a and 13b, the upper and lower ends of cylindrical cells 13a and 13b are joined by welding.

FIG. 10 shows a - cross section of FIG. 3, and shows a state in which the fuel rods 1 are inserted into each cylindrical cell 13 of FIG. 5.

The fuel spacer 14 used in the fuel assembly of this embodiment has a plurality of cylindrical cells 13, and holes 21 are formed between the cylindrical cells 13 that are diagonally adjacent to each other in a lattice-like arrangement of the cylindrical cells 13. Since the fuel spacer 2 does not have a lantern box 9 unlike the conventional fuel spacer 2, the projected area in the axial direction is reduced compared to the conventional fuel spacer, and pressure loss can be reduced. The fuel spacer 14 has a structure in which the fuel support rigid protrusion 15 forms an opening 26 in the fuel axis direction, so that the projected area of the fuel support protrusion is smaller than that of the conventional fuel spacer 2 having an S-bend. Furthermore, the fact that the fuel supporting elastic member 16 is a cylindrical elastic body also contributes to reducing the projected area of the fuel spacer 14 in the axial direction. Such a fuel supporting rigid protrusion 1
5 and the use of fuel supporting elastic members 16 contribute to further reducing pressure losses.

Furthermore, the fuel assembly of this embodiment has a cylindrical cell 13
2, there are no lantern boxes 9 and lattice intersections 22 (Fig. 2) between diagonally adjacent fuel rods as in the conventional fuel spacer 2, and there are holes as shown in Fig. 3. 21 is formed, so
No stirring of the coolant flowing into the cylindrical cell 13 occurs. Therefore, the occurrence of film boiling on the fuel rod surface in the area where the output fluctuation is large near the control rod insertion part around the fuel assembly is suppressed, and the MCPR of the fuel assembly is reduced.
is improved. Therefore, this embodiment can improve core stability and thermal-hydraulic characteristics.

The fuel supporting rigid protrusion 15 may be provided as shown in FIG. That is, the fuel supporting rigid protrusion 15
is provided at the center in the axial direction of the cylindrical cells 13a and 13b. As in the case of Fig. 5, cylindrical cell 1
An opening 26 is formed by the side wall of 3a (cylindrical cell 13b) and the fuel supporting rigid protrusion 15.

Another example of the fuel spacer used in this example will be described based on FIG. 12. This fuel spacer has all the cylindrical cells 13a each having a U-shaped slit 16a as shown in FIG.
Two of these cylindrical cells 13a are assembled with their tops and bottoms reversed, one fuel support elastic member 16 is inserted into each slit, and the top and bottom ends of the cylindrical cells 13a are welded. The fuel support elastic member 16 can also be supported in this way, and it is only necessary to manufacture one type of cylindrical cell 13a.

The main structure of the fuel spacer 14 of the fuel assembly having the structure shown in FIG. 3 is as described above. The configuration for supporting the fuel spacer 14 in this fuel assembly will be described below with reference to FIGS. 13, 14, and 15. FIG. 13 is an enlarged view of the central part of FIG.

The spacer support rod 17 has a plurality of cylindrical cells 1
It is inserted into the space surrounded by 3. This space is formed within the fuel spacer 14 and is large enough to accommodate the cylindrical cell 13. The circular elastic holding portion 18 is inserted between diagonally adjacent cylindrical cells 13 at the center and attached (joined) to the cylindrical cells 13 . This circular elastic holding part 18
has a cylindrical shape similar to the fuel supporting elastic member 16 which is made of a circular cylindrical elastic body.

Since the fuel assembly of this embodiment uses a cylindrical cell for the fuel spacer 14, the spacer is supported in the diagonal direction of one square of a rectangular lattice, like the conventional fuel spacer 2 shown in FIG. Point the spacer support protrusion 6 of the rod toward this spacer support protrusion 6.
It is not possible to use a fuel spacer support method in which the spacer is inserted into the square, and then the spacer support rod is rotated 45 degrees to lock the divider of the spacer to the spacer support rod.
-Publication No. 106183, page 2, from line 14 in the top right column to line 3 in the left column of the same page and Figure 4). This is because the radial length of the spacer support protrusion 6 that supports the fuel spacer cannot be increased because the spacer support rod is inserted into the cylindrical cell corresponding to the square.

Therefore, in this embodiment, the circular elastic holding part 18 is provided as described above, and the spacer is supported in the space formed by removing the cylindrical cells located between the circular elastic holding parts 18. Insert the rod 17 into the outer surface of one of the spacer support rods 17.
A spacer support protrusion 6 is provided in the same manner as the spacer support rod 3 shown in the figure. The spacer support rod 17 has a plurality of spacer support protrusions 6 in its axial direction, as shown in FIG. The spacer support protrusion 6 supports the circular elastic holding portion 18 . Therefore, the fuel spacer 14 is attached to the spacer support rod 1.
It is appropriately supported in the axial direction by spacer support protrusions 6 of 7.

A plurality of cylindrical cells, for example, the cylindrical cell 13c, located around the spacer support rods 17, 17 and forming the above-mentioned space have a rigid support portion 15 that protrudes outward.
a is provided. The spacer support rod 17 inserted into the space is supported by two spacer rod rigid holding parts 15a and one circular elastic holding part 18 in a direction perpendicular to the axis of the fuel assembly.

When inserting the spacer support rod 17 into the space of the fuel spacer 14, first, the circular elastic holding part 18 and the cylindrical cell 13 adjacent thereto are inserted.
The spacer support protrusion 6 is positioned in the space formed between the spacer support protrusion 6 (position 6' in FIG. After moving to the opposite end of the circular elastic holding part 18, the spacer support rod 17 is rotated to lock the spacer support protrusion 6 to the circular elastic holding part 18. FIG. 15 shows the - cross section of FIG. 13 in a state where the spacer support protrusion 6 is locked to the circular elastic holding part 18. A pair of spacer support protrusions 6 are located above and below the circular elastic holding portion 18 . As described above, the circular elastic holding portions 18 attached to the plurality of cylindrical cells 13 in the fuel spacer 14 are supported by the spacer support protrusions 6 of the spacer support rods 17 arranged in the space, so that the fuel spacer 14 Support work can be done easily.

Furthermore, the fact that the spacer support projections 6 of the spacer support rods 17 hold the circular elastic holding portions 18 attached to the plurality of cylindrical cells 13 in the axial direction of the fuel assembly means that the spacer support projections 6 Since it is located below the coolant passage formed between the fuel rod 1 and the cylindrical cell 13 and does not disturb the coolant flowing into this coolant passage, the minimum critical power ratio of the fuel assembly can be further increased. .

Note that when the cylindrical cells 13 of the fuel spacer 14 are supported by the spacer support protrusions 6, the spacer support protrusions 6 are positioned below the coolant passages formed between the fuel rods 1 and the inner surfaces of the cylindrical cells 13. (inevitably, the spacer support protrusion 6 is connected to the cylindrical cell 13
The spacer support protrusion 6 protrudes toward the fuel rod 1 side from the inner surface of the spacer support protrusion 6), and the cooling water flowing into the coolant passage is disturbed by the spacer support protrusion 6. In a boiling water nuclear reactor, there is a two-layer gas-liquid flow (a two-layer flow of steam and cooling water) within the fuel assembly, especially in its upper part, and the above phenomenon is caused by the flow of coolant from the surface of the fuel rods within the cylindrical cells. There is a possibility that the liquid film will peel off and film boiling will occur. this is,
This leads to a decrease (deterioration) of the minimum output ratio. This embodiment can solve such problems as described above.

The fuel assembly of this embodiment having the above-described structure reduces the pressure loss within the fuel assembly, especially the pressure loss at the fuel spacer 14 portion, and increases the thermal margin of the fuel assembly, that is, increases the minimum output power. can be achieved. Furthermore, the fuel assembly of this example is
Supporting work for the fuel spacer 14 can be easily performed.

Furthermore, since the spacer support rod 17 is inserted into the space, the outer diameter of the spacer support rod 17 can be made larger than that of the fuel rod 1.

The operation of locking the spacer support protrusion 6 to the circular elastic holding portion 18 can also be performed as follows.
That is, the spacer support protrusion 6 is positioned in the space 23 between two adjacent cylindrical cells 13 forming a space, and in this state, the spacer support protrusion 6 is moved to the opposite end of the circular elastic holding part 18. and rotate as described above.

Examples of the spacer support rod 17 include those disclosed in JP-A-52-13982, JP-A-55-72388, JP-A-55-83881, JP-A-55-94184;
It is possible to use water rods having the function of supporting a fuel spacer as described in JP-A-56-106183 and JP-A-56-120976.

FIG. 16 shows a fuel assembly according to another embodiment of the present invention in which a fuel spacer 24 is used instead of the fuel spacer 14. The fuel assembly of this example differs from the fuel assembly shown in FIG. 3 only in the structure of the fuel spacer. In the fuel spacer 24 used in this embodiment, the circular elastic holding part 18 of the fuel spacer 14 is replaced with a square elastic holding part 25. The square elastic holding part 25 has a cylindrical shape like the circular elastic holding part 18. The spacer support rod 17 inserted into the space of the fuel spacer 24 has two spacer rod rigid holding parts 15a and 1 in the direction perpendicular to the axis of the fuel assembly.
It is held at three points: four rectangular elastic holding parts 25. The operation for locking the spacer support protrusion 6 of the spacer support rod 17 to the rectangular elastic holding portion 25 is the same as that for the fuel spacer 14.

This embodiment can achieve the same effects as the embodiment shown in FIG. Furthermore, in this embodiment, since the engaging portion between the spacer support protrusion 6 of the spacer support rod 17 and the rectangular elastic holding portion 25 is large, the fuel spacer support protrusion 6 by the spacer support protrusion 6 of the fuel assembly during the reactor usage period of the fuel assembly is increased. 24 support becomes more reliable.

As described above, the fuel assembly of the present invention can reduce pressure loss by reducing the projected area of the fuel spacer in the axial direction. Further, it is possible to improve the thermal margin of the fuel assembly, that is, increase its minimum critical output ratio. The increase in the minimum output ratio is due to the fact that holes are formed between diagonally adjacent cylindrical cells in the lattice arrangement of cylindrical cells in the fuel spacer;
and spacer support projections are obtained by supporting rod holders attached to a plurality of cylindrical cells in the axial direction of the fuel assembly. The fact that the spacer support protrusion of the spacer support rod supports the rod holding portion simplifies the fuel operation of the fuel spacer.

[Brief explanation of drawings]

Fig. 1 is a structural diagram showing a partial longitudinal section of a conventional fuel assembly used in a boiling water reactor, Fig. 2 is a plan view of the fuel spacer shown in Fig. 4 is a side view of the fuel spacer of FIG. 3, and FIG. 5 is a cross-sectional view of the fuel spacer of FIG. 3, showing two adjacent cylinders in the fuel spacer of FIG. 6 and 7 are side views of the cylindrical cells 13a and 13b in FIG. An explanatory diagram showing a state of insertion into the mold slit 16a,
FIG. 9 shows the U-shaped slit 16 of the cylindrical cell 13a.
An explanatory view showing a state in which the fuel support elastic member 16 inserted into the inside of the cylindrical cell 13b is inserted into the rectangular slit 16b of the cylindrical cell 13b. FIG. 5 is a vertical cross-sectional view of another embodiment of the assembled pair of cylindrical cells, FIG. 13 is an enlarged view of the central part of the fuel spacer in FIG. 3, and FIG. 14 is a spacer support rod in FIG. 3. 15 is a cross-sectional view taken from FIG. 13, and FIG. 16 is a cross-sectional view of an upper portion of a fuel spacer with a fuel assembly according to another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Fuel rod, 4... Upper tie plate, 5... Lower tie plate, 13... Cylindrical cell, 14, 24... Fuel spacer, 15... Fuel supporting rigid protrusion, 16... Fuel supporting elastic member, 16a... U-shaped slit ,1
6b... Rectangular slit, 17... Spacer support rod, 18... Circular elastic holding part, 21... Hole, 2
5... Rectangular elastic holding part.

Claims (1)

[Scope of Claims] 1. A fuel assembly including a plurality of fuel rods and a fuel spacer for maintaining mutual spacing between the fuel rods, which includes: (a) arranged in a square lattice and spaced apart from each other in the diagonal direction of the lattice; multiple cylindrical cells forming pores in the
(b) a spacer support rod having a spacer support protrusion on a side wall; (c) the fuel spacer is configured to cause the fuel rod inserted into the cylindrical cell to be inserted into the cylindrical cell; (d) the spacer support rod is inserted into a space surrounded by the plurality of cylindrical cells; e) a plurality of rod holding portions for holding the spacer support rods in the space in a direction perpendicular to the axis of the fuel assembly are provided in separate cylindrical cells forming the space; (f) the spacer support; A fuel assembly characterized in that a protrusion supports, in the axial direction of the fuel assembly, the rod holder that is attached to a plurality of cylindrical cells among the rod holders. 2. The fuel assembly according to claim 1, wherein the fuel support protrusion is a part of the cylindrical cell protruding inward. 3. The fuel assembly according to claim 1, wherein the fuel supporting elastic member has an opening facing in the axial direction of the cylindrical cell.