JPH0214675B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel assembly for a nuclear reactor, and more particularly to a fuel assembly in which the fuel rods are laterally supported by an elastic device that maintains the spacing between the fuel rods. .

U.S. Reissue Patent No. 28,079 discloses a typical structure for this type of fuel assembly. The fuel assembly disclosed in this patent includes a grid having spring fingers or bands that laterally engage and position the fuel rods. The fuel rods also rest on the lower end plate. In nuclear reactors currently being manufactured, each fuel assembly is disposed between an upper nozzle and a lower nozzle.
During operation of a nuclear reactor, the fuel rods grow, or expand, due to both increased temperature and neutron flux irradiation. To prevent the fuel rods from being subjected to excessive pressure, it is desirable to suspend the fuel rods above the lower nozzle. The prior art relied on friction between the aforementioned spring fingers and the fuel rod to keep the fuel rod suspended above the lower nozzle. However, this turned out to be unsatisfactory, since irradiation with a neutron flux causes relaxation of the spring fingers and a reduction in their force. This phenomenon is particularly evident in spring fingers made of Zircaloy (commercial name) alloy, ie zirconium, which is preferred because of its small neutron absorption cross section. To overcome laxity, so-called support devices have been proposed. In such support devices, springs such as Inconel alloy are embedded in Zircaloy strips, but fuel assemblies containing support devices made of two metals are expensive and difficult to manufacture. is difficult. Another proposal to combat spring relaxation is to provide large spring fingers with sufficient contact area to the fuel rods, but the resulting grid is very large.

Manufacturing the prior art fuel assemblies described above is also an expensive process. Although the spacing between fuel rods, i.e. the channel spacing in each fuel assembly, must be maintained within predetermined limits, prior art practice was to measure the spacing after the fuel assembly was completed; If the measurements are outside the specified limits, the fuel assembly must be rebuilt.

The main objective of the present invention is to overcome the drawbacks of the prior art mentioned above and to maintain the lateral position of the fuel rods as well as the tendency of the fuel rods to move downwards and engage the lower nozzle. It is an object of the present invention to provide a fuel assembly of relatively uncomplicated and low cost construction and moderate size that provides support for fuel rods that suppress

To this end, the present invention provides a nuclear fuel assembly for a nuclear reactor having groups of fuel rods supported in parallel in a plurality of axially spaced grid structures, each extending longitudinally from a plurality of cylinder units. A plurality of blocks are provided, and a set of corresponding cylinder units of the plurality of blocks are coaxial, and each cylinder unit includes one cylinder having an elastic device inside, and the elastic device is at least three curved spring strips spaced equidistantly around the inner periphery of the tube, the opposite ends of each of the at least three spring strips being connected to the tube in circumferentially displaced relation to each other; The at least three spring strips are arranged near the ends of the unit and arranged in a radially symmetrical manner, with the center portions of the spring strips being located near the ends of each set of coaxial cylindrical units. said at least three spring strips in said tube unit to elastically force said one fuel rod along a plurality of contact areas of considerable length. supporting and centrally positioning the fuel rod;
Upon relaxation of said spring strip by irradiation with a neutron flux,
The fuel rod is characterized by restricting displacement in the longitudinal direction of the fuel rod along the cylinder unit.

The cross section of the tube will vary depending on the desired arrangement of the fuel rods and may be circular, oval, oval, polygonal, etc. The spring strip is formed from a wire having a circular, oval, oval or polygonal cross section. Typically, a plurality or set of tube units are uniformly spaced along a fuel rod, and the rod has spring strips in the tube of considerable length as described above. Engage along the contact area. Such a structure is referred to herein as a fuel rod module. Also within the scope of the invention are fuel rod modules in which the fuel rods pass through a single long tube unit having at least three long spring strips therein. Again, at least three long spring strips configured in the same manner as described above engage the fuel rod along a contact area of considerable length to keep it centered. Because the contact area on each fuel rod module is of considerable length, the spring strips are effective in maintaining the lateral and longitudinal position of the fuel rods despite relaxation during operation.

Preferably, the fuel assembly includes a plurality of straps with a plurality of fuel rod modules mounted inside the straps. The plurality of fuel rod modules includes a plurality of tube units (often referred to herein as a set of tube units or a block of tube units) located at each longitudinal position of each fuel rod module in a band-like strap. It is installed in layers in a rectangular shape. The cylindrical unit of each block and the belt-like strap to which the block is attached are formed into a rigid structure. The joints between the walls of each strip and at least some (usually all) of the tubes around the blocks in the strip are welded, and
Some (usually all) of the joints between the cylinders of the contact layer within the block are also welded. Although fuel assemblies according to the present invention, which have notable advantages, typically include a plurality of strips, fuel assemblies having a single long strip containing a plurality of blocks of cylinders are also within the scope of the invention. to go into.

The fuel assembly includes a plurality of control rod thimbles arranged at predetermined positions between rows of fuel rods. The thimble is a continuous tube of somewhat smaller diameter than the tube and has a radial bulge with a plurality of tubes surrounding the thimble supporting the thimble at the bulge.

In the normal practice of the invention, the tube, spring strip, and fuel rod cladding are constructed of Zircaloy alloy. The spring strip relaxes under neutron irradiation. However, as mentioned above, the spring strip engages the fuel rod over a contact area of considerable length so that the lateral and longitudinal position of the fuel rod is maintained despite the relaxation.

The invention will become more readily apparent from the following description of preferred embodiments thereof, illustrated in the accompanying drawings.

The illustrated fuel assembly 21 includes a plurality of fuel rod modules 23 surrounded by a grid structure or strap 25. As shown in FIG. Each module 23 (FIG. 9) includes a plurality of cylinder units 27 mounted on a fuel rod 29 at equal intervals. The diameter of one fuel rod is 9.5mm
(0.374in) is common. If non-uniform spacing is desired, that is also within the scope of the invention. Each tube unit 27 (FIGS. 4, 6, and 8) has a tube or cylinder 31 containing a plurality (three in this example) of curved spring strips 33. These spring strips are uniformly spaced around the inner circumference of each tube 31, and each tube 31 is provided with a plurality of openings 35 uniformly spaced around its circumference. It is being The openings 35 serve to eliminate parasitic neutron absorbing materials, allow attachment of the spring strips 33, and facilitate the passage of coolant to ensure good mixing of the reactor coolant. The ends of each spring strip 33 are welded in circumferentially displaced relation to each other near the end of the tube 31, more specifically to the opposing ends 39 of the plurality of openings. That is, each spring strip has the lower end 3 of one opening.
9 and to the upper end of another opening circumferentially spaced from this opening. The amount of spacing depends on the amount of curvature of the spring strips in the circumferential direction; if the spring strips were completely wrapped around the inner circumferential surface of the tube 31, there would be no spacing.

Typically, each tube 31 has an outer diameter of 12.7 mm and a diameter of 0.76 mm.
It has a wall thickness of mm and a length of approximately 5 cm. The diameter of the wire forming the spring strip 33 is 0.76 mm. As shown, there are three spring strips 33 around the inner circumference of the tube 31. There are three openings 35, the longitudinal centers of which surround the tube 31 at intervals of 120 degrees. Further, the upper and lower ends of the spring strip 33 also surround the cylinder 31 at a distance of 120° from each other in the circumferential direction. Each spring strip 33 is curved approximately one third of a turn, and each fuel rod 29 has eight tube units 27. The three spring strips 33 are also arranged radially symmetrically so that their central portions engage one fuel rod 29 extending through the center of the tube 31.

Band-like strap 2 surrounding fuel rod module 23
5 has a generally rectangular shape. In each fuel assembly, the plurality of straps 25 are aligned, and the fuel rod modules are located inside the straps.
The cylinder units are arranged in layers as shown in FIG. A plurality of cylindrical units form a generally rectangular block inside each band-like strap. Walls 4 of tubes 31 and strip-shaped straps 25 along the periphery of each block
1 is tack welded.
The welds are generally fusion welded using a laser. The joints 42 between the tubes 31 within each block are also fusion welded. Preferably, fusion welding is performed from the top of the fuel assembly. In place, a thimble 43 for a control rod (not shown) passes through the block. Typically, the thimble has an outer diameter of 12.2mm,
The wall thickness is 0.76mm. The thimble 43 is a tube having a bulge 45 which engages with the adjacent tube 31 and is supported above the tube. The bulge 45 below the adjacent cylinder prevents the thimble from being displaced in the vertical direction. The bulge 45 can be formed before or after assembly. The band-shaped strap 25 is provided with an earthquake-resistant section 47 along its wall 41. The seismic portion is a protrusion extending outwardly from the wall. Adjacent fuel assemblies are loaded into the core of the nuclear reactor with seismic parts 47 interposed therebetween.

The distance between the tubes 31 is set relative to the fuel rods. Fuel rod center distance is 45-50
cm away.

The fuel rod module 23, thimble 43 and strip strap 25 are mounted on a plurality of mounting jigs 51 (13th
(Fig.) to form the fuel assembly 21. Each mounting jig 51 is a yoke having a cross member 53 and legs 55 and 57, the cross member 53 having two fingers 59 on each side. Fingers 5
9 and the fingers 59 there is centrally located a movable member 61 which is movable with respect to them. The finger-like portion 59 is the leg portion 5
5, and the movable member 61 is integral with the leg portion 57, that is, fixed to the leg portion 57. The corner at the joint between the leg portion 55 and the finger portion 59 is connected to the pivot pin 6.
3 passes through the pin 63, and the movable member 61 and the leg 57 pivot together about this pin 63. At the end opposite the pin 63, the finger 59 and the movable member 61 are provided with a concentric hole 65 through which a locking pin 67 passes. Removal of locking pin 67 causes movable member 61 and leg 57 to pivot downwardly to remove leg 57, thereby allowing removal of the complete fuel assembly within mounting jig 51.

When manufacturing a fuel assembly by carrying out the present invention, the cylinder 31 having the opening 35, the thimble 43 having the bulge 45, the spring strip 33, the fuel rod 29, and the top side of the belt-like strap 25 are used. and plates 71 and 73 that will become the sides. Boards 71 and 73
is provided with an earthquake-resistant part 47, and all parts including the cladding tube of the fuel rod 29 are made of Zircaloy alloy. However, fuel assemblies in which the cladding is constructed of other materials, such as stainless steel, are also within the scope of the present invention.

The spring strip 33 is mounted in a tube 31 forming the tube unit 27. The ends of the spring strip are welded to the lower and upper ends 39 of spaced apart openings 35. As shown in the drawing, if there are three spring strips and three openings, and the spring strip is 1/3 turn long, then each spring strip has one opening at its lower end. 35, and its upper end is welded to the upper end 39 of the next opening in the counterclockwise direction of the above-mentioned one opening when viewed from the top surface of the cylindrical unit 27. A plurality of cylinder units 27 are combined into one fuel rod 2
9 by attaching it to the fuel rod module 23.
is formed. The tube units are uniformly spaced along the fuel rods 29. The spring strips 33 are uniformly spaced around the circumference of each cylinder 31 and positioned so that the fuel rod 29 is centered in all cylinders mounted thereon. Lock pin 67 into hole 65
Align the plurality of mounting jigs 51 (Fig. 13) placed in the case so that their horizontal members 53 are on the same plane,
Plates 71 and 73 are provided on the inner surfaces of the transverse member 53 and legs 55 and 57 of each mounting jig (FIG. 12). 1st
As shown in FIG. 2, plates 71 and 73 are preferably joined along their sides to form a U-shaped groove. However, it is also within the scope of the invention to provide separate plates 71 and 73 in the mounting jig. A first row of fuel rod modules 23 is placed along the horizontal plate 71 such that the tube units 27 are in the first layer on the horizontal plate 71. The joint 40 between the tube 31 and the plate 71 is welded. The joint between the tube 31 and the plate 73 at the end of the first layer is also welded. A second row of fuel rod modules 23 is placed on top of the first layer such that the cylinder units 27 of this row become a second layer above the cylinder units of the first layer. The joint portion 42 of the first and second layer tubes is welded. Furthermore, the joints 40 between the tube and the plate 73 at each end of the first and second layers are also welded. Other rows of fuel rod modules are placed in the same manner in third, fourth, fifth, etc. rows such that the tube units 27 are layered in a U-shaped structure. The cylindrical units 27 of each other layer are located on the cylindrical units of the layer immediately below. The joints 42 between the cylinders of each of the other layers and the cylinders of the layer below are welded. Also, board 7
3 and the tubes at the ends of each other layer are also welded. In this way a substantially rigid structure is created. Typically, such assembled rows are square with 17 tube unit positions in each direction, or 289 tube unit positions in total. The expression "tube unit position" is used considering that the tube unit 27 is not located at all positions. When fabricating a fuel assembly, when assembling the above-mentioned rigid structure, thimble 4
Leave space for inserting 3. Board 7
1 and 73, the fuel rod module 23, and the thimble are completed, the upper plate 71 is attached to the horizontal plate 73 to complete the belt-like strap 25.
A plate 71 in which a row of fuel rod modules 23 is formed into a U shape.
Although there are advantages to forming a fuel assembly by arranging the fuel rods 29 into the coaxial tube units 27 after the rectangular rows of tubes have been assembled to the plates 71 and 73, it is also practical. within the scope of the invention. After the fuel assembly 21 is completely configured, the locking pin 67 is removed, the cross member 61 and leg 57 are pivoted downward, and the fuel assembly is removed. During fuel assembly formation, the spacing between control rods, or channel spacing, is repeatedly measured to ensure that the spacing is within limits before the fuel assembly is completed.

The function of the ribbon straps 25 is to frame the rows of fuel rod modules 23 and the thimbles 43. The straps 25 are provided with guide tabs 81 which assist during loading of the fuel assembly into the reactor core (not shown). Seismic sections 47 on the straps 25 dampen lateral movements between the fuel assemblies 21 during seismic disturbances.

The advantages of the fuel assembly according to the invention are as follows.

1 At least three curved spring strips spaced equidistantly around the inner periphery of the tube are provided near the end of the tube unit in such a way that their ends are circumferentially displaced from each other; The spring strips are arranged in a radially symmetrical arrangement such that their midsections engage one fuel rod extending through the center of each pair of coaxial tube units. ,
Three spring strips in each tube unit elastically support a fuel rod along multiple contact areas of considerable length, centering the fuel rod and directing the neutron flux. When the spring strips relax due to irradiation, the longitudinal displacement of the fuel rods along the cylinder unit is restricted.

2 Since the spring strips create turbulence in the coolant,
The mixing vanes required in prior art fuel assemblies are eliminated or the number of mixing vanes is reduced.

3 fuel rod modules 23 can be first assembled and then assembled into fuel assemblies 21.

4. Scratches on fuel rod cladding tubes are eliminated or reduced.

5. Since the work involves stacking the rods one row at a time, the channel spacing, that is, the spacing between control rods, can be easily inspected during work.

[Brief explanation of drawings]

Fig. 1 is a plan view of a fuel assembly according to an embodiment of the present invention, Fig. 2 is a partial side view of the fuel assembly shown in Fig. 1, and Fig. 3 is a part of the fuel assembly extending from a belt-like strap. 4 is a perspective view of a tube unit of a fuel rod module included in a fuel assembly according to the present invention, FIG. 5 is a plan view of the tube unit, and FIG. 6 is a side view of the tube unit. Fig. 7 is an end view of only the cylinder in Fig. 6, and Fig. 8 is a - of Fig. 6.
9 is a side view of a fuel rod module according to the present invention, and FIG. 10 is a cross-sectional view taken along line -
11 is a partial side view for explaining the method of manufacturing a fuel assembly according to the present invention, FIG. 12 is a partial plan view of the apparatus shown in FIG. 11, and FIG. 1 is a perspective view of a mounting jig used in fabricating a fuel assembly according to the present invention; FIG. 21...Fuel assembly, 25...Grid structure (belt-shaped strap), 27...Cylinder unit, 29...
... fuel rod, 31 ... cylinder, 33 ... spring strip.

Claims (1)

[Claims] 1. A nuclear fuel assembly for a nuclear reactor having groups of fuel rods supported in parallel in a plurality of axially spaced grid structures, comprising a plurality of blocks each extending vertically from a plurality of cylinder units. , the corresponding set of cylinder units of the plurality of blocks are coaxial, and each cylinder unit includes one cylinder having an elastic device inside, and the elastic device is
at least three curved spring strips spaced equidistantly around the inner periphery of the tube, the ends of each of the at least three spring strips being in circumferentially displaced relation to each other. The at least three spring strips are arranged near the ends of the cylindrical units, and are arranged radially symmetrically so that the spring strips in each set of the coaxial cylindrical units are arranged at the center of the spring strips. engaging a fuel rod extending through the center, the at least three spring strips in the tube unit pulling the fuel rod along a plurality of contact areas of considerable length; an atom that elastically supports, centrally positions the fuel rod, and restricts longitudinal displacement of the fuel rod along the cylinder unit when the spring strip is relaxed by irradiation with a neutron flux; Furnace fuel assembly.