JP2901685B2 - Fuel assemblies and reactors - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for loading in a light water moderation reactor.

[Conventional technology]

Currently, a plutonium multiplied BWR (hereinafter abbreviated as Pu multiplied reactor) is being developed as a next-generation light water reactor with the aim of significantly reducing the fuel cycle cost by increasing the burnup and making effective use of plutonium (Japanese Patent Application No. Sho 59). No. 251389). In the Pu multiplication reactor, the amount of fissile Pu produced and the amount extinguished during reactor operation are almost the same, and the amount of fissile Pu in the fuel assemblies loaded in the reactor core and the fissile Pu ratio is
It is almost 1.0. In order to achieve such core characteristics, the fuel rods are arranged in a triangular array and densely packed to reduce the water-to-fuel volume ratio (H / U ratio) in the fuel assembly.

A conventional fuel assembly used in a Pu multiplication furnace is composed of an upper tie plate, a fuel spacer, a plurality of fuel rods, a lower tie plate, and a channel box for storing the same. In order to reduce the H / U ratio of the fuel assembly, the fuel rods 4 have a triangular close-packed structure. Therefore, the horizontal cross section of the fuel assembly has a hexagonal shape. The upper and lower tie plates hold both ends of the fuel rod. A plurality of fuel spacers are provided at intervals in the axial direction to align and support the fuel rods in the lateral direction. The channel box surrounding the bundle of fuel rods has a cylindrical shape with a hexagonal horizontal section.

In such a fuel assembly of a Pu multiplication furnace, when the clump clearance between fuel rods is reduced to reduce the H / U ratio, the pressure loss between the upper and lower portions of the fuel assembly increases. This pressure loss is reduced.

An example of a dense lattice fuel assembly is described in Japanese Patent Application No. 62-273485. In this fuel assembly, the length of a fuel rod is reduced to half of that of a conventional fuel rod, and an intermediate tie plate is provided. The upper end of the fuel rod is supported on an intermediate tie plate. The intermediate tie plate is connected to the upper tie plate by a connecting pipe.

As a control rod used for the fuel assembly of the Pu multiplication furnace, a control element assembly including a plurality of cylindrical control elements (clusters) is used. As a method of inserting the cluster type control rod, (1) a type inserted from the upper part of the core (type 1) and (2) a type inserted from the lower part of the core (type 2) are being studied. The fuel assembly shown in FIG.
It is a structural example based on the type 1 control rod insertion method. When the type 1 control rod shape and the insertion method are applied to the Pu multiplication furnace, the raised rod guide tube has a structure penetrating through the steam-water separator and the dryer above the core. Such a reactor structure
In addition to being complicated, there is a disadvantage that the entire furnace vessel is also large.
Therefore, as a method for inserting the cluster type control rods into the fuel assembly, type 2 in which the control rods are inserted from the lower part of the core is desirable.

Furthermore, in the core of the Pu multiplication reactor, the gap between the fuel assemblies is set to 1
Approximately / 10. Therefore, unlike the current BWR, the upper grid plate cannot be inserted between the assemblies above the fuel assembly, and it is difficult to position the fuel assembly in the core radial direction.

[Problems to be solved by the invention]

The fuel assembly disclosed in Japanese Patent Application Laid-Open No. 62-273485 requires a new connecting pipe for supporting the intermediate tie plate, and the structure becomes complicated.

An object of the present invention is to provide a fuel assembly and a nuclear reactor that have a simple structure and can increase the conversion ratio. The cluster control rod with the follower at the tip has a structure that can be inserted into the fuel bundle from the lower part of the fuel assembly, and is mechanically and thermally stable against various loads generated during the core loading period It is an object of the present invention to provide a simple fuel assembly and a nuclear reactor.

[Means for solving the problem]

The object is to provide a plurality of fuel rods, a plurality of hollow control rod guide tubes disposed between the fuel rods and having a longer axial length than the fuel rods, and lower ends of the fuel rods and the control rod guide tubes. A lower tie plate for supporting a portion, an upper tie plate to which an upper end of the control rod guide tube is attached, and an upper end of the fuel rod provided on the control rod guide tube below the upper tie plate. And a fuel rod upper supporting member for supporting the portion.

[Action]

Since the fuel rod upper support member that supports the upper end of the fuel rod is provided on the control rod guide tube that is a component of the fuel assembly, it is not necessary to provide another member to support that member, and the fuel assembly The structure becomes simple. Further, since the control rod guide tube having a longer length than the fuel rod is provided, the control rod having a follower portion can be inserted into the control rod guide tube, and the plutonium conversion ratio is improved. The cluster control rod with the follower at the tip has a structure that can be inserted into the fuel bundle from the lower part of the fuel assembly, and is mechanically and thermally stable against various loads generated during the core loading period It is.

〔Example〕

Hereinafter, a fuel assembly according to an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a fuel assembly according to this embodiment.

The fuel assembly 50 of the present embodiment has the control rod thimble tube 3 into which the cluster control rod is inserted because the cluster control rod provided with a follower of a fixed length at the tip is inserted from the lower end. The control rod thimble tube 3 penetrates from the thimble tube attachment portion 7a of the lower tie plate 7 to the thimble tube attachment portion 1a of the upper tie plate 1 and is attached to each thimble tube attachment portion 7a. The control rod thimble tubes 3 are dispersedly arranged in the fuel rods 4 as shown in FIG. In the present embodiment, 18 control rod thimble tubes 3 are provided for 151 fuel rods 4. The lower end of the fuel rod 4 is supported by the fuel holding portion 7b of the lower tie plate 7. The fuel holding unit 7b
It is located above the thimble tube mounting portion 7a.

In the Pu multiplication furnace, the pressure loss increases because the interval between the fuel rods 4 becomes narrow, and the upward lift on the fuel rods 4 increases. It is difficult to hold each fuel rod at a predetermined position only by the restraining force of the fuel spacer as in a conventional fuel assembly. For this reason, the fuel assembly 50 moves the fuel rod end plug support plate 2 into which the upper end plug of each fuel rod 3 is inserted at a position lower than the thimble pipe mounting portion 1a of the upper tie plate 1 so that the control rod thimble pipe 3 can be inserted. Attach to The fuel rod end plug support plate 2 supports an upper end plug of the fuel rod 3. By providing the fuel rod end plug support plate 2, it is possible to prevent the fuel rod 3 from rising upward due to lift. The fuel rod end plug support plate 2 is held on the upper tie plate 2 via the control rod thimble tube 3.

The engagement structure of the control rod thimble tube 3 with the upper tie plate 1, the fuel rod end plug support plate 2, and the lower tie plate 7 will be described with reference to FIGS. 3 and 4. FIG.

FIG. 3 shows the thimble tube mounting portion 1a of the upper tie plate 1.
3 shows an engagement structure between the fuel rod end plug support plate 2 and the control rod thimble tube 3.

The control rod thimble tube 3 has a hollow portion for inserting one neutron absorption rod provided in the cluster type control rod inside. This hollow portion opens at the lower end of the control rod thimble tube 3. The upper end of the control rod thimble tube 3 is solid, and the diameter of the solid portion is large. A screw is provided outside this part. The solid part is inserted into the thimble tube mounting part 1a. In this state, the nut 51 is attached to the upper end of the control rod thimble tube 3, and the control rod thimble tube 3 is connected to the thimble tube attachment portion 1a. The control rod thimble tube 3 and the fuel rod end plug support plate 2 are connected by a stopper 27 provided on the control rod thimble tube 3 and an expansion spring 30 mounted on the upper end plug 29 of the fuel rod 4. Note that the control rod thimble tube 3 and the fuel rod end plug support plate 2 can be joined by welding.

FIG. 5 shows the thimble tube mounting portion 7a of the lower tie plate 7.
2 shows a cross-section of FIG. The thimble tube mounting portion 7a is provided with a through hole 33 of the control rod thimble tube 3 and a coolant circulation hole 32. For some of the through holes 33, grooves 34 are provided on the outer peripheral portion of the through hole 33 so that the stoppers 35A and 35B provided at the lower end of the control rod thimble tube 3 can pass through.

FIG. 4 shows a state in which the control rod thimble tube 3 provided with the catches 35A and 35B is connected to the lower tie plate 7. The stopper portion 35A is located at a position in contact with the lower surface 7a of the upper portion of the lower tie plate of the control rod thimble tube 3 in contact with the lower surface of the fuel holding portion 7b of the lower tie plate and the upper surface 7b of the bottom surface of the lower tie plate.
And two locations. The stopper part 35B
The control rod is provided on the thimble tube 3 so as to be in contact with the upper surface of the thimble tube mounting portion 7a. The control rod thimble tube 3 provided with the catches 35A and 35B is inserted into the through hole 33 provided with the groove 34. The catches 35A and 35B move through the groove 34. By rotating the control rod thimble tube 3 by 90 ° at a position where the stopper portion 35B has passed through the thimble tube attaching portion 7a, the stopper portion 35B is rotated.
The lower tie plate 7 and the control rod thimble tube 3 are connected by A and 35B. Since the control rod thimble tube 3 is attached to the upper tie plate 1 in this state as described above, rotation is prevented. A lower positioning pin hole 25 is provided on a side surface of the lower tie plate 7. The lower end of the control rod thimble tube 3 is located below the fuel holding portion 7b, that is, the thimble tube mounting portion 7a located at the lower end of the lower tie plate 7.
Neutron absorption rod
It is easy to insert the ten control rod thimble tubes 3 from below.

Further, a channel box is provided so as to include the fuel rods 4 bundled from the upper portion of the lower tie plate 7 to the bottom surface of the upper tie plate 1.

The structure of the upper tie plate 1 of the fuel assembly 50 will be described in detail with reference to FIGS. Upper tie plate 1
Has a hexagonal ring-shaped handling portion 1b above the thimble tube mounting portion 1a. The thimble tube mounting portion 1a and the handling portion 1b are connected by a column 1c located at a corner. Thus the upper tie plate 1
Has a box-shaped hexagonal shape, and each side face is cut out except for corners and upper and lower ends. This is to reduce the weight of the upper tie plate 1 and at the same time to form the handling portion 1b when moving the fuel assembly. FIG. 7 shows a state in which the thimble tube attaching portion 1a is viewed from above with the control rod thimble tube 3 removed. Like the thimble tube attaching portion 7a, the thimble tube attaching portion 1a also has a coolant circulation hole 32 and a through hole 33 into which the control rod thimble tube 3 is inserted. A hold-down spring 20 that extends from the bottom surface of the upper tie plate 1 to the uppermost surface is provided on some of the columns 1c. When the fuel assemblies 50 are loaded in the core, the hold-down springs 20 contact the upper core lattice plate 13 (FIG. 10) which presses the uppermost surface of the upper tie plate 1 from above, and the fuel assemblies 50 It plays the role of pressing the whole down in the axial direction by its spring force.

Further, a conical pin hole 21 is provided at a predetermined corner of the uppermost surface of the upper tie plate 1 as shown in FIG. In this pin hole 21, a conical positioning pin 14 (FIG. 10) installed on the lower surface of the upper core support plate 13 is inserted. Each fuel assembly 50 is positioned in the core radial direction by the positioning pin 14.

FIG. 8 shows a detailed structure of the fuel rod end plug support plate 2. The fuel rod end plug support plate 2 is formed by connecting boss holes 23 into which all the fuel rods 4 and the end plugs of the control rod thimble tube 3 are inserted by a connecting grid 24. The fuel spacer 6 used in the fuel assembly 50 is shown in FIG. In order to minimize pressure loss, it is made by joining a plurality of cylinders having a small projected area.

FIG. 10 shows a boiling water reactor in which a fuel assembly 50 is loaded in a core. The reactor pressure vessel 19 has a core shroud 16 inside, and an upper shroud is provided above the core shroud 16.
Covered with 15.

Each fuel assembly 50 is loaded above the control rod guide tube 12 via the lower core support plate 11. The upper core lattice plate 13 is provided at the upper end of the core shroud 16, and the fuel assembly 50 is supported by its upper end contacting the upper core lattice plate 13. Positioning pins 14 are provided on the lower surface of the upper core lattice plate 13 so that they can be inserted into the pin holes 21 on the uppermost surface of the upper tie plate 1 of each fuel assembly 50. Also, the upper core lattice plate 13
Is connected to the upper shroud 15 and
Connect to the top of 16.

By providing the upper core lattice plate 13, the fuel assembly 50
However, even if an excessive lift is applied upward due to an increase in pressure loss between the upper and lower portions, the fuel assembly 50 can be prevented from floating.

Although not shown, a positioning pin provided on a fuel support bracket supporting a lower end of the fuel assembly 50 is inserted into a positioning hole 25 of the lower tie plate 7. This allows
The positioning of the lower tie plate 7 is performed. The fuel support fitting is supported by the control rod guide tube 12.

The cluster type control rod 8 is moved up and down in the control rod guide tube 12 by the control rod driving device 60. Cluster type control rod 8
Is one for three fuel assemblies 50 as shown in FIG.
Body provided. The neutron absorbing rods 10 (FIG. 11) provided on one cluster type control rod 8 are inserted from the lower end of the thimble pipe 3 into each control rod thimble pipe 3 of the three fuel assemblies 50. FIG. 11 shows a state in which the neutron absorbing rod 10 has been inserted into the control rod thimble tube 3. The plurality of neutron absorbing rods 10 inserted into one fuel assembly 50 are attached to the control rod 8 and the arm 26 at the lower end to form one cluster. Three clusters are mounted on one drive shaft 9.
The cluster type control rod 8 is configured as described above. The drive shaft 9 is connected to a control rod drive 60.

The neutron absorbing rod 10 of the cluster type control rod 8 has a neutron absorbing part and a follower part located above the neutron absorbing part. The neutron absorbing portion is filled with a neutron absorbing material (for example, B ₄ C). With the neutron absorbing rods 10 fully inserted, the follower occupies the space inside the control rod thimble tube 3 above the fuel rods 4. The follower section is a control rod thimble tube 3
It has the function of eliminating cooling water inside and is made of a substance with a small neutron absorption cross section. The lengths of the neutron absorber and the follower are equal to the length of the fuel rod 4.

FIG. 12 shows a state in which the control rod 8 is pulled out of the fuel assembly 50. The neutron absorbing part of the control rod 8 is withdrawn from the fuel assembly 50, but the follower part is the control rod thimble tube 3
Located within. For this reason, the water to uranium ratio in the region where the follower portion remains decreases, and the plutonium conversion ratio can be increased.

According to the present embodiment, since the length of the control rod thimble tube 3 is longer than the length of the fuel rod 4, the control rod having a follower portion at the tip end can be easily connected to the fuel assembly 50 below.
Can be inserted inside. For control rods that are inserted into the reactor core only when the reactor is started and shut down and the neutron absorber is completely removed during normal power operation (for example, during rated power operation), the follower section remains in the fuel assembly ,
The plutonium conversion ratio is improved. Further, since the fuel rod end plug support plate 2 supporting the upper end of the fuel rod 4 is attached to the control rod thimble tube 3, it is not necessary to newly provide a member for holding the fuel rod end plug support plate 2. The structure of the fuel assembly 50 is a simple structure. Since the fuel rod 4 is shorter than the control rod thimble tube 3, the pressure loss of the fuel assembly 50 is reduced. Further, since the upper end of the fuel assembly 50 is supported by the upper core lattice plate 13, the pressure loss between the upper and lower portions of the fuel assembly increases, and even if an excessive lift is generated in the entire fuel assembly, mechanical stability is maintained. The structure can be kept. Further, during the normal operation of the nuclear reactor, the neutron absorbing rod 8 for the control rod 8 for power control is used.
10 is inserted into the control rod thimble tube 3, so that the follower portion is located outside the control rod thimble tube 3 even if the follower is located above the fuel rod end plug support plate 2. Does not come into contact with the two-phase flow. Therefore, flow vibration of the follower portion can be prevented, and damage to the neutron absorbing rod 10 can be avoided.

When replacing the fuel assembly 50, the fuel assembly 50 is taken out of the core and moved while one cluster is inserted into the fuel assembly 50. Since the follower is inserted into the control rod thimble tube 3, the follower does not shake during the movement of the fuel assembly 50. Therefore, the cluster can be reused without damage to the neutron absorbing rod 10. This cluster is loaded into the core with the new fuel assembly inserted into the control rod thimble tube (the cluster and the fuel assembly are suspended by the refueling machine). In JP-A-62-273485, it is necessary to connect the lower tie plate and the intermediate tie plate with a connecting fuel rod, but in this embodiment, this connecting fuel rod is not required. This also simplifies the structure of the assembly resistance.

Further, in the present embodiment, since a control element assembly having a plurality of thin plate-like control elements radially from the center of the fuel assembly is inserted into the fuel assembly from the lower part of the core, it occurs in Type 1 of the conventional example. The complexity of the upper core structure and the enlargement of the upper part of the reactor vessel can be avoided. Further, since the core upper structure of the current BWR is used as it is, the same structural stability as that of the current reactor can be secured.

The configuration of the fuel assembly and the core in the above-described embodiment is as follows.
Currently, it can be applied to various high conversion reactors that are being developed as next-generation light water reactors in addition to the Pu multiplication reactor. These furnaces use a dense grid fuel assembly with a dense fuel rod array to increase the conversion ratio during operation. In such a fuel assembly, it is indispensable to take measures to prevent the fuel assembly from rising due to an increase in pressure loss, as in the case of the Pu multiplier.
Further, in various high conversion furnaces, there is a high possibility that a follower is attached to the control rod in order to increase the conversion ratio when the control rod is pulled out. In this case, it is necessary to largely change the fuel assembly structure from the existing one. However, if the fuel assembly and the reactor structure of the above-described embodiment are used, the above problems can be solved even in these various high conversion reactors.

〔The invention's effect〕

According to the present invention, a fuel assembly having a small pressure loss can be obtained with a simple structure. Further, since the follower portion is inserted into the control rod guide tube, damage to the neutron absorption rod can be prevented and the plutonium conversion ratio can be improved.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of one embodiment of a fuel assembly according to the present invention, FIG. 2 is a transverse sectional view of three fuel assemblies in FIG.
The figure is a detailed longitudinal sectional view of the upper part of FIG. 1, FIG. 4 is a detailed longitudinal sectional view of the lower tie plate, FIG. 5 is a sectional view taken along line VV of FIG. 4, and FIG. Side view of rate, seventh
FIG. 8 is a plan view of the upper tie plate, FIG. 8 is a cross-sectional view of the fuel rod end plug support plate of FIG. 1, FIG. 9 is a cross-sectional view of the fuel spacer of FIG. 1, and FIG. FIG. 11 is a longitudinal sectional view of a reactor loaded with the fuel assembly of FIG. 11, FIG. 11 is an explanatory view showing a state in which a cluster type control rod is inserted into the fuel assembly, and FIG. It is explanatory drawing which shows the state which was pulled out. DESCRIPTION OF SYMBOLS 1 ... Upper tie plate, 1a ... Thimble tube mounting part, 2 ... Fuel rod end plug support plate, 3 ... Control rod thimble tube, 4 ... Fuel rod,
5 ... Channel box, 7 ... Lower tie plate, 7a ...
Thimble tube mounting part, 7b: fuel holding part, 8: cluster type control rod, 10: neutron absorption rod, 11: lower core support plate, 12 ...
Control rod guide tube, 13 ... Upper core lattice plate, 19 ... Reactor pressure vessel, 20 ... Hold down spring.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G21C 3/32 G21C 3/30

Claims (6)

(57) [Claims] 1. A plurality of fuel rods, a plurality of hollow control rod guide tubes disposed between the fuel rods and having a longer axial length than the fuel rods, the fuel rods and the control rod guide tubes A lower tie plate that supports a lower end of the control rod guide tube, an upper tie plate to which an upper end of the control rod guide tube is attached, and a lower tie plate provided on the control rod guide tube below the upper tie plate; And a fuel rod upper support member for supporting the upper end of the fuel assembly. 2. The upper tie plate has a hollow hexagonal column shape, and an upper end of the control rod guide tube is attached to a bottom of the upper tie plate, and reaches a corner of the upper tie plate from a bottom surface to an upper surface. The fuel assembly according to claim 1, further comprising a spring. 3. The fuel assembly according to claim 1, wherein the upper tie plate has a flat top surface and a hollow interior, and a pin hole is provided at each hexagonal corner of the top tie plate. 4. The fuel assembly according to claim 1, wherein the lower tie plate has a structure in which an upper end supports a lower end of the fuel rod and a lower end supports the control rod guide tube. 5. A nuclear reactor having a core loaded with the fuel assembly of claim 1 and having a member in contact with an upper tie plate upper surface of the fuel assembly. 6. A nuclear reactor according to claim 5, wherein said member in contact with said upper surface has a plurality of positioning pins which can be inserted into pin holes provided in the upper surface of the upper tie plate of each fuel assembly.