JPH0452913B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a combustion assembly and a core structure of a nuclear reactor, and in particular to a combustion assembly and a core of a nuclear reactor suitable for application to a boiling water reactor. It's about structure.

[Background of the invention]

In a boiling water reactor, as shown in Japanese Patent Publication No. 57-476, a large number of fuel assemblies are arranged in a lattice with gaps formed between them, and four adjacent fuel assemblies are It is configured to insert a cross-shaped control rod between them. A fuel assembly consists of multiple fuel rods filled with a large number of fuel pellets.
It has upper and lower tie plates and a channel box. The upper and lower ends of the fuel rods are held by upper and lower tie plates, respectively.
A channel box is attached to the tie plate and surrounds the outside of the bundle of fuel rods held by the upper and lower tie plates.

Cooling water flows within the channel box from the lower end to the upper end of the fuel assembly. As the cooling water rises in the channel box, it is heated by the fuel rods and becomes steam. Therefore, in the upper part of the fuel assembly, the cooling water flows in a two-phase flow state of liquid phase and steam. In the gaps formed between adjacent fuel assemblies, no boiling takes place and saturated water is present.

This uneven distribution of the two-phase flow part and the saturated water part makes the core non-homogeneous, which incurs some sacrifices in terms of fuel economy and power peaking.

As a solution to the above problems, JP-A-52-
There is a reactor core shown in Publication No. 112096. In this core, control rods are inserted and gaps between combustion assemblies are eliminated, promoting homogenization of the core.

Further, in the fuel assembly used in the boiling water reactor shown in FIG. 2 of Japanese Patent Application Laid-open No. 47-649, control rod guide tubes are arranged between the combustion rods. Each neutron absorption rod constituting the control rod is inserted into this control rod guide tube, as shown in FIG. 1 of Japanese Patent Application Laid-Open No. 47-649.

[Purpose of the invention]

An object of the present invention is to provide a fuel assembly and a nuclear reactor core structure that have a simple core lower structure and can improve thermal neutron utilization.

[Summary of the invention]

A feature of the fuel assembly of the present invention is that a plurality of control rod guide tubes are attached near one side of the lower and upper tie plates.

The core structure of the nuclear reactor of the present invention is characterized by having partition plates extending from the lower end of the core toward the upper end, having a plurality of fuel rods, and having a plurality of control guide tubes near one side. 4 fuel assemblies, 1
The control rod guide tubes of one fuel assembly are arranged in one square of the lattice-like partition plate so as to face the fuel rods of the adjacent fuel advancing assembly, and the control rod guide tubes of four fuel assemblies are inserted into the control rod guide tubes of each of the four fuel assemblies. A control rod having a plurality of neutron absorption rods is provided.

Next, the principle of improving fuel economy according to the present invention will be described. Figure 1 shows the relationship between the infinite neutron multiplication factor and the atomic ratio of water to uranium. When the water-to-uranium ratio is increased, the neutron moderating effect of water becomes effective, and the infinite neutron multiplication factor generally increases. However, when the water-to-uranium ratio exceeds a certain value, the increase in the infinite neutron multiplication factor due to the moderating effect and the decrease due to the neutron absorption effect of water are approximately equal, and the infinite neutron multiplication factor no longer increases. In a conventional reactor core,
Gap water exists between each fuel assembly. Therefore, the water-to-uranium ratio differs greatly between the periphery and the center of the fuel assembly. Points A, B, and C shown in FIG. 1 indicate the neutron infinite multiplication factors of the fuel assembly average, the periphery of the fuel assembly, and the center of the fuel assembly, respectively. In conventional fuel assemblies, the number of fuel rods at the periphery of the fuel assembly facing the gap water and the number of fuel rods at the center of the fuel assembly other than that are almost the same, and the average neutron multiplication factor of the fuel assembly is (Point A) is Point B
and the average value of point C.

From FIG. 1, it can be seen that in order to keep the local power peaking coefficient low and improve fuel economy, the difference in water to uranium ratio between the center and the periphery of the fuel assembly should be reduced. In other words, the combustion rods may be uniformly arranged in the reactor core. When the channel boxes are removed and control rod cluster guide tubes are provided as in the present invention, and the spacing between the fuel rods is expanded to arrange the fuel rods in a square grid of 8 x 8, for example, the control rod cluster guide tubes are The infinite neutron multiplication factors of the facing part and the central part of the fuel assembly are points D and E. Therefore, in this case, the average neutron infinite multiplication factor of the fuel assembly is also at point F, which is higher than the conventional point A, improving neutron utilization efficiency and fuel economy.

[Embodiments of the invention]

A preferred embodiment of the present invention applied to a boiling water reactor will be described with reference to FIGS. 2 and 3. FIG. 2 shows a part of a core cross section of a boiling water reactor. The reactor core is composed of a lattice-shaped partition plate 1 and a large number of fuel assemblies 2. Four fuel assemblies 2A to 2D are installed in one square of the partition plate 1.

The structure of the fuel assembly 2 will be explained based on FIGS. 3 and 4. The fuel assembly 2 includes an upper tie plate 3, a lower tie plate 5, a plurality of fuel rods 6,
It consists of a plurality of spacers 7, a water rod 8, and a plurality of control rod guide tubes 9. Upper tie plate 3
has a handle 4. Both ends of each fuel rod 6 are held by an upper tie plate 3 and a lower tie plate 5. Both ends of the water rod 8 are also held by the upper tie plate 3 and the lower tie plate 5. The shape of the lower tie plate 5 is rectangular as shown in FIG. 4 when viewed from above. The upper tie plate 3 also has a rectangular shape like the lower tie plate 5 when viewed from above.

A plurality of (eight in this embodiment) control rod guide tubes 9
is attached to each tie plate near one side of the upper tie plate 3 and lower tie plate 5, as shown in FIGS. 3 and 4. The arrangement of the fuel rods 6, water rods 8, and control rod guide tubes 9 is as shown in FIG. A plurality of spacers 7 are arranged in the axial direction of the fuel assembly 2 to maintain a predetermined spacing between the fuel rods 6 arranged in a child-like manner.
The spacer 7 also maintains the mutual spacing between the control rod guide tubes 9 at a predetermined value. Spacer 7 is attached to water rod 8.

The fuel assembly 2 does not have a channel box as in the conventional case.

The lattice-shaped partition plate 1 extends from the lower end of the reactor core toward the upper end, and divides the inside of the reactor core into a large number of squares. As shown in FIG. 5, the lower end of the partition plate 1 is attached to a core support plate 10 installed in a reactor pressure vessel (not shown) with bolts (not shown). A hole extending in the axial direction of the core is provided at the intersection point of the partition plate 1, and a neutron detector 11 is installed within the hole. In each square of the partition plate 1, there are four fuel assemblies 2A to 2D.
(The structure is shown in FIG. 3) are arranged. These four fuel assemblies are arranged such that all the control rod guide tubes 9 of one fuel assembly face the fuel rods of the fuel assembly adjacent to that fuel assembly. For example, the control rod guide tube 9 located on one side of the fuel assembly 2A is different from the fuel rod 6 located on one side of the adjacent fuel assembly 2B.
are facing each other. That is, the fuel rods provided in one fuel assembly have a control rod guide tube 9 between them.
The fuel rods are interposed between the fuel rods and are opposed to the fuel rods provided in other fuel assemblies. Therefore, within the squares of the partition plate 1, the control rod guide tubes 9 are arranged in a cross shape as shown in FIG.

The fuel assembly 2 is held by a fuel support fitting 14 installed at the upper end of a control rod guide tube 12 that extends through the core support plate 10, as shown in FIG. That is, the fuel assembly 2 is held by inserting its lower tie plate 5 into an upper opening 15 provided at the upper part of the fuel support fitting 14. The control rod guide tube 12 and fuel support fittings 14 are
One cell is arranged for each square of the partition plate 1. The four upper openings 15 are connected to the fuel support fitting 1.
4 is provided. Each upper opening 15 has a fuel assembly 2 arranged in one square of the partition plate 1.
The lower ends of A to 2D are inserted. The fuel support fitting 14 connects four upper openings 15 and four lower openings 16, and one upper opening 15 and one lower opening 16 to guide the cooling water of one fuel assembly 2. It has four cooling water passages 18. An orifice 17 is provided in the lower opening 16. Four cooling water passages 1 are provided in the center of the fuel support fitting 14.
A cross-shaped gap 19 surrounded by 8 is formed. A partition member 20 is arranged at the upper end of this gap 19 so as to close it. The partition member 20 has a cross shape as shown in FIG.
It has a large number of through holes 22 through which each neutron absorption rod 26 passes. A tapered hole portion 23 is provided at the upper end of the through hole 22 . The partition member 20 is attached to the fuel support fitting 14. The lower end of the control rod guide tube 9 provided in the fuel assembly 2 is inserted into the through hole 22 of the partition member 20, as shown in FIG. The tapered hole 23 has a guide function for inserting the control rod guide tube 9 into the through hole 22 .

The control rod 25 moves vertically within the control rod guide tube 12 , and each neutron absorption rod 26 making up the control rod 25 is inserted into the control rod guide tube 9 through a gap 19 . The detailed structure of this control rod 25 will be explained based on FIG. 7. In this embodiment, the control rod 25 is constructed by mounting 32 neutron absorption rods 26 in a cross shape on a cross-shaped fixed plate 27. Inside each neutron absorption rod 26, neutron absorption material B ₄ C
is filled. The coupling ring 28 is the fixing plate 2
It is provided at the bottom of 7. Coupling 28 is removably coupled to a control rod drive (not shown).

During operation of a boiling water reactor, cooling water flows into the fuel assembly through the opening 13 provided in the control rod guide tube 12, the lower opening 16 of the fuel support fitting 14, the cooling water passage 18, and the upper opening 15. Supplied within 2 days. During this operation, the upper tie plate 3 and lower tie plate 5 of the fuel assemblies 2A to 2D arranged in one square of the partition plate 1 come into contact with each other and also with the partition plate 1 due to thermal expansion. . When the cooling water led to the fuel assemblies 2A to 2D flows out from the lower tie plate 5 and reaches the part where the fuel 6 is arranged, it rises within one square defined by the partition plate 1. . That is,
The cooling water supplied to the fuel assemblies 2A to 2D is mixed with each other in one square of the partition plate 1. The partition plate 1 performs the function of a channel box provided in a conventional fuel assembly.

Output control of the boiling water reactor is performed by moving the control rods 25 up and down. control rod 2
By moving up and down the neutron absorption rod 26
moves inside the control rod guide tube 9. Even when the control rods 25 are completely withdrawn from the core, the upper ends of the neutron absorption rods 26 remain inserted into the lower ends of the control rod guide tubes 9.

In the core of this example, there is no water gap region,
Furthermore, the space for inserting and withdrawing the control rods is limited to the area occupied by the control rod guide tube 13, which alleviates the non-uniformity of the cooling water in the horizontal plane of the reactor core, thereby making the power distribution more uniform. Also, the spacing between each fuel rod can be made larger than in conventional fuel assemblies because there is no water gap region or channel box. When this embodiment is applied to an 8×8 fuel assembly, the center spacing of the fuel rods increases from the conventional 16.3 mm to 17.9 mm. As a result, the ratio of the cross-sectional area of the coolant flow path in the horizontal cross-section of the unit fuel rod grid to the cross-sectional area of uranium dioxide in the fuel rod increases approximately 1.5 times from 1.75 to 2.65. As a result, the water body uranium ratio increases.
Since the average energy of neutrons in the center of the fuel assembly is reduced, the fuel assembly becomes more homogeneous, improving fuel economy. Furthermore, since absorption of neutrons by the channel box is also eliminated, the amount of natural uranium required per unit output can be reduced by 11% according to this embodiment. In addition, in this example, the cross-sectional area of the coolant flow path increases by 50%, so when the coolant flow rate is constant, the flow velocity decreases to 67% of the conventional one, and the pressure loss decreases compared to the conventional one.
It drops to 45%. In the structure shown in FIGS. 1 and 10 of JP-A-52-112096, a gap is formed between adjacent channel boxes surrounding four fuel assemblies, and eventually It has a double-walled channel box structure. In this embodiment, since the partition plate 1 has a single wall, the amount of neutrons absorbed is smaller than that in JP-A-52-112096. Therefore, in this embodiment, thermal neutrons can be used effectively, and the amount of natural uranium required per unit output in this embodiment can be lower than that in JP-A-52-112096.

In this embodiment, there is no gap between the fuel assemblies 2 to which the control rod guide tubes 9 are attached, and adjacent fuel assemblies are connected to each other directly (tie plates contact each other) or through the grid-like partition plate 1. Because the structure is pushed against each other, abnormal vibrations will not occur even if horizontal loads are applied to the reactor due to earthquakes, etc. Further, since the control rod guide tube 9 is fixedly supported on one side of the fuel assembly 2, insertability of the control rod 25 can also be ensured. Since the control rods 25 can be configured in a cross shape as in the past, the structure of the lower part of the reactor core, that is, the fuel support fittings 14
It can be inserted into the reactor core using the gap 19 without changing the structure of the reactor. When the control rod guide tube 9 is provided between the fuel rods 6 as in JP-A-47-649, it becomes troublesome to insert the neutron absorption rod into the control guide tube 9 through the lower tie plate 5 and the fuel support fitting 14. Become. Therefore, it is necessary to change the core substructure, resulting in structural complexity. The core lower structure in this example has an extremely simple structure. Further, since the lattice-shaped partition plate 1 is pressed down by the fuel assembly from both sides, there is no need for the partition plate 1 itself to have seismic strength, and the thickness of the partition plate 1 can be reduced. This also allows the amount of absorbed thermal neutrons to be reduced.

As described above, according to this embodiment, fuel economy can be improved by homogenizing the reactor core, and seismic strength can be improved with fewer reactor internal structures. Also, local output peaking is reduced, so
There is no need to create an enrichment distribution of the fuel rods within the horizontal section of the fuel assembly, which was conventionally done in order to reduce local power peaking. Furthermore, since there is a margin in the cooling water flow path area, it is possible to increase the fuel rod diameter and increase the uranium loading amount, and the continuous operation time can be extended without increasing the burnup. Further, in the present invention, fuel assemblies can be converted one by one as in the conventional method.

To replace the control rods 25, the four fuel assemblies 2A to 2D and the fuel support fittings 14 in one square of the partition plate 1 are removed from the square, and the connection with the control rod drive device is released. It is done by folding.

In the fuel assembly 2, control rod guide tubes 9 are provided all over the fuel assembly 2 and one side, but some of them are replaced with fuel rods 14 as shown in the fuel assembly 2E shown in FIG. The amount of fuel loaded in the tank may be increased. Four such fuel assemblies 2E are loaded into one square of the partition plate 1. In this case, the core becomes more homogenized, the amount of fuel loaded in the core increases, and the period of continuous operation can be extended without increasing the fuel content.

〔Effect of the invention〕

According to the present invention, the lower core structure has a simple structure and the thermal neutron utilization rate can be improved.

[Brief explanation of the drawing]

Fig. 1 is a characteristic diagram showing the relationship between the water to uranium ratio and the infinite neutron multiplication factor, Fig. 2 is a horizontal cross-sectional view of the core of a boiling water absorption reactor that is a preferred embodiment of the present invention, and Fig. 3 The figure is a structural diagram of a fuel assembly, which is a preferred embodiment of the present invention, which is loaded in the core of FIG. 2, FIG. 4 is a cross-sectional view of FIG. 3, and FIG. FIG. 6 is a plan view of the partition member shown in FIG. 5, FIG. 7 is a perspective view of the control rod shown in FIG. 5, and FIG. 8 is a detailed sectional view of the lower part of the cross section. FIG. 2 is a cross-sectional view of a fuel assembly. 1... Partition plate, 2, 2A to 2E... Fuel assembly, 3... Upper tie plate, 5... Lower tie plate, 6... Fuel rod, 9... Control rod guide tube, 1
1...Neutron detector, 14...Fuel support fittings.

Claims (1)

[Scope of Claims] 1. In a fuel assembly consisting of upper and lower tie plates and a plurality of fuel rods having both ends held by each of the tie plates, a plurality of control rod guide tubes are connected to the upper tie plate. and a fuel assembly, characterized in that the fuel assembly is disposed near one side of the lower tie plate, and both ends of each of the control rod guide tubes are attached to the upper and lower tie plates. 2. In a nuclear reactor and core structure in which a plurality of fuel assemblies each consisting of an upper and a lower tie plate and a plurality of fuel rods each having both ends held by the tie plate are arranged, from the lower end of the core to the upper end. A lattice-like partition plate is provided extending toward the top, and four combustion assemblies each having a plurality of control rod guide tubes with both ends attached near one side of the upper and lower tie plates are connected to one of the partition plates. Installed within the square,
The four combustion assemblies are arranged such that a plurality of control rod guide pipes provided in one combustion assembly face the combustion rods of the adjacent combustion assembly, and each of the four combustion assemblies is A nuclear reactor core structure comprising a control rod having a plurality of neutron absorption rods inserted into a control rod guide tube.