JP7495871B2 - Fast reactor and method for operating control rods in fast reactor - Google Patents

Description

The present invention relates to a fast reactor and a control rod operation method for a fast reactor, and more particularly to a fast reactor and a control rod operation method for a fast reactor suitable for realizing load following operation in the fast reactor.

A fast breeder reactor, which is a type of fast reactor, is described in JP-B-6-56426. The fast breeder reactor has a reactor vessel filled with liquid sodium as a coolant, and a core that is placed in the reactor vessel and is loaded with multiple fuel assemblies. The fuel assemblies are arranged in a trumpet tube with a regular hexagonal cross section, and multiple fuel rods that contain nuclear fuel material including plutonium and depleted uranium. An entrance nozzle with an opening through which liquid sodium flows is provided at the lower end of the trumpet tube. The fuel assemblies are supported by the core support plate by inserting the trumpet tube into the core support plate installed in the reactor vessel. A coolant outlet portion that allows liquid sodium to flow out of the fuel assemblies is formed at the upper end of the trumpet tube.

The core of a fast breeder reactor has a core fuel region having an inner core region and an outer core region surrounding the inner core region, a blanket fuel region surrounding the core fuel region, and a shield region surrounding the blanket region. In the case of a standard homogeneous core, the plutonium enrichment of the fuel assemblies loaded in the outer core region is higher than the plutonium enrichment of the fuel assemblies loaded in the inner core region. As a result, the power distribution in the radial direction of the core is flattened.

A rotating plug is rotatably attached to the upper end of the reactor vessel over the core. A plurality of lower control rod guide tubes, each having a regular hexagonal cross section, are disposed between the fuel assemblies in the core, and the lower end of each lower control rod guide tube is supported by the core support plate. A plurality of upper control rod guide tubes, each having a regular hexagonal cross section, are disposed above the core directly above each lower control rod guide tube, and the upper end of each upper control rod guide tube is attached to the rotating plug. A plurality of control rod drive mechanisms are disposed at the upper end of each upper control rod guide tube and are attached to the rotating plug.

In the fast breeder reactor, two independent systems of control rods are used: main reactor shutdown system control rods (adjustment rods) and backup reactor shutdown system control rods (safety rods). The main reactor shutdown system control rods are used to adjust the change in reactivity and power distribution accompanying the combustion of nuclear fuel material. The backup reactor shutdown system control rods are installed as a backup in case the main reactor shutdown system control rods fail. The fast breeder reactor can be shut down in an emergency by either the main reactor shutdown system control rods or the backup reactor shutdown system control rods. Each of the main reactor shutdown system control rods and the backup reactor shutdown system control rods has a plurality of neutron absorbing rods in which boron carbide ( _B4C ) pellets are enclosed in a stainless steel cladding tube, and these neutron absorbing rods are bundled into a cluster and stored in a cylindrical protective tube.

The main reactor shutdown system control rods are arranged in some of the lower control rod guide tubes and are connected to the lower ends of drive extension shafts that are connected to the control rod drive mechanism and extend downward through the upper and lower control rod guide tubes. The backup reactor shutdown system control rods are arranged in the remaining lower control rod guide tubes and are connected to the lower ends of other drive extension shafts that are connected to the control rod drive mechanism and extend downward through the upper and lower control rod guide tubes. The main reactor shutdown system control rods and backup reactor shutdown system control rods are moved axially of the core within the upper and lower control rod guide tubes by the control rod drive mechanism.

Pellets of mixed oxide fuel (MOX fuel) made of a mixture of oxides of Pu and depleted uranium are loaded in the axial center of the fuel rod. Furthermore, within the fuel rod, axial blanket regions loaded with a plurality of uranium dioxide pellets made of depleted uranium are disposed above and below the MOX fuel loading region, respectively. The inner core fuel assembly loaded in the inner core region and the outer core fuel assembly loaded in the outer core region thus have a plurality of fuel rods loaded with a plurality of MOX fuel pellets.

The blanket fuel region surrounding the core fuel region is loaded with blanket fuel assemblies, which have multiple fuel rods filled with multiple pellets made of depleted uranium. The fuel rods in the blanket fuel assemblies do not contain plutonium. Of the neutrons generated by the nuclear fission reaction that occurs in the fuel assemblies loaded in the core fuel region, those that leak from the core fuel region are absorbed by U-238 in each fuel rod of the blanket fuel assemblies loaded in the blanket fuel region. As a result, Pu-239, a fissile nuclide, is newly produced in each fuel rod of the blanket fuel assembly.

In boiling water reactors, control rods (gray nose control rods) are used, in which a region formed from a material (e.g., stainless steel) with a smaller neutron absorption cross section than boron carbide is located at the tip. The region below the tip is filled with boron carbide, which has a larger neutron absorption cross section. By using such control rods, the increase in core power near the tip of the control rod when the control rod is withdrawn can be suppressed, and the power distribution in the axial direction of the core can be flattened.

Japanese Patent Publication No. 6-56426

As mentioned above, by using control rods whose tips are made of a material with a small neutron absorption cross section (e.g., stainless steel), power peaking near the tip of the control rod is reduced when the control rod is withdrawn, and the power distribution in the axial direction of the core is made flatter.

However, unlike BWRs, which can adjust the reactor power by controlling the control rods and adjusting the core flow rate, fast reactors must adjust the reactor power using only the main reactor shutdown system control rods. In load-following operation of such fast reactors, part of the axial direction of the main reactor shutdown system control rods is inserted into the active fuel length of the core. In this case, the axial power distribution of the core becomes distorted, similar to the reactor power shown by the dashed line in Figure 9 of the Hitachi Review mentioned above, which may increase power peaking and reduce the thermal margin in the core.

An object of the present invention is to provide a fast reactor and a control rod operation method for the fast reactor, which can increase the thermal margin of the fast reactor during load following operation.

The present invention achieves the above object by providing a fast reactor comprising a reactor vessel and a rotary plug rotatably installed at an upper end of the reactor vessel and covering the reactor vessel,
a reactor core disposed in the reactor vessel, the reactor core being a main reactor shutdown system control rod having an inner control rod cluster and an annular outer control rod cluster surrounding the inner control rod cluster, the main reactor shutdown system control rod having an inner control rod cluster and an outer control rod cluster, the ...
a control rod moving device for moving the inner control rod cluster of the main reactor shutdown system control rods in the axial direction of the core is provided on the rotating plug;
another control rod movement device for moving the outer control rod cluster of the main reactor shutdown system control rods in the axial direction of the core is provided on the rotating plug;
A core support plate arranged below the core within the reactor vessel is installed on the reactor vessel, a lower end of a first inner control rod guide tube arranged in the core is attached to the core support plate, a lower end of a second outer control rod guide tube arranged in the core and surrounding the first inner control rod guide tube is attached to the core support plate, the inner control rod cluster of the main reactor shutdown system control rod is arranged in the first inner control rod guide tube, and the outer control rod cluster of the main reactor shutdown system control rod is arranged in an annular region formed between the first inner control rod guide tube and the second outer control rod guide tube .

When the outer control rod cluster is fully withdrawn from the core fuel region of the core, and the inner control rod cluster, which has a smaller control rod worth, is fully inserted into the core fuel region, the power distribution in the axial direction of the core fuel region becomes a cosine distribution, and the power peak of the power distribution becomes smaller. Therefore, when this control rod is used, the thermal margin of the fast reactor during load following operation increases.

Furthermore, when the outer control rod cluster is fully withdrawn from the core fuel region and part of the inner control rod cluster is inserted into the core fuel region, a power peak is formed near the lower end of the inner control rod cluster. However, since the control rod worth of the inner control rod cluster is small, the power peak is smaller than the power peak formed near the lower end of a conventional main reactor shutdown system control rod when part of the conventional main reactor shutdown system control rod that does not have the inner control rod cluster and the annular outer control rod cluster is withdrawn from the core fuel region. Therefore, even when part of the inner control rod cluster is inserted into the core fuel region, the thermal margin of the fast reactor during load following operation is increased.

The present invention makes it possible to increase the thermal margin of a fast reactor during load following operation.

1 is a vertical sectional view of a control rod of a fast reactor according to a first embodiment of the present invention, that is, a main reactor shutdown system control rod. FIG. 2 is an explanatory diagram showing the outer control rod cluster shown in FIG. 1 engaged with a control rod retaining unit; 2 is a vertical cross-sectional view of the inner and outer control rod clusters of the control rods shown in FIG. 1 fully inserted into a core fuel region of a fast reactor. FIG. IV-IV cross-sectional view of FIG. 3. FIG. 2 is a vertical cross-sectional view of a backup reactor shutdown system control rod in a fast reactor in which the control rod of the first embodiment is used. FIG. 2 is a cross-sectional view of a backup reactor shutdown system control rod. FIG. 6 is a vertical cross-sectional view of a fast reactor using the control rod shown in FIG. 1 and the backup reactor shutdown system control rod shown in FIG. 5. FIG. 2 is a configuration diagram of a control rod control system that operates the control rod shown in FIG. FIG. 6 is a configuration diagram of a control rod control system for operating the backup reactor shutdown control rod shown in FIG. 5 . FIG. 8 is a cross-sectional view of half the core of the fast reactor shown in FIG. 7. 6 is an explanatory diagram showing the state in which the control rod shown in FIG. 1 and the backup reactor shutdown system control rod shown in FIG. 5 are inserted into the core during load following operation of a fast reactor. FIG. FIG. 1 is an explanatory diagram showing a state in which a conventional main reactor shutdown system control rod is inserted into a core fuel region (active fuel length). This is a cross-sectional view taken along the line XX in FIG. FIG. 13 is a characteristic diagram showing the relationship between the power at each axial position and the axial position of the active fuel length when the main reactor shutdown system control rod shown in FIG. 1 and the conventional main reactor shutdown system control rod shown in FIG. 12 are inserted into the core fuel region. FIG. 7 is a vertical sectional view of a fast reactor using the control rod shown in FIG. 1 and the backup reactor shutdown system control rod shown in FIG. 5 in embodiment 2 which is another preferred embodiment of the present invention. FIG. 16 is an explanatory diagram showing the insertion state of the inner control rod cluster and the outer control rod cluster of the main reactor shutdown system control rods shown in FIG. 15 into the core fuel region during load following operation of a fast reactor. FIG. 17 is an explanatory diagram showing the inner control rod cluster and the outer control rod cluster shown in FIG. 16 in a fully inserted state into the core fuel region. FIG. 16 is an explanatory diagram showing the operation states of the inner control rod cluster and the outer control rod cluster of the backup reactor shutdown system control rods shown in FIG. 15 . FIG. 18 is an explanatory diagram showing a fully inserted state of the inner control rod cluster and the outer control rod cluster of the backup reactor shutdown system control rods shown in FIG. 17 into the core fuel region during load following operation of a fast reactor. FIG. 11 is an explanatory diagram showing an insertion state of a main reactor shutdown system control rod into a core fuel region during load following operation of a fast reactor in a control rod operation method for a fast reactor in a third embodiment which is another preferred embodiment of the present invention. FIG. 21 is an explanatory diagram showing a fully inserted state of each of the inner control rod cluster and the outer control rod cluster in the main reactor shutdown system control rod shown in FIG. 20 into the core fuel region. FIG. 11 is an explanatory diagram showing an insertion state of a backup reactor shutdown system control rod into a core fuel region during load following operation of the fast breeder reactor in the control rod operation method of the fast breeder reactor of the third embodiment. FIG. 23 is an explanatory diagram showing a fully inserted state of the inner control rod cluster and the outer control rod cluster of the backup reactor shutdown system control rods shown in FIG. 22 into the core fuel region.

An embodiment of the present invention is described below.

The control rod of a fast reactor according to embodiment 1, which is a preferred embodiment of the present invention, will be described with reference to Figures 1, 2, and 4.

In the fast breeder reactor 8 (see Figure 7) described below, the control rods are configured in two independent systems: control rod 1 (Figure 1), which is the main reactor shutdown system control rod, and backup reactor shutdown system control rod 23 (Figure 5). Control rod 1, which is the main reactor shutdown system control rod, is used to adjust the change in reactivity and power distribution associated with the burning of fissile material contained in the nuclear fuel material when the fast breeder reactor 8 is in operation. Backup reactor shutdown system control rod 23 is installed as a backup in case control rod 1, which is the main reactor shutdown system control rod, fails. The fast breeder reactor 8 can be shut down in an emergency using only either control rod 1 or backup reactor shutdown system control rod 23.

The control rod 1 of this embodiment will be described in detail with reference to Figs. 1, 2 and 4. The control rod 1 has an inner control rod cluster (first inner control rod cluster) 2 and an outer control rod cluster (first outer control rod cluster) 5. The outer control rod cluster 5 has an annular cross section and surrounds the inner control rod cluster 2 (see Fig. 4). The inner control rod cluster 2 has a plurality of rod-shaped control rod elements 3 arranged in a cylindrical control rod protection tube 4 with both ends sealed (see Figs. 1 and 4). The control rod elements 3 are filled with a plurality of pellets of boron carbide (B ₄ C), which is a neutron absorbing material, in a sealed cladding tube (not shown). The outer control rod cluster 5 has a plurality of rod-shaped control rod elements 6 arranged in an annular control rod protection tube 7 with both ends sealed (see Figs. 1 and 4). The control rod elements 6 are also filled with a plurality of pellets of boron carbide in a sealed cladding tube (not shown). The axial length of each of the control rod elements 3 and 6 is the same as the active fuel length, which is the axial length of the core fuel region 12 described below.

The backup reactor shutdown control rod 23 will be described in detail with reference to Figures 5 and 6. The backup reactor shutdown control rod 23 has an inner control rod cluster (second inner control rod cluster) 36 and an outer control rod cluster (second outer control rod cluster) 55. The outer control rod cluster 55 has an annular cross section and surrounds the inner control rod cluster 36 (see Figure 6). The inner control rod cluster 36 has a plurality of rod-shaped control rod elements 37 arranged in a cylindrical control rod protection tube 38 with both ends sealed (see Figure 6). The control rod element 37 is filled with a plurality of boron carbide pellets in a sealed cladding tube (not shown). The outer control rod cluster 55 has a plurality of rod-shaped control rod elements 56 arranged in an annular control rod protection tube 57 with both ends sealed (see Figure 6). The control rod element 56 is also filled with a plurality of boron carbide pellets in a sealed cladding tube (not shown). The axial length of each of the control rod elements 37 and 56 is the same as the active fuel length, which is the axial length of the core fuel region 12 described below.

The structure of a fast breeder reactor, which is a type of fast reactor to which the control rod 1 is applied, will be described with reference to FIG. 7. The electric output of the fast breeder reactor 8 is 750,000 kW. The fast breeder reactor 8 includes a reactor vessel 9, a rotating plug 10, a core 11, a control rod 1, a backup reactor shutdown system control rod 23, lower inner control rod guide tubes 26 and 58, and lower outer control rod guide tubes 27 and 59. In addition to these components, the fast breeder reactor 8 includes an upper core support plate 50, a winding device 30, a control rod drive mechanism 32, upper inner control rod guide tubes 28 and 60, upper outer control rod guide tubes 29 and 61, a winding device 62, and a control rod drive mechanism 64. The winding devices 30 and 62 are the first and third control rod movement devices, and the control rod drive mechanisms 32 and 64 are the second and fourth control rod movement devices. Liquid sodium 24, which is a liquid metal, is used as a coolant and is filled in the reactor vessel 9. The upper core support plate 50 is disposed within the reactor vessel 9 and attached to the inner surface of the reactor vessel 9. A core support structure 52 is attached to the lower surface of the upper core support plate 50, and a lower core support plate 51 disposed within the core support structure 52 is attached to the inner surface of the core support structure 52. The lower core support plate 51 is located below the upper core support plate 50.

The rotating plug 10 is rotatably attached to the upper end of the reactor vessel 9. The core 11 is disposed in the reactor vessel 9 below the rotating plug 10 and has a core fuel region 12 and a gas plenum region 13 in the axial direction. The gas plenum region 13 is located above the core fuel region 12 (see FIG. 11). A cross-section of the core 11 at the core fuel region 12 is shown in FIG. 10. The core fuel region 12 includes an inner core fuel region 14 and an outer core fuel region 15, with the outer core fuel region 15 surrounding the inner core fuel region 14. Furthermore, in the core 11, a radial blanket region 16 surrounds the outer core fuel region 15, and a reflector region 17 surrounds the radial blanket region 16.

A plurality of inner core fuel assemblies 18 are loaded in the inner core fuel region 14, and a plurality of outer core fuel assemblies 19 are loaded in the outer core fuel region 15. Each of the inner core fuel assemblies 18 and the outer core fuel assemblies 19 uses a ternary alloy (metallic fuel) of U-Pu-10Zr as a nuclear fuel material, and has a plurality of fuel rods (not shown) in which a plurality of fuel pellets made of U-Pu-10Zr are filled in a cladding tube. Each fuel rod has a nuclear fuel filling region filled with the fuel pellets in the cladding tube, and a gas plenum region formed above the nuclear fuel filling region. When the burnup of each of the inner core fuel assemblies 18 and the outer core fuel assemblies 19 is 0 GWd/t, the Pu enrichment in the outer core fuel assemblies 19 is higher than that of the inner core fuel assemblies 18, and the average Pu enrichment of the outer core fuel region 15 is higher than that of the inner core fuel region 14. This flattens the power distribution in the radial direction of the core fuel region 12. In the inner core fuel assembly 18 and the outer core fuel assembly 19, multiple fuel rods are arranged in a cylindrical trumpet tube (not shown) with a regular hexagonal cross section.

A number of blanket fuel assemblies 20 are loaded in the radial blanket region 16. The 0 GWd/t blanket fuel assembly 20 has a number of fuel rods with a number of fuel pellets made of depleted uranium sealed in a cladding tube. The blanket fuel assembly 20 arranges the fuel rods in a cylindrical bell tube (not shown) with a regular hexagonal cross section. A number of reflectors are loaded in the reflector region 17. The reflectors are made of stainless steel with a regular hexagonal cross section.

An entrance nozzle (not shown) is provided at the lower end of each of the trumpet tubes of the inner core fuel assembly 18, the outer core fuel assembly 19, and the blanket fuel assembly 20. The entrance nozzle of each fuel assembly penetrates the upper core support plate 50 and reaches the lower core support plate 51. Each of the inner core fuel assembly 18, the outer core fuel assembly 19, and the blanket fuel assembly 20 is supported by the upper core support plate 50. A high-pressure plenum 41 is formed between the upper core support plate 50 and the lower core support plate 51 in the core support structure 52. A low-pressure plenum 42 is formed below the lower core support plate 51 in the core support structure 52. A lower plenum 40 is formed below the upper core support plate 50 between the bottom of the reactor vessel 9 and the core support structure 52. An opening 25 formed in the side wall of the core support structure 52 connects the lower plenum 40 and the high-pressure plenum 41.

The inlet pipe 9A is attached to the reactor vessel 9. The reactor vessel 9 is filled with liquid sodium 24 as a coolant, and the inlet pipe 9A penetrates the side wall of the reactor vessel 9 above the liquid level of the liquid sodium 24 to reach the inside of the reactor vessel 9. Inside the reactor vessel 9, the inlet pipe 9A descends between the inner surface of the reactor vessel 9 and the core 11, penetrates the upper core support plate 50, and reaches the lower plenum 40. The lower end of the inlet pipe 9A is attached to the side of the core support structure 52, and the inlet pipe 9A communicates with the high-pressure plenum 41 through the above-mentioned opening 25. The outlet pipe 9B attached to the side wall of the reactor vessel 9 is connected to the upper plenum 53 formed in the reactor vessel 9 above the core 11. One end of the primary cooling system pipe (not shown), in which a primary main coolant pump (not shown) and an intermediate heat exchanger (not shown) are installed, is connected to the inlet pipe 9A, and the other end of the primary cooling system pipe is connected to the outlet pipe 9B.

Each of the control rods 1 is disposed between the inner core fuel assemblies 18 in the inner core fuel region 14 and between the outer core fuel assemblies 19 in the outer core fuel region 15 (see FIG. 10). A control rod guide tube 46 (see FIG. 4) having a regular hexagonal cross section for the control rod 1 is disposed between the inner core fuel assemblies 18 in the inner core fuel region 14 and between the outer core fuel assemblies 19 in the outer core fuel region 15. The lower end of the control rod guide tube 46 is attached to the upper core support plate 50, and the upper end of the control rod guide tube 46 is located above the upper end of the core 11. The cylindrical lower outer control rod guide tube 27 is disposed in the control rod guide tube 46, and the cylindrical lower inner control rod guide tube 26 is disposed in the lower outer control rod guide tube 27. The lower ends of the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27 are attached to the upper core support plate 50. The upper ends of the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27 are also located above the upper end of the core 11.

The cylindrical upper outer control rod guide tube 29 is disposed directly above the lower outer control rod guide tube 27, and the upper end of the upper outer control rod guide tube 29 is attached to the rotating plug 10 through the rotating plug 10. A support plate 54 is attached to the upper surface of the rotating plug 10, covering the upper end of the upper outer control rod guide tube 29. The cylindrical upper inner control rod guide tube 28 is disposed directly above the lower inner control rod guide tube 26 and disposed within the upper outer control rod guide tube 29, and the upper end of the upper inner control rod guide tube 28 is attached to the support plate 54. The control rod drive mechanism 32, which is a second control rod moving device, is installed on the support plate 54. The drive extension shaft 33 connected to the control rod drive mechanism 32 extends downward from the rotating plug 10 and is disposed within the upper inner control rod guide tube 28 and the lower inner control rod guide tube 26. The inner control rod cluster 2 of the control rod 1 is disposed within the lower inner control rod guide tube 26 and is connected to the lower end of the drive extension shaft 33.

A winding device 30, which is a first control rod moving device, is installed on the upper surface of the rotating plug 10, and the lower end of a stainless steel rope 31 extending downward from the winding device 30 passes through an annular region formed between the upper outer control rod guide tube 29 and the upper inner control rod guide tube 28 and is attached to a control rod holding unit 44 arranged between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27. The annular outer control rod cluster 5 of the control rod 1 arranged between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27 is held by the control rod holding unit 44. Specifically, the outer control rod cluster 5 is connected to the control rod holding unit 44 and held by the control rod holding unit 44 by inserting a rotating hook 45 provided on the control rod holding unit 44 into the ring of the hanging hardware 22 provided at the upper end of the outer control rod cluster 5. The rotating hook 45 is a connecting means for detachably connecting the outer control rod cluster 5 and the control rod holding unit 44. As described below, the control rod holding unit 44 operates the rotating hook (connection means) 45 by inputting a scram signal, and the outer control rod cluster 5 and the control rod holding unit 44 are separated.

In order to facilitate the up and down movement of the outer control rod cluster 5, at least three hoisting fixtures 22 are attached at equal intervals to the upper end of the outer control rod cluster 5. Three winding devices 30, the same number as the number of hoisting fixtures 22, are installed on the upper surface of the rotating plug 10 directly above each of the three hoisting fixtures 22. The lower ends of stainless steel ropes 31 extending downward from the winding devices 30 are attached to each of the three control rod holding units 44 arranged between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27. The rotating hooks 45 provided on each of the three control rod holding units 44 are inserted into the loops of the three hoisting fixtures 22. The outer control rod cluster 5 is held by the three control rod holding units 44.

Alternatively, the inner control rod cluster 2 may be moved in the axial direction of the core 11 by the winding device 30, and the outer control rod cluster 5 may be moved in the axial direction of the core 11 by the control rod drive mechanism 32, which is the second control rod moving device. In this case, one winding device 30 and three control rod drive mechanisms 32 are installed on a support plate 54 attached to the upper surface of the rotating plug 10. The three control rod drive mechanisms 32 are arranged around the winding device 30. The lower end of a stainless steel rope 31 extending downward from the winding device 30 passes through the upper inner control rod guide tube 28 and the lower inner control rod guide tube 26, and is attached to a control rod holding unit 44 arranged in the lower inner control rod guide tube 26. The inner control rod cluster 2 is connected to and held by the control rod holding unit 44 by inserting a rotating hook 45 provided on the control rod holding unit 44 into a loop of a hanging fitting 22 provided at the upper end of the inner control rod cluster 2 arranged in the lower inner control rod guide tube 26.

The lower ends of the drive extension shafts 33 connected to each of the three control rod drive mechanisms 32 and extending downward are connected to the upper end of the annular outer control rod cluster 5 arranged in the annular region formed between the lower outer control rod guide tube 27 and the lower inner control rod guide tube 26 through the annular region formed between the upper outer control rod guide tube 29 and the upper inner control rod guide tube 28. The positions at which each drive extension shaft 33 of the three control rod drive mechanisms 32 is connected to the upper end of the outer control rod cluster 5 are arranged at equal intervals at the upper end of the annular outer control rod cluster 5. The three control rod drive mechanisms 32 synchronously move the outer control rod cluster 5 in the axial direction of the core 11.

The outer control rod cluster 5 that has been withdrawn from the core fuel region 12 is emergency inserted into the core fuel region 12 by the control rod drive mechanism 32, and the inner control rod cluster 2 that has been withdrawn from the core fuel region 12 is emergency inserted into the core fuel region 12 by disconnecting the rotating hook 45 of the control rod holding unit 44 from the hanging bracket 22 attached to the upper end of the inner control rod cluster 2.

The backup reactor shutdown system control rods 23 are also arranged between the inner core fuel assemblies 18 in the inner core fuel region 14 and between the outer core fuel assemblies 19 in the outer core fuel region 15 (see FIG. 10). Control rod guide tubes 73 (see FIG. 6) with a regular hexagonal cross section for the backup reactor shutdown system control rods 23 are arranged between the inner core fuel assemblies 18 in the inner core fuel region 14 and between the outer core fuel assemblies 19 in the outer core fuel region 15. The lower end of the control rod guide tube 73 is attached to the upper core support plate 50, and the upper end of the control rod guide tube 73 is located above the upper end of the core 11. The cylindrical lower outer control rod guide tube 59 is arranged in the control rod guide tube 73, and the cylindrical lower inner control rod guide tube 58 is arranged in the lower outer control rod guide tube 59. The lower ends of the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59 are attached to the upper core support plate 50. The upper ends of the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59 are also located above the upper end of the core 11.

The cylindrical upper outer control rod guide tube 61 is disposed directly above the lower outer control rod guide tube 59, and the upper end of the upper outer control rod guide tube 61 is attached to the rotating plug 10 through the rotating plug 10. The support plate 54A is attached to the upper surface of the rotating plug 10, covering the upper end of the upper outer control rod guide tube 61. The cylindrical upper inner control rod guide tube 60 is disposed directly above the lower inner control rod guide tube 58 and disposed within the upper outer control rod guide tube 61, and the upper end of the upper inner control rod guide tube 60 is attached to the support plate 54A. The control rod drive mechanism 64, which is the fourth control rod moving device, is installed on the support plate 54A. The drive extension shaft 65 connected to the control rod drive mechanism 64 extends downward from the rotating plug 10 and is disposed within the upper inner control rod guide tube 60 and the lower inner control rod guide tube 58. The inner control rod cluster 36 of the backup reactor shutdown system control rod 23 is disposed within the lower inner control rod guide tube 58 and is connected to the lower end of the drive extension shaft 65.

The lower inner control rod guide tube 26 is disconnected from the upper inner control rod guide tube 28, the lower outer control rod guide tube 27 is disconnected from the upper outer control rod guide tube 29, and further, the lower inner control rod guide tube 58 is disconnected from the upper inner control rod guide tube 60, and the lower outer control rod guide tube 59 is disconnected from the upper outer control rod guide tube 61, so that the rotation of the rotating plug 10 is not hindered by those control rod guide tubes.

A winding device 62, which is a third control rod moving device, is installed on the upper surface of the rotating plug 10, and the lower end of a stainless steel rope 63 extending downward from the winding device 62 is attached to a control rod holding unit 66 arranged between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59 through an annular region formed between the upper inner control rod guide tube 60 and the upper outer control rod guide tube 61. The annular outer control rod cluster 55 of the backup reactor shutdown system control rod 23 arranged between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59 is held by the control rod holding unit 66. Specifically, the outer control rod cluster 55 is held by the control rod holding unit 66 by inserting a rotating hook 67 provided on the control rod holding unit 66 into the ring of a hanging bracket 75 provided at the upper end of the outer control rod cluster 55.

In order to facilitate the up and down movement of the outer control rod cluster 55, at least three hoisting fixtures 75 are attached at equal intervals to the upper end of the outer control rod cluster 55. Three winding devices 62, the same number as the number of hoisting fixtures 75, are installed on the upper surface of the rotating plug 10 directly above each of the three hoisting fixtures 75. The lower ends of the ropes 63 extending downward from the winding devices 62 are attached to each of the three control rod holding units 66 arranged between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59. The rotating hooks 67 provided on each of the three control rod holding units 66 are inserted into the loops of the three hoisting fixtures 75. The outer control rod cluster 55 is held by the three control rod holding units 66.

The fast breeder reactor 8 has a control rod system including an outer control rod cluster, a first control rod moving device for moving the outer control rod cluster in the axial direction of the core 11, an inner control rod cluster, and a second control rod moving device for moving the inner control rod cluster in the axial direction of the core 11. The first control rod moving device and the second control rod moving device have different mechanisms for moving the control rod cluster in the axial direction of the core 11. Specifically, the control rod system in the fast breeder reactor 8 includes an outer control rod cluster 5 of the control rod 1 (main reactor shutdown system control rod), a first control rod moving device for moving the outer control rod cluster 5 in the axial direction of the core 11, an inner control rod cluster 2 of the control rod 1, a second control rod moving device for moving the inner control rod cluster 2 in the axial direction of the core 11, an outer control rod cluster 55 of the backup reactor shutdown system control rod 23, a third control rod moving device for moving the outer control rod cluster 55 in the axial direction of the core 11, an inner control rod cluster 36 of the backup reactor shutdown system control rod 23, and a fourth control rod moving device for moving the inner control rod cluster 36 in the axial direction of the core 11. The first and third control rod moving devices for moving the outer control rod cluster 5 of the control rod 1 and the outer control rod cluster 55 of the backup reactor shutdown system control rod 23 in the axial direction, respectively, are winding devices. The second and fourth control rod moving devices, which move the inner control rod cluster 2 of the control rod 1 and the inner control rod cluster 36 of the backup reactor shutdown system control rod 23 in the axial direction, respectively, are control rod drive mechanisms.

Furthermore, in the backup reactor shutdown system control rod 23, the inner control rod cluster 36 may be moved in the axial direction of the core 11 by the winding device 62, and the outer control rod cluster 55 may be moved in the axial direction of the core 11 by the control rod drive mechanism 64. In this case, one winding device 62 and three control rod drive mechanisms 64 are each installed on a support plate 54 attached to the upper surface of the rotating plug 10. The three control rod drive mechanisms 64 are arranged around the winding device 62. The lower end of a rope 63 extending downward from the winding device 62 passes through the upper inner control rod guide tube 60 and the lower inner control rod guide tube 58 and is attached to a control rod holding unit 66 arranged in the lower inner control rod guide tube 58. The inner control rod cluster 36 is connected to the control rod holding unit 66 and held by the control rod holding unit 66 by inserting the rotating hook 67 on the control rod holding unit 66 into the ring of the hanging fitting 22 on the upper end of the inner control rod cluster 36 arranged in the lower inner control rod guide tube 58.

The lower ends of the drive extension shafts 65 connected to each of the three control rod drive mechanisms 64 and extending downward are connected to the upper end of the annular outer control rod cluster 5 arranged in the annular region formed between the lower outer control rod guide tube 59 and the lower inner control rod guide tube 58 through the annular region formed between the upper outer control rod guide tube 61 and the upper inner control rod guide tube 60. The positions at which each drive extension shaft 65 of the three control rod drive mechanisms 64 is connected to the upper end of the outer control rod cluster 55 are arranged at equal intervals on the upper end of the annular outer control rod cluster 55. The three control rod drive mechanisms 64 synchronously move the outer control rod cluster 55 in the axial direction of the core 11.

The outer control rod cluster 55 that has been withdrawn from the core fuel region 12 is emergency inserted back into the core fuel region 12 by the control rod drive mechanism 64, and the inner control rod cluster 36 that has been withdrawn from the core fuel region 12 is emergency inserted back into the core fuel region 12 by disconnecting the rotating hook 67 of the control rod holding unit 66 from the lifting bracket 22 attached to the upper end of the inner control rod cluster 36.

In FIG. 7, the lower inner control rod guide tubes 26 and 58, the lower outer control rod guide tubes 27 and 59, the upper inner control rod guide tubes 28 and 60, the upper outer control rod guide tubes 29 and 61, the inner control rod cluster 2 of the control rod 1, and the outer control rod cluster 5 are shown enlarged to facilitate understanding of the mutual relationship of their configurations. Furthermore, the lower inner control rod guide tube 58, the lower outer control rod guide tube 59, the upper inner control rod guide tube 60, the upper outer control rod guide tube 61, the inner control rod cluster 36 of the backup reactor shutdown system control rod 23, and the outer control rod cluster 55 are also shown enlarged to facilitate understanding of the mutual relationship of their configurations.

The control unit 43 for the control rod 1 is connected to each winding device 30, the control rod drive mechanism 32, and the control rod holding unit 44, as shown in FIG. 8. The control unit 43A for the backup reactor shutdown system control rod 23 is connected to each winding device 62, the control rod drive mechanism 64, and the control rod holding unit 66, as shown in FIG. 9. It is also possible to not provide the control unit 43A, and to connect the control unit 43 to the winding device 62, the control rod drive mechanism 64, and the control rod holding unit 66 as well.

During operation of the fast breeder reactor 8, the main coolant pump provided in the primary cooling system piping described above is driven, and liquid sodium 24 present in the upper plenum 53 in the reactor vessel 9 is discharged from the outlet piping 9B to the primary cooling system piping. This liquid sodium 24 is pressurized by the main coolant pump and supplied to the intermediate heat exchanger. The liquid sodium 24 flows on the shell side of the intermediate heat exchanger and exchanges heat with the secondary liquid sodium flowing in the heat transfer tube of the intermediate heat exchanger. The liquid sodium 24 discharged from the intermediate heat exchanger to the primary cooling system piping is supplied from the inlet piping 9A to the lower plenum 40 in the reactor vessel 9. The liquid sodium 24 in the lower plenum 40 is led to the high-pressure plenum 41 through the opening 25.

The liquid sodium 24 in the high pressure plenum 41 reaches the insides of the inner core fuel assembly 18, the outer core fuel assembly 19 and the blanket fuel assembly 20 through the coolant inlets formed in the side walls of the entrance nozzles of the inner core fuel assembly 18, the outer core fuel assembly 19 and the blanket fuel assembly 20. The liquid sodium 24 rising in the inner core fuel assembly 18 and the outer core fuel assembly 19 is heated by the heat generated by the nuclear fission of the fissile material (e.g. Pu-239) contained in the nuclear fuel material filled in the fuel rods in each fuel assembly, and the temperature increases. The liquid sodium 24 with the increased temperature flows out from the upper ends of the inner core fuel assembly 18 and the outer core fuel assembly 19 into the upper plenum 53. This high-temperature liquid sodium 24 is supplied to the intermediate heat exchanger as described above. The liquid sodium 24 supplied into the blanket fuel assembly 20 also rises within the blanket fuel assembly 20 and flows out from the top end of the blanket fuel assembly 20 into the upper plenum 53.

The liquid sodium 24 in the high pressure plenum 41 is guided into the lower inner control rod guide tube 26 provided for the control rod 1 and into the annular region formed between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27, and rises in the lower inner control rod guide tube 26 and the annular region. The liquid sodium 24 rising in the lower inner control rod guide tube 26 cools the inner control rod cluster 2 present in the lower inner control rod guide tube 26 and is discharged from the upper end of the lower inner control rod guide tube 26 to the upper plenum 53. The liquid sodium 24 rising in the annular region between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27 cools the outer control rod cluster 5 present in the annular region and is discharged from the annular region to the upper plenum 53.

Furthermore, the liquid sodium 24 in the high pressure plenum 41 is guided into the lower inner control rod guide tube 58 provided for the backup reactor shutdown system control rods 23, and into the annular region formed between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59, and rises in the lower inner control rod guide tube 58 and the annular region. The liquid sodium 24 rising in the lower inner control rod guide tube 58 cools the inner control rod cluster 36 present in the lower inner control rod guide tube 58, and is discharged from the upper end of the lower inner control rod guide tube 58 to the upper plenum 53. The liquid sodium 24 rising in the annular region between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59 cools the outer control rod cluster 55 present in the annular region, and is discharged from the annular region to the upper plenum 53.

In a certain operating cycle of the fast breeder reactor 8 equipped with the control rod 1 of this embodiment, load following operation is performed. The method of operating the control rod in the load following operation of this fast breeder reactor 8 is described below.

When the fast breeder reactor 8 is shut down, the inner control rod cluster 2 and the outer control rod cluster 5 of all the control rods 1 are fully inserted into the core fuel region 12 (the part of the active fuel length) as shown in FIG. 3. The outer control rod cluster 5 is fully inserted into the core fuel region 12 by reversing the rotation drum around which the rope 31 of the winding device 30 is wound, unwinding the rope 31 from the rotation drum, and lowering the control rod holding unit 44 holding the outer control rod cluster 5. In FIG. 3, the outer control rod cluster 5 is detached from the control rod holding unit 44 and fully inserted into the core fuel region 12, but during normal shutdown of the fast breeder reactor 8, as described above, the outer control rod cluster 5 fully inserted into the core fuel region 12 is held by the control rod holding unit 44. The inner control rod cluster 2 is fully inserted into the core fuel region 12 by the control rod drive mechanism 32.

After the operation of the fast breeder reactor 8 is started in a certain operation cycle, an outer control rod cluster extraction signal is output from the control unit 43 to each winding device 30 based on an outer control rod cluster extraction command, which is a control command output from a control panel (not shown) in the control room, by an operator in a control room (not shown) located outside the reactor containment vessel (not shown) in which the fast breeder reactor 8 is installed. This extraction signal causes each rotating drum of the three winding devices 30 to rotate forward. Each rotating drum is rotated forward by synchronously rotating a motor provided for each rotating drum. The rotation speed of the motor is reduced to a predetermined speed by a reduction gear, so that each rotating drum is rotated forward at that predetermined speed. By rotating each rotating drum of the three winding devices 30 forward, each of the three ropes 31 is wound separately around each rotating drum, and the outer control rod cluster 5 of the control rod 1 is gradually raised within the annular region between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27.

During that certain operating cycle, all control rods 1 operate normally, so all backup reactor shutdown system control rods 23 are not operated and are fully withdrawn from the core fuel region 12 (see Figure 11).

As the outer control rod cluster 5 rises, the fissile plutonium contained in the nuclear fuel material in the inner core fuel assemblies 18 and the outer core fuel assemblies 19 loaded in the core 11 of the fast breeder reactor 8 undergoes nuclear fission, and the fast breeder reactor 8 eventually reaches criticality. Furthermore, the outer control rod cluster 5 is withdrawn from the core fuel region 12, promoting the nuclear fission of the fissile material in the inner core fuel assemblies 18 and the outer core fuel assemblies 19 loaded in the core 11, and the temperature of the liquid sodium 24 rising within those fuel assemblies increases. In conjunction with this, the reactor power of the fast breeder reactor 8 also increases.

As described above, the outer control rod cluster 5 is withdrawn from the core fuel region 12 by rotating the rotating drum of each winding device 30 in the forward direction and winding the rope 31 around the rotating drum. When the outer control rod cluster 5 is completely withdrawn from the core fuel region 12, at least a part of the control rod holding unit 44 is positioned between the upper end of the lower inner control rod guide tube 26 and the upper end of the lower outer control rod guide tube 27 (see FIG. 1).

When the outer control rod clusters 5 of a predetermined number of control rods 1 are fully withdrawn from the core fuel area 12 while the inner control rod clusters 2 of all control rods 1 remain fully inserted in the core fuel area 12, the forward rotation of the rotating drums of those winding devices 30 is stopped. In a state in which the outer control rod clusters 5 of a predetermined number of control rods 1 are fully withdrawn from the core fuel area 12 (see FIG. 1), the reactor power of the fast breeder reactor 8 is raised to the first target reactor power in its load following operation (a certain reactor power less than the rated power (100% reactor power) during rated operation of the fast breeder reactor 8). When those outer control rod clusters 5 are fully withdrawn from the core fuel area 12 and the reactor power is raised to the first target reactor power, the rotation of the rotating drums of each winding device 30 is stopped as follows. That is, a drive stop signal is output from the control unit 43 to each winding device 30 based on an outer control rod cluster withdrawal stop command output from the control panel to the control unit 43. In each winding device 30 to which a drive stop signal has been input, the rotation of the rotating drum is stopped. After the reactor power reaches the first target reactor power, the load following operation of the fast breeder reactor 8 at the first target reactor power continues until the end of the aforementioned certain operating cycle.

If the reactor power falls below the first target reactor power during a certain operation cycle, the control unit 43 outputs an inner control rod cluster withdrawal signal to the control rod drive mechanism 32 based on the inner control rod cluster withdrawal command output from the control panel. This inner control rod cluster withdrawal signal activates the control rod drive mechanism 32. The inner control rod cluster 2 of the control rod 1 is gradually withdrawn from the core fuel region 12 by the operation of the control rod drive mechanism 32, increasing the reactor power of the fast breeder reactor 8. When the reactor power reaches the first target reactor power, the control unit 43 outputs an inner control rod cluster withdrawal stop signal to the control rod drive mechanism 32 based on the inner control rod cluster withdrawal stop command output from the control panel, and the lifting of the inner control rod cluster 2 by the control rod drive mechanism 32 is stopped. By repeating such lifting and stopping of the inner control rod cluster 2, the operation of the fast breeder reactor 8 is continued with the reactor power maintained at the first target reactor power throughout the operation cycle.

The excess reactivity of the fast breeder reactor 8 changes when the burnup reactivity fluctuates due to the burnup of fissile material, such as fissile plutonium, contained in the nuclear fuel material of each of the inner and outer core fuel assemblies 18 and 19, or when the temperature of the liquid sodium 24, which is the coolant of the fast breeder reactor 8, changes and the Doppler reactivity and expansion reactivity of the nuclear fuel material are introduced. When such a change in the excess reactivity of the fast breeder reactor 8 occurs, the operation of the fast breeder reactor 8 can be continued in response to the change in excess reactivity by adjusting the number of control rods 1 inserted into the inner control rod cluster 2 of the core fuel region 12.

When the operation of the fast breeder reactor 8 in its operation cycle is completed, the operation of the fast breeder reactor 8 is stopped. A command to stop the operation of the fast breeder reactor 8 is output from the control panel to the control unit 43. The control unit 43, which has input the command to stop the operation, outputs a stop operation signal to all the winding devices 30 and all the control rod drive mechanisms 32. The three winding devices 30, which have input this stop operation signal, rotate their respective rotating drums in the reverse direction to unwind the ropes 31 wound around each rotating drum, and lower the outer control rod cluster 5 within the annular region between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27. The control rod drive mechanism 32, which has input the stop operation signal, lowers the inner control rod cluster 2 within the lower inner control rod guide tube 26. When the outer control rod cluster 5 and the inner control rod cluster 2 of all the control rods 1 are fully inserted into the core fuel region 12, that is, when each of the outer control rod cluster 5 and the inner control rod cluster 2 reaches the position shown in FIG. 3, the operation of the fast breeder reactor 8 is stopped and the cold shutdown state is entered.

The target reactor power in load-following operation may change within one operation cycle, or may change between different operation cycles. In addition to load-following operation at the first target reactor power, load-following operation may be performed at a second target reactor power higher than the first target reactor power and lower than the rated power in an operation cycle. In this case, the outer control rod clusters 5 of the control rods 1, the number of which is greater than the number of control rods 1 in the case of load-following operation at the first target reactor power, are fully withdrawn from the core fuel area 12. It is also possible to perform load-following operation at a third target reactor power, which is lower than the first target reactor power. In load-following operation at the third target reactor power, the outer control rod clusters 5 of the control rods 1, the number of which is less than the number of control rods 1 in the case of load-following operation at the first target reactor power, are fully withdrawn from the core fuel area 12.

An abnormality occurs in the fast breeder reactor 8, and the fast breeder reactor 8 must be shut down. At this time, a scram signal is output from the control panel in the control room to the control unit 43. The control unit 43, which has input the scram signal, outputs the scram signal to all control rod holding units 44 and all control rod drive mechanisms 32. The control rod holding unit 44, which has input the scram signal, rotates the rotating hook 45 in the reverse direction to remove the rotating hook 45 from the hanging bracket 22 provided on the outer control rod cluster 5. As a result, the outer control rod cluster 5 is separated from the control rod holding unit 44, and the outer control rod cluster 5 falls toward the upper core support plate 50. That is, the outer control rod cluster 5 is rapidly inserted into the core fuel region 12. In addition, the control rod drive mechanism 32, which has input the scram signal, releases the connection between the inner control rod cluster 2 and the drive extension shaft 33 of the control rod drive mechanism 32. As a result, the inner control rod cluster 2 also falls toward the upper core support plate 50 and is rapidly inserted into the core fuel region 12. In this way, the fast breeder reactor 8 is shut down. The rapidly inserted inner control rod cluster 2 and outer control rod cluster 5 are present in the core fuel region 12 (see FIG. 3).

If an abnormality occurs in the fast breeder reactor 8 with all inner control rod clusters 2 fully inserted into the core fuel region 12, the control unit 43, which receives a scram signal output from the control panel, outputs a scram signal only to all control rod holding units 44. At this time, all outer control rod clusters 5 are detached from each control rod holding unit 44 and rapidly inserted into the core fuel region 12.

Since the backup reactor shutdown control rods 23 used in this embodiment have the inner control rod cluster 36 and the outer control rod cluster 55, the range of load-following operation of the fast breeder reactor 8 can be expanded. That is, by completely withdrawing the outer control rod clusters 5 of all the control rods 1 from the core fuel area 12, and further completely withdrawing the outer control rod clusters 55 of a predetermined number (plurality) of backup reactor shutdown control rods 23, which are a part of all the backup reactor shutdown control rods 23 provided in the fast breeder reactor 8, from the core fuel area 12, the fast breeder reactor 8 can be operated at a fourth target reactor power lower than the reactor power reached when the outer control rod clusters 5 are completely withdrawn from the core fuel area 12. The load-following operation at the fourth target reactor power in the fast breeder reactor 8 using both all the control rods 1 and the predetermined number of backup reactor shutdown control rods 23 will be described below.

In this load following operation, as described above, the outer control rod clusters 5 of all the control rods 1 are operated by the winding devices 30, and the inner control rod clusters 2 of all the control rods 1 are operated by the control rod drive mechanisms 32. Therefore, here, the operations of the outer control rod cluster 55 and the inner control rod cluster 36 of the backup reactor shutdown system control rods 23 will be described.

After the operation of the fast breeder reactor 8 in a certain operation cycle is started, the three winding devices 30 provided for each outer control rod cluster 5 are operated to completely withdraw all the outer control rod clusters 5 of all the control rods 1 from the core fuel area 12. In conjunction with the complete withdrawal of all the outer control rod clusters 5 from the core fuel area 12, the rotating drums of the three winding devices 62 are rotated in the forward direction to wind the rope 63 around each rotating drum, and the outer control rod clusters 55 of a predetermined number of backup reactor shutdown system control rods 23 are gradually raised within the annular area between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59. Based on the outer control rod cluster withdrawal command, which is a control command output from the control panel in the control room by the operator, an outer control rod cluster withdrawal signal is output from the control unit 43A to each winding device 62. This withdrawal signal causes the rotating drums of each winding device 62 to rotate in the forward direction, and the outer control rod clusters 55 of a predetermined number of backup reactor shutdown system control rods 23 are raised.

As the outer control rod cluster 55 rises, the fissile plutonium contained in the nuclear fuel material in the inner core fuel assembly 18 (or outer core fuel assembly 19) near the backup reactor shutdown system control rod 23 to be operated, which is loaded in the core 11 of the fast breeder reactor 8, undergoes nuclear fission, and together with the rise of the outer control rod clusters 5 of each control rod 1, the fast breeder reactor 8 eventually reaches criticality. Furthermore, a predetermined number of outer control rod clusters 55 are withdrawn from the core fuel region 12, promoting the nuclear fission of the fissile material in the inner core fuel assembly 18 (or outer core fuel assembly 19), and the temperature of the liquid sodium 24 rising within those fuel assemblies increases. As the outer control rod cluster 55 and each outer control rod cluster 5 are withdrawn, the reactor power of the fast breeder reactor 8 also increases.

When the outer control rod cluster 55 is fully withdrawn from the core fuel region 12, at least a portion of the control rod holding unit 66 is positioned between the upper end of the lower inner control rod guide tube 58 and the upper end of the lower outer control rod guide tube 59 (see FIG. 5).

When the outer control rod clusters 55 of a predetermined number of backup reactor shutdown control rods 23 are fully withdrawn from the core fuel area 12, the forward rotation of the rotating drum of the winding device 62 is stopped. With the outer control rod clusters 5 of all control rods 1 and the outer control rod clusters 55 of a predetermined number of backup reactor shutdown control rods 23 fully withdrawn from the core fuel area 12, the reactor power of the fast breeder reactor 8 is increased to the aforementioned fourth target reactor power in the load following operation. When the reactor power has increased to the fourth target reactor power, the rotation of the rotating drum of the winding device 62 is stopped as follows. That is, based on the outer control rod cluster withdrawal stop command output from the control panel to the control unit 43A, a drive stop signal is output from the control unit 43A to the winding device 62. In the winding device 62 to which the drive stop signal has been input, the rotation of the rotating drum is stopped. After the reactor power reaches the fourth target reactor power, the load-following operation of the fast breeder reactor 8 at the fourth target reactor power continues until the end of the aforementioned certain operating cycle.

In the middle of a certain operation cycle, if the reactor power falls below the fourth target reactor power, an inner control rod cluster withdrawal signal is output from the control unit 43 to the control rod drive mechanism 32 and from the control unit 43A to the control rod drive mechanism 64 based on the inner control rod cluster withdrawal command output from the control panel. This inner control rod cluster withdrawal signal operates the control rod drive mechanisms 32 and 64. All the inner control rod clusters 2 are gradually withdrawn from the core fuel region 12 by the operation of the control rod drive mechanism 32, and a predetermined number of inner control rod clusters 36 are gradually withdrawn from the core fuel region 12 by the operation of the control rod drive mechanism 32, thereby increasing the reactor power of the fast breeder reactor 8. When the reactor power becomes the fourth target reactor power, an inner control rod cluster withdrawal stop signal is output from the control unit 43 to the control rod drive mechanism 32 based on the inner control rod cluster withdrawal stop command output from the control panel, and an inner control rod cluster withdrawal stop signal is output from the control unit 43A to the control rod drive mechanism 64. As a result, the withdrawal of the inner control rod cluster 2 by the control rod drive mechanism 32 and the withdrawal of the inner control rod cluster 36 by the control rod drive mechanism 64 are stopped. By repeating such withdrawal and stopping of the withdrawal of the inner control rod clusters 2 and 36, the load following operation of the fast breeder reactor 8 is continued with the reactor power being maintained at the fourth target reactor power throughout that certain operation cycle.

When the operation of the fast breeder reactor 8 in a certain operation cycle is completed, the operation of the fast breeder reactor 8 is stopped. A command to stop the operation of the fast breeder reactor 8 is output from the control panel to the control units 43 and 43A. The control unit 43 that inputs the operation stop command outputs an operation stop signal to the winding devices 30 corresponding to all the control rods 1 and to all the control rod drive mechanisms 32. The control unit 43A that inputs the operation stop command outputs an operation stop signal to the winding devices 62 and the control rod drive mechanisms 64 corresponding to a predetermined number of backup reactor shutdown system control rods 23.

The operation of the outer control rod cluster 5 and the inner control rod cluster 2 by the three winding devices 30 and the control rod drive mechanism 32 to which the operation stop signal has been input is as described above, so a description thereof will be omitted here.

When the inner control rod clusters 2 and outer control rod clusters 5 of all control rods 1 are fully inserted into the core fuel region 12, the operation of the fast breeder reactor 8 is stopped.

When the reactor power of the fast breeder reactor 8 is controlled using multiple normal control rods 1 and one backup reactor shutdown system control rod 23, if an abnormality occurs in the fast breeder reactor 8, a scram signal is output from the control panel in the control room to the control unit 43, and the scram signal is also output to the control unit 43A. The control unit 43 outputs the scram signal to the control rod holding unit 44 of the normal control rod 1. As described above, the outer control rod cluster 5 is detached from the control rod holding unit 44 and rapidly inserted into the core fuel area 12. The control unit 43A outputs the scram signal to the control rod holding unit 66 of the backup reactor shutdown system control rod 23. As a result, the outer control rod cluster 55 is detached from the control rod holding unit 66 and rapidly inserted into the core fuel area 12, and the fast breeder reactor 8 is brought to an emergency shutdown.

The configuration of a conventional main reactor shutdown system control rod 68 will be described with reference to Figures 12 and 13. The main reactor shutdown system control rod 68 has multiple control rod elements 69 arranged in a cylindrical control rod protection tube 70 with the upper and lower ends sealed. The control rod elements 69 are filled with multiple pellets of boron carbide in a sealed cladding tube (not shown). The main reactor shutdown system control rod 68 is arranged in a control rod guide tube 72 arranged between fuel assemblies in the core fuel region 12. The main reactor shutdown system control rod 68 is attached to a drive extension shaft 71 connected to a control rod drive mechanism (not shown).

The number of control rod elements 69 included in the main reactor shutdown system control rod 68 is 21 (see FIG. 4), which is the same as the total number of control rod elements 3 included in the inner control rod cluster 2 and the control rod elements 6 included in the outer control rod cluster 5 of the control rod 1 used in this embodiment. Therefore, of the inner control rod cluster 2 and the outer control rod cluster 5, the reactivity worth of the control rod 1 when the inner control rod cluster 2 of the control rod 1 is fully inserted into the core fuel region 12 is smaller than the reactivity worth of the main reactor shutdown system control rod 68 when the main reactor shutdown system control rod 68 is fully inserted into the core fuel region 12.

In this embodiment, since the inner control rod cluster 2 having a small control rod worth is inserted into the core fuel region 12, the peak of the power distribution in the axial direction of the core fuel region 12 into which the inner control rod cluster 2 is inserted is lower than the peak of the power distribution in the axial direction of the core fuel region 12 into which the conventional main reactor shutdown system control rod 68 is inserted. In FIG. 14, the power distribution in the axial direction of the core fuel region 12 in the state in which the outer control rod cluster 5 is fully withdrawn from the core fuel region 12 and the inner control rod cluster 2 is fully inserted into the core fuel region 12 in the control rod 1 of this embodiment is shown by a solid line. The power distribution in the axial direction of the inner control rod cluster 2 when fully inserted into the core fuel region 12 is a cosine distribution as shown by the solid line. In contrast, the power distribution in the axial direction of the core fuel region 12 in the case in which a part of the conventional main reactor shutdown system control rod 68 is withdrawn from the core fuel region 12 forms a large power peak near the lower end of the conventional main reactor shutdown system control rod, as shown by the dashed line in FIG. 14. When the outer control rod cluster 5 is fully withdrawn from the core fuel region 12 and the inner control rod cluster 2 is fully inserted into the core fuel region 12, the peak power in the axial power distribution of the core fuel region 12 is smaller than the peak power in the axial power distribution of the core fuel region 12 when a part of the conventional main reactor shutdown system control rod 68 is withdrawn from the core fuel region 12. Therefore, when the control rod 1 of this embodiment is used, the thermal margin of the fast breeder reactor 8 during load following operation is increased.

Next, the axial power distribution of the core fuel region 12 in a state where the outer control rod cluster 5 is fully withdrawn from the core fuel region 12 and a part of the inner control rod cluster 2 is inserted into the core fuel region 12 will be described. The axial power distribution of the core fuel region 12 in a state where a part of the inner control rod cluster 2 is inserted into the core fuel region 12 is not a cosine distribution as shown by the solid line in FIG. 14, but has a power peak near the lower end of the inner control rod cluster 2. For this reason, when the outer control rod cluster 5 is fully withdrawn from the core fuel region 12 and a part of the inner control rod cluster 2 is inserted into the core fuel region 12, the power peak formed near the lower end of the inner control rod cluster 2 is larger than the power peak of the cosine distribution in the axial direction of the core fuel region 12 in a state where the outer control rod cluster 5 is fully withdrawn from the core fuel region 12 and the inner control rod cluster 2 is fully inserted into the core fuel region 12. However, since the control rod worth of the inner control rod cluster 2 is smaller than that of the conventional main reactor shutdown system control rod 68, the power peak formed near the lower end of the inner control rod cluster 2 is smaller than the power peak formed near the lower end of the conventional main reactor shutdown system control rod 68. In this way, even if a portion of the inner control rod cluster 2 is inserted into the core fuel region 12, the thermal margin of the fast breeder reactor 8 during load following operation is increased.

In this embodiment, the backup reactor shutdown system control rod 23 also has an inner control rod cluster and an outer control rod cluster, just like the control rod 1. Therefore, by using the backup reactor shutdown system control rod 23 in place of the failed control rod 1, the fast breeder reactor 8 can be operated in a load following manner, just like when only the control rod 1 is used.

In load-following operation of the fast breeder reactor 8 using the backup reactor shutdown system control rods 23, when the outer control rod cluster 55 of the backup reactor shutdown system control rods 23 is fully withdrawn from the core fuel region 12 and the inner control rod cluster 36 is fully inserted into the core fuel region 12, the power distribution in the axial direction of the core fuel region 12 becomes a cosine distribution, as in the case where only the control rod 1 is used, and the power peak of the power distribution becomes smaller than the power peak of the axial power distribution of the core fuel region 12 when a part of the conventional main reactor shutdown system control rod 68 is withdrawn from the core fuel region 12. Therefore, when the backup reactor shutdown system control rods 23 are used, the thermal margin of the fast breeder reactor 8 during load-following operation increases.

Furthermore, in a state where the outer control rod cluster 55 of the backup reactor shutdown system control rod 23 is fully withdrawn from the core fuel region 12 and a portion of the inner control rod cluster 36 is fully inserted into the core fuel region 12, the axial power distribution of the core fuel region 12 has a large power peak near the lower end of the inner control rod cluster 36. However, since the control rod worth of the inner control rod cluster 36 is smaller than that of the conventional main reactor shutdown system control rod 68, the power peak formed near the lower end of the inner control rod cluster 36 is smaller than the power peak formed near the lower end of the conventional main reactor shutdown system control rod 68. In this way, even when a portion of the inner control rod cluster 36 is inserted into the core fuel region 12, the thermal margin of the fast breeder reactor 8 during load following operation is increased.

In the fast breeder reactor 8, the outer control rod cluster 5 of the control rod 1 is moved in the axial direction of the core 11 by the winding device 30, and the outer control rod cluster 55 of the backup reactor shutdown system control rod 23 is moved in the axial direction of the core 11 by the winding device 62. Therefore, the outer control rod clusters 5 and 55 can be moved in the axial direction of the core 11 by the winding device, which has a simpler structure than the control rod drive mechanism.

In the fast breeder reactor 8, a lower inner control rod guide tube 26 and a lower outer control rod guide tube 27 surrounding the lower inner control rod guide tube 26 are arranged in the core 11, and the inner control rod cluster 2 of the control rod 1 is moved in the axial direction of the core 11 within the lower inner control rod guide tube 26, and the outer control rod cluster 5 is moved in the axial direction of the core 11 between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27. This makes it possible to avoid interference between the inner control rod cluster 2 and the outer control rod cluster 5, which move in the axial direction of the core 11, and allows the inner control rod cluster 2 and the outer control rod cluster 5 to move smoothly in the axial direction of the core 11.

Furthermore, in the fast breeder reactor 8, a lower inner control rod guide tube 58 and a lower outer control rod guide tube 59 surrounding the lower inner control rod guide tube 58 are arranged in the core 11. Therefore, the inner control rod cluster 36 of the backup reactor shutdown system control rod 23 is moved in the axial direction of the core 11 within the lower inner control rod guide tube 58, and the outer control rod cluster 55 is moved in the axial direction of the core 11 between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59. Therefore, it is possible to prevent the inner control rod cluster 36 and the outer control rod cluster 55, which move in the axial direction of the core 11, from interfering with each other, and the inner control rod cluster 36 and the outer control rod cluster 55 can each move smoothly in the axial direction of the core 11.

As the control rods 1 to be operated during load following operation, it is desirable to use control rods 1 located closer to the center of the core 11, for example, control rods 1 located in the inner core fuel region 14 rather than in the outer core fuel region 15, and further, in the inner core fuel region 14, control rods 1 located as close to the center of the inner core fuel region 14 as possible. The control rod worth of the control rods 1 is higher the closer the control rods 1 are located to the center of the core 11. Therefore, by using control rods 1 located closer to the center of the core 11 during load following operation, the fluctuation range of the reactor power during load following operation can be increased.

Even if it becomes necessary to use the backup reactor shutdown control rods 23 during load following operation, the range of fluctuation in reactor power during load following operation can be increased by using the backup reactor shutdown control rods 23 located near the center of the core 11.

The outer control rod cluster 5 of the control rod 1 is detachably connected via a rotating hook 45 to a control rod holding unit 44 attached to a rope 31 wound around a rotating drum of the winding device 30. When an abnormality occurs in the fast breeder reactor 8, the rotating hook 45 is activated by the control rod holding unit 44 that has input a scram signal, and the outer control rod cluster 5 can be detached from the control rod holding unit 44. Therefore, even the outer control rod cluster 5 that is moved in the axial direction of the core 11 by the winding device 30 can be easily and quickly inserted into the core fuel region 12 in the event of an abnormality in the fast breeder reactor 8.

In addition, the outer control rod cluster 55 of the backup reactor shutdown system control rod 23 is detachably connected to the control rod holding unit 66 attached to the rope 63 wound around the rotating drum of the winding device 62 via the rotating hook 45. Therefore, when an abnormality occurs in the fast breeder reactor 8, the rotating hook 45 is activated by the control rod holding unit 66 that inputs a scram signal, and the outer control rod cluster 55 of the backup reactor shutdown system control rod 23 used to control the reactor output in place of the failed control rod 1 can be detached from the control rod holding unit 66. Therefore, even the outer control rod cluster 55 that is moved in the axial direction of the core 11 by the winding device 62 can be easily and quickly inserted into the core fuel region 12 in the event of an abnormality in the fast breeder reactor 8.

When the fast breeder reactor 8 has backup reactor shutdown system control rods 23 including the inner control rod clusters 36 and the outer control rod clusters 55, and the outer control rod clusters 5 of all control rods 1 and the outer control rod clusters 55 of a predetermined number of backup reactor shutdown system control rods 23 are fully withdrawn from the core fuel area 12, the inner control rod clusters 2 of all control rods 1 and the inner control rod clusters 36 of a predetermined number of backup reactor shutdown system control rods 23 are withdrawn from a fully inserted state into the core fuel area 12, and load-following operation can be performed in a certain operating cycle while maintaining the fourth target reactor power. In the fast breeder reactor 8 of this embodiment, the range of load-following operation can be expanded by using all control rods 1 and a predetermined number of backup reactor shutdown system control rods 23 in combination.

Since the inner control rod clusters 2 of all control rods 1 (or the outer control rod clusters 5 of all control rods 1) are used, the radial power distribution in the core fuel region 12 is flatter than when only a portion of the control rods 1 are inserted, improving the thermal margin.

The configuration of a fast breeder reactor in embodiment 2, which is another preferred embodiment of the present invention, will be described with reference to Figures 15 to 19. The fast breeder reactor 8A of this embodiment has a configuration in which the winding device 30, which is the first control rod moving device that moves the outer control rod cluster 5 of the control rod 1 in the axial direction of the core 11, in the fast breeder reactor 8 described in embodiment 1, is replaced with a control rod drive mechanism 32A, and the winding device 62, which is the third control rod moving device that moves the outer control rod cluster 55 of the backup reactor shutdown system control rod 23 in the axial direction of the core 11, is replaced with a control rod drive mechanism 64A. The control rod drive mechanisms 32A and 64A are the first and third control rod moving devices. Three control rod drive mechanisms 32A are installed on the upper surface of a support plate 54 provided on the upper surface of the rotating plug 10, and are arranged around the control rod drive mechanism 32, which is the second control rod moving device . The upper end of the outer control rod cluster 5 existing in the annular region between the lower inner control rod guide tube 26 and the lower outer control rod guide tube 27 is connected to the lower end of each of the drive extension shafts 33A of the three control rod drive mechanisms 32A. The three control rod drive mechanisms 64A are installed on the upper surface of a support plate 54A provided on the upper surface of the rotating plug 10, and are arranged around the control rod drive mechanism 64. The upper end of the outer control rod cluster 55 existing in the annular region between the lower inner control rod guide tube 58 and the lower outer control rod guide tube 59 is connected to the lower end of each of the drive extension shafts 65A of the three control rod drive mechanisms 64A.

The fast breeder reactor 8A has a control rod system including an outer control rod cluster, a first control rod moving device for moving the outer control rod cluster in the axial direction of the core 11, an inner control rod cluster, and a second control rod moving device for moving the inner control rod cluster in the axial direction of the core 11. The control rod system in the fast breeder reactor 8A specifically includes an inner control rod cluster 2 of the control rod 1 (main reactor shutdown system control rod), a second control rod moving device for moving the inner control rod cluster 2 in the axial direction of the core 11, an outer control rod cluster 5 of the control rod 1, a first control rod moving device for moving the outer control rod cluster 5 in the axial direction of the core 11, an inner control rod cluster 36 of the backup reactor shutdown system control rod 23, a fourth control rod moving device for moving the inner control rod cluster 36 in the axial direction of the core 11, an outer control rod cluster 55 of the backup reactor shutdown system control rod 23, and a third control rod moving device for moving the outer control rod cluster 55 in the axial direction of the core 11. The first to fourth control rod moving devices, which move the inner control rod cluster 2 and outer control rod cluster 5 of the control rod 1 and the inner control rod cluster 36 and outer control rod cluster 55 of the backup reactor shutdown system control rod 23 in the axial direction, respectively, are control rod drive mechanisms.

The fast breeder reactor 8A has a control rod system including an outer control rod cluster, a first control rod moving device for moving the outer control rod cluster in the axial direction of the core 11, an inner control rod cluster, and a second control rod moving device for moving the inner control rod cluster in the axial direction of the core 11. The control rod system in the fast breeder reactor 8A specifically includes an outer control rod cluster 5 of a control rod 1 (main reactor shutdown system control rod), a first control rod moving device for moving the outer control rod cluster 5 in the axial direction of the core 11, an inner control rod cluster 2 of the control rod 1, a second control rod moving device for moving the inner control rod cluster 2 in the axial direction of the core 11, an outer control rod cluster 55 of a backup reactor shutdown system control rod 23, a third control rod moving device for moving the outer control rod cluster 55 in the axial direction of the core 11, an inner control rod cluster 36 of a backup reactor shutdown system control rod 23, and a fourth control rod moving device for moving the inner control rod cluster 36 in the axial direction of the core 11. The first to fourth control rod moving devices included in the control rod system are control rod drive mechanisms.

In this embodiment, in addition to the inner control rod cluster 2 and the inner control rod cluster 36, the outer control rod cluster 5 is moved in the axial direction of the core 11 by the control rod drive mechanism 32A, and the outer control rod cluster 55 is moved in the axial direction of the core 11 by the control rod drive mechanism 64A.

In a certain operation cycle of the fast breeder reactor 8A, load following operation is performed at the first target reactor power described above. When the fast breeder reactor 8A is started in that certain operation cycle, an outer control rod cluster withdrawal command output from a control panel in the control room is input to the control unit 43, and an outer control rod cluster withdrawal signal is output from the control unit 43 to three control rod drive mechanisms 32A provided for each outer control rod cluster 5 of a predetermined number of control rods 1. As a result, each of the outer control rod clusters 5 of the predetermined number of control rods 1 is withdrawn from the core fuel region 12 by the three control rod drive mechanisms 32A, and the reactor power of the fast breeder reactor 8 is increased to the first target reactor power. When the reactor power reaches the first target reactor power, a drive stop signal is output from the control unit 43 to each of the three control rod drive mechanisms 32A based on the outer control rod cluster withdrawal stop command output to the control unit 43. As a result, the axial movement of the outer control rod clusters 5 of the predetermined number of control rods 1 is stopped. Load-following operation of the fast breeder reactor 8A at the first target reactor power continues until the end of the aforementioned certain operating cycle, with the inner control rod cluster 2 being withdrawn as necessary, as in Example 1.

When the load following operation in a certain operation cycle is completed, the inner control rod cluster 2 and the outer control rod cluster 5 are fully inserted into the core fuel region 12 (see Figure 17), and the operation of the fast breeder reactor 8A is stopped.

In this embodiment, as in the first embodiment, the fast breeder reactor 8A can be operated under load by using all the control rods 1 and a predetermined number of backup reactor shutdown control rods 23 in combination. In this load following operation, there exists a state in which the outer control rod cluster 55 of the backup reactor shutdown control rods 23 is fully withdrawn from the core fuel region 12, and the inner control rod cluster 36 of the backup reactor shutdown control rods 23 is fully inserted into the core fuel region 12 (see FIG. 18).

When an abnormal event occurs in the fast breeder reactor 8A and the fast breeder reactor 8A is scrammed, the inner control rod cluster 36 and the outer control rod cluster 55 of the backup reactor shutdown system control rods 23 are fully inserted into the core fuel region 12 (Figure 19).

This embodiment can obtain the effects obtained in the first embodiment, excluding the effects obtained by installing the winding device 30 and the control rod holding unit 44, and the effects obtained by installing the winding device 62 and the control rod holding unit 66. Furthermore, in this embodiment, since the outer control rod cluster 5 is moved in the axial direction of the core 11 by the control rod drive mechanism 32A, it is not necessary to change the number of inserted control rods 1 during load following operation due to differences in the combustion state of the nuclear fuel material in the fuel assemblies in the core 11 and differences in the temperature of the coolant (liquid sodium 24), as in the first embodiment, and the operation of the outer control rod cluster 5 of the control rods 1 is simplified. In addition, power peaking in the radial direction of the core fuel region 12 can also be suppressed.

A method for operating a control rod in a fast breeder reactor in embodiment 3, which is another preferred embodiment of the present invention, will be described with reference to Figures 15 and 20 to 23. The fast breeder reactor to which the control rod operating method in this embodiment is applied is the fast breeder reactor 8A described in embodiment 2, and has a control rod 1 including an inner control rod cluster 2 and an outer control rod cluster 5, and a backup reactor shutdown system control rod 23 including an inner control rod cluster 36 and an outer control rod cluster 55.

In the control rod operation method of the fast breeder reactor of this embodiment, when the fast breeder reactor 8A is in load following operation, there exists a state in which the inner control rod cluster 2 of the control rod 1 is fully withdrawn from the core fuel region 12 and the outer control rod cluster 5 of the control rod 1 is fully inserted into the core fuel region 12. The control rod operation method of the fast breeder reactor of this embodiment, in which such a state exists, differs from the control rod operation methods of the fast breeder reactors of each of the embodiments 1 and 2, in which there exists a state in which the outer control rod cluster 5 of the control rod 1 is fully withdrawn from the core fuel region 12 and the inner control rod cluster 2 of the control rod 1 is fully inserted into the core fuel region 12 during load following operation.

When load-following operation in a certain operating cycle is started, the outer control rod clusters of all control rods 1 are left fully inserted into the core fuel region 12, while the inner control rod clusters 2 of a predetermined number of control rods 1 are operated by the corresponding control rod drive mechanisms 32 to be fully withdrawn from the core fuel region 12 (see FIG. 20). As a result, the reactor power of the fast breeder reactor 8A increases to the target reactor power during load-following operation. Thereafter, the outer control rod clusters 5 of the predetermined number of control rods 1 are withdrawn from the core fuel region 12 by the control rod drive mechanisms 32A as necessary, while the load-following operation in which the reactor power is maintained at the target reactor power is continued until the end of the aforementioned certain operating cycle.

In this embodiment, as in the first embodiment, all the control rods 1 and a predetermined number of backup reactor shutdown control rods 23 are used in combination to perform load following operation of the fast breeder reactor 8A in a certain operation cycle. In the predetermined number of backup reactor shutdown control rods 23 used in combination, the inner control rod clusters 36 are fully withdrawn from the core fuel area 12 by the control rod drive mechanism 64 as in the control rods 1 (see FIG. 22), and then, when the reactor power of the fast breeder reactor 8A falls below the target reactor power during load following operation, the outer control rod clusters 55 of the predetermined number of backup reactor shutdown control rods 23 are withdrawn from the core fuel area 12 by the control rod drive mechanism 64A, and the reactor power is increased to the target reactor power. In the above certain operation cycle, the outer control rod clusters 55 are withdrawn from the core fuel area 12 as necessary, while the load following operation of the fast breeder reactor 8A at the target reactor power can be continued until the end of the certain operation cycle.

In this embodiment, as in the first embodiment, the load-following operation of the fast breeder reactor 8A can be performed by using all the control rods 1 and a predetermined number of backup reactor shutdown system control rods 23 in combination. In this load-following operation, there exists a state in which the inner control rod cluster 36 of the backup reactor shutdown system control rods 23 is fully withdrawn from the core fuel region 12, and the outer control rod cluster 55 of the backup reactor shutdown system control rods 23 is fully inserted into the core fuel region 12 (see FIG. 22).

When an abnormal event occurs in the fast breeder reactor 8A and the fast breeder reactor 8A is scrammed, the inner control rod cluster 36 and the outer control rod cluster 55 of the backup reactor shutdown system control rods 23 are fully inserted into the core fuel region 12 (Figure 23).

When the inner control rod cluster 2 is fully withdrawn from the core fuel region 12, the control rod worth of the outer control rod cluster 5 withdrawn from the core fuel region 12 is greater than the control rod worth of the inner control rod cluster 2 withdrawn from the core fuel region 12 in FIG. 16.

This embodiment can obtain the effects obtained in embodiment 1, excluding the effects obtained by installing the winding device 30 and the control rod holding unit 44, and the effects obtained by installing the winding device 62 and the control rod holding unit 66. Furthermore, since the inner control rod cluster 2 and the outer control rod cluster 5 have different reactivity worths as described above, the range (type) of reactor power fluctuations can be doubled. Therefore, by moving the inner control rod cluster 2 and the outer control rod cluster 5, which have different control rod worths, independently in the axial direction of the core 11, frequent load following operation can be performed.

In this embodiment, when load-following operation is performed, the outer control rod cluster 5 is withdrawn instead of the inner control rod cluster 2 being withdrawn from the core 11 as in embodiments 1 and 2. By withdrawing the outer control rod cluster 5, the reactor power during load-following operation can be lowered.

Each of the first to third embodiments describes the case where, during load-following operation, the inner control rod cluster 2 is withdrawn when the outer control rod cluster 5 of the control rod 1 is fully withdrawn, and the outer control rod cluster 5 is withdrawn when the inner control rod cluster 2 of the control rod 1 is fully withdrawn. However, even when performing burnup compensation of reactivity during normal operation (e.g., rated operation) rather than during load-following operation of the fast breeder reactor 8 (or fast breeder reactor 8A), it is also possible to withdraw the inner control rod cluster 2 when the outer control rod cluster 5 of the control rod 1 is fully withdrawn, or to withdraw the outer control rod cluster 5 when the inner control rod cluster 2 of the control rod 1 is fully withdrawn. In particular, if more control rods 1 are used in the core fuel region 12 when performing burnup compensation of reactivity, the power distribution in the radial direction of the core fuel region 12 can be flattened, and the thermal margin can be increased.

In Examples 1 to 3, the inner core fuel assembly 18 and the outer core fuel assembly 19 loaded into the core 11 each use a ternary alloy U-Pu-10Zr as the nuclear fuel material. Instead of U-Pu-10Zr, MOX fuel may be used as the nuclear fuel material for each of the inner core fuel assembly 18 and the outer core fuel assembly 19.

In addition, in Examples 1 to 3, liquid sodium is used as the coolant for the fast breeder reactor, but liquid metals other than sodium, such as lead and lead bismuth, may be used instead of liquid sodium.

1...control rod (main reactor shutdown system control rod), 2, 36...inner control rod cluster, 5, 55...outer control rod cluster, 8, 8A...fast breeder reactor, 9...reactor vessel, 10...rotating plug, 11...core, 12...core fuel area, 14...inner core fuel area, 15...outer core fuel area, 18...inner core fuel assembly, 19...outer core fuel assembly, 23...backup reactor shutdown system control rod, 24...liquid sodium, 26, 58...lower inner control rod guide tube, 27, 59...lower outer control rod guide tube, 30, 62...winding device, 31, 63...rope, 32, 32A, 64, 64A...control rod drive mechanism, 33, 33A, 65, 65A...drive extension shaft, 44, 66...control rod holding unit, 45...rotating hook, 50...upper core support plate.

Claims (12)

a reactor vessel; and a rotary plug rotatably installed at an upper end of the reactor vessel and covering the reactor vessel,
a reactor core disposed in the reactor vessel, the reactor core being a main reactor shutdown system control rod having an inner control rod cluster and an annular outer control rod cluster surrounding the inner control rod cluster, the main reactor shutdown system control rod having an inner control rod cluster and an outer control rod cluster, the ...
a control rod moving device for moving the inner control rod cluster of the main reactor shutdown system control rods in the axial direction of the core is provided on the rotating plug;
another control rod movement device for moving the outer control rod cluster of the main reactor shutdown system control rods in the axial direction of the core is provided on the rotating plug;
a core support plate disposed below the core within the reactor vessel is installed on the reactor vessel, a lower end of a first inner control rod guide tube disposed in the core is attached to the core support plate, a lower end of a second outer control rod guide tube disposed in the core and surrounding the first inner control rod guide tube is attached to the core support plate, the inner control rod cluster of the main reactor shutdown system control rod is disposed in the first inner control rod guide tube, and the outer control rod cluster of the main reactor shutdown system control rod is disposed in an annular region formed between the first inner control rod guide tube and the second outer control rod guide tube. In addition to the main reactor shutdown control rods, backup reactor shutdown control rods are used.
The backup reactor shutdown system control rod, like the main reactor shutdown system control rod, has an inner control rod cluster and an annular second outer control rod cluster surrounding the inner control rod cluster,
each of the inner control rod cluster and the second outer control rod cluster of the backup reactor shutdown system control rods is disposed in a reactor core disposed in the reactor vessel;
a control rod moving device for moving the inner control rod cluster of the backup reactor shutdown system control rod in the axial direction of the core is provided on the rotating plug;
2. The fast reactor according to claim 1, further comprising another control rod moving device disposed in the rotating plug for moving the second outer control rod cluster of the backup reactor shutdown system control rods in the axial direction of the core. The fast reactor according to claim 2, wherein the lower end of the second inner control rod guide tube arranged in the core is attached to the core support plate, the lower end of the second outer control rod guide tube arranged in the core and surrounding the second inner control rod guide tube is attached to the core support plate, the inner control rod cluster of the backup reactor shutdown system control rod is arranged in the second inner control rod guide tube, and the second outer control rod cluster of the backup reactor shutdown system control rod is arranged in an annular region formed between the second inner control rod guide tube and the second outer control rod guide tube. The fast reactor according to any one of claims 1 to 3, wherein the other control rod moving device that moves the outer control rod cluster has a different moving mechanism from the control rod moving device that moves the inner control rod cluster. the other control rod moving device for moving the outer control rod cluster is a winding device, the outer control rod cluster is suspended by a rope that is wound around the winding device,
5. The fast reactor of claim 4, wherein the control rod movement device for moving the inner control rod cluster is a control rod drive mechanism. The fast reactor of claim 5, wherein the outer control rod cluster is detachably connected to a control rod holding unit attached to the rope and is suspended from the rope via the control rod holding unit. The fast reactor according to any one of claims 1 to 3, wherein the control rod moving device that moves the inner control rod cluster and the other control rod moving device that moves the outer control rod cluster are each a control rod drive mechanism. A method for operating a control rod of a fast reactor having an inner control rod cluster and an outer control rod cluster of a main reactor shutdown system control rod, the control rod having an inner control rod cluster and an annular outer control rod cluster surrounding the inner control rod cluster, the control rod being disposed in a core fuel region of a reactor core in a reactor vessel, the method comprising:
either the inner control rod cluster or the outer control rod cluster is withdrawn from the fuel core region, and among the inner control rod cluster and the outer control rod cluster, the control rod clusters other than the control rod cluster withdrawn from the fuel core region remain fully inserted in the fuel core region;
A method for operating a control rod of a fast reactor, comprising the steps of: withdrawing either the inner control rod cluster or the outer control rod cluster from the fuel core region, thereby increasing a reactor power of the fast reactor to a target reactor power . 9. The method for operating a control rod of a fast reactor according to claim 8, wherein when a reactor power of the fast reactor falls below the target reactor power after either the inner control rod cluster or the outer control rod cluster has been completely withdrawn from the fuel core region, one of the inner control rod cluster and the outer control rod cluster other than the control rod cluster completely withdrawn from the fuel core region is withdrawn from the fuel core region from the fuel core region, thereby maintaining the reactor power of the fast reactor at the target reactor power . The fast reactor has a backup reactor shutdown system control rod including an inner control rod cluster and an annular second outer control rod cluster surrounding the inner control rod cluster, similar to the main reactor shutdown system control rod;
9. The control rod operation method for a fast reactor according to claim 8, wherein both the inner control rod cluster and the second outer control rod cluster of the backup reactor shutdown system control rods are withdrawn from the core fuel region, and among the inner control rod cluster and the second outer control rod cluster, control rod clusters other than the control rod cluster withdrawn from the core fuel region remain fully inserted in the core fuel region. The fast reactor includes a rotary plug that is rotatably installed at an upper end of the reactor vessel and covers the reactor vessel, and a winding device that is installed on the rotary plug,
Either the inner control rod cluster or the outer control rod cluster to be withdrawn from the core fuel region is moved in the axial direction of the core fuel region by a winding device;
10. The method for operating a control rod of a fast reactor according to claim 8 or 9, wherein either the inner control rod cluster or the outer control rod cluster to be withdrawn from the fuel core region and detachably connected to a control rod holding unit attached to a rope wound around the winding device falls in the fuel core region when detached from the control rod holding unit by a scram signal. The fast reactor includes a rotating plug that is rotatably installed at an upper end of the reactor vessel and covers the reactor vessel, and a control rod drive mechanism that is installed on the rotating plug,
10. The method for operating a control rod in a fast reactor according to claim 8 or 9 , wherein either the inner control rod cluster or the outer control rod cluster to be withdrawn from the fuel core region is moved in the axial direction of the fuel core region by the control rod drive mechanism.

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