Fast reactor and neutron detection unit

Abstract

To provide a fast reactor and a neutron detection unit, each of which even when an apparatus for measuring neutron flux is damaged, enables the apparatus to be easily inspected, replaced and repaired, and can excellently measure the neutron flux.SOLUTION: A fast reactor S100 comprises: a reactor core 20 housing nuclear fuel; a main vessel 10 having an open top and housing a reactor core 20 together with liquid sodium being cooling material; a roof slab structure 11 disposed on a top of the main vessel 10 and closing the main vessel 10; and a neutron detection unit 40 measuring neutron flux from the reactor core 20. The neutron detection unit 40 includes: a hollow tubular member 41 with lower end side located in the main container 10 and upper end side disposed in such a way that the hollow tubular member 41 penetrates the roof slab structure 11; and a neutron detector 42 which is arranged at a position lower than a liquid level of the liquid sodium and higher than the reactor core 20 inside the tubular member 41 and measures the neutron flux.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a fast reactor and a neutron detection unit. [Background technology]

[0002] Conventionally, in order to operate a nuclear reactor safely, neutrons generated from the core have been measured. For example, Patent Document 1 discloses a structure in which a hollow cylinder filled with a gas such as argon, which has a low neutron shielding ability, is arranged inside the main vessel in a tank-type fast reactor. A neutron detector is arranged outside the main vessel, and measures the neutron flux of neutrons that pass through the hollow cylinder and reach the neutron detector. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 4-248497 Summary of the Invention [Problem to be solved by the invention]

[0004] In the configuration of Patent Document 1, the hollow cylinder is disposed in a reactor and is immersed in a coolant. Since the coolant is at a high temperature, it is difficult to inspect, replace, or repair the hollow cylinder in a configuration in which the hollow cylinder is immersed in the coolant, and if the hollow cylinder is damaged, the coolant will flow in, making it difficult to accurately measure the neutron flux.

[0005] Therefore, the present invention has been made in consideration of these points, and aims to provide a fast reactor and a neutron detection unit that can easily inspect, replace, or repair the equipment for measuring neutron flux even if the equipment is damaged, and that can measure neutron flux well. [Means for solving the problem]

[0006] One form of a fast reactor of the present invention comprises a core that contains nuclear fuel, a main vessel that is open at the top and contains the core together with liquid sodium as a coolant, a roof slab structure that is located on top of the main vessel and closes the main vessel, and a neutron detection unit that measures neutron flux from the core, wherein the neutron detection unit has a hollow tubular member whose lower end is located within the main vessel and whose upper end is arranged to penetrate the roof slab structure, and a neutron detector that is located inside the tubular member below the liquid sodium level and above the core to measure the neutron flux.

[0007] The neutron detection unit may include a moderator disposed within the tubular member to slow down neutrons traveling towards the neutron detector.

[0008] The neutron detection unit may have a shielding material disposed within the tubular member that attenuates gamma rays from the liquid sodium activated by neutrons from the reactor core, and the shielding material may be formed of bismuth oxide.

[0009] The neutron detection unit may include a moderator disposed within the tubular member for slowing down neutrons moving toward the neutron detector, and a shielding material disposed within the tubular member for attenuating gamma rays from the liquid sodium activated by neutrons from the core, wherein the moderator is disposed below the neutron detector, and the shielding material surrounds the neutron detector at a position above the moderator.

[0010] A shielding plug for blocking heat and radiation from within the main vessel may be removably provided at an upper end of the tubular member.

[0011] The neutron detection unit has a guide means inside the tubular member that determines the position of the neutron detector relative to the tubular member, and the guide means may be a roller member provided on a side of the neutron detector or a side of a member that houses the neutron detector and in contact with the inner surface of the tubular member.

[0012] The reactor may further include a core upper structure provided in a region above the core and forming a storage space in which measuring instruments are placed, the core upper structure having a cylindrical body surrounding the storage space and extending in the height direction of the main vessel and a support plate provided below the cylindrical body, the tubular member being disposed within the cylindrical body and a portion of the tubular member being fixed to the support plate.

[0013] One form of neutron detection unit of the present invention is a neutron detection unit used in a fast reactor that includes a core that contains nuclear fuel, a main vessel that is open at the top and contains the core together with liquid sodium as a coolant, and a roof slab structure that is located on top of the main vessel and closes the main vessel, and the unit has a hollow tubular member whose lower end is located within the main vessel and whose upper end is arranged to penetrate the roof slab structure, and a neutron detector that is located inside the tubular member below the liquid sodium level and above the core, and measures neutron flux from the core. Effect of the Invention

[0014] According to the present invention, it is possible to provide a fast reactor and a neutron detection unit that can easily inspect, replace, or repair the equipment for measuring neutron flux even if the equipment is damaged, and that can measure neutron flux well. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of a fast reactor. [Diagram 2] FIG. 1 is a schematic diagram showing an example of a fast reactor viewed from above. [Diagram 3] FIG. 2 is a diagram showing a schematic example of the arrangement of a neutron detector, a shielding material, and a moderator. [Figure 4] FIG. 13 is a diagram showing a schematic diagram of a neutron detection unit according to a modified example of the present invention. [Diagram 5] 2 is a cross-sectional view of the tubular member cut in a direction perpendicular to the longitudinal direction of the tubular member. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A fast reactor according to one embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing the configuration of a fast reactor. FIG. 2 is a schematic diagram showing an example of a fast reactor viewed from above. Arrows in FIG. 1 indicate the flow of liquid sodium, which is a coolant. In the following, terms such as "upper" and "lower" are used according to the orientation of objects depicted in the drawings, but these terms are not intended to limit the present invention. The "vertical direction" corresponds to the height direction of the main vessel.

[0017] The fast reactor S100 is a tank-type fast reactor that uses, for example, uranium or plutonium as fuel to control and sustain a nuclear fission chain reaction to extract energy. The fast reactor S100 includes, as its main components, a main vessel 10, a roof slab structure 11, a core 20, a core upper structure 30, a neutron detection unit 40, an intermediate heat exchanger 50, a circulation pump 60, and a direct core cooling system heat exchanger 70 (see FIG. 2).

[0018] One of the features of the fast reactor S100 of this embodiment is that the neutron detection unit 40 for measuring neutron flux has a tubular member 41 penetrating the roof slab structure 11, and a neutron detector 42 and the like are provided in the tubular member 41. The tubular member 41 is not entirely provided in liquid sodium, but is provided so as to penetrate the roof slab structure 11, so that workers can access the tubular member 41 and the neutron detector 42 therein from the upper surface of the roof slab structure 11. Therefore, even if the equipment for measuring neutrons is damaged, it is possible to obtain an effect that the equipment can be easily inspected, replaced, and the like.

[0019] [Configuration of each part] The main vessel 10 is a cylindrical vessel with an open top and a closed bottom. The main vessel 10 has a diameter of, for example, about 15 m to 20 m. The main vessel 10 accommodates a core 20 together with liquid sodium, which is a primary system coolant. A core upper structure 30, an intermediate heat exchanger 50, a circulation pump 60, and a direct core cooling system heat exchanger 70 (see FIG. 2) are provided within the main vessel 10. The top of the main vessel 10 is closed by a roof slab structure 11.

[0020] A partition plate 15 is provided inside the main vessel 10. The partition plate 15 is, for example, a circular ring-shaped member, and is provided so as to divide the inside of the main vessel 10 into an upper plenum and a lower plenum.

[0021] The core 20 is a portion that contains nuclear fuel. The core 20 has fuel assemblies, shielding assemblies, and control rods (none of which are shown). Although not shown, the core 20 is located, for example, in the center of the main vessel 10 when viewed from above. The core 20 is contained inside a core barrel 10a provided in the main vessel 10. The core barrel 10a is a structure that supports the core constituent elements and the core 20 in the horizontal direction.

[0022] The core upper structure 30 is a structure extending vertically from the top of the main vessel 10 toward the region directly above the core 20. Various drive mechanisms, control devices, and measuring instruments are provided in the core upper structure 30. Specifically, the core upper structure 30 is provided with a control rod drive mechanism, as well as measuring instruments such as thermometers and fuel damage detectors.

[0023] 1, the core upper structure 30 has a cylindrical body 31 and a support plate 32. The cylindrical body 31 has a shape that surrounds an accommodation space in which a drive mechanism, a control device, measuring instruments, etc. are arranged. Specifically, the cylindrical body 31 is a cylinder extending in the vertical direction. The upper end of the cylindrical body 31 is located above the roof slab structure 11, and the lower end of the cylindrical body 31 is located below the liquid sodium level and above the reactor core 20.

[0024] The support plate 32 is a plate-like member disposed below the cylindrical body 31. The support plate 32 is fixed to a part of the cylindrical body 31. As an example, the support plate 32 may be a member that supports at least a part of a predetermined mechanism, measuring instrument, etc. of the core upper structure 30. The support plate 32 also supports a part of a tubular member 41 of the neutron detection unit 40, as described later.

[0025] (Roof slab structure 11) The roof slab structure 11 is a lid that closes the main vessel 10. Regarding the cross-sectional structure of the roof slab structure 11, as shown in Fig. 1, the roof slab structure 11 has a heat shielding layer 11a, a radiation shielding layer 11b, and a cooling gas space 11c.

[0026] The thermal shielding layer 11a is formed on the lower surface side of the roof slab structure 11 and is a layer that blocks heat from inside the main vessel 10. The radiation shielding layer 11b is formed on the upper surface side of the roof slab structure 11 and is a layer that blocks radiation. The cooling gas space 11c is a space formed between the thermal shielding layer 11a and the radiation shielding layer 11b. The cooling gas space 11c is filled with a cooling gas supplied from a gas supply source (not shown). The temperature of the cooling gas space 11c during operation of the fast reactor S100 is, for example, 100°C or less, specifically, 70°C or less.

[0027] (The structure of the roof slab structure 11 as viewed from above) Next, a description will be given of the structure of the roof slab structure 11 viewed from above. The roof slab structure 11 includes a roof slab member 12 and a rotating plug 13, as shown in FIG.

[0028] The roof slab member 12 is a structure that covers the upper part of the main vessel 10, specifically, it covers the upper part of the main vessel 10 in the area around the rotating plug 13.

[0029] The rotating plug 13 is disposed in a central region of the roof slab structure 11, inside the roof slab member 12. The rotating plug 13 has a large rotating plug 13-1 and a small rotating plug 13-2.

[0030] Both the large rotating plug 13-1 and the small rotating plug 13-2 are rotatable around a vertical axis. The small rotating plug 13-2 supports the refueling machine 14. The refueling machine 14 is a mechanism for replacing nuclear fuel in the reactor core 20. The large rotating plug 13-1 and the small rotating plug 13-2 are eccentrically disposed relative to each other. The large rotating plug 13-1 and the small rotating plug 13-2 each rotate to position the refueling machine 14 at a predetermined position on the reactor core 20. During refueling, the refueling machine 14 moves to a predetermined position and replaces the fuel at that position.

[0031] Referring again to Fig. 1, the intermediate heat exchanger 50 is a heat exchanger that cools liquid sodium, the temperature of which has been raised by heat from the reactor core 20, by exchanging heat between the liquid sodium and a secondary system coolant (not shown) inside the intermediate heat exchanger 50. The intermediate heat exchanger 50 is formed in a cylindrical shape. The intermediate heat exchanger 50 is disposed in the vertical direction so as to penetrate the roof slab structure 11 and the partition plate 15.

[0032] The intermediate heat exchanger 50 has an inlet window 51 located in the upper plenum and an outlet window 52 located in the lower plenum. The inlet window 51 is an opening through which high-temperature liquid sodium in the upper plenum flows in. The outlet window 52 is an opening through which liquid sodium that has passed through the interior of the intermediate heat exchanger 50 flows out to the lower plenum.

[0033] The circulation pump 60 is a pump for circulating liquid sodium. Like the intermediate heat exchanger 50, the circulation pump 60 is also disposed vertically so as to penetrate the roof slab structure 11 and the partition plate 15. The circulation pump 60 pumps the liquid sodium to the diagrid 16 via piping 61 located in the lower plenum. The liquid sodium pumped to the diagrid 16 by the circulation pump 60 flows upward through the core components such as fuel assemblies, shielding assemblies, and control rods (none of which are shown) supported in the core barrel 10a.

[0034] The liquid sodium, which has received heat from the core 20 and reached a temperature of, for example, about 500°C to 550°C, then flows into the upper plenum. The liquid sodium that has flowed into the upper plenum flows into the intermediate heat exchanger 50 from the inlet window 51 of the intermediate heat exchanger 50. The liquid sodium then flows downward inside the intermediate heat exchanger 50 and is cooled to, for example, about 400°C. The cooled liquid sodium flows out from the outlet window 52.

[0035] The liquid sodium that flows out from the exit window 52 and into the lower plenum is sucked in by the circulation pump 60. The sucked in liquid sodium is pumped again to the diagrid 16 by the action of the circulation pump 60. In this way, the liquid sodium cools the reactor core 20 while circulating inside the main vessel 10.

[0036] 2 shows a specific example of the arrangement of the intermediate heat exchanger 50, the circulation pump 60, and the direct core cooling system heat exchanger 70, but the number and arrangement positions of these components may be appropriately changed depending on the specifications of the fast reactor S100, etc. The direct core cooling system heat exchanger 70 is a conventionally known one, so a detailed description thereof will be omitted.

[0037] (Neutron Detection Unit 40) The neutron detection unit 40 is a unit for measuring the neutron flux of neutrons from the reactor core 20. The neutron detection unit 40 has a tubular member 41, a neutron detector 42, and a shielding plug 43. A shielding material and a moderator may be provided inside the tubular member 41, which will be described later with reference to other drawings.

[0038] The number of neutron detection units 40 is arbitrary, but in this embodiment, four neutron detection units 40 are provided as shown in Fig. 2. Since the neutron detection units 40 all have the same configuration, hereinafter, one neutron detection unit 40 will be described.

[0039] The tubular member 41 is, for example, a hollow cylindrical member made of metal. The cross-sectional shape of the tubular member 41 may be any shape, such as a circle, a rectangle, or a polygon, but is a circle in this embodiment. The tubular member 41 is disposed so that its upper end passes through the roof slab structure 11 and its lower end is located in a region directly above the reactor core 20. A neutron detector 42 is disposed in the tubular member 41. An inert gas, such as argon, is sealed inside the tubular member 41.

[0040] The upper end of the tubular member 41 is an opening exposed on the upper surface of the roof slab structure 11, and a worker can access the inside of the tube through this opening. A shielding plug 43 is removably attached to the upper end of the tubular member 41 as a lid. The tubular member 41 is inserted into the roof slab structure 11 from above the roof slab structure 11. As an example, the tubular member 41 is removable from the roof slab structure 11. The tubular member 41 is removed from the roof slab structure 11 as necessary, for example, when the tubular member 41 is damaged or when equipment or the like arranged inside breaks down. Note that it is not essential in the present invention that the tubular member 41 is removable from the roof slab structure 11, and the tubular member 41 may be fixedly attached to the roof slab structure 11.

[0041] The tubular member 41 may be arranged in such a manner that only a part of the upper end side is supported by the roof slab structure 11, but in this embodiment, the tubular member 41 is arranged in such a manner that a part of the upper end side is supported by the roof slab structure 11 and a part of the lower end side is supported by the support plate 32. According to this configuration, not only the upper end side of the tubular member 41 is supported, but both the upper end side and the lower end side are supported, so that the tubular member 41 is stably supported even if the tubular member 41 is long. In addition, since the support plate 32 is arranged as a component of the core upper structure 30, according to the configuration in which the tubular member 41 is fixed to the support plate 32 in this manner, it is not necessary to newly provide a member for fixing the tubular member 41. The tubular member 41 may be fixed to the support plate 32 by passing a part of the lower end side of the tubular member 41 through a through hole formed in the support plate 32, and specifically, the tubular member 41 may be horizontally restrained by passing a part of the lower end side of the tubular member 41 through a through hole of the support plate 32. In such a configuration, fixing means such as fixing screws may be used, but as an example, it is preferable that the tubular member 41 is fixed to the support plate 32 by mechanically connecting the tubular member 41 and the support plate 32 without using such fixing means. With such a configuration, since fixing means such as fixing screws are not used, there is no problem that the fixing means becomes detached and is lost.

[0042] The lower end side of the tubular member 41 is immersed in the liquid sodium in the main vessel 10. The lower end of the tubular member 41 is closed, but neutrons enter the inside of the tubular member 41 from this lower end and the peripheral wall.

[0043] The shielding plug 43 is detachably provided as a lid for the tubular member 41. The shielding plug 43 is a member for blocking heat and radiation from inside the main vessel 10. As the shielding plug 43, for example, stainless steel is used in consideration of the shielding effect against neutrons and gamma rays.

[0044] The neutron detector 42 is a detector that measures the neutron flux of neutrons from the core 20. The neutron detector 42 is disposed inside the tubular member 41. The neutron detector 42 is provided so as to be removable from the tubular member 41. Specifically, the neutron detector 42 is disposed inside the tubular member 41 at a position below the liquid level of the liquid sodium and above the core 20. The neutron detector 42 is preferably located in an area above the core barrel 10a, and more preferably located in an area above the core 20 within the area above the core barrel 10a. The neutron detector 42 is preferably provided at a position at least lower than the inlet window 51 of the intermediate heat exchanger 50.

[0045] In the example of Figure 1, the neutron detector 42 is located above the support plate 32, but if the lower end of the tubular member 41 protrudes below the support plate 32, the neutron detector 42 may be located below the support plate 32.

[0046] The neutron detector 42 is a high-temperature-resistant grade detector that can operate in a high-temperature environment of, for example, 500° C. or higher. According to the configuration of this embodiment, the neutron detector 42 is provided at a position relatively close to the core 20 in the tubular member 41, so that the neutron flux of neutrons from the core 20 can be efficiently detected.

[0047] The neutron detector 42 outputs a detection value to a control device (not shown) arranged outside the main vessel 10 through wiring (not shown) passing through the tubular member 41. The detection value is displayed on a display unit of the control device, for example. The control device may determine whether an output value of a sensor measuring the pressure of the gas sealed in the tubular member 41 is within a predetermined range, and may issue an alert if the output value is not within the predetermined range. With this configuration, the soundness of the tubular member 41 can be monitored by measuring the pressure of the gas inside the tubular member 41.

[0048] 3 is a diagram showing a schematic example of the arrangement of the neutron detector 42, the shielding material 44, and the moderator 45. The neutron detection unit 40 is provided with one or both of the shielding material 44 and the moderator 45 as necessary.

[0049] The shielding material 44 is a member that attenuates gamma rays from liquid sodium activated by neutrons from the core 20. The shielding material 44 is made of, for example, bismuth oxide, which has a high density and a high melting point of 817° C. and absorbs little neutrons. With this configuration, the gamma rays are attenuated by the shielding material 44, so that the noise components in the output of the neutron detector 42 can be reduced.

[0050] The moderator 45 is a member that slows down neutrons moving from the core 20 toward the neutron detector 42. The moderator 45 slows down neutrons moving at high or medium speeds and converts them into thermal neutrons that are highly efficient to detect in the neutron detector 42. The moderator 45 is, for example, zirconium hydride, yttrium hydride, or a combination of these, which are types of metal hydrides produced by absorbing hydrogen between the lattices of metal crystals. By using such a material, it is possible to obtain a good neutron moderation effect similar to that of polyethylene, even in a high-temperature environment.

[0051] Various arrangements of the shielding material 44 and the moderator 45 are possible, but in this embodiment, the arrangements shown in Figures 3(a) and 3(b) are shown as examples. In the arrangement shown in Figure 3(a), the neutron detector 42, the shielding material 44, and the moderator 45 are arranged inside a tubular member 41, the shielding material 44 surrounds the neutron detector 42, and the neutron detector 42 surrounded by the shielding material 44 is further surrounded by the moderator 45. With this configuration, neutrons moving toward the neutron detector 42 are slowed down by the moderator 45 and converted into thermal neutrons, so that the neutron flux can be detected well by the neutron detector 42. In addition, the gamma rays are attenuated by the shielding material 44, so that the noise components in the output of the neutron detector 42 can be reduced.

[0052] In the embodiment of FIG. 3(b), the moderator 45 is disposed below the neutron detector 42, and the shielding material 44 surrounds the neutron detector 42 at a position above the moderator 45. The moderator 45 is for slowing down the neutrons moving from the core 20 toward the neutron detector 42, and is therefore preferably disposed below the neutron detector 42. On the other hand, since gamma rays are emitted from activated liquefied sodium surrounding the tubular member 41, it is preferable that the neutron detector 42 is surrounded by the shielding material 44. As shown in FIG. 3(b), the moderator 45 does not necessarily need to be disposed on the side of the neutron detector 42. According to such a configuration in which the moderator 45 is not disposed on the side of the neutron detector 42, the dimension of the neutron detector 42 in the width direction (horizontal direction in FIG. 3) can be reduced, and therefore the size of the structure including the neutron detector 42 can be reduced.

[0053] (Effects of the S100 fast reactor) As described above, in the fast reactor S100 of this embodiment, the neutron detection unit 40 for measuring the neutron flux has a tubular member 41 penetrating the roof slab structure 11, and the neutron detector 42 and the like are provided in the tubular member 41. In this manner, the entire tubular member 41 is not provided in the liquid sodium, and the tubular member 41 is provided so as to penetrate the roof slab structure 11. This allows an operator to access the tubular member 41 and the neutron detector 42 therein from the upper surface of the roof slab structure 11. Therefore, even if the neutron detector 42 for measuring the neutron flux is damaged or the tubular member 41 itself is damaged, it is easy to perform work such as inspection, replacement, and repair, and as a result, the neutron flux can be measured well in the fast reactor S100. In addition, since the neutron detector 42 is disposed below the liquid sodium level and relatively close to the core 20, the neutron flux can be measured with high accuracy.

[0054] 3, when the shielding material 44 is provided, the gamma rays from the activated liquid sodium are attenuated by the shielding material 44, thereby reducing the noise components in the output of the neutron detector 42. When the moderator material 45 is provided, neutrons moving at high or medium speeds can be decelerated and converted into thermal neutrons, allowing the neutron detector 42 to measure the neutron flux well.

[0055] Moreover, the configuration as shown in FIG. 3(b) is advantageous in reducing the size of the structure surrounding the neutron detector 42, as described above.

[0056] <Modification> Fig. 4 is a diagram showing a schematic diagram of a neutron detection unit according to a modified example of the present invention. Fig. 5 is a cross-sectional view of a tubular member cut in a direction perpendicular to the longitudinal direction of the tubular member. The neutron detection unit 40A in Figs. 4 and 5 has the same configuration as the above embodiment in that it includes a tubular member 41 and a neutron detector 42. In this neutron detection unit 40A, the neutron detector 42 is housed in a cylindrical housing 46, for example.

[0057] In the neutron detection unit according to the present invention, in which the neutron detector 42 is disposed inside the tubular member 41, the position of the neutron detector 42 is not stable if the neutron detector 42 is simply suspended from the upper part of the roof slab structure 11 inside the tubular member 41. Therefore, in the example of Figures 4 and 5, a guide means is provided that defines the position of the neutron detector 42 relative to the tubular member 41.

[0058] Specifically, roller members 48 are provided as the guide means. The roller members 48 are attached to the side of a housing 46, which is a member that houses the neutron detector 42. Although one roller member 48 may be provided, in this example, a plurality of roller members 48 are provided. As shown in FIG. 5, a plurality of roller members 48 are provided along the circumferential direction between the side of the housing 46 and the inner surface of the tubular member 41. The roller members 48 are configured to contact the inner surface of the tubular member 41, and by providing such roller members 48, the position of the housing 46 with respect to the tubular member 41 in the horizontal direction and the radial direction of the tubular member 41 is defined, and thus the position of the neutron detector 42 in the housing 46 is stabilized.

[0059] The roller members 48 are rotatable while in contact with the inner surface of the tubular member 41. Therefore, for example, when the housing 46 is inserted into the tubular member 41 and when the housing 46 is removed from the tubular member 41, the housing 46 can be moved smoothly.

[0060] In Fig. 4, a configuration in which the housing 46 is provided is illustrated, but the housing 46 may be omitted. Alternatively, the neutron detector 42, the shielding material 44, and the moderator 45 may be arranged in the housing 46 in the manner as shown in Fig. 3(a) or 3(b). The housing 46 may be suspended by a wire (not shown). In Fig. 4, the housing 46 is located at a position a predetermined distance above the bottom surface of the tubular member 41, but the housing 46, or the neutron detector 42 when the housing 46 is not provided, may be placed on the bottom surface of the tubular member 41.

[0061] The present invention is not limited to the above specific configuration. For example, the neutron detection unit 40 may be provided in the roof slab member 12. Also, one or more first neutron detection units and one or more second neutron detection units may be arranged at different radial positions of the roof slab structure 11 (for example, on a first reference circle and a second reference circle having different diameters).

[0062] Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments, and various modifications and changes are possible within the scope of the gist of the present invention. For example, all or part of the device can be configured by distributing or integrating functionally or physically in any unit. In addition, new embodiments resulting from any combination of multiple embodiments are also included in the embodiments of the present invention. The effect of the new embodiment resulting from the combination combines the effect of the original embodiment. [Explanation of symbols]

[0063] 10 Main vessel 10a Core tank 11 Roof slab structure 11a Heat shield layer 11b Radiation shielding layer 11c Cooling gas space 12 Roof slab members 13 Rotating plug 13-1 Giant Rotation Plug 13-2 Small rotating plug 14 Fuel exchange machine 15 Partition 16 Diagrid 20 Core 30 Core Superstructure 31 Cylinder 32 Support plate 40, 40A Neutron Detection Unit 41 Tubular member 42 Neutron Detector 43 Shielded plug 44 Shielding material 45 Moderator 46 Case 48 Roller member 50 Intermediate heat exchanger 51 Entrance window 52 Exit Window 60 Circulation Pump 61 Piping 70 Direct core cooling system heat exchanger S100 fast reactor

Claims

1. a reactor core containing nuclear fuel; a main vessel having an open top and containing the reactor core together with liquid sodium as a coolant; a roof slab structure disposed on top of the main vessel and closing the main vessel; a neutron detection unit for measuring neutron flux from the reactor core; Equipped with The neutron detection unit includes: a hollow tubular member having a lower end side located within the main vessel and an upper end side disposed to pass through the roof slab structure; a neutron detector disposed inside the tubular member at a position below the liquid level of the liquid sodium and above the reactor core for measuring the neutron flux; A fast reactor.

2. The neutron detection unit includes: a moderator disposed within the tubular member for slowing down neutrons traveling toward the neutron detector; 2. The fast reactor of claim 1.

3. The neutron detection unit includes: a shielding material disposed within the tubular member for attenuating gamma rays from the liquid sodium activated by neutrons from the reactor core; The shielding material is made of bismuth oxide.

3. A fast reactor according to claim 1 or 2.

4. The neutron detection unit includes: a moderator disposed within the tubular member for slowing down neutrons moving toward the neutron detector, and a shielding material disposed within the tubular member for attenuating gamma rays from the liquid sodium activated by neutrons from the reactor core, the moderator is disposed below the neutron detector, The shielding material surrounds the neutron detector at a position above the moderator.

2. The fast reactor of claim 1.

5. A shielding plug is removably provided at the upper end of the tubular member to block heat and radiation from inside the main vessel.

3. A fast reactor according to claim 1 or 2.

6. The neutron detection unit includes: a guide means within the tubular member for defining a position of the neutron detector relative to the tubular member; The guide means is A roller member is provided on a side surface of the neutron detector or a side surface of a member accommodating the neutron detector, and is in contact with an inner surface of the tubular member.

3. A fast reactor according to claim 1 or 2.

7. a core upper structure provided in a region above the core and defining a storage space in which instrumentation is arranged; The core upper structure includes: A cylindrical body that surrounds the storage space and extends in a height direction of the main container; A support plate provided below the cylindrical body; having The tubular member is disposed within the cylindrical body, and a portion of the tubular member is fixed to the support plate.

3. A fast reactor according to claim 1 or 2.

8. A neutron detection unit for use in a fast reactor, the fast reactor comprising: a core for accommodating nuclear fuel; a main vessel having an open top and accommodating the core together with liquid sodium as a coolant; and a roof slab structure disposed on top of the main vessel and closing the main vessel, the neutron detection unit comprising: A hollow tubular member having a lower end side located within the main vessel and an upper end side disposed to pass through the roof slab structure; a neutron detector disposed inside the tubular member at a position below the liquid level of the liquid sodium and above the reactor core, the neutron detector measuring neutron flux from the reactor core; A neutron detection unit having:

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