Neutron measurement method for fast reactors and fast reactors

Abstract

To provide a fast reactor capable of improving the accuracy in neutron flux measurement by a neutron detector 45.SOLUTION: A fast reactor 1 includes: a reactor core 20; a main container 10 with a bottom formed in a cylindrical shape with an open top for housing the reactor core together with coolant; a roof slab structure 11 placed at the top of the main container 10 to close the main container 10; and a neutron detection unit 40 that measures the neutron flux of neutrons from the reactor core. The neutron detection unit 40 includes: a neutron guide tube 41 with the lower edge located inside the main container 10 in a region above the reactor core 20 and extends along the height direction of the main container so that the top edge is located within the roof slab structure to guide neutrons from nuclear fuel above the main container 10; a moderator 42 that is placed in roof slab structure 11 for reducing the speed of neutrons guided through the neutron guide tube 41; and a neutron detector 45 placed in the roof slab structure 11 measuring the neutron flux of the decelerated neutrons.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a neutron measurement method for a fast reactor and a fast reactor. [Background technology]

[0002] Conventionally, neutrons emitted from the reactor core have been measured to ensure safe operation of a nuclear reactor. Shielding design for neutrons emitted from the reactor core must satisfy two conflicting requirements: the first requirement is to keep the neutron flux incident on the in-core equipment, which is the equipment inside the main vessel, below a limit value, and the second requirement is to keep the neutron flux incident on the neutron detector, which is the neutron instrumentation for monitoring the reactor core, above a limit value.

[0003] For example, Patent Document 1 discloses a structure in which a hollow cylinder is disposed that is filled with a gas such as argon or helium, which has a low neutron shielding ability, or is evacuated, in order to satisfy the first and second requirements in a tank-type fast reactor. Specifically, the hollow cylinder is disposed in a position within the reactor between the core and the neutron detector, and the hollow cylinder promotes the transmission of neutrons to the outside of the main vessel, and the neutron flux is measured by the neutron detector. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3041058 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, a hollow cylinder is placed inside a reactor and is immersed in liquid sodium, which is a coolant. During operation of a fast reactor, the liquid sodium becomes very hot, making it difficult to inspect the hollow cylinder using inspection equipment placed inside the liquid sodium. As a result, a problem occurs in which neutron flux cannot be measured accurately due to damage to the hollow cylinder.

[0006] In response to this, a configuration is also conceivable in which the neutron shielding material in the shielding assembly is replaced with liquid sodium and the neutron flux is measured with a neutron detector installed outside the main vessel. However, in fast reactors, spent fuel may be stored in place of part of the shielding assembly near the core. In this case, neutrons may be emitted from the spent fuel or neutrons from the core may be absorbed by the spent fuel. As a result, it is difficult to accurately measure the neutron flux with a neutron detector installed outside the reactor, and there is room for improvement.

[0007] The present invention has been made in view of these points, and has as its object to improve the accuracy of neutron flux measurements made by a neutron detector. [Means for solving the problem]

[0008] One aspect of the present invention provides a fast reactor comprising: a core that accommodates nuclear fuel; a main vessel formed in a cylindrical shape with a bottom and an open top that accommodates the core together with a coolant; a roof slab structure that is disposed on top of the main vessel and closes the main vessel; and a neutron detection unit that measures neutron flux of neutrons from the core, wherein the neutron detection unit extends in the height direction of the main vessel in a region above the core so that its lower end is located within the main vessel and its upper end is located within the roof slab structure, and the fast reactor comprises: a neutron guide tube that guides neutrons from the core to above the main vessel; a moderator that is disposed within the roof slab structure and that slows down the speed of the neutrons guided through the neutron guide tube; and a neutron detector that is disposed within the roof slab structure and that measures the neutron flux of the slowed down neutrons.

[0009] The neutron detection unit may be removably attached to the roof slab structure from above the roof slab structure, and may have a first pipe extending toward the inside of the main vessel, a portion of the first pipe being configured as the neutron guide tube, and the moderator being arranged within the first pipe.

[0010] The fast reactor may further include a superstructure provided in a region above the core and forming an accommodation space in which instrumentation equipment is arranged, the superstructure having a cylindrical body surrounding the accommodation space and extending in the height direction of the main vessel, and an instrumentation mounting plate provided at the lower end of the cylindrical body, the first pipe being arranged within the cylindrical body, and the lower end of the first pipe being fixed to the instrumentation mounting plate and the middle of the first pipe being fixed to at least one horizontal plate arranged in the height direction within the core superstructure.

[0011] In the fast reactor, the neutron detection unit may further include a shielding plug that is disposed above the moderator in the first pipe and is a member that shields against heat and radiation from the core side.

[0012] The neutron detection unit may have a second pipe detachably attached to the roof slab structure from above the roof slab structure, and the neutron detector may be disposed on the second pipe.

[0013] The roof slab structure may include a roof slab covering the top of the main vessel, and a rotating plug disposed in the center of the roof slab for moving a refueling machine for exchanging the nuclear fuel, and the second pipe may be disposed in the rotating plug.

[0014] The roof slab structure may have a heat shielding layer formed on the underside of the roof slab structure and a radiation shielding layer formed on the upper side of the roof slab structure, and a cooling gas space, which is a space filled with cooling gas, may be formed between the heat shielding layer and the radiation shielding layer, and the second pipe may be arranged so that the neutron detector is located in the cooling gas space.

[0015] The fast reactor is provided with a plurality of the neutron detection units and further includes a control device that acquires output values ​​from the plurality of neutron detectors, and the control device may compare the output value from a predetermined neutron detector with the output value from one or more other neutron detectors, and if the difference in output values ​​is equal to or greater than a predetermined threshold, output an alert indicating an abnormality in the predetermined neutron detector.

[0016] A neutron measurement method for a fast reactor according to one aspect of the present invention is a neutron measurement method for a fast reactor comprising: a core that contains nuclear fuel; a main vessel formed in a cylindrical shape with a bottom and an open top and that contains the core together with a coolant; a roof slab structure that is disposed on top of the main vessel and closes the main vessel; and a neutron detection unit that measures neutron flux of neutrons from the core, wherein the neutron detection unit is positioned at a height of the main vessel so that a lower end of the neutron detection unit is located within the main vessel and an upper end of the neutron detection unit is located within the roof slab structure in a region above the core. the neutron guide tube extending in a direction perpendicular to the main vessel wall and guiding neutrons from the reactor core to above the main vessel; a moderator disposed within the roof slab structure for slowing down the velocity of the neutrons guided through the neutron guide tube; and a neutron detector disposed within the roof slab structure for measuring the neutron flux of the slowed down neutrons, and the method comprises the steps of: slowing down the velocity of the neutrons guided through the neutron guide tube with the moderator; and measuring the neutron flux of the neutrons slowed down by the moderator with the neutron detector. [Effects of the Invention]

[0017] According to the present invention, the accuracy of the neutron flux measured by the neutron detector can be improved. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of a fast reactor. [Figure 2] FIG. 2 is an enlarged view of a portion of FIG. [Figure 3] 1 is a schematic diagram showing an example of a fast reactor viewed from above. [Figure 4]FIG. 4 is an enlarged view of a portion of FIG. [Figure 5] 10 is a graph showing neutron attenuation in the case where a neutron guide tube is provided and in the case where no neutron guide tube is provided. DETAILED DESCRIPTION OF THE INVENTION

[0019] The configuration of a fast reactor 1 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 the fast reactor 1. Fig. 2 is an enlarged view of a portion of Fig. 1. Fig. 3 is a schematic view showing an example of the fast reactor 1 as viewed from above. Fig. 4 is an enlarged view of a portion of Fig. 3. Fig. 1 is a cross-sectional view taken along line AA in Fig. 3. Arrows in Fig. 2 schematically show the direction in which neutrons move.

[0020] In the following, terms indicating directions such as "upper" and "lower" are used in accordance with 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.

[0021] The fast reactor 1 is, for example, a tank-type fast reactor, which extracts energy by controlling and sustaining a nuclear fission chain reaction using, for example, uranium or plutonium as fuel. The fast reactor 1 mainly comprises 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.

[0022] One of the features of the fast reactor 1 of this embodiment is that the neutron detection unit 40 that measures neutron flux is provided in a region above the core 20, rather than on the radially outer side of the main vessel 10. Specifically, the neutron detection unit 40 has a neutron guide tube 41 that guides neutrons toward the upper side of the main vessel 10, a moderator 42 that slows down the velocity of the neutrons, and a neutron detector 45 that measures the neutron flux. The moderator 42 and the neutron detector 45 are arranged inside the roof slab structure 11. Neutrons from the core 20 are guided to the upper side of the main vessel 10 through the neutron guide tube 41, slowed down by the moderator 42, and measured by the neutron detector 45 within the roof slab structure 11, which has a relatively low temperature.

[0023] According to this configuration, the neutron detection unit 40 is disposed above the reactor core 20, and is therefore less susceptible to the attenuation of neutrons by sodium, which is the coolant, compared to a configuration in which the neutron detectors are disposed radially outside the main vessel 10. Therefore, the accuracy of the neutron flux measurements by the neutron detectors 45 of the neutron detection unit 40 can be improved.

[0024] [Configuration of each part] Below, we will explain each part of the fast reactor 1. Note that the characteristic configuration of this embodiment is the neutron detection unit 40 and its peripheral structure, but before explaining these in detail, we will explain the overall configuration and operation of the fast reactor 1.

[0025] The main vessel 10 is, for example, a cylindrical shape with a bottom and an open top, and has a diameter of, for example, about 15 to 20 meters. As shown in Fig. 1, the main vessel 10 contains a reactor core 20, a reactor core upper structure 30, an intermediate heat exchanger 50, a circulation pump 60, a direct core cooling system heat exchanger 70 (see Fig. 3), and sodium and the like as a primary system coolant. The upper end of the main vessel 10 is closed by a roof slab structure 11. A partition plate 15 is provided inside the main vessel 10.

[0026] The partition plate 15 is, for example, a circular member, and is arranged horizontally so as to divide the interior of the main vessel 10 into an upper plenum and a lower plenum. The upper plenum and the lower plenum communicate with each other via the core barrel 10a, the intermediate heat exchanger 50, the circulation pump 60, and the like. As a result, by operating the circulation pump 60 as described below, the coolant circulates between the upper plenum and the lower plenum.

[0027] The core 20 is a portion that contains nuclear fuel and includes fuel assemblies, shielding assemblies, and control rod assemblies (none of which are shown). The core 20 is contained inside a core barrel 10a provided in the center of the main vessel 10. Specifically, the core barrel 10a is provided so as to open to both the upper plenum and the lower plenum at the center of the partition plate 15, allowing coolant to pass through. The specific configuration of the core 20 is not particularly limited, but as an example, fuel assemblies are arranged in the central region of the core, and shielding assemblies are arranged around the fuel assemblies. The control rod assemblies are arranged near the fuel assemblies.

[0028] The core upper structure 30 is provided with a control rod drive mechanism and various measuring instruments such as thermometers and fuel damage detectors (none of which are shown). The core upper structure 30 is disposed above the reactor core 20. The core upper structure 30 has a cylindrical body 31 and an instrumentation mounting plate 32. The cylindrical body 31 is, for example, a cylindrical member and extends in the height direction of the main vessel 10. The cylindrical body 31 forms a storage space in which instrumentation instruments and the like are disposed.

[0029] The instrumentation mounting plate 32 is a plate-like member arranged in the core upper structure 30. Specifically, the instrumentation mounting plate 32 is located at the lower end of the core upper structure 30, and is arranged, for example, horizontally above the core 20. The instrumentation mounting plate 32 supports a portion of the first pipe P1 of the neutron detection unit 40 together with the core upper structure 30, as will be described later.

[0030] (Roof slab structure 11) The roof slab structure 11 is placed on top of the main vessel 10 and functions as a lid that closes the main vessel 10. As for 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.

[0031] The heat-shielding layer 11a is formed on the lower surface 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 of the roof slab structure 11 and is a layer that blocks radiation.

[0032] The cooling gas space 11c is a sealed 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. The temperature of the cooling gas space 11c during operation of the fast reactor 1 is, for example, 100°C or less, and specifically, 70°C or less.

[0033] Next, the structure of the roof slab structure 11 viewed from above will be described. As shown in Figures 3 and 4, the roof slab structure 11 includes a roof slab 12 and a rotary plug 13. The roof slab 12 is a structure that covers the top of the main vessel 10, and as an example, covers the top of the main vessel 10 in an annular region around the rotary plug 13.

[0034] The rotating plug 13 is disposed in the center of the roof slab 12. The rotating plug 13 is composed of a large rotating plug 13-1 and a small rotating plug 13-2. The small rotating plug 13-2 supports a refueling machine 14 (only some of the components are shown) for exchanging nuclear fuel, and the core superstructure 30.

[0035] The large rotation plug 13-1 and the small rotation plug 13-2 are provided eccentrically to each other, and the large rotation plug 13-1 and the small rotation plug 13-2 rotate to position the refueling machine 14 at a predetermined position above the reactor core 20. When refueling, the refueling machine 14 moves to the predetermined position and performs the refueling work.

[0036] Referring again to FIG. 1 , the intermediate heat exchanger 50 is a heat exchanger that cools the primary system coolant, whose temperature has been raised by heat from the reactor core 20, by exchanging heat between the primary system coolant and the secondary system coolant (not shown) inside the intermediate heat exchanger 50. The intermediate heat exchanger 50 is formed in a cylindrical shape and is arranged vertically so as to penetrate the roof slab structure 11 and the partition plate 15. The intermediate heat exchanger 50 has an entrance window 51 located in the upper plenum and an exit window 52 located in the lower plenum. The entrance window 51 is an opening through which the high-temperature coolant in the upper plenum flows in. The exit window 52 is an opening through which the coolant that has passed through the interior of the intermediate heat exchanger 50 flows out into the lower plenum.

[0037] The circulation pump 60 is a pump for circulating the coolant, and extends vertically so as to penetrate the roof slab structure 11 and the partition plate 15. The circulation pump 60 pumps the coolant to the core barrel 10a through piping 61 located in the lower plenum.

[0038] The flow of coolant during operation of the fast reactor 1 is well known and will be briefly described below. The coolant, pressure-fed to the core barrel 10a by the circulation pump 60, receives heat from the core 20 in the core barrel 10a and reaches a temperature of, for example, approximately 500°C to 550°C. The coolant then flows upward inside the core barrel 10a and into the upper plenum. The coolant that has flowed into the upper plenum flows into the intermediate heat exchanger 50 through the inlet window 51 of the intermediate heat exchanger 50 and is cooled to, for example, approximately 400°C inside the intermediate heat exchanger 50. The cooled coolant flows downward inside the intermediate heat exchanger 50 and flows out through the outlet window 52.

[0039] The coolant that flows out of the outlet window 52 and into the lower plenum is sucked in by the circulation pump 60. The sucked coolant is pumped back into the core barrel 10a by the action of the circulation pump 60. In this way, the coolant cools the core 20 while circulating inside the main vessel 10.

[0040] 3 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 changed as appropriate depending on the specifications of the fast reactor 1. The direct core cooling system heat exchanger 70 is a conventionally known direct core cooling system heat exchanger, and therefore a detailed description thereof will be omitted.

[0041] (Neutron Detection Unit 40) The neutron detection unit 40 is a unit that measures the neutron flux of neutrons from the reactor core 20. As shown in Figures 1 and 2, the neutron detection unit 40 mainly includes a neutron guide tube 41, a moderator 42, a first shielding plug 43, a neutron detector 45, a second shielding plug 46, a first pipe P1, and a second pipe P2. Specifically, the neutron guide tube 41, the moderator 42, and the first shielding plug 43 are provided in the first pipe P1, and the neutron detector 45 and the second shielding plug 46 are provided in the second pipe P2.

[0042] As shown in Fig. 4, a plurality of neutron detection units 40 are arranged in the circumferential direction of the roof slab 12. Specifically, as an example, four neutron detection units 40 are arranged at 90° intervals in the circumferential direction. Since the configuration of each neutron detection unit 40 is the same, one neutron detection unit 40 will be described below.

[0043] The first pipe P1 is a cylindrical member made of metal. The cross-sectional shape of the first pipe P1 can be any shape, such as a circle, a rectangle, or a polygon, but in this embodiment, it is a circle as an example. The first pipe P1 is disposed within the cylindrical body 31 of the core upper structure 30, and is located in an area above the core 20. Here, the area above the core 20 is not limited to being directly above the core 20, but in this embodiment, it is, as an example, an area directly above the core 20 (see reference symbol S1 in FIG. 2).

[0044] The first pipe P1 is detachably attached to the roof slab structure 11 from above the roof slab structure 11. The first pipe P1 is configured so that it can be replaced with another first pipe P1, for example, in the event that equipment arranged inside it fails. The upper end of the first pipe P1 is supported by the roof slab structure 11, and extends vertically from the roof slab structure 11 toward the inside of the main vessel 10. The lower end of the first pipe P1 does not necessarily need to be supported by a predetermined member, but in this embodiment, it is supported by the instrumentation mounting plate 32 of the core superstructure 30.

[0045] The first pipe P1 is a relatively long member extending from the roof slab structure 11 to the vicinity of the reactor core 20, and the configuration in which the lower end is fixed to the instrumentation mounting plate 32 prevents the first pipe P1 from bending. As a result, the first pipe P1 and the neutron guide tube 41 and the like arranged in the first pipe P1 are less likely to be damaged.

[0046] The neutron guide tube 41 is configured as part of the first pipe P1, and guides neutrons from the vicinity of the relatively high temperature reactor core 20 to the relatively low temperature roof slab structure 11 side. Specifically, the neutron guide tube 41 extends so that its lower end is located inside the main vessel 10 and its upper end is located inside the roof slab structure 11. The neutron guide tube 41 is a hollow member, and is filled with an inert gas such as argon gas so that neutrons can pass through without being significantly attenuated.

[0047] The moderator 42 is disposed above the neutron guide tube 41 in the first pipe P1. As an example, the moderator 42 is located inside the roof slab structure 11. The moderator 42 reduces the speed of the neutrons guided through the neutron guide tube 41. The neutrons are slowed down by the moderator 42 in this way and become thermal neutrons, which can be detected by the neutron detector 45.

[0048] The first shielding plug 43 is disposed in the first pipe P1 above the moderator 42. The first shielding plug 43 is a member that shields against heat and radiation from the core 20 side.

[0049] The second pipe P2 is a metallic tubular member, and like the first pipe P1, is detachably attached to the roof slab structure 11 from above the roof slab structure 11. The cross-sectional shape of the second pipe P2 may be any shape, such as circular, rectangular, or polygonal, but in this embodiment, it is circular as an example. As an example, the second pipe P2 is arranged parallel to the first pipe P1. As shown in FIG. 4, in this embodiment, the second pipe P2 is arranged radially outward of the first pipe P1 from the main vessel 10 (see FIG. 1).

[0050] The second pipes P2 may be arranged at any position in the roof slab structure 11, but in this embodiment, the second pipes P2 are provided in the rotating plugs 13 as shown in Fig. 4. Although the present invention is not limited to the configuration in Fig. 4, in the example in Fig. 4, three second pipes P2 are provided in the small rotating plug 13-2 and one second pipe P2 is provided in the large rotating plug 13-1.

[0051] The neutron detector 45 is a detector that measures the neutron flux of neutrons, and is provided in the second pipe P2. Specifically, the neutron detector 45 measures the neutron flux of neutrons that have been slowed down by passing through the moderator 42. The neutron detector 45 is disposed inside the roof slab structure 11, specifically inside the cooling gas space 11c of the roof slab structure 11.

[0052] As described above, in this embodiment, the first pipe P1 and the second pipe P2 are detachably inserted from above into the roof slab structure 11 (specifically, the rotating plug 13 of the roof slab structure 11). With this configuration, the first pipe P1 and the second pipe P2 can be easily pulled out, and wiring cables to various devices is also easy.

[0053] 5 is a graph showing the state of neutron attenuation with and without a neutron guide tube. In FIG. 5, the horizontal axis represents the axial distance [cm] from the top surface of the roof slab structure 11, and the vertical axis represents the neutron flux [n / (cm 2 ·sec). As shown in Figure 5, when there is no neutron guide tube, the amount of neutron flux decreases as one approaches the top surface of the roof slab structure from the instrumentation mounting plate 32 on the core side (as the value of "axial distance from the top surface" approaches "0"), but when there is a neutron guide tube, the rate of decrease in neutron flux is smaller than when there is no neutron guide tube. From this, it can be seen that by using a neutron guide tube, neutrons can be guided to the top surface of the main vessel without significant attenuation.

[0054] (Control device 80) The control device 80 (see FIG. 1) is a device that displays predetermined information on a display unit 81 in accordance with the detection results of the neutron detector 45. The control device 80 is configured, for example, by a computer having a CPU and a memory unit. The display unit 81 is, for example, a display. The control device 80 is electrically connected to the neutron detectors 45 of each neutron detection unit 40, and acquires output values ​​from each neutron detector 45.

[0055] As described above, in the fast reactor 1 of this embodiment, the neutron detector 45 measures the neutron flux of neutrons that are guided into the roof slab structure 11 by the neutron guide tube 41 of the first pipe P1 and moderated by the moderator 42. The lower end of the first pipe P1 is inserted into the coolant of the main vessel 10, and since the coolant becomes hot during operation of the fast reactor 1, it is expected that the neutron guide tube 41 will be damaged. If such damage occurs, the neutron flux will not be able to be detected accurately because it will be significantly attenuated by the infiltrating sodium.

[0056] Therefore, in this embodiment, the control device 80 estimates the occurrence of damage to the neutron guide tube 41 based on the detection results of the neutron detectors 45. Specifically, the control device 80 acquires output values ​​from the multiple neutron detectors 45, and compares the output value from one predetermined neutron detector 45 with the output values ​​from multiple other neutron detectors 45. The output value from the multiple other neutron detectors 45 is, for example, an average value of the output values ​​from three neutron detectors 45.

[0057] When the difference between the output value from a predetermined neutron detector 45 and the output value from another neutron detector 45 is equal to or greater than a predetermined threshold, the control device 80 outputs an alert indicating an abnormality in the predetermined neutron detector 45. Specifically, as one example, the control device 80 displays the alert on the display unit 81. The alert may be, for example, a message to prompt an operator to inspect the neutron detection unit 40, or a message urging the operator to replace the equipment.

[0058] With this configuration, when an operator sees the alert displayed on the display unit 81, the operator can know that an abnormality has occurred in the neutron detection unit 40, and can prevent the operation of the neutron detection unit 40 and take measures such as inspection or equipment replacement.

[0059] The control device 80 may perform the above-described abnormality detection by comparing the output value from one predetermined neutron detector 45 with the output value from any other neutron detector 45. The control device 80 may also compare the output value from one predetermined neutron detector 45 with a reference value that is set to a predetermined value in advance, and output an alert if the difference is equal to or greater than a predetermined threshold.

[0060] (Action and effect) In the fast reactor 1 of this embodiment described above, a neutron guide tube 41, a moderator 42, and a neutron detector 45 are provided as the neutron detection unit 40, and neutrons from the core 20 are guided by the neutron guide tube 41 to above the main vessel 10, and the neutron flux is measured by the neutron detector 45 inside the roof slab structure 11, which has a relatively low temperature. The neutron measurement method in the fast reactor 1 of this embodiment includes a step of slowing down the velocity of the neutrons guided through the neutron guide tube 41 with the moderator 42, and a step of measuring the neutron flux of the neutrons slowed down by the moderator 42 with the neutron detector 45.

[0061] With this configuration, the neutron detector 45 is less susceptible to the attenuation of neutrons by sodium, which is the coolant, compared to a configuration in which the neutron detector 45 is disposed radially outside the main vessel 10, thereby improving the accuracy of the neutron flux measurements by the neutron detector 45. Furthermore, with the configuration of this embodiment in which the neutron detection unit 40 is provided above the core 20, even if spent fuel is stored in place of part of the shielding assembly, the neutron detector 45 is less susceptible to the influence of neutrons emitted from the spent fuel and neutrons from the core that are absorbed by the spent fuel, allowing for accurate detection of the neutron flux.

[0062] Furthermore, in the fast reactor 1 of this embodiment, the neutron guide tube 41 is configured as a part of the first pipe P1 that is detachably attached to the roof slab structure 11. Therefore, for example, if the neutron guide tube 41 is damaged, it is easy to replace the neutron guide tube 41.

[0063] In the fast reactor 1 of this embodiment, the lower end of the first pipe P1 is supported and fixed to the instrumentation mounting plate 32, and the middle of the first pipe P1 is supported and fixed to several horizontal plates in the height direction within the core upper structure 30 (the latter not shown). One or more horizontal plates may be arranged within the core upper structure 30, and in one embodiment, it is preferable that the horizontal plates support an intermediate region between the upper and lower ends of the first pipe P1. This configuration prevents the first pipe P1 from bending, and as a result, the first pipe P1 and the neutron guide tube 41 arranged in the first pipe P1 are less likely to be damaged. This makes it possible to accurately measure neutron flux over a long period of time.

[0064] Furthermore, in the fast reactor 1 of this embodiment, the first pipe P1 is further provided with a first shielding plug 43. By providing the first shielding plug 43 in the first pipe P1 in this way, the upper part of the roof slab structure 11 is shielded from radiation such as neutrons during reactor operation, and when it becomes necessary to replace instrumentation equipment installed in the neutron guide tube 41, the first shielding plug 43 is removed and replaced when the reactor is shut down.

[0065] Furthermore, in the fast reactor 1 of this embodiment, a neutron detector 45 is arranged in a second pipe P2 that is detachably attached to the roof slab structure 11. The neutron detector 45 may be arranged at any position inside the roof slab structure 11 (specifically, in the cooling gas space 11c), but when the neutron detector 45 is provided in the second pipe P2 in this manner, there is an advantage that the neutron detector 45 can be easily replaced when it becomes necessary to replace it.

[0066] In the fast reactor 1 of this embodiment, the neutron detector 45 is disposed in the cooling gas space 11c of the roof slab structure 11, which is thermally insulated by the thermal shielding layer 11a. With this configuration, the neutron detector 45 is less likely to be damaged by heat, and there is no need to use a special detector that can be used under high-temperature conditions.

[0067] Furthermore, in the fast reactor 1 of this embodiment, if some of the multiple neutron detection units 40 are damaged and an abnormality is detected based on the output value of the neutron detector 45, the control device 80 outputs an alert to the display unit 81. Therefore, an operator can know that an abnormality has occurred in the neutron detection unit 40, and can prevent the operation of the neutron detection unit 40 in question and take measures such as inspection or equipment replacement.

[0068] (Variation) 3, in the above embodiment, the second pipes P2 provided with the neutron detectors 45 are arranged in the large rotation plug 13-1 and the small rotation plug 13-2 of the rotation plug 13. However, in the present invention, for example, all of the second pipes P2 may be arranged in the large rotation plug 13-1 or the small rotation plug 13-2.

[0069] The present invention has been described above using embodiments, but 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 functionally or physically distributing or integrating any unit. Furthermore, new embodiments resulting from any combination of multiple embodiments are also included in the embodiments of the present invention. The effects of the new embodiments resulting from the combination also have the effects of the original embodiments. [Explanation of symbols]

[0070] 1 Fast reactor 10 Main vessel 10a Core vessel 11 Roof slab structure 11a Heat shield layer 11b Radiation shielding layer 11c Cooling gas space 12 Roof slab 13 Rotating plug 13-1 Giant Slalom Plug 13-2 Small rotation plug 14 Fuel exchange machine 15 Divider 20 Core 30 Core superstructure 31 Cylinder 32 Instrument mounting plate 40 Neutron Detection Unit 41 Neutron guide tube 42 Moderator 43 First shielding plug 45 Neutron Detector 46 Second shielding plug 50 Intermediate heat exchanger 51 Entrance window 52 Exit window 60 Circulation Pump 61 Piping 70 Direct core cooling system heat exchanger 80 Control device 81 Display section P1 First pipe P2 Second pipe S1 area

Claims

1. a reactor core containing nuclear fuel; a main vessel formed in a cylindrical shape with an open top and a closed bottom, which accommodates the reactor core together with a coolant; a roof slab structure disposed above the main vessel and enclosing the main vessel; a neutron detection unit for measuring the flux of neutrons from the reactor core; A fast reactor comprising: The neutron detection unit includes: a neutron guide tube extending in a height direction of the main vessel in a region above the reactor core so that its lower end is located within the main vessel and its upper end is located within the roof slab structure, and which guides neutrons from the reactor core to above the main vessel; a moderator disposed within the roof slab structure and configured to reduce the velocity of the neutrons guided through the neutron guide tube; a neutron detector disposed within the roof slab structure for measuring the neutron flux of the moderated neutrons; A fast reactor.

2. The neutron detection unit includes: a first pipe detachably attached to the roof slab structure from above the roof slab structure and extending toward the inside of the main container; a part of the first pipe is configured as the neutron guide tube, and the moderator is disposed within the first pipe; The fast reactor of claim 1.

3. a core upper structure provided in an area above the core and defining a storage space in which instrumentation equipment is disposed; The core upper structure includes: a cylindrical body that surrounds the storage space and extends in a height direction of the main container; an instrumentation mounting plate provided at a lower end of the cylindrical body; and the first pipe is disposed within the cylinder, and a lower end of the first pipe is fixed to the instrumentation mounting plate, and an intermediate portion of the first pipe is fixed to at least one horizontal plate disposed in a height direction within the core superstructure; The fast reactor according to claim 2.

4. The neutron detection unit includes: a shielding plug disposed above the moderator in the first pipe and serving as a member for shielding against heat and radiation from the reactor core side; The fast reactor according to claim 2 or 3.

5. The neutron detection unit includes: a second pipe detachably attached to the roof slab structure from above the roof slab structure; The neutron detector is disposed in the second pipe.

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

6. The roof slab structure comprises: a roof slab covering the top of the main vessel; a rotating plug disposed in a central portion of the roof slab and configured to move a refueling machine for replacing the nuclear fuel; wherein the second pipe is disposed in the rotary plug. The fast reactor according to claim 5.

7. The roof slab structure comprises: a heat shielding layer formed on the underside of the roof slab structure; a radiation shielding layer formed on the upper surface side of the roof slab structure; a cooling gas space, which is a space filled with a cooling gas, is formed between the thermal shielding layer and the radiation shielding layer; The second pipe is The neutron detector is arranged to be located in the cooling gas space. The fast reactor according to claim 6.

8. A plurality of the neutron detection units are provided, a control device that acquires output values ​​from the plurality of neutron detectors; The control device comparing an output value from a predetermined neutron detector with an output value from one or more other neutron detectors, and outputting an alert indicating an abnormality in the predetermined neutron detector if the difference in the output values ​​is equal to or greater than a predetermined threshold value; 3. The fast reactor according to claim 1 or 2.

9. A neutron measurement method for a fast reactor comprising: a core that contains nuclear fuel; a main vessel formed in a cylindrical shape with a bottom and an open top, that contains the core together with a coolant; a roof slab structure that is disposed on top of the main vessel and closes the main vessel; and a neutron detection unit that measures the neutron flux of neutrons from the core, the neutron detection unit includes: a neutron guide tube extending in a height direction of the main vessel in a region above the reactor core such that its lower end is located within the main vessel and its upper end is located within the roof slab structure, and guiding neutrons from the core to above the main vessel; a moderator disposed within the roof slab structure and slowing down the velocity of the neutrons guided through the neutron guide tube; and a neutron detector disposed within the roof slab structure and measuring the neutron flux of the slowed down neutrons, a step of reducing the velocity of the neutrons guided through the neutron guide tube by the moderator; measuring a neutron flux of the neutrons moderated by the moderator with the neutron detector; A neutron measurement method for a fast reactor, comprising:

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