Radiation measurement device and radiation measurement method

Abstract

To provide a radiation measurement device and radiation measurement method, which enable measurement of locations and distribution of nuclear materials.SOLUTION: A radiation measurement device 1 of the present invention comprises a first neutron detector 2 for measuring neutrons emitted from a measurement target 101, a first neutron moderator 3a provided next to the first neutron detector 2 in an arbitrary direction, a layer of neutron absorber 4 provided outside the first neutron moderator 3a, and a second neutron moderator 3b provided next to the outside of the layer of neutron absorber 4.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a radiation measurement device and a radiation measurement method. [Background technology]

[0002] Facilities that handle radioactive materials, including nuclear materials, include nuclear power plants, nuclear fuel processing and manufacturing facilities, waste treatment facilities, accelerator facilities, facilities with controlled areas for radioactive materials, etc. In some of these facilities, nuclear fuel materials such as uranium and plutonium are used for power generation and basic research. In facilities that handle nuclear fuel materials, the safety management of nuclear fuel materials is required because the materials emit radiation that is harmful to the human body. When processing radioactive materials, including nuclear fuel, it is necessary to ensure sufficient safety so that criticality does not occur. When processing materials that contain nuclear fuel materials but whose composition is unknown, special care is required to ensure safety because the conditions for reaching criticality are unknown.

[0003] Therefore, in Patent Document 1, a radiation measuring device using a neutron detector is installed on the surface of a material to measure the radiation intensity distribution. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-121896 A Summary of the Invention [Problem to be solved by the invention]

[0005] According to Patent Document 1, when processing or removing materials including molten and solidified nuclear fuel, a neutron moderator and a neutron detector are arranged adjacent to each other in the axial direction of a guide tube into which neutrons are injected, and a neutron shielding coating that blocks neutrons is provided on the outer periphery of the guide tube, thereby measuring the radiation intensity distribution on the surface of the material. However, in the method described in Patent Document 1, although thermal neutrons can be shielded by the neutron shielding coating, it cannot shield fast neutrons.

[0006] Therefore, high-speed neutrons with high penetrating power coming from an area outside the measurement area anticipated by the guide tube enter the neutron moderator, and thermal neutrons that are thermalized by the neutron moderator are measured by the neutron detector, resulting in the problem that neutron information outside the intended area is included and the radiation intensity distribution cannot be measured accurately. This means that it is not guaranteed that nuclear fuel is present at the measured position.

[0007] The present invention has been devised in view of the above circumstances, and an object of the present invention is to provide a radiation measurement device and a radiation measurement method capable of measuring the position and distribution of nuclear material. [Means for solving the problem]

[0008] In order to solve the above problem, the radiation measurement device of the present invention comprises a first neutron detector that measures neutrons emitted from a measurement object, a first neutron moderator arranged adjacent to the first neutron detector in any direction, a layer of neutron absorbing material provided on the outside of the first neutron moderator, and a second neutron moderator arranged adjacent to the outside of the layer of neutron absorbing material. Effect of the Invention

[0009] According to the radiation measurement device and the measurement method of the present invention, it is possible to provide a radiation measurement device and a radiation measurement method capable of measuring the position and distribution of nuclear material. According to the present invention, it is possible to provide: [Brief description of the drawings]

[0010] [Figure 1] 1 is a longitudinal sectional view of a main part of an example of a radiation measurement device according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing a radiation measurement apparatus as an example of another form of the radiation measurement apparatus according to the first embodiment of the present invention. [Diagram 3] FIG. 11 is a diagram showing an example of a scanning device that is equipped with a radiation measurement device according to a second embodiment of the present invention and scans and measures a measurement object. [Figure 4] FIG. 13 is a diagram showing a process flow for creating a distribution map of nuclear fuel positions. [Diagram 5] FIG. 2 is a diagram showing an example of a distribution map of nuclear fuel positions. [Figure 6] FIG. 2 is a schematic diagram of an example of a scanning device equipped with a radiation measurement device for measuring changes over time in the distribution of nuclear fuel positions during a nuclear fuel processing operation; [Figure 7] FIG. 13 is a diagram showing a process flow for measuring the change over time in the distribution of nuclear fuel positions during a work process and determining whether or not processing / removal work can be continued. [Figure 8A] FIG. 1 is an example of a diagram that shows a schematic representation of neutron flux time change information in a case where the measurement area expected by a radiation measurement device does not include nuclear fuel, but is assumed to include nuclear fuel in the adjacent surrounding area. [Figure 8B] FIG. 1 is an example of a diagram that shows a schematic representation of neutron flux time change information in a case where the measurement area expected by a radiation measurement device does not include nuclear fuel, but is assumed to include nuclear fuel in the adjacent surrounding area. [Figure 9A] This is an example of a diagram that shows a schematic representation of neutron flux time change information in a case where it is assumed that nuclear fuel is included in the area observed by a radiation measurement device, but that no nuclear fuel is included in the surrounding area. [Figure 9B] This is an example of a diagram that shows a schematic representation of neutron flux time change information in a case where it is assumed that nuclear fuel is included in the area observed by a radiation measurement device, but that no nuclear fuel is included in the surrounding area. [Figure 10]FIG. 13 is a diagram showing a process flow for updating a distribution map of nuclear fuel positions based on the measurement values ​​of a radiation measurement device before and after removing a portion of a measurement object. [Figure 11] FIG. 13 is a diagram showing an example of a distribution map updated by a process for updating the distribution map of nuclear fuel positions. [Figure 12] FIG. 13 is a diagram showing an example of a measurement method of a radiation measurement apparatus in a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The present invention relates to a radiation measurement device and a measurement method thereof, and more particularly to a measurement device and a method thereof for identifying the position of nuclear material, etc. during the operation of a radiation measurement device or during construction or dismantling work of a nuclear power plant or the like during decommissioning.

[0012] <<First embodiment>> A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a longitudinal sectional view of a main part of an example of a radiation measurement device 1 according to a first embodiment of the present invention. The radiation measurement device 1 of the first embodiment includes a cylindrical or columnar neutron detector 2 (first neutron detector), a neutron moderator 3a (first neutron moderator) adjacent to one axial end of the neutron detector 2, a neutron moderator 3c adjacent to the cylindrical side of the neutron detector 2, a neutron absorber 4 (a layer of neutron absorber) arranged to cover the entire periphery thereof, and a neutron moderator 3b adjacent to the outside of the neutron absorber 4 covering the neutron moderator 3a, and is stored in a container 5. Supplementally, in this embodiment, the neutron moderator 3a and the neutron moderator 3b are arranged adjacent to one longitudinal end (one axial end) of the neutron detector 2. Note that one side (one side) of the longitudinal end (one axial end) of the neutron detector 2 is the side facing the neutron source 100.

[0013] The neutron moderator 3a has a cylindrical shape. The neutron moderator 3c has an elongated circular shape. The neutron absorber 4 has a cylindrical shape with a bottom surface and a top surface. The neutron moderator 3b has a short torus shape. As the material for the neutron moderators 3a to 3c, a material having a high neutron moderation effect, such as polyethylene, acrylic, or water, is used. Supplementally, as shown in Fig. 1, the cylindrical neutron detector 2 has a cylindrical side covered with a neutron moderator 3c, and a neutron moderator 3a is disposed adjacent to one end in the longitudinal direction. The structure formed by the neutron detector 2, the neutron moderators 3c, and the neutron moderators 3a is entirely covered with a neutron absorbing material 4. Incidentally, the shape of the neutron detector 2 is not limited to a cylindrical or columnar shape, and may be a prismatic prism (a triangular prism, a square prism, a pentagonal prism, or a polygonal prism with a larger angle), etc. When the shape of the neutron detector 2 is a prismatic prism, the shapes of the neutron moderator 3a, etc. and the neutron absorber 4 can be shapes corresponding to the shape of the neutron detector 2.

[0014] The container 5 is provided in consideration of the convenience of handling the radiation measuring device 1. When the radiation measuring device 1 is used in an air environment, in particular, the container 5 is not necessary. On the other hand, when the radiation measurement device 1 is used in an underwater environment, it is desirable to use a hollow container 5 . The hollow container 5 is provided to prevent the neutron detector 2 from being damaged due to being submerged in water, and also to suppress thermalization of fast neutrons.

[0015] As shown in Fig. 1, measurement by the radiation measurement device 1 is performed by orienting the neutron moderator 3a in the direction of a neutron source 100, which is a radiation source. A measurement signal from a neutron detector 2 that detects neutrons emitted from the neutron source 100 is amplified in a signal amplifier 7 via a cable 6a. The amplified measurement signal is sent to a data processor 8 via a cable 6b. The neutrons emitted from the neutron source 100 are neutrons emitted when spontaneous fission material contained in nuclear fuel undergoes nuclear fission (hereinafter referred to as spontaneous fission neutrons). The neutrons have a wide energy distribution from high energy (several MeV) to low energy (1 eV or less). Most of the neutrons are fast neutrons. Fast neutrons are not scattered by materials or are scattered only a small number of times, and have directionality.

[0016] On the other hand, thermal neutrons are fast neutrons that scatter with matter, transferring their energy to the matter while losing their own energy. As a result, thermal neutrons lose their directionality. For this reason, it is possible to identify the location of nuclear fuel by measuring the direction of fast neutrons, which have directionality. Neutron detector 2 is a neutron detector that has sensitivity to thermal neutrons, such as a He-3 proportional counter or a B-10 proportional counter, which have a proven track record. In order to measure fast neutrons with these neutron detectors, the fast neutrons need to be converted into thermal neutrons.

[0017] Therefore, the neutron moderator 3a thermalizes (hereinafter, thermalizes) the fast neutrons coming from the neutron source 100 of the radiation source to identify the direction of the fast neutrons. It is desirable that the neutron moderator 3a has a sufficient length in order to improve the measurement sensitivity of the neutron detector 2 by increasing the amount of thermalized neutrons. The neutron moderator 3c is provided to guide the neutrons thermalized by the neutron moderator 3a to the neutron detector 2. Although there is no problem if the neutron moderator 3c is not provided, it is desirable to provide it from the viewpoint of measurement sensitivity. It is desirable that the thickness of the neutron moderator 3c is set to a level that is not affected by the thermalization of fast neutrons entering from the lateral direction in FIG. 1 (outside the side peripheral surface of the neutron detector 2).

[0018] The neutron absorbing material 4, which covers the entire surface (whole) around the neutron moderator 3a and the neutron moderator 3c, blocks thermal neutrons in the measurement environment. Cadmium, which has a large neutron absorption cross section, can be used as the material for the neutron absorbing material 4 (as long as the neutron absorbing material 4 has a thickness that can block thermal neutrons present in the measurement environment). However, although the neutron absorbing material 4 can block thermal neutrons, fast neutrons pass through it. The fast neutrons that pass through the neutron absorbing material 4 are partially thermalized by the neutron moderating material 3a. Therefore, fast neutrons coming from directions other than the direction of the neutron source 100 shown in FIG. 1, especially from the lateral direction in FIG. 1 (outside the side surface of the neutron detector 2), are also measured.

[0019] Therefore, as shown in Fig. 1, an annular neutron moderator 3b is disposed outside the neutron absorber 4 that covers the neutron moderator 3a. The neutron moderator 3b thermalizes the fast neutrons coming from the lateral direction in Fig. 1 (outside the side peripheral surface of the neutron moderator 3a) and absorbs the neutrons thermalized by the neutron absorber 4. This blocks the fast neutrons coming from the lateral direction in Fig. 1 (outside the side peripheral surface of the neutron moderator 3a). For this reason, it is desirable that the neutron moderator 3b has a thickness sufficient to sufficiently thermalize the fast neutrons coming from the lateral direction in Fig. 1.

[0020] However, there are cases where the neutron moderator 3b cannot be made sufficiently thick (long) due to restrictions such as the measurement environment. In such cases, it is possible to use radiation transport analysis such as the Monte Carlo method in combination to calculate the amount or rate of transmission of fast neutrons through the set thickness of the neutron moderator 3b, i.e., the amount or rate of fast neutrons that enter the neutron moderator 3a, and to carry out correction processing such as subtracting this from the measurement value measured by the neutron detector 2.

[0021] FIG. 2 shows a radiation measurement device 1A as an example of another configuration of the radiation measurement device 1 of the first embodiment. The components of the radiation measurement device 1A are the same as those of the radiation measurement device 1 shown in Fig. 1, but neutron moderators 3a to 3c are arranged on the side of the cylindrical part of the neutron detector 2 placed horizontally. In the radiation measurement device 1 shown in Fig. 1, the neutron detector 2 is upright on the paper surface of Fig. 1. In other words, the neutron detector 2 is oriented in the vertical direction of Fig. 1, while in Fig. 2, the neutron detector 2 is oriented in the horizontal direction on the paper surface of Fig. 2. That is, in the first embodiment, the "arbitrary direction" of the neutron detector 2 is the vertical direction, and in one example of another form of the first embodiment, the "arbitrary direction" of the neutron detector 2 is the horizontal direction. In other words, in the first embodiment of Fig. 1, the neutron detector 2 is oriented so as to stand upright (vertical) with respect to the neutron source 100, and in one example of another form of Fig. 2, the neutron detector 2 is oriented so as to lie down (horizontal) with respect to the neutron source 100.

[0022] It is possible to select the radiation measurement device 1 shown in FIG. 1 or the radiation measurement device 1A shown in FIG. 2 depending on the installation environment at the measurement site, such as the condition of narrow spaces in the measurement environment. According to the first embodiment of the present invention described above, the position of nuclear fuel can be measured by measuring the direction in which fast neutrons from the nuclear fuel come (the direction of the neutron source 100).

[0023] <<Second embodiment>> A second embodiment of the present invention will be described with reference to FIGS. FIG. 3 shows an example of a scanning device 50 that is equipped with the radiation measurement device 1 according to the second embodiment of the present invention and scans and measures a measurement object 101. The scanning device 50 of the second embodiment is configured to have a scanning device body 55 and a scanning arm 51 installed on the scanning device body 55. The scanning device body 55 moves (remotely moves) in the direction indicated by the white arrow α11 from the front side to the back side of FIG. 3. The radiation measurement device 1 can be moved to a neutron measurement position by the scanning device 50. Also, the radiation measurement device 1 can be moved by attaching wheels to the underside of the scanning device 50.

[0024] The scanning arm 51 moves up and down (indicated by the white arrow α12 in FIG. 3) as it approaches the scanning device main body 55 or by the movement of the scanning arm 51 itself, or moves left and right in FIG. 3 (indicated by the white arrow α13 in FIG. 3) by contracting and extending the joints of the scanning arm 51 (indicated by the white arrow α13 in FIG. 3). The movement of scanning arm 51 and scanning device main body 55 enables three-dimensional scanning of measurement object 101 in three axial directions.

[0025] A gripping device 51h is provided at the tip of the scanning arm 51. The gripping device 51h grips the radiation measurement device 1. The gripping device 51h is capable of rotating as indicated by a white arrow α14 in FIG. By the above-mentioned three-axis movement (white arrows α11, α12, α13) and rotational movement (white arrow α14 in FIG. 3), the entire surface of the measurement object 101 can be scanned, and the radiation of the measurement object 101 can be measured.

[0026] 3 is amplified by a signal amplifier 7 and then transmitted to a data processor 8 for data processing. At that time, time information at which the measurement signal was acquired is also recorded. Meanwhile, in the scanning device 50, the scanning position, specifically, information on the measurement position by the radiation measurement device 1 is recorded in the scanning position recording device 52 as the scanning position of the scanning device 50 based on each operation information of the scanning device main body 55 and the scanning arm 51. By recording the scanning position of the scanning device 50, the neutron measurement value is associated with the scanning position at the time of measurement.

[0027] A distribution map of the positions of nuclear fuel is created based on the measurement values ​​related to the direction of travel of fast neutrons by the radiation measurement device 1 and the information on the scanning positions recorded in the scanning position recording device 52. As shown in FIG. 3, an optical camera 11 can be installed in the scanning device 50 to confirm the positional relationship between the measurement object 101 and the radiation measurement device 1, or to confirm whether an unexpected situation has occurred. The optical camera 11 photographs the positional relationship between the measurement object 101 and the radiation measurement device 1, or photographs the situation of the measurement object 101. This makes it possible to confirm the positional relationship between the measurement object 101 and the radiation measurement device 1, or to confirm whether an unexpected situation has occurred near the measurement object 101.

[0028] It is also possible to improve the accuracy of identifying the nuclear fuel position by using a gamma ray measurement device 12 capable of nuclide analysis in combination with the gamma ray measurement device 12 to measure gamma rays emitted from the fission products. In Fig. 3, the gamma ray measurement device 12 has a gamma ray detector 13 and a shielding body 14 equipped with a collimator 15. For example, a scintillation or semiconductor detector capable of measuring the gamma ray energy spectrum can be used as the gamma ray detector 13. By using these detectors, it is possible to identify the nuclides that emit gamma rays from the energy of the gamma rays, and among those nuclides, those originating from nuclear fuel or fission products can be specified.

[0029] This makes it possible to evaluate these positions and distributions by combining with the collimator 15 described below. The collimator 15 is for narrowing the field of view (controlling the direction of incidence of the gamma rays) in order to identify the direction from which the gamma rays come. The shield 14 is for suppressing gamma rays entering the gamma ray detector 13 from directions other than those focused on by the collimator 15.

[0030] <Creating distribution map 53 of nuclear fuel locations> FIG. 4 shows a process flow for creating a distribution map 53 of nuclear fuel positions. In the data processing unit 8 (see FIG. 3), a process for creating a distribution map 53 of nuclear fuel positions is carried out. First, information such as the measurement position, the time required for measurement, and the operation method related to the scanning order such as how to scan is input (processing step S1001 in FIG. 4). Based on the input information, the scanning device 50 operates the scanning device main body 55 and the scanning arm 51 to move the radiation measurement device 1 to a specified measurement position near the measurement object 101 (processing step S1002 in FIG. 4).

[0031] When the movement of the radiation measurement device 1 is completed, the position of the radiation measurement device 1 is recorded by the scanning position recording device 52 (processing step S1003 in FIG. 4). Next, the radiation measurement device 1 measures fast neutrons at the measurement time specified as input information (processing step S1004 in FIG. 4). During measurement by the radiation measurement device 1, the measurement values ​​are sequentially transmitted to the data processing unit 8 and recorded in the scanning position recording device 52 (processing step S1005 in FIG. 4). In processing step S1004, after the measurement for the measurement time designated in processing step S1001 is completed and further the recording in step S1005 is completed, the information on the scanning method etc. at the time of input is compared with the current measurement position to determine whether it is the final position or not (processing step S1006 in FIG. 4).

[0032] If it is not the final position (NO in processing step S1006 in FIG. 4), the process returns to processing step S1002, and movement and measurement are continued. If it is the final position (Yes in processing step S1006 in FIG. 4), a distribution map of nuclear fuel positions is created from the information on the measurement position and the information on the measurement values ​​acquired up to that point (processing step S1007 in FIG. 4). By creating a distribution map of nuclear fuel locations 53 (see Figure 4), safe processing and removal work can be carried out.

[0033] <An example of a distribution map of nuclear fuel locations> FIG. 5 shows an example of a distribution map 53 of nuclear fuel positions. The distribution of nuclear fuel positions is expressed as contour lines according to the intensity of radiation (fast neutrons), as shown in Figure 5. Dark areas are areas with high radiation doses, and light areas are areas with low radiation doses. In creating the distribution map 53, as described above, the information on the measurement positions and the measurement values ​​may be simply given as point information, and contour lines may be created based on the information. If necessary, in order to achieve high accuracy, the map may be created by performing image reconstruction processing based on known maximum likelihood estimation.

[0034] It should be noted that the above-mentioned scanning device 50 in Fig. 3 is one example, and is not limited or restricted to the configuration shown in Fig. 3. In addition, in Fig. 3, the surface of the measurement object 101 is flat for simplicity, but in reality, it is considered that the surface has a three-dimensionally varying shape having a slope, a step, etc. However, with the method of this embodiment (the present invention), it is possible to measure even a surface having such a three-dimensionally varying shape.

[0035] As shown in Figure 5, by creating a distribution map of nuclear fuel locations in advance, it is possible to distinguish between locations that contain a large amount of nuclear fuel and therefore require careful processing, and locations that contain a small amount of nuclear fuel and can be processed relatively roughly, when carrying out processing work to dismantle the nuclear fuel. Therefore, it is possible to use the distribution map of nuclear fuel locations when planning processing work. According to the second embodiment of the present invention described above, the distribution of nuclear fuel positions can be measured by using the scanning device 50 (see FIG. 3) in combination with the radiation measurement device 1. Therefore, it becomes possible to utilize the measured distribution of nuclear fuel positions in a processing work plan for dismantling nuclear fuel.

[0036] <<Third embodiment>> A third embodiment of the present invention will be described with reference to FIGS. The third embodiment of the present invention is intended to measure a time change in the distribution of nuclear fuel positions during a work process in which a part of a measurement object 101 is processed and removed.

[0037] FIG. 6 is a schematic diagram showing an example of a scanning device 50A equipped with a radiation measurement device 1 for measuring the change over time in the distribution of nuclear fuel positions during a nuclear fuel processing operation process. A scanning device 50A of the third embodiment is provided with a second scanning arm 51a in addition to the scanning arm 51 shown in Fig. 3, in comparison with the scanning device 50 equipped with the radiation measurement device 1 shown in Fig. 3. A neutron detector 2A is mounted on the second scanning arm 51a. Further, the device has a time change recording device 62 and a measurement value time change information generating and determining device 63. The time change recording device 62 records the measurement values ​​at each time measured by the radiation measurement device 1 and the neutron detector 2A. The time change recording device 62 can store the measurement values ​​at each time.

[0038] The measurement value time change information generation and determination device 63 uses the time changes of the measurement values ​​recorded by the time change recording device 62 as input to generate measurement value time change information, and determines the neutron increase rate based on the generated information values.

[0039] However, in FIG. 6, the optical camera 11 and the gamma ray measurement device 12 shown in FIG. 3 are omitted in order to ensure ease of viewing of FIG. 6. It is of course possible to provide the optical camera 11 and the gamma ray measurement device 12 in the same manner as in FIG. 3 and operate in the same manner as in the second embodiment. The neutron detector 2A measures the change in neutron flux locally at the work site in order to monitor information relating to the criticality of nuclear fuel during the work process.

[0040] In the processing and removal work of the measurement object 101 containing nuclear fuel, the state of the measurement object 101 is changed. There is a concern that the shape and state of the measurement object 101 may change due to the processing and removal work, which may increase the number of incoming neutrons. An increase in the number of incoming neutrons leads to an increase in the neutron multiplication factor, and even if the measurement object 101 was in a subcritical state before processing, the possibility cannot be denied that the nuclear fuel contained in the measurement object 101 may approach a critical state. In order to safely carry out the processing and removal work of the measurement object 101, it is necessary to monitor the rate of increase in the number of incoming neutrons, and if a rapid increase in the rate of neutrons that predicts criticality is expected, an alarm must be issued and the work must be stopped.

[0041] Therefore, the neutron detector 2A can use a detector that has sensitivity to thermal neutrons, similar to the radiation measurement device 1. However, the neutron detector 2A has a different configuration from the radiation measurement device 1, since it measures neutron flux in a local field. For example, when processing or removal work is performed underwater, the neutron detector 2A is sealed in a water-sealed container, and the increase or decrease in neutron flux is determined by measuring thermal neutrons using the neutron moderation effect of water.

[0042] In addition, when processing / removal work is performed in air, the neutron detector 2A is entirely covered with a moderator of an appropriate thickness, and the increase or decrease in neutron flux is determined by measuring the increased thermal neutrons, although this is not shown in the drawings. The neutron detector 2A may be located anywhere in the vicinity of the processing / removal area in order to measure the neutron flux in a local field. In addition, although not shown in FIG. 6, the scanning device 50A may be provided with an optical camera 11 (see FIG. 3) to confirm the positional relationship between the measurement object 101 and the radiation measurement device 1, or to confirm whether an unexpected situation has occurred, as in the second embodiment. In particular, by photographing the processing / removal work status of the measurement object 101 with the optical camera 11 and acquiring the time information, it becomes possible to associate the progress of the work with the information obtained by the measurement.

[0043] Also, similarly to the second embodiment, it is possible to improve the accuracy of identifying the nuclear fuel position by using a gamma ray measurement device 12 (see FIG. 3) capable of nuclide analysis in combination to measure gamma rays emitted from the fission products.

[0044] The radiation measurement device 1 shown in FIG. 6 is disposed so as to anticipate the area of ​​the measurement object 101 to be processed or removed, and measures the state of the measurement object 101 before and after the processing or removal. If the processed / removed area of ​​the measurement object 101 contains a large amount of nuclear fuel, the amount of fast neutrons measured after processing / removal will decrease. On the other hand, if there is no significant change in the amount of fast neutrons measured after processing / removal, it can be estimated that the amount of nuclear fuel contained in the processed / removed area is relatively small.

[0045] In addition, by updating the distribution map 53 (see Figure 5) of the nuclear fuel positions described in the second embodiment based on the measurement values ​​of the processed and removed areas of the measurement object 101, the processing work plan can be updated from the newly acquired information.

[0046] <Process flow for measuring time-dependent changes in distribution of nuclear fuel positions and determining whether work can continue or not> Figure 7 shows the process flow for measuring the time change in the distribution of nuclear fuel positions during the work process and determining whether or not processing and removal work can be continued. The process of determining whether or not the processing / removal work of the measurement object 101 can be continued is performed by the data processing unit 8 (see FIG. 6).

[0047] First, conditions such as the measurement position and measurement time of the measurement object 101 are input based on a plan for processing and removing the measurement object 101 (processing step S1101 in FIG. 7). The measurement position is determined based on the size of the work area for the processing work, etc. The measurement time is determined based on the lower detection limit, etc. Next, the times of the radiation measurement device 1 and the neutron detector 2A shown in Fig. 6 are synchronized (processing step S1102 in Fig. 7). The time synchronization is performed to obtain a time correlation between the radiation measurement device 1 and the neutron detector 2A.

[0048] Next, the radiation measurement device 1 in Fig. 6 is moved to a designated position for measuring fast neutrons of the measurement object 101 (processing step S1103 in Fig. 7). Furthermore, the neutron detector 2A is placed near the radiation measurement device 1 and the measurement object 101 (processing step S1104 in Fig. 7). Thereafter, the radiation measurement device 1 and the neutron detector 2A start measurement (processing step S1105 in FIG. 7).

[0049] During measurements by the radiation measurement device 1 and the neutron detector 2A, the time history of the measurement values ​​at each time is sequentially transmitted to the data processing unit 8 and recorded (processing step S1106 in FIG. 7). Next, a processing / removal operation is performed on a part of the measurement object 101 (processing step S1107 in FIG. 7). From the time of collection until the end of the processing / removal operation, measurement by the radiation measurement device 1 and the neutron detector 2A is continuously performed.

[0050] Next, the measurement value time history of the radiation measurement device 1 and the time history of the neutron detector 2A are input to the measurement value time change information generation process (processing step S1108 in FIG. 7). Next, a neutron flux time change information value is generated from the time history of the respective measurement values ​​of the radiation measurement device 1 and the neutron detector 2A by a measurement value time change information generation and determination process (processing step S1109 in FIG. 7). Furthermore, the measurement value time change information generation and determination process performs a process of determining whether or not to issue a work stop warning by comparing the neutron flux time change information value with a preset threshold value (processing step S1110 in FIG. 7). The threshold value is a threshold value that determines whether or not to stop the processing / removal work, and is set in advance as a value to be used in the determination process.

[0051] If the neutron flux time change information value is equal to or less than the threshold value (NO in processing step S1110 in FIG. 7), the process returns to processing step S1107 and continues the operation. On the other hand, if the neutron flux time change information value is greater than the threshold value (YES in process step S1110 in FIG. 7), a work stop warning is issued to prompt the user to stop work (process step S1111 in FIG. 7). Thereafter, the measurement by the radiation measurement device 1 and the neutron detector 2A is terminated (processing step S1112 in FIG. 7).

[0052] It should be noted that the process step S1112 may be stopped after measuring the progress of the change in the neutron flux measurement values ​​of the radiation measurement device 1 and the neutron detector 2A for a while after the work stop warning is issued and the measurement values ​​have sufficiently converged. By the above-mentioned process of determining whether or not the processing / removal work of the measurement object 101 can be continued, the processing / removal work can be performed safely.

[0053] <An example of neutron flux time change information> 8A and 8B show examples of images of neutron flux time change information generated in the measurement value time change information generation determination process.

[0054] 8A and 8B are examples of diagrams that show neutron flux time change information in a case where the measurement area expected by the radiation measurement device 1 does not include nuclear fuel, but is assumed to include nuclear fuel in the adjacent surrounding area.

[0055] 8A and 8B, the horizontal axis represents time T, and the vertical axis represents neutron count rate C. In the neutron measurement value time history 65 by the radiation measurement device 1 shown in Fig. 8A, no change is observed in the neutron count rate even after time T0 when removal of a portion of the measurement object 101 started. In the case of Fig. 8A, the area seen by the radiation measurement device 1 does not include nuclear fuel, so there is almost no change in fast neutrons before and after the start of removal.

[0056] On the other hand, in the neutron measurement value time history 66 of the neutron detector 2A shown in Fig. 8B, for example, the time interval is dT, the neutron count rate that has changed during dT is dC, and dC / dT is the neutron flux time change information value. Referring to Fig. 8B, a situation is shown in which the neutron count rate C rises (dC / dT increases suddenly) after the start of removing a part of the measurement object 101. This indicates that, for example, when the removal work is performed underwater, water enters the removed space, the neutrons are thermalized by the water, and react with the nuclear fuel contained in the surroundings adjacent to the area observed by the radiation measurement device 1, causing a nuclear fission reaction to proceed, and as a result, fission neutrons are generated.

[0057] The neutron flux time change information value dC / dT is compared with a preset threshold value to determine whether or not to issue a work stop warning. Criticality may occur during processing and removal work, which is dangerous, so a threshold value for stopping processing work is preset. In order to ensure sufficient safety in the operation of the measurement value time change information generation determination process, the time interval dT can be shortened, or the threshold can be set low, or the time interval dT can be shortened and the threshold can be set low.

[0058] 9A and 9B show other examples of images of neutron flux time change information generated in the measurement value time change information generation determination process. 9A and 9B are examples of diagrams that show schematic representations of neutron flux time change information in a case where it is assumed that nuclear fuel is included in the area observed by the radiation measurement device 1, but no nuclear fuel is included in the surrounding area.

[0059] 9A shows that the neutron count rate decreases after time T0 when removal of a portion of the measurement object 101 is started. This shows that the radiation source disappears when the area seen by the radiation measurement device 1 contains nuclear fuel.

[0060] Similarly, in the neutron measurement value time history 66 of the neutron detector 2A shown in Fig. 9B, the neutron count rate decreases after the time T0 when the removal of a portion of the measurement object 101 started. This indicates that, contrary to the case explained in Fig. 8B above, even if water enters the space from which a portion of the measurement object 101 has been removed, no nuclear reaction is induced because there is no nuclear fuel in the adjacent surroundings, and there is no increase in neutrons. In this case, the neutron flux time change information value dC / dT becomes a negative value, and no decision is made to issue a work stoppage warning.

[0061] In this way, by checking the updated status of processing and removal work while monitoring information that could lead to criticality, it is possible to safely proceed with processing and removal work. The above-mentioned process is particularly important in a situation where the composition of the measurement object 101 is unknown, even if the distribution of the nuclear fuel position is known. In a situation where the composition is unknown, it is difficult to estimate the absolute value of the amount of nuclear fuel, etc., because the situation of radiation shielding, etc. is also unknown. It is important to ensure safety by monitoring information that leads to criticality, especially in the early stages of such processing and removal work. By analyzing the removed material, data on the composition, etc. of the measurement object 101 is gradually accumulated, and by feeding back this information, the information on the distribution of the composition can be made more accurate. As a result, the construction plan can be made safer and more accurate.

[0062] <Process to update distribution map 53 of nuclear fuel locations> FIG. 10 shows a process flow for updating the distribution map 53 of nuclear fuel positions based on the measurement values ​​of the radiation measurement device 1 before and after removing a portion of the measurement object 101 described above. In the update process, first, the previously created distribution map 53 (see FIG. 5) is input (processing step S1011 in FIG. 10). Next, information such as the measurement position and measurement time for which data is to be updated is input (processing step S1001a). In this process, it is possible to reuse the information in processing step S1101 in the processing flow diagram shown in FIG. 7.

[0063] Furthermore, the neutron flux time change information value shown in processing step S1109 in Fig. 7 is acquired (processing step S1012 in Fig. 10). The distribution map 53 is updated by a distribution map update process based on the input information on the measurement position and measurement value, and the information on the measurement value that is considered to be in a steady state among the neutron flux time change information values ​​(processing step S1013 in Fig. 10). Fig. 11 shows an example of a distribution map 53A updated by the process of updating the distribution map of nuclear fuel positions.

[0064] In FIG. 11, the measurement value at the removed portion 53b has decreased. According to the third embodiment of the present invention described above, by simultaneously monitoring the removal status of the radiation source and the increase or decrease in neutron flux, it is possible to safely proceed with work by checking the update status of the work while monitoring information leading to criticality.

[0065] <<Fourth embodiment>> FIG. 12 shows a diagram illustrating an example of a measurement method of the radiation measurement device 1 in the fourth embodiment of the present invention.

[0066] A fourth embodiment of the present invention will be described with reference to FIG. The fourth embodiment of the present invention relates to a method for calibrating the radiation measurement device 1 in a measurement environment.

[0067] The radiation measurement device 1 described above has directivity by giving sensitivity to a specific direction of the neutron detector 2 in order to capture the direction of arrival of fast neutrons. Therefore, in order to calibrate the environmental background information, it is necessary to carry out the calibration based on information from all directions in the measurement environment, not just information from a specific direction. Figure 12 is a diagram showing an example of a measurement method for calibration based on this idea.

[0068] The radiation measurement device 1 described above changes its posture by rotating from the upper left state in FIG. 12 along the thick white arrow to the lower left state. In each posture, the radiation measurement device 1 is rotated at least once in the direction indicated by the thin curved arrow β11 in the figure. Rotation is repeated by measuring in a certain direction for a certain period of time, then rotating a specified rotation angle and measuring at the next position. For this reason, there is no particular need to rotate the neutron detector 2 at high speed. The specified angle for rotation in the direction indicated by the arrow β11 may be an angle determined by the diameter of the neutron moderator 3a (the angle formed by an arc that is the diameter of the neutron moderator 3a), or it may be an angle smaller than that.

[0069] In this way, the radiation measurement device 1 is rotated at least once in a certain attitude, and then the attitude of the radiation measurement device 1 is tilted by a specified angle as shown in FIG. 12, and the device is repeatedly rotated at least once in the direction indicated by the thin curved arrow β11 in the figure. Similarly, the orientation angle of the attitude may be an angle determined by the diameter of the neutron moderator 3a (an angle formed by the diameter of the neutron moderator 3a as an arc) or an angle smaller than that. It is desirable to perform measurements for calibrating the radiation measurement device 1 at a position or location where the influence of the fast neutron source (neutron source 100) is expected to be as small as possible. The information measured in this way is evaluated as environmental background information by subtracting it from the measurement value in the measurement of the nuclear fuel position.

[0070] According to the fourth embodiment of the present invention described above, even if the radiation measurement device 1, which is a detector, has sensitivity in a specific direction and has directivity, it becomes possible to calibrate the environmental background information. [Industrial Applicability]

[0071] By using the radiation measurement device and measurement method of the present invention, it is possible to measure the location and distribution of nuclear material. <<Other embodiments>> 1. The present invention is not limited to the above-described embodiment and modified configurations, and various modifications and specific forms are possible within the scope of the appended claims. [Explanation of symbols]

[0072] 1. 1A Radiation measurement device 2 Neutron detector (first neutron detector) 2A Neutron Detector (Second Neutron Detector) 3a Neutron moderator (first neutron moderator) 3b Neutron moderator (second neutron moderator) 4. Neutron absorbers 8 Data Processing Section 11 Optical Camera 12 Gamma ray measuring device 13 Gamma ray detector 14 Shield 15 Collimator 50 Scanning Device 51 Scanning arm (scanning device) 52 Scanning position recording device 53, 53A distribution map 55 Scanning device body 62 Time Change Recording Device 63 Measurement value time change information generation and determination device 65, 66 Neutron measurement time history 100 neutron source 101 Measurement object S1001~S1111 Processing steps

Claims

1. A first neutron detector that measures neutrons emitted from the measurement object; A first neutron moderator disposed adjacent to the first neutron detector in any direction; a layer of neutron absorbing material disposed on the exterior of the first neutron moderator; and a second neutron moderator disposed adjacent to the outer side of the layer of neutron absorbing material.

2. 2. The radiation measurement device according to claim 1, A radiation measurement device characterized in that the first neutron moderator, the neutron absorbing material, and the second neutron moderator are arranged adjacent to one longitudinal end of the first neutron detector.

3. 2. The radiation measurement device according to claim 1, A radiation measurement device, characterized in that the first neutron moderator, the neutron absorbing material, and the second neutron moderator are arranged adjacent to each other in a short side direction of the first neutron detector.

4. The radiation measurement device according to any one of claims 1 to 3, A scanning device that is equipped with at least one of the radiation measurement devices and scans in an arbitrary direction; and a scanning position recorder for recording a position scanned by the scanning device.

5. The radiation measurement device according to any one of claims 1 to 3, a second neutron detector for measuring neutrons in the environment; a time change recorder that records changes over time in the measurement values ​​of the first neutron detector and the measurement values ​​of the second neutron detector.

6. The radiation measurement device according to any one of claims 1 to 3, A radiation measuring device comprising a gamma ray measuring device for measuring gamma rays emitted from an object to be measured.

7. The radiation measurement device according to any one of claims 1 to 3, A radiation measurement device comprising an optical camera for photographing the condition of an object to be measured.

8. A radiation measurement method for a radiation measurement device including a scanning device, a first neutron detector that measures neutrons emitted from a measurement object, a scanning position recording device, and a data processing device, comprising: the scanning device scans the first neutron detector; the scanning position recorder records a scanning position of the first neutron detector; The first neutron detector measures neutrons to obtain a measurement value; The radiation measuring method, wherein the data processing unit creates a distribution map based on the recorded scanning positions and the measured values.

9. The radiation measurement method according to claim 8, the scanning device newly scans the first neutron detector; the scan position recorder records a new scan position of the first neutron detector; the first neutron detector newly measures neutrons to obtain a new measurement value; The radiation measuring method, wherein the data processing unit updates the distribution map based on the new measurement value and the new scanning position that has been recorded.

10. The radiation measurement method according to claim 8 or 9, The radiation measurement device includes a time change recorder and a second neutron detector that measures neutrons in the environment; the second neutron detector measures neutrons to obtain a second measurement value; The time change recorder records a time change of a measurement value of the first neutron detector, The radiation measurement method is characterized in that the data processing unit generates measurement value time change related information by correlating the measurement value time change of the first neutron detector with the measurement value time change of the second neutron detector, and performs a determination process based on the measurement value time change related information.

11. The radiation measurement method according to claim 8 or 9, The radiation measurement device includes a gamma ray measurement device, A radiation measuring method, characterized in that the gamma ray measuring device measures gamma rays emitted from the object to be measured.

12. The radiation measurement method according to claim 8 or 9, The radiation measurement device includes an optical camera, The radiation measuring method, wherein the optical camera photographs the condition of the object to be measured.

13. A radiation measurement method using the radiation measurement device according to any one of claims 1 to 3 and a scanning device, comprising: The scanning device includes: rotating the first neutron detector as a first rotation and performing a second rotation on the first neutron detector about a rotation axis perpendicular to a rotation plane of the first rotation; A radiation measurement method, characterized in that the first neutron detector measures neutrons during each of the first rotation and the second rotation.

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