Radiation assessment method, nuclear fuel debris removal method, and radiation assessment device

Abstract

To provide a radiation evaluation method, a method for removing nuclear fuel debris, and a radiation evaluation device that can evaluate more easily than before intensity of a radiation source in a target area where removal work is performed.SOLUTION: A radiation evaluation method includes the steps of: installing radiation detectors 120, 130 having directivity at two or more different positions to measure a radiation counting rate c in two or more places in an area 101 where removal work is performed; estimating detection efficiency ε of the radiation detectors 120, 130 for the target area 101; and from results of measurement at the different positions, comparing values obtained by dividing the radiation counting rate c by the detection efficiency ε to evaluate intensity of a radiation source present in the area 101.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a radiation assessment method, a nuclear fuel debris removal method, and a radiation assessment device. [Background technology]

[0002] As an example of a criticality detection technology that is quick and reliable and does not depend on changes in the state of the object to be detected, a criticality detection device is described that includes a first neutron detector that detects neutrons passing a position a first distance away from the intersection on a first half-line whose intersection is the destruction position that destroys the molten solidified nuclear fuel, a second neutron detector that detects neutrons passing a position a second distance away from the intersection on this first half-line, a third neutron detector that detects neutrons passing a position a first distance away from the intersection on a second half-line that is different from the first half-line and whose intersection is the destruction position, and a fourth neutron detector that detects neutrons passing a position a second distance away from the intersection on the second half-line. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-78585 Summary of the Invention [Problem to be solved by the invention]

[0004] Background art in this technical field is the technology described in Patent Document 1. As described in the above Patent Document 1, in order to derive the effective multiplication factor k, which is used as a basis for determining whether to continue or stop the removal work, it is necessary to go through a complicated process, such as deriving a correction coefficient in advance. This makes the removal work take time, and increases the radiation dose to workers.

[0005] On the other hand, in Patent Document 1, the premise for proceeding with the steps of the disclosed flowchart is that the effective multiplication factor k is directly derived using a gas monitor, a reactor noise method, etc., and the degree of subcriticality is measured. Therefore, in actual nuclear fuel debris removal work, a measurement method for directly deriving the effective multiplication factor k is essential, which poses the problem that the process of deriving the effective multiplication factor k, which has been a conventional problem, cannot be omitted.

[0006] The present invention provides a radiation assessment method, a nuclear fuel debris removal method, and a radiation assessment device that can more easily assess the radiation source intensity of a target area where removal work is to be performed than conventional methods. [Means for solving the problem]

[0007] The present invention includes multiple means for solving the above-mentioned problems, and one example thereof is characterized by comprising the steps of installing directional radiation detectors at two or more different positions and measuring the radiation count rate in a nuclear fuel debris area that is the target of removal work at two or more positions; estimating the detection efficiency of the radiation detectors for the target nuclear fuel debris area; and evaluating the strength of the radiation source present in the nuclear fuel debris area by comparing the values obtained by dividing the radiation count rate by the detection efficiency from the measurement results at the different positions. [Effects of the Invention]

[0008] According to the present invention, the radiation source intensity near the surface of the target area where removal work is to be performed can be evaluated more easily than in the past, which makes it possible to efficiently proceed with the removal work and is effective in reducing radiation exposure of workers. Problems, configurations, and effects other than those described above will become clear from the description of the following examples. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing an outline of a radiation evaluation device according to a first embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the structure of a radiation detector in the radiation evaluation device according to the first embodiment. [Figure 3]FIG. 2 is a diagram showing an example of the structure of a radiation detector in the radiation evaluation device according to the first embodiment. [Figure 4] FIG. 2 is a diagram showing an example of a radiation evaluation method using the radiation evaluation device according to the first embodiment. [Figure 5] FIG. 2 is a diagram showing an example of a flow for evaluating the intensity of radiation emitted by nuclear fuel debris, as a specific example of a radiation evaluation method according to the first embodiment. [Figure 6] FIG. 4 is a diagram showing another example of the radiation evaluation method using the radiation evaluation apparatus according to the first embodiment. [Figure 7] FIG. 4 is a diagram showing another example of a flow for evaluating the intensity of radiation emitted by nuclear fuel debris, as a specific example of the radiation evaluation method according to the first embodiment. [Figure 8] FIG. 10 is a diagram showing an example of the structure of a radiation detector in a radiation evaluation apparatus according to a second embodiment. [Figure 9] FIG. 10 is a diagram showing an example of a radiation evaluation method using the radiation evaluation apparatus according to the second embodiment. [Figure 10] FIG. 10 is a diagram showing an example of a flow for evaluating the intensity of radiation emitted by nuclear fuel debris, as a specific example of a radiation evaluation method according to a second embodiment. [Figure 11] FIG. 10 is a diagram showing an example of the flow of nuclear fuel debris removal work according to a third embodiment, which is an application example of the radiation evaluation method according to the present invention. [Figure 12] FIG. 10 is a diagram showing an example of the flow of nuclear fuel debris removal work according to a third embodiment, which is an application example of the radiation evaluation method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] The radiation assessment method, nuclear fuel debris removal method, and radiation assessment device of the present invention will be described below with reference to the accompanying drawings. In the drawings used in this specification, identical or corresponding components are designated by the same or similar reference numerals, and repeated explanations of these components may be omitted.

[0011] Example 1 A radiation evaluation method and a radiation evaluation apparatus according to a first embodiment of the present invention will be described with reference to FIGS.

[0012] First, the overall configuration of the radiation evaluation apparatus will be described with reference to Fig. 1 to Fig. 3. Fig. 1 is a diagram showing an outline of the radiation evaluation apparatus according to Example 1, Fig. 2 is a diagram showing an example of the structure of a radiation detector in the radiation evaluation apparatus according to Example 1, and Fig. 3 is a diagram showing an example of the structure of the radiation detector in the radiation evaluation apparatus according to Example 1.

[0013] The radiation evaluation device shown in FIG. 1 is composed of directional radiation detectors 120 and 130, a camera 140, a processing device 150, and the like.

[0014] In the removal work of the nuclear fuel debris 100, the area that can be removed in one operation is limited, so in this embodiment, the area of the nuclear fuel debris 100 that is the target of the removal work is set as area 101.

[0015] Here, it is desirable that this area 101 exists near the surface of the nuclear fuel debris 100. Note that the "area 101" here may be considered to be approximately 10 cm x 10 cm x 10 cm or 20 cm x 20 cm x 20 cm in size, but the definition can be changed as appropriate depending on the procedure for removing the nuclear fuel debris 100, etc.

[0016] FIG. 2 is a diagram illustrating an example of the structure of the radiation detectors 120 and 130 according to the first embodiment.

[0017] The radiation detectors 120, 130 shown in Figure 2 are single-channel detectors using collimators 122, 132, so-called pinhole collimators, and each have collimators 122, 132 that block radiation other than in a specific direction 105 around the radiation sensor portion 121, 131 so as to increase radiation sensitivity (increase directivity) to the intersection lines 123, 133.

[0018] These radiation detectors 120 and 130 are intended to mainly measure neutrons, particularly fast neutron components. When measuring fast neutrons, thermal neutron components are generally also measured at the same time, and neutrons generated secondarily using an external neutron source are also measured as necessary.

[0019] In addition, the neutron collimator used to provide directionality is broadly defined as one that has the function of increasing the sensitivity of the detector contained within in a specific direction, regardless of the material used (e.g., polyethylene, cadmium) or the structure (e.g., collimator diameter).

[0020] Additionally, gamma ray measurements will also be combined as necessary. For gamma rays, we will consider measuring them by placing detectors with the same directivity as neutrons in different positions.

[0021] The radiation detector 120 measures the radiation emitted from a radiation source in the area 101 of the nuclear fuel debris 100 that is the target of the removal work and its surrounding area 102, and the radiation detector 130 measures the radiation emitted from a radiation source in the area 101 of the nuclear fuel debris 100 that is the target of the removal work and its surrounding area 103.

[0022] The two radiation detectors 120, 130 are not particularly limited in their placement other than that they are placed so that the intersection lines 123, 133 indicating the directions of their respective directivities intersect inside the region 101, particularly at the center 110, and are placed at any angle relative to the center 110 of the target region 101 of the nuclear fuel debris 100. Note that the measurement region changes depending on the combination of angles.

[0023] Generally, the closer the angle between the intersection lines 123, 133, which are in the same direction as the directivity of the radiation detectors 120, 130, is to 90 degrees, the closer it is to a cube. For example, since the region 101 of the target nuclear fuel debris 100 is likely to be defined as a shape close to a cube with uniform side lengths, it is desirable to install the radiation detectors at an angle close to 90 degrees. However, on the other hand, if the radiation detectors 120, 130 get too close to the nuclear fuel debris 100 itself, they will be significantly affected by radiation from sources other than the region 101 of the target nuclear fuel debris 100, so it is desirable to adjust them to be a certain distance away from the nuclear fuel debris 100.

[0024] The camera 140 is expected to be used for observing the surface condition of the nuclear fuel debris 100, understanding the positional relationship between the radiation detectors 120, 130 and the nuclear fuel debris 100, or estimating the properties of the target nuclear fuel debris 100. Note that the camera does not have to be a camera 140, and may be replaced with a device having equivalent functions.

[0025] The processing device 150 evaluates the strength of the radiation source present in the area 101 by comparing the value obtained by dividing the radiation count rate c by the detection efficiency ε based on the radiation count rate c of the area 101 to be removed, which is measured at two or more different positions by the radiation detectors 120, 130, and the detection efficiency ε of the radiation detectors 120, 130 estimated for the target area 101.

[0026] The processing device 150 evaluates the detection efficiency ε by an analytical model or Monte Carlo simulation based on past investigation results based on composition analysis by sample recovery or on direct observation results of the region 101 using the camera 140 or the like.

[0027] This processing device 150 can be realized by a PC (Personal Computer) equipped with hardware such as an arithmetic unit such as a CPU, a main memory such as a semiconductor memory, an auxiliary memory such as a hard disk, an input device such as a keyboard or a USB port, and an output device consisting of a monitor, and the control of the operation of each device and various arithmetic processing described below are executed based on various programs.

[0028] The programs may be stored in an internal storage unit, an external recording medium, a data server (all not shown), or the like, and may be read and executed by the CPU. The control processes may be integrated into a single program, or may be separated into multiple programs, or a combination thereof. Some or all of the programs may be implemented using dedicated hardware or may be modularized. Furthermore, the various programs may be installed from a program distribution server, an internal storage medium, or an external recording medium.

[0029] FIG. 3 is a diagram illustrating another example of the structure of the radiation detector according to the first embodiment.

[0030] The radiation detector 410 shown in FIG. 3 is a so-called long detector, in which detection elements 400a, 400b, ... are arranged one-dimensionally so that the detection response of each of the detection elements 400a, 400b, ... can be compared, and has a signal processing board 401 that amplifies signals from each of the one-dimensionally arranged detection elements 400a, 400b, ... and converts them into digital signals.

[0031] The directivity of the radiation detector 410 is set to coincide with the intersection line passing through the center 110 of the target nuclear fuel debris area 101 in Figure 1. The signal processing board 401 may be structurally separated into detection elements 400a, 400b, ...

[0032] The radiation detectors 120, 130 shown in Fig. 2 require collimators 122, 132 for radiation, making the radiation detectors 120, 130 heavy and increasing the operational burden and risk of remote operation. On the other hand, the radiation detector 410 shown in Fig. 3 does not require a collimator for radiation, making it possible to reduce the size and weight accordingly. However, since measurements are required using multiple detection elements 400a, 400b, ..., measures to combat noise in the measurements and processing such as sensitivity calibration between elements are required, so it is desirable to use them appropriately by taking advantage of their respective characteristics.

[0033] Next, a radiation evaluation method according to this embodiment, which is preferably performed by a radiation evaluation device, will be described with reference to FIGS.

[0034] First, an example of a radiation evaluation method for measuring from two different angles will be described with reference to Fig. 4 and Fig. 5. Fig. 4 is a diagram showing an example of a radiation evaluation method using a radiation evaluation device according to Example 1, and Fig. 5 is a specific example of the radiation evaluation method according to Example 1, showing an example of a flow for evaluating the intensity of radiation emitted by nuclear fuel debris.

[0035] The radiation detector used in FIG. 4 is the radiation detector 120 or 130 shown in FIG. 2, which has directivity in one direction, or the radiation detector 410 shown in FIG.

[0036] As shown in Fig. 4, directional radiation detectors 120, 130, 410 are installed at two or more different positions, and the radiation count rate c in the area 101 that is the target of the removal work is measured at two or more locations. Here, two radiation detectors 120, 130 may be installed at different positions as shown in Fig. 1 and Fig. 2, or one of the radiation detectors 410 shown in Fig. 3 or 120, 130 shown in Fig. 1 etc. may be moved to perform measurement at a different position.

[0037] Then, the detection efficiency ε of the radiation detectors 120, 130, 410 for the target region 101 is estimated, and the strength of the radiation source present in the region 101 is evaluated by comparing the value obtained by dividing the radiation count rate c by the detection efficiency ε from the measurement results at different positions.

[0038] Next, the flow of evaluating a radiation source near a surface using the method of Fig. 4 will be described with reference to Fig. 5. The processing device 150 is a suitable entity for executing each of the following steps. The same applies to the other flows shown below.

[0039] First, as shown in FIG. 5, the radiation detection efficiencies ε1, ε1 of the radiation detectors 120, 130 installed at different angles are estimated based on the results of past surveys, the camera 140, or sampling analysis (S501).

[0040] Next, the radiation count rates C1 and C2 in the region 101 of the nuclear fuel debris 100 are measured by the radiation detectors 120 and 130, respectively (S502).

[0041] Thereafter, the processing device 150 compares the values obtained by dividing the radiation counting rates C1 and C2 of each radiation detector 120 and 130 estimated and measured in the previous steps S501 and S502 by their respective detection efficiencies ε1 and ε2, and determines whether either C1 / ε1 or C2 / ε2 is above a standard (S503).

[0042] If it is determined in S503 that both the values C1 / ε1 and C2 / ε2 obtained by dividing the radiation counting rates C1, C2 of each radiation detector 120, 130 by the detection efficiencies ε1, ε1 are below the standard, it is determined that there is no radiation source in the target area 101 or its surroundings (S506), and the processing is completed.

[0043] On the other hand, if it is determined in S503 that either the value C1 / ε1 or C2 / ε2 is above the standard, the processing device 150 then compares the values C1 / ε1 and C2 / ε2 and determines whether C1 / ε1=C2 / ε2 (S504).

[0044] If it is determined in S504 that C1 / ε1 and C2 / ε2 are equal, it is determined that a radiation source exists in the region 101 of the target nuclear fuel debris 100 (S505), and the process is completed.

[0045] On the other hand, if it is determined in S504 that C1 / ε1 and C2 / ε2 are not equal, it is determined that a radiation source exists within the surrounding area 102 or 103 of the area 101 of the target nuclear fuel debris 100 (S507), and the processing is completed.

[0046] Here, when C1 / ε1 > C2 / ε2, it can be determined that the radiation source exists in the peripheral region 103 located on the extension line of the intersection line 123 of the radiation detector 120. When C1 / ε1 < C2 / ε2, it can be determined that the radiation source exists in the peripheral region 103 located on the extension line of the intersection line 133 of the radiation detector 130.

[0047] The reason why the determination in FIG. 5 holds is as follows.

[0048] The radiation detectors 120 and 130 installed at different angles are here respectively referred to as radiation detector A and radiation detector B, the region 101 of the nuclear fuel debris 100 to be removed, the peripheral region 102 of the nuclear fuel debris 100 around it, and the peripheral region 103 are respectively expressed as nuclear fuel debris A, nuclear fuel debris B, and nuclear fuel debris C.

[0049] At this time, the detection efficiencies of the radiation detectors 1 and 2 for the region A of the nuclear fuel debris 100 to be removed are respectively ε 2C , 2A , C , A , B , ε<00OO002>and are defined as such. The detection efficiencies of the radiation detectors 1 and 2 for the region B of the nuclear fuel debris 100 around the object to be removed are ε 1A , ε 2A and are defined as such. The radiation source intensities in the regions A, B, and C of the nuclear fuel debris are respectively S A , S B , S C and are defined as such.

[0050] [[ID=3o]]When the counting rates measured by the radiation detectors 1 and 2 are respectively C1 and C2, C1 = (ε 1A S A + ε 1B S B ) / (1 - k) (1) C2 = (ε 2A S A + ε 2C S C ) / (1 - k) (2) Here, k is the effective multiplication factor.

[0051] The detection sensitivity of radiation detectors is higher for nuclear fuel debris near the surface, so ε 1A ≫ε 1B , ε 2A ≫ε 2B holds, so we can change equation (1) to ε 1A , Equation (2) is ε 2A Dividing by, C1 / ε 1A =(S A +S B (ε 1B / ε 1A )) / (1-k) (3) C2 / ε 2A =(S A +S C (ε 2C / ε 2A )) / (1-k) (4) This becomes:

[0052] Here, if a strong radiation source exists in the area A of the nuclear fuel debris 100 of interest, that is, S A >S B Katsu S A >S C If this holds, the left-hand sides of equations (3) and (4) become approximately equivalent (C1 / ε 1A =C2 / ε 2A ).

[0053] On the other hand, the radiation source intensity S A More peripheral area S B and S C If the radiation source strength of the target nuclear fuel debris 100 is high, or if the radiation source strength of the target area A of the nuclear fuel debris 100 is the same as that of its surroundings, the left side of equation (3) and equation (4) are not equivalent (C1 / ε 1A ≠C2 / ε 2A ).

[0054] Because the nuclear fuel debris 100 is melted nuclear fuel mixed with the surrounding concrete and metal, it is assumed that it is not homogeneously mixed with the surrounding materials but is often unevenly distributed. Therefore, it can be assumed that the nuclear fuel debris 100 is likely to contain a mixture of areas with extremely high and low radiation source strength depending on the area.

[0055] Therefore, when measurements are carried out from a location where the criticality safety of the nuclear fuel debris 100 is high, that is, a location where it is unlikely that nuclear fuel has fallen from the bottom of the RPV, it can be assumed that nuclear fuel has not melted down into the target area and all of its surrounding areas.

[0056] From these facts, C1 / ε 1A =C2 / ε 2A is established when the radiation source strength S A is high.

[0057] On the other hand, C1 / ε 1A ≠C2 / ε 2A is established when there is no strong radiation source in the area A of the nuclear fuel debris 100 in question, but there is a strong radiation source in the surrounding areas B and C, or when there is no radiation source in the area A of the nuclear fuel debris 100 in question, or in the surrounding areas B and C. In other words, C1 / ε 1A ≠C2 / ε 2A If the above equation holds, it means that there is no strong radiation source in the region 101 of the nuclear fuel debris 100 in question.

[0058] Next, an example of a radiation evaluation method in which measurements are taken in a circular manner from a plurality of different angles using one radiation detector will be described with reference to Figures 6 and 7. Figure 6 is a diagram showing an example of a radiation evaluation method using a radiation evaluation device according to Example 1, and Figure 7 is a specific example of the radiation evaluation method according to Example 1, showing an example of a flow for evaluating the intensity of radiation emitted by nuclear fuel debris.

[0059] The radiation detector used in FIG. 6 is also the radiation detector 120, 130 shown in FIG. 2, which has directivity in one direction, or the radiation detector 410 shown in FIG. 3, and one radiation detector 120, 130, 410 is installed in the area 101 and rotated around an axis 810 passing through the center of the area 101, thereby changing the installation position and performing measurements at multiple positions.

[0060] Next, the flow of evaluating a radiation source near the surface using the method of FIG. 6 will be explained with reference to FIG.

[0061] First, as shown in FIG. 7, based on the results of past surveys, the camera 140, or sampling analysis, the radiation detection efficiency ε iA (i=1, 2, ...., N) is estimated (S701).

[0062] Next, the radiation count rate C i (i=1, 2, ...., N) are measured at each position (S702).

[0063] Thereafter, the processing device 150 calculates the radiation count rate C of the radiation detectors 120, 130, and 410 estimated and measured in the previous steps S701 and S702. i The respective detection efficiencies ε i Compare the values divided by C i / ε i It is determined whether or not one or more of the above is equal to or greater than a reference value (S703).

[0064] In S703, any C at all measurement positions i i / ε i If it is determined that the radiation level is also below the standard, it is determined that there is no radiation source in the target area 101 or its surroundings (S706), and the process is completed.

[0065] In contrast, in S703, either C i / ε i If it is determined that C is equal to or greater than the standard, then the processing device 150 calculates C for all measurement positions i. i / ε i The magnitude relationship of C for all measurement positions i is compared. i / ε i It is determined whether or not all are equal (S704).

[0066] C in S704 i / ε iIf it is determined that all of the values are equal, it is determined that a radiation source exists within the area 101 of the nuclear fuel debris 100 in question (S705), and the process is completed.

[0067] In contrast, in S704, C i / ε i If it is not determined that all of the values are equal, it is determined that a radiation source exists in the surrounding area of the area 101 of the nuclear fuel debris 100 in question (S707), and the process is completed.

[0068] The methods of FIGS. 6 and 7 require more measurement time than the methods of FIGS. 4 and 5 due to the increased number of measurements. However, for example, i / ε i If there are locations where the angles (i=1, 2, ..., N) are different, specifically where the angle is high for a particular angle i, this may suggest that there is a strong radiation source in the directional direction at that angle i.

[0069] 4 and 5 assume that there is a high possibility that there are places where the radiation source strength is extremely high and places where it is extremely low depending on the region 101 of the nuclear fuel debris 100, but if we consider exceptional cases, we cannot deny the possibility that a situation may arise in which the equation holds by chance even when there is no strong radiation source in the region 101 of the nuclear fuel debris 100 in question. In that case, the methods of Figures 6 and 7 have a greater number of equation comparisons in S704, so it can be said that there is a lower probability that the equation will hold by chance even when there is no strong radiation source in the region 101 of the nuclear fuel debris 100.

[0070] It is also possible to arrange two or more detectors and rotate each around its axis of rotation for measurement, as shown in Figure 4. In this case, the more detectors that are prepared, the fewer the number of rotations that are required.

[0071] Next, the effects of this embodiment will be described.

[0072] The radiation evaluation method of the first embodiment of the present invention described above includes the steps of installing directional radiation detectors 120, 130, 410 at two or more different positions and measuring the radiation count rate c in the area 101 that is the target of removal work at two or more locations; estimating the detection efficiency ε of the radiation detectors 120, 130, 410 for the target area 101; and evaluating the intensity of the radiation source present in the area 101 by comparing the values obtained by dividing the radiation count rate c by the detection efficiency ε from the measurement results at the different positions.

[0073] This makes it possible to determine whether removal work is possible without deriving the effective multiplication factor k, which requires complex processes such as deriving a correction coefficient, and when carrying out work to remove molten solidified nuclear fuel (nuclear fuel debris), it becomes possible to determine in advance whether removal work is possible so that the removal work does not cause nuclear fission reactions to reach criticality. Therefore, it is possible to provide a speedy removal work of nuclear fuel debris 100 while ensuring criticality safety.

[0074] Furthermore, the process of estimating the detection efficiency ε is based on the results of previous investigations based on composition analysis of sample recovery or on the results of direct observation of region 101, and is evaluated using an analytical model or Monte Carlo simulation, making it possible to estimate the detection efficiency ε with high accuracy.

[0075] Furthermore, by using as the radiation detectors 120, 130, 410 either single-channel detectors using collimators 122, 132, or detectors in which detection elements 400a, 400b, ... are arranged one-dimensionally so that the detection response of each detection element 400a, 400b, ... can be compared, a detector with a simple structure and excellent directionality can be used.

[0076] In addition, the process of measuring the radiation count rate c can be performed in a short time by installing one radiation detector 120, 130, 410 for the region 101 and installing different radiation detectors 120, 130, 410 at different angles from the center of the region 101 to perform measurements.

[0077] Furthermore, the process of measuring the radiation count rate c involves installing one radiation detector 120, 130, 410 in the region 101 and rotating it around an axis 810 passing through the center of the region 101 to change the installation position and perform measurements at multiple positions, thereby eliminating the need to prepare multiple radiation detectors and making it easier to identify the relevant region when a radiation source is present in the surrounding region of the target region 101.

[0078] In addition, in the process of estimating the detection efficiency ε, the detection efficiencies ε1 and ε2 of the respective radiation detectors 120, 130, and 410 are estimated, and in the process of evaluating the radiation source strength, the values obtained by dividing the radiation count rates C1 and C2 of the respective radiation detectors 120, 130, and 410 by the respective detection efficiencies ε1 and ε2 are compared, and if the values are the same, it is determined that a radiation source is present in the region 101, thereby enabling evaluation in a short period of time.

[0079] Furthermore, in the process of estimating the detection efficiency ε, the detection efficiencies ε of the radiation detectors 120, 130, and 410 installed at different angles are calculated. i In the process of estimating the radiation source intensity, the radiation count rate C i The respective detection efficiencies ε i By comparing the values divided by 101 and determining that a radiation source is present in the region 101 if the values are the same, it is not necessary to prepare multiple radiation detectors and it becomes easier to identify the region in question when a radiation source is present in the region surrounding the target region 101.

[0080] <Example 2> Second Embodiment A radiation evaluation method and a radiation evaluation device according to a second embodiment of the present invention will be described with reference to FIGS.

[0081] FIG. 8 is a diagram showing an example of the structure of a radiation detector in a radiation evaluation device according to Example 2, FIG. 9 is a diagram showing an example of a radiation evaluation method using the radiation evaluation device, and FIG. 10 is a specific example of the radiation evaluation method, showing an example of a flow for evaluating the intensity of radiation emitted by nuclear fuel debris.

[0082] As shown in FIG. 8, the radiation detector of this embodiment is a multi-channel detector array 510 using a collimator 501 .

[0083] In this detector array 510, detector elements 500 are arranged in respective storage areas arranged in parallel two-dimensionally or one-dimensionally in a collimator 501 made of a radiation-shielding material. Therefore, the detector elements 500 constituting the detector array 510 have directivity 503a, 503b, ... in a plurality of directions as shown in FIG.

[0084] Next, an example of a radiation evaluation method using the detector array 510 will be described with reference to FIGS.

[0085] As shown in FIG. 9, the detector array 510 is moved horizontally across the surface of the area 101 of the nuclear fuel debris 100 to be removed, so as to cross directly above the area 101, and radiation is measured multiple times at different positions.

[0086] Next, the flow of evaluating a radiation source near the surface using the method of FIG. 9 will be explained with reference to FIG.

[0087] 9, the area position of the nuclear fuel debris 100 to be measured is j=1, 2, ..., N, and the detector array detection elements are k=1, 2, ..., M. Since detection element k has directivity in different directions, this is equivalent to measuring radiation from M different angles with respect to area j of the nuclear fuel debris 100.

[0088] First, as shown in FIG. 10, based on the results of past investigations, the camera 140, or sampling analysis, the radiation detection efficiency ε jk (j=1,2,...N, k=1,2,...,M) is estimated (S1001).

[0089] Next, while moving the detector array 510 horizontally, the radiation count rate C of each detector element k for each position j including above the region 101 of the nuclear fuel debris 100 is calculated. jk is measured at each position (S1002).

[0090] Thereafter, the processing device 150 calculates the counting rate C of the detection element k for the region j of the nuclear fuel debris 100, which was estimated and measured in the previous steps S1001 and S1002. jk The detection efficiency ε jk Compare the values divided by C jk / ε jk It is determined whether or not one or more of the above is equal to or greater than a reference value (S1003).

[0091] In S703, C jk / ε jk If it is determined that the radiation level is also below the standard, it is determined that there is no radiation source in the target area 101 or its surroundings (S1006), and the process is completed.

[0092] In contrast, in S1003, either C jk / ε jk If it is determined that C is equal to or greater than the standard, then the processing device 150 calculates C for all measurement positions j. jk / ε jk Compare the magnitude of C for all measurement positions j jk / ε jk It is determined whether or not they are all equal (S1004).

[0093] C in S1004 jk / ε jk If it is determined that all of the values are equal, it is determined that a radiation source exists within the region 101 of the nuclear fuel debris 100 in question (S1005), and the process is completed.

[0094] In contrast, in S1004, C jk / ε jkIf it is not determined that all of the values are equal, it is determined that a radiation source exists in the surrounding area of the area 101 of the target nuclear fuel debris 100 (S1007), and the process is completed.

[0095] The other configurations and operations are substantially the same as those of the radiation evaluation method and radiation evaluation device of the first embodiment, and details thereof will be omitted.

[0096] The radiation evaluation method and radiation evaluation device according to the second embodiment of the present invention also provide substantially the same effects as those of the radiation evaluation method and radiation evaluation device according to the first embodiment described above.

[0097] Furthermore, when using radiation detectors 120, 130, 410 having directivity in one direction as in Example 1, the measurement system can be easily made small and lightweight, whereas in Example 2 the radiation detector is a multi-channel detector array 510 using a collimator 501, which means that the measurement system can easily become large and heavy, but since it is possible to measure multiple positions simultaneously, a wide range can be evaluated in a short time.

[0098] Furthermore, the process of measuring the radiation count rate c involves placing one detector array 510 for the region 101 and measuring while moving the detector array 510 horizontally relative to the region 101, thereby enabling measurements to be performed over a wide area in a short period of time.

[0099] In addition, in the step of measuring the radiation count rate c at two or more locations, the radiation count rate C of the detection element 500 for each detection position j is jk and estimating the detection efficiency ε, the detection efficiency ε of the detection element 500 for each detection position j is jk and the radiation source strength is evaluated by estimating the respective radiation count rates C for each position j. jk The detection efficiency ε jk Divided by C jk / ε jk Compare C for all k jk / ε jkIf the values are the same, it is determined that the radiation source is in the region 101 where the position j exists, thereby making it possible to perform evaluation over a wide range in a short time.

[0100] Example 3 A radiation evaluation method and a nuclear fuel debris removal method using a radiation evaluation device according to a third embodiment of the present invention will be described with reference to Figures 11 and 12. Figures 11 and 12 are diagrams showing an example of a flow of nuclear fuel debris removal work according to the third embodiment.

[0101] 11 shows an example of nuclear fuel debris removal work using the radiation evaluation method according to Example 1. This can be applied to all detection structures, measurement methods, and evaluations of radiation source strength, regardless of whether it is Example 1 or 2.

[0102] In the method for removing nuclear fuel debris 100, as shown in FIG. 11, first, radiation in an area 101 to be removed is measured.

[0103] Specifically, when determining whether or not nuclear fuel debris removal work is possible based on the reference value of the radiation counting rate of the area 101 to be removed, the area 101 of the target nuclear fuel debris 100 is selected (S1100), and radiation detectors 120, 130, 410 and detector array 510 are placed near the area 101 of the target nuclear fuel debris 100 (S1101).

[0104] Then, the detection efficiency ε 1A ,ε 2A This step S1102 corresponds to step S501 in FIG. 5, step S701 in FIG. 7, and step S1001 in FIG.

[0105] Next, the radiation count rates C1 and C2 are measured from different angles or different positions (S1103) to evaluate the radiation source intensity in the target region 101 (S1104). This S1102 corresponds to S502 and S503 in Fig. 5, S702 and S703 in Fig. 7, and S1002 and S1003 in Fig. 10 described above.

[0106] Next, it is determined whether the radiation source intensity in the region 101 is below the criterion based on criticality safety (S1105). If it is determined that it is below the criterion based on criticality safety, the process proceeds to S1108, and if it is determined that it is equal to or greater than the criterion based on criticality safety, the process proceeds to S1106.

[0107] If it is determined in S1105 that the radiation source intensity in the region 101 is equal to or greater than the criterion based on criticality safety, then the effective multiplication factor k due to the criticality approach monitor criticality is evaluated (S1106), and it is determined whether the evaluated effective multiplication factor k is less than the criterion based on criticality safety (S1107). If it is determined that the effective multiplication factor k is less than the criterion based on criticality safety, the process proceeds to S1108.

[0108] When it is determined in S1105 that the radiation source strength of the area 101 is less than the standard based on criticality safety, or when it is determined in S1107 that the effective multiplication factor k is less than the standard based on criticality safety, the target area 101 is determined to be suitable for removal work and is removed (S1108), and the process returns to S1100 to select the next area 101.

[0109] On the other hand, if it is determined in S1107 that the effective multiplication factor k is equal to or greater than the standard based on criticality safety, the process returns to S1100 to change the position of the target region 101 without carrying out removal work.

[0110] Here, the steps that fall below the criticality safety criteria in the evaluation of the radiation source strength (Yes in S1105) are S506 or S507 in FIG. 5, S706 or S707 in FIG. 7, and S1006 or S1007 in FIG.

[0111] 5, S705 in FIG. 7, and S1005 in FIG. 10 correspond to the judgment standard or higher based on criticality safety in the evaluation of the radiation source strength (No in S1105).

[0112] Next, another example of nuclear fuel debris removal work will be described with reference to Fig. 12. As with Fig. 11, this can be applied to all detection structures, measurement methods, and radiation source strength evaluation methods, regardless of whether they are in the first or second embodiment.

[0113] S1201 to S1204 shown in FIG. 12 are the same as S1101 to S1104 shown in FIG.

[0114] Next, it is determined whether the radiation source intensity in the region 101 is below the standard based on criticality safety (S1205). If it is determined that the radiation source intensity is below the standard based on criticality safety, the process proceeds to S1206.

[0115] On the other hand, if it is determined that the level is equal to or greater than the standard based on criticality safety, the process returns to S1200 to change the position of the target area 101 without carrying out removal work.

[0116] Next, the effective multiplication factor k due to the criticality approach monitor criticality is evaluated (S1206), and it is determined whether the evaluated effective multiplication factor k is less than the criterion based on criticality safety (S1207). If it is determined that it is less than the criterion based on criticality safety, the process proceeds to S1208, where the target area 101 is determined to be removable and is removed (S1208), and the process returns to S1200 to select the next area 101.

[0117] On the other hand, if it is determined in S1207 that the effective multiplication factor k is equal to or greater than the standard based on criticality safety, the process returns to S1200 to change the position of the target region 101 without carrying out removal work.

[0118] The configuration and operation of the radiation evaluation method and radiation evaluation device are substantially the same as those of the first or second embodiment described above, and details thereof will be omitted.

[0119] According to the nuclear fuel debris removal method of Example 3 of the present invention, by utilizing the radiation evaluation method and radiation evaluation device of Example 1 or Example 2 described above, it becomes possible to remove nuclear fuel debris 100 in a shorter time than conventionally.

[0120] Furthermore, in the procedure shown in FIG. 11, if the radiation evaluation judgment in Example 1 etc. is below the judgment criterion based on criticality safety, it is judged that the area of nuclear fuel debris in question can be removed; in this case, however, the judgment criterion based on S1105 is a standard value linked to sub-criticality, and therefore is estimated to be a strict value.

[0121] On the other hand, in the procedure shown in Figure 12, removal is judged to be possible only if both the evaluation and judgment of the radiation source strength satisfy the criteria based on criticality safety, and the effective multiplication factor in criticality approach monitoring also satisfy the criteria based on criticality safety, and this can be said to be safer than the procedure shown in Figure 11, but it is expected that the number of processes for safety confirmation will increase. However, since the evaluation of the effective multiplication factor in criticality approach monitoring takes time in the procedures of Figures 11 and 12, by using a flow that does not evaluate all criticality safety in this process, it is possible to remove nuclear fuel debris from positions with high criticality safety, thereby speeding up the removal work.

[0122] <Other> It should be noted that the present invention is not limited to the above-described embodiment, and includes various modifications. For example, the above-described embodiment has been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to an embodiment having all of the described configurations.

[0123] It is also possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment, or to add, delete, or replace part of the configuration of each embodiment with the configuration of another embodiment.

[0124] For example, some or all of the above-described configurations, functions, processing units, processing means, etc. may be implemented in hardware, for example, by designing them as integrated circuits. Furthermore, the above-described configurations, functions, etc. may be implemented in software by a processor interpreting and executing a program that implements each function. Information such as the programs, tables, and files that implement each function can be stored in a memory, a recording device such as a hard disk or SSD (Solid State Drive), or a recording medium such as an IC card, SD card, or DVD.

[0125] In addition, the control lines and information lines shown are those that are considered necessary for the explanation, and do not necessarily show all the control lines and information lines in the product. In reality, it can be assumed that almost all components are interconnected.

[0126] Furthermore, although the accuracy of the intensity is reduced in principle, the region 101 that can be evaluated is not limited to the vicinity of the surface of the nuclear fuel debris 100. [Explanation of symbols]

[0127] 100...Nuclear fuel debris 101…Area 102, 103... Surrounding areas 105…Specific direction 110…center 120,130...Radiation detector 121, 131...Radiation sensor part 122,132...Collimator 123,133…intersection line 140...camera 150...Processing device (arithmetic unit) 400a, 400b...detection element 401...Signal processing board 410...Radiation detector 500...Detection element 501...Collimator 503a,503b…Directivity 510...detector array 810...Axis

Claims

1. a step of installing directional radiation detectors at two or more different positions and measuring the radiation count rate of the nuclear fuel debris area that is the target of removal work at two or more locations; estimating the detection efficiency of the radiation detector for the nuclear fuel debris region of interest; and evaluating the intensity of the radiation source present in the nuclear fuel debris region by comparing the values obtained by dividing the radiation count rate by the detection efficiency from the measurement results at different positions. Radiation assessment methods.

2. The radiation evaluation method according to claim 1, The step of estimating the detection efficiency shall be evaluated using analytical models or Monte Carlo simulations based on the results of previous investigations based on compositional analysis of sample recovery or direct observations of the nuclear fuel debris area. Radiation assessment methods.

3. The radiation evaluation method according to claim 1, The radiation detector is a single-channel detector using a collimator, a multi-channel detector using the collimator, or a detector in which detection elements are arranged one-dimensionally so that the detection response of each detection element can be compared. Radiation assessment methods.

4. The radiation evaluation method according to claim 3, The step of measuring the radiation count rate involves installing one radiation detector in the nuclear fuel debris area, and installing different radiation detectors at different angles from the center of the nuclear fuel debris area to perform measurements. Radiation assessment methods.

5. The radiation evaluation method according to claim 3, The step of measuring the radiation count rate involves installing one of the radiation detectors in the nuclear fuel debris area and rotating it around an axis passing through the center of the nuclear fuel debris area to change the installation position, thereby performing measurements at multiple positions. Radiation assessment methods.

6. The radiation evaluation method according to claim 3, The step of measuring the radiation count rate involves installing one radiation detector relative to the nuclear fuel debris area, and measuring while moving the radiation detector horizontally relative to the nuclear fuel debris area. Radiation assessment methods.

7. The radiation evaluation method according to claim 4, In the step of estimating the detection efficiency, the detection efficiency ε １ , ε ２ Estimate In the step of evaluating the radiation source intensity, the radiation count rate C １ , C ２ The detection efficiency ε １ , ε ２ If the values are the same, it is determined that there is a radiation source in the nuclear fuel debris area. Radiation assessment methods.

8. The radiation evaluation method according to claim 5, In the step of estimating the detection efficiency, the detection efficiency ε ｉ (i=1, 2, ..., N (N is a natural number)) is estimated, In the step of evaluating the radiation source intensity, the radiation count rate C ｉ The detection efficiency ε ｉ If the values are the same, it is determined that there is a radiation source in the nuclear fuel debris area. Radiation assessment methods.

9. The radiation evaluation method according to claim 6, In the step of measuring the radiation count rate at two or more locations, the radiation count rate C of the detection element k for each detection position j is ｊｋ Measure each of In the step of estimating the detection efficiency, the detection efficiency ε of the detection element k for each detection position j is ｊｋ Estimate In the step of evaluating the radiation source intensity, the radiation count rate C for each position j is calculated. ｊｋ The detection efficiency ε ｊｋ The value C divided by ｊｋ / ε ｊｋ and compare C for all k. ｊｋ / ε ｊｋ If the values are the same, it is determined that a radiation source exists in the nuclear fuel debris area where position j exists. Radiation assessment methods.

10. The radiation evaluation method according to any one of claims 1 to 9, determining whether the radiation source strength of the nuclear fuel debris region is below a criticality safety-based standard; a step of determining whether or not the effective multiplication factor k is less than the standard based on criticality safety when the radiation source strength of the nuclear fuel debris region is determined to be equal to or greater than the standard based on criticality safety; a step of determining that removal work is possible for the nuclear fuel debris area when it is determined that the radiation source intensity of the nuclear fuel debris area is less than the standard based on criticality safety or when it is determined that the effective multiplication factor k is less than the standard based on criticality safety; and when it is determined that the effective multiplication factor k is equal to or greater than the standard based on criticality safety, changing the position of the target nuclear fuel debris region. Methods for removing nuclear fuel debris.

11. The radiation evaluation method according to any one of claims 1 to 9, determining whether the radiation source strength of the nuclear fuel debris region is below a criticality safety-based standard; When it is determined that the radiation source strength of the nuclear fuel debris region is less than the standard based on criticality safety, determining whether or not the effective multiplication factor k is less than the standard based on criticality safety; a step of determining that removal work is possible for the nuclear fuel debris area when it is determined that the effective multiplication factor k is less than a standard based on criticality safety; and changing the position of the target nuclear fuel debris region when the radiation source intensity of the nuclear fuel debris region is determined to be equal to or greater than the standard based on criticality safety, or when the effective multiplication factor k is determined to be equal to or greater than the standard based on criticality safety. Methods for removing nuclear fuel debris.

12. a directional radiation detector; a calculation unit that evaluates the intensity of the radiation source present in the nuclear fuel debris area by comparing a value obtained by dividing the radiation count rate of the nuclear fuel debris area that is the target of removal work, the radiation count rate being measured at two or more different positions by the radiation detector, and the detection efficiency of the radiation detector estimated for the target nuclear fuel debris area. Radiation evaluation equipment.

13. The radiation evaluation apparatus according to claim 12, The calculation unit evaluates the detection efficiency using an analytical model or Monte Carlo simulation based on past investigation results based on composition analysis of sample recovery or direct observation results of the nuclear fuel debris region. Radiation evaluation equipment.

14. The radiation evaluation apparatus according to claim 12, The radiation detector is either a single-channel detector using a collimator, a multi-channel detector using the collimator, or a detector in which detection elements are arranged one-dimensionally so that the detection response of each detection element can be compared. Radiation evaluation equipment.

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