JP2019032208A - Radiation evaluation method and radiation evaluation device - Google Patents

Abstract

To provide a radiation evaluation method and a radiation evaluation device for improving precision of a radiation evaluation regardless of a shape and a size of a measurement target.SOLUTION: A radiation evaluation device 50 includes: a mesh division processing part 11 for dividing an inside of a storage container 100 for storing radioactive waste into a plurality of meshes; an initial condition setting part 12 for setting a radiation source for each of the plurality of meshes and setting density of the radioactive waste; a counting rate detection part 10 for detecting a counting rate for indicating the number of detected radiations per unit time from each of a plurality of positions around the storage container 100; and a radiation distribution evaluation part 13 for evaluating a radiation distribution in the storage container 100 by performing statistical operation, based on strength of the radiation source set by the initial condition setting part 12, and the density and a counting rate of the radioactive waste.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a radioactivity evaluation method and a radioactivity evaluation apparatus used when measuring radioactivity radiated from a measurement object such as radioactive waste, for example.

  Generally, radioactive waste generated from nuclear facilities is disposed of according to radioactivity level. When disposing of this type of radioactive waste, a clearance level is provided that it is not necessary to treat it as a radioactive material if it is sufficiently smaller than the natural radioactivity level. In addition, in the decommissioning of the nuclear facilities described above, it is expected that a large amount of radioactive waste having a sufficiently low level of radioactivity such as metal waste and concrete (below the clearance level) will be generated. If it is possible to easily classify such a large amount of generated radioactive waste, the disposal cost and labor (time) of the radioactive waste can be greatly reduced.

  Conventionally, in order to determine whether or not the radioactive level of a radioactive waste (measuring object) is equal to or less than a clearance level, a radioactivity measuring device including a radiation detector is known (for example, Patent Document 1). reference). In this type of radioactivity measurement device, when measuring radioactivity, the count rate (cps; count per second) of the radiation (eg, γ (gamma) rays) detected by the radiation detector and the radioactivity conversion factor Radioactivity (Bq) is evaluated using (Bq / cps).

Japanese Patent No. 4268970

  Here, radioactive waste (measuring object) is separated according to material and location, and this classified radioactive waste is further cut into an equal shape and size and stored in a measurement container (storage container). When the pretreatment is performed, the measurement accuracy of radioactivity can be increased, but the burden of this pretreatment work has been increased. In addition, when evaluating the radioactivity, when the representative points are measured on the assumption that the above-mentioned radioactive waste is uniformly and uniformly stored in the measurement container, the radioactive waste is unevenly distributed in the measurement container. Considering the possibility, a safety factor will be introduced into the radioactivity conversion factor. For this reason, processing is performed assuming that the amount of radioactivity is higher than that of the actual radioactive waste, and unnecessary processing is performed.

  This invention solves the subject mentioned above, and provides the radioactivity evaluation method and the radioactivity evaluation apparatus which aimed at the improvement of the precision of radioactivity evaluation irrespective of the shape and size of a measuring object. For the purpose.

  In order to achieve the above-mentioned object, the radioactivity evaluation method according to the present invention includes a mesh division step for dividing a measurement space in which a measurement object exists into a plurality of meshes, and a radiation source is set for each of the plurality of meshes. An initial condition setting step for setting the density of the measurement object, a count rate detection step for detecting a count rate indicating the number of radiation detected per unit time from each of a plurality of positions around the measurement space, and an initial condition And a radioactivity distribution evaluation step for evaluating the radioactivity distribution in the measurement space by performing a statistical calculation based on the intensity and density of the radiation source set in the setting step and the count rate.

  According to this configuration, by evaluating the radioactivity distribution in the measurement space based on the intensity of the radiation source of each mesh and the density and counting rate of the measurement object, regardless of the shape and size of the measurement object, The accuracy of radioactivity evaluation can be improved. Moreover, since the process which arranges a measuring object into the same shape and magnitude | size becomes unnecessary, the burden of the pre-process in radioactivity evaluation can be aimed at.

  In addition, the density set in the center of gravity position determination step for determining the center of gravity position of the plurality of radiation sources based on the radiation source set in each of the plurality of meshes, and the density set in the initial condition setting step based on the center of gravity position of the radiation source It is good also as a structure containing the density correction step which correct | amends. According to this configuration, by correcting the density based on the position of the center of gravity of the radiation source, it is possible to further increase the accuracy of the radioactivity distribution in the measurement space evaluated in the radioactivity distribution evaluation step. Further improvement in accuracy can be realized.

  The plurality of radiation sources set in the initial condition setting step may have the same intensity and a uniform density. According to this configuration, the statistical calculation in the radioactivity distribution evaluation step can be performed more easily.

  The initial condition setting step may include a step of measuring the weight and height of the measurement object. According to this configuration, the density can be calculated from the weight and height of the measurement object.

  In addition, the measurement object may be a radioactive waste generated by a nuclear facility or decommissioning of the nuclear facility. According to this configuration, the generated radioactive waste can be easily classified and disposed of according to the radioactivity level. For example, it is accurate whether the radioactivity level of the radioactive waste (measurement object) is below the clearance level. It can be discriminated well and rationally and efficiently.

  Further, the radioactivity evaluation apparatus according to the present invention includes a mesh division processing unit that divides a measurement space in which a measurement object exists into a plurality of meshes, a radiation source is set for each of the plurality of meshes, and the density of the measurement object An initial condition setting unit that sets a count rate detecting unit that is arranged at a plurality of positions around the measurement space and detects a count rate indicating the number of detected radiation per unit time from each of the plurality of positions; A radioactivity distribution evaluation unit that evaluates the radioactivity distribution in the measurement space by performing statistical calculation based on the intensity and density of the radiation source set by the condition setting unit and the count rate. According to this configuration, the radioactivity distribution evaluation unit evaluates the radioactivity distribution in the measurement space based on the intensity of the radiation source of each mesh and the density and count rate of the measurement object, thereby determining the shape of the measurement object and Regardless of the size, the accuracy of radioactivity evaluation can be improved.

  According to the present invention, by evaluating the radioactivity distribution in the measurement space based on the intensity of the radiation source of each mesh and the density and count rate of the measurement object, regardless of the shape and size of the measurement object, The accuracy of radioactivity evaluation can be improved. Moreover, since the process which arranges a measuring object into the same shape and magnitude | size becomes unnecessary, the burden of the pre-process in radioactivity evaluation can be aimed at.

FIG. 1 is a perspective view of the radioactivity measurement apparatus according to the present embodiment. FIG. 2 is a cross-sectional view as viewed in the y direction in FIG. FIG. 3 is a cross-sectional view taken in the x direction in FIG. FIG. 4 is a diagram illustrating a state in which the inside of the storage container is divided into a plurality of meshes. FIG. 5 is a diagram illustrating a state in which a radiation source is set for each of the divided meshes. FIG. 6 is a flowchart showing a procedure for evaluating the radioactivity distribution of the radioactive waste in the storage container. FIG. 7 is a diagram for explaining variables in the ML-EM method, which is an example of a method for estimating the radioactivity distribution. FIG. 8 is a diagram for explaining a procedure for correcting the density distribution in the storage container. FIG. 9 is a diagram illustrating a procedure for obtaining a new detection probability based on the corrected density distribution.

  Embodiments according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. In addition, constituent elements in the following embodiments include those that can be easily replaced by those skilled in the art or those that are substantially the same.

  FIG. 1 is a perspective view of the radioactivity measuring apparatus according to the present embodiment, FIG. 2 is a cross-sectional view as viewed in the y direction in FIG. 1, and FIG. 3 is a cross-sectional view as viewed in the x direction in FIG. . Note that the x direction, y direction, and z direction indicated by arrows in the figure are directions that intersect each other at 90 degrees, the x direction indicates the left-right direction (horizontal direction), and the y direction is the front-rear direction (horizontal direction). The z direction indicates the vertical direction (vertical direction).

  The radioactivity measurement apparatus 1 measures the radioactivity amount of radioactive waste (measurement object) stored in a storage container (measurement space) 100 without destroying the storage container 100. The storage container 100 has a rectangular parallelepiped shape (rectangular shape) made of iron having surfaces perpendicular to the x direction, the y direction, and the z direction. The storage container 100 is, for example, a regular hexahedron whose x direction × y direction × z direction is 160 [cm] × 160 [cm] × 160 [cm] and the wall thickness of iron is 0.23 [cm]. There is no. Further, the storage container 100 includes a lid that can be opened and closed on the upper surface in the z direction, and the lid is closed after the radioactive waste is stored in the storage container 100.

  In this embodiment, the radioactive waste is stored in the storage container 100 regardless of the shape or size. For this reason, the pretreatment operation | work which cuts radioactive waste in advance and arranges a shape and a size is not essential.

  As shown in FIG. 1, the radioactivity measurement apparatus 1 includes a radiation detection unit 2, a slide movement unit 3, a rotation movement unit 4, a height detection unit 5, and an arithmetic control unit 6.

  The radiation detection unit 2 detects radiation (γ rays) of radioactive waste stored in the storage container 100. The radiation detector 2 has the same distance (for example, a clearance 1 [cm] between the storage container 100 and the four surfaces in the x direction and the z direction, excluding two opposite surfaces of the storage container 100 (two surfaces opposite to each other in the y direction). ).

  As illustrated in FIG. 2, the radiation detection unit 2 includes two detectors 2 a for one surface. Specifically, the radiation detector 2 facing the surface in the x direction of the storage container 100 is provided with two detectors 2a arranged in the z direction. The radiation detector 2 facing the z-direction surface of the storage container 100 is provided with two detectors 2a arranged in the x direction. For this reason, as shown by the broken line in FIG. 2, the four radiation detection units 2 each divide the surface of the storage container 100 into two parts in the x direction and the z direction by two detectors 2a and store them in the y direction view. Radiation is detected by dividing the cross section of the container 100 into four ranges. Furthermore, the radiation detection unit 2 detects radiation by dividing the storage container 100 into four ranges in a slide movement direction (y direction) described later by the slide movement unit 3 as indicated by a broken line in FIG. As a result, each of the four radiation detection units 2 detects the radiation by dividing the cross section of the storage container 100 into a total of 16 by the two detectors 2a.

  Thus, in order to detect the radiation by dividing the cross section of the storage container 100 into a plurality of parts, the radiation detection unit 2 has a collimator 2b that narrows the field of view of the detector 2a facing the storage container 100. In addition, the radioactivity measuring apparatus 1 of this embodiment has the flame | frame 1a for ensuring the area | region which arrange | positions the radiation detection part 2 in the bottom side (surface of the bottom side of az direction) of the storage container 100. FIG.

  As the detector 2a of the radiation detector 2, a Ge detector is preferably used. The Ge detector has a characteristic of high energy resolution and easy identification of the peak position. The detector 2a is not limited to a Ge detector, and for example, a plastic scintillation detector, a NaI detector, or a CdTe detector may be used.

  The slide moving unit 3 relatively slides the storage container 100 and each radiation detection unit 2 in the y direction, which is a direction orthogonal to two opposite surfaces (two surfaces opposite to each other in the y direction) of the storage container 100 described above. It is to be moved. The slide moving unit 3 in the present embodiment slides the storage container 100 in the y direction with each radiation detection unit 2 stationary. For example, the slide moving unit 3 includes a pair of rails 3a extending in the y direction so as to support the bottom surface of the storage container 100 as shown in FIG. Moving means for moving the storage container 100 along the present direction. The slide moving unit 3 includes a weight measuring unit 3b that measures the weight of the radioactive waste stored in the storage container 100 when the bottom surface of the storage container 100 is supported. In this case, since the weight (empty weight) of the storage container 100 is previously measured and known, the weight of the radioactive waste can be measured by subtracting the weight of the storage container 100 from the detected weight. For example, a load cell is used as the weight measuring unit 3b.

  The rotational movement unit 4 holds the storage container 100 with a vertical axis perpendicular to two opposite surfaces (two surfaces opposite to each other in the z direction in the present embodiment) of the four surfaces of the storage container 100 that the radiation detection unit 2 faces. It is rotated 90 degrees. The rotational movement unit 4 in the present embodiment is provided at an end of the slide movement of the storage container 100 by the slide movement unit 3 (end of the rail 3a) and is rotationally driven by an axis along the z direction. have.

  The height detection unit 5 detects the height of the storage container 100 supported by the slide moving unit 3. Specifically, the height detection unit 5 includes a distance between the detection surface of the height detection unit 5 and the rail 3a, and a distance between the detection surface of the height detection unit 5 and the lid (upper surface) of the storage container 100. From the above, the height of the storage container 100 is detected. Moreover, when the cover part of the storage container 100 is open | released, the height of the radioactive waste in the storage container 100 is detected.

  The calculation control unit 6 executes calculation processing for evaluating the radioactivity distribution of the radioactive waste in the storage container 100 based on the radiation detected by the radiation detection unit 2, and includes a count rate detection unit 10 and a mesh A division processing unit 11, an initial condition setting unit 12, a radioactivity distribution evaluation unit 13, and a storage unit 14 are provided. In this configuration, the radiation evaluation unit 50 includes the radiation detection unit 2 and the calculation control unit 6.

  The count rate detection unit 10 obtains a radiation count rate (cps: count per second) based on the number of detected radiations detected by the detector 2a of the radiation detection unit 2. This count rate is a value indicating the number of radiations detected by each detector 2a per unit time, and is stored in the storage unit 14 together with relative position information between the storage container 100 and the detector 2a.

  The mesh division processing unit 11 performs a process of dividing the inside of the storage container (measurement space) 100 into a mesh M that is a predetermined virtual space. In this embodiment, as shown in FIG. 4, the inside of the storage container 100 is divided into two upper and lower stages, and the upper and lower spaces are set by being divided into 16 × 4 × 4 meshes M, respectively. . That is, in this embodiment, the inside of the storage container 100 is virtually divided into 32 meshes M in total. The number and size of the meshes M can be appropriately changed according to the position of the detector 2a of the radiation detection unit 2, for example. The set position information of the mesh M is output to the radioactivity distribution evaluation unit 13.

  The initial condition setting unit 12 sets initial conditions for evaluating the radioactive distribution of the radioactive waste in the storage container 100. The set initial condition is stored in the storage unit 14. Specifically, the initial condition setting unit 12 sets the radiation source S to each of the plurality of meshes M as an initial condition, as shown in FIG. In FIG. 5, upper or lower meshes M are shown, but radiation sources S are set for all the meshes M. This radiation source S is a virtual radiation source arranged in each mesh M, and in this embodiment, a radiation source S having the same intensity (size) is set in each mesh M. Moreover, the initial condition setting part 12 sets the density of the radioactive waste accommodated in the storage container 100 as an initial condition. In the present embodiment, the density (bulk density) of the radioactive waste is calculated from the weight of the radioactive waste measured by the weight measuring unit 3b and the volume of the storage container 100 calculated from the height of the storage container 100. . In this case, the radioactive waste is not unevenly distributed in the storage container 100 and is set to have a uniform density.

  The radioactivity distribution evaluation unit 13 performs statistical calculation based on the intensity of the radiation source S and the density of radioactive waste set in each mesh M by the initial condition setting unit 12 and the count rate detected by the count rate detection unit 10. The radioactivity distribution of the radioactive waste in the storage container 100 is evaluated. A specific procedure will be described later.

  The storage unit 14 stores the count rate detected by the count rate detection unit 10, the intensity of the radiation source S and the density of radioactive waste, which are the initial conditions set by the initial condition setting unit 12. The storage unit 14 can be configured by, for example, a nonvolatile memory such as a hard disk device, a volatile memory such as a RAM (Random Access Memory), or a combination thereof.

  Moreover, it is preferable that the radioactivity measuring apparatus 1 of this embodiment is provided with the detection part shielding part 7 and the storage container shielding part 8 as shown in FIGS. The detection unit shielding unit 7 includes the radiation detection unit 2 and covers a range where the radiation detection unit 2 faces the surface of the storage container 100. The detection unit shielding unit 7 is configured as a rectangular box body in which only the side of the radiation detection unit 2 facing the storage container 100 is opened by a lead plate made of lead having a thickness of 3 [cm], for example, The radiation detection unit 2 is disposed inside. With this configuration, the detection unit shielding unit 7 shields radiation that reaches the radiation detection unit 2 from the outside of the apparatus.

  The storage container shielding unit 8 covers the storage container 100 when the radiation detection unit 2 detects radiation. The storage container shielding unit 8 is a range of sliding movement of the storage container 100 by the slide moving unit 3 when the radiation detection unit 2 detects radiation, for example, an iron plate having a thickness of 4 [cm] made of iron, The storage container 100 is configured as a rectangular box that covers four surfaces of the storage container 100 in the x direction and the z direction with a predetermined distance (for example, a gap of 1 [cm] from the storage container). With this configuration, the storage container shielding unit 8 shields radiation from the outside of the apparatus to the storage container 100 when the radiation detection unit 2 detects radiation. In addition, the storage container shielding unit 8 is configured to not cover the storage container 100 so as not to prevent the radiation detection unit 2 from detecting radiation at the portion where the detection unit shielding unit 7 described above is provided. Further, when the storage container 100 is carried into the apparatus from the end in the slide movement direction (y direction) of the slide movement unit 3 that is not on the rotational movement unit 4 side, both sides in the y direction are opened. It is configured as a cylinder. When the storage container shielding unit 8 carries the storage container 100 into the apparatus from the rotational movement unit 4 side, the end of the slide movement unit 3 not in the rotational movement unit 4 side in the slide movement direction (y direction) is It may be occluded.

  Next, the radiation detection operation by the radioactivity measurement apparatus 1 will be described. First, as shown in FIG. 3, the radioactivity measuring apparatus 1 slides the storage container 100 along the y direction by the slide moving unit 3. In this case, the slide moving unit 3 slides the storage container 100 in four stages in the y direction along the mesh M (FIG. 4) set in the storage container 100. The radiation detection unit 2 detects the radiation at each stage of sliding movement.

  Next, the radioactivity measuring apparatus 1 moves the storage container 100 to the position of the rotary table 4 a of the rotary moving unit 4 by the slide moving unit 3. And the radioactivity measuring apparatus 1 rotates the storage container 100 90 degree | times by the rotational movement part 4 with the axial center which follows az direction. For this reason, in the storage container 100, the surface facing the y direction faces the x direction, and the surface facing the x direction faces the y direction.

  Next, the radioactivity measurement apparatus 1 slides the storage container 100 in four stages along the y direction by the slide moving unit 3 in the opposite direction. For this reason, the surface in the y direction that did not face the radiation detection unit 2 in the previous slide movement faces the x direction and the radiation detection unit 2 faces, and radiation is newly detected from the surface. As a result, γ rays emitted from the six surfaces of the storage container 100 are detected. The radiation detected by each detector 2a of the radiation detection unit 2 is sent to the count rate detection unit 10 to detect the count rate. This count rate is stored in the storage unit 14 in association with the relative position of the detector 2a and the storage container 100.

  Next, a procedure for evaluating the radioactive distribution of the radioactive waste in the storage container 100 will be described. FIG. 6 is a flowchart showing a procedure for evaluating the radioactivity distribution of the radioactive waste in the storage container. First, the mesh division processing unit 11 divides a space in the storage container 100 in which radioactive waste is stored into a plurality of meshes M (step Sa1; mesh division step). Specifically, as shown in FIG. 4, the inside of the storage container 100 is divided into upper and lower stages, and the upper and lower spaces are divided into 4 × 4 16 meshes M, respectively. For this reason, in this embodiment, the space in the storage container 100 is virtually divided into 32 meshes M.

  Next, the initial condition setting unit 12 sets the radiation source S to each of the divided meshes M (step Sa2; initial condition setting step). Specifically, as shown in FIG. 5, the radiation sources S are arranged on all the meshes M, respectively. These radiation sources S are virtual radiation sources having the same radiation source intensity.

  Next, the initial condition setting unit 12 measures the weight of the radioactive waste in the storage container 100 supported by the slide moving unit 3 via the weight measuring unit 3b (step Sa3). In this case, since the weight (empty weight) of the storage container 100 is known in advance, the weight W of the radioactive waste can be measured by subtracting the weight of the storage container 100 from the detected total weight. The initial condition setting unit 12 measures the height of the storage container 100 via the height detection unit 5 (step Sa4). Then, the initial condition setting unit 12 calculates the density ρ (bulk density) of the radioactive waste from the weight of the radioactive waste and the volume V of the storage container 100 calculated from the height of the storage container 100 (step) Sa5: initial condition setting step). In this case, it is assumed that radioactive waste is stored in the storage container 100 so as to have a uniform density ρ without uneven distribution of the radioactive waste. In this way, the storage container 100 is divided into a plurality of meshes M, the radiation sources S are arranged on the meshes M, and the density ρ of the radioactive waste in the storage container 100 is assumed to be uniform. Initial conditions (preconditions) for evaluating the radioactivity distribution of the radioactive waste that is unknown how it is stored in the container 100 are set.

  Next, the count rate detection unit 10 detects the count rate (step Sa6; count rate detection step). Specifically, the storage container 100 is slid along the y direction by the slide moving unit 3. In this case, the slide moving unit 3 moves the storage container 100 in four stages in the y direction along the mesh M (FIG. 4) set in the storage container 100. Then, each detector 2a of the radiation detection unit 2 detects the radiation dose at each stage of sliding movement.

  Further, when the storage container 100 slides in four steps in the y direction, the radioactivity measurement apparatus 1 moves the storage container 100 to the position of the rotary table 4 a of the rotational movement unit 4 by the slide moving unit 3. And the radioactivity measuring apparatus 1 rotates the storage container 100 90 degree | times by the rotational movement part 4 with the axial center which follows az direction. Then, the storage container 100 is again slid in four stages along the y direction by the slide moving unit 3. For this reason, the surface in the y direction that did not face the radiation detection unit 2 in the previous slide movement faces the radiation detection unit 2 in the x direction, and a radiation dose is newly detected from the surface. The detected radiation dose is output to the count rate detection unit 10 as needed to detect the count rate. This count rate is stored in the storage unit 14 in association with the relative position of the storage container 100 for each detector 2a.

  Next, the radioactivity distribution evaluation unit 13 evaluates the radioactivity distribution with the highest possibility on the assumption that the density ρ of the radioactive waste in the storage container 100 is constant (step Sa7; radioactivity distribution evaluation). Step). In this embodiment, ML-EM (Maximum Likelihood-Expectation Maximization) method is used for this radioactivity distribution evaluation. This ML-EM method is said to be the maximum likelihood (maximum) estimation / expectation maximization method, and it is assumed that the most probable radiation is statistically based on the assumption that the measured data follow a Poisson distribution. It is a successive approximation method for estimating the performance distribution.

FIG. 7 is a diagram for explaining variables in the ML-EM method. In FIG. 7, y ₁ , y ₂ , y ₃ ..., Y _i indicate the count rates of the radiation detected by the detector 2a. Further, C _ij is a detection probability for radiation from an arbitrary mesh M of the storage container 100. Further, λ ₁ , λ ₂ , λ ₃ ..., Λ _j indicate the value of the source intensity of each mesh M. In FIG. 7, the positions of the count rates y ₁ , y ₂ , y ₃ ..., Y _i indicate the relationship between the mesh M of the storage container 100 and the count rate detection position. Further, the detection probability C _ij indicates a correspondence relationship between the count rate y _i in an arbitrary mesh M and the source intensity λ _j . This detection probability C _ij can be obtained by Equation 1.

In Equation 1, ε ₀ is a value indicating the reference efficiency (cps / Bq), and this reference efficiency ε ₀ is measured when a standard radiation source is placed in front of the detector 2a at a certain distance. Efficiency. Further, η _ij is a solid angle correction coefficient in the target mesh M whose detection probability is to be calculated, and is a coefficient for correcting the difference in solid angle between the detector 2a and the mesh M at the time of measuring the reference efficiency ε ₀ described above. . μ is a mass attenuation coefficient, and ρ is a density of radioactive waste set as an initial condition. Further, l (el) is a distance between the detector 2a and the target mesh M. According to Equation 1, the detection probability C _ij in each mesh M can be obtained.

Next, the source intensity λ _j in each mesh M is estimated by the ML-EM method using the obtained detection probability C _ij and the count rate y _i of the radiation detected by the detector 2a. In this case, the source intensity λ _j can be obtained by Equation 2.

Thereby, as an initial condition, from the state where the radiation sources S having the same intensity are arranged on each mesh M, the tendency of the radioactivity distribution in the storage container 100 gradually increases using the detection probability C _ij and the count rate y _i. I understand. In this example, for simplification of description, the count rate y _i detected by one detector 2a and the detection probability C _ij corresponding to the detector 2a have been described. Then, the source intensity λ _j is obtained, and these are combined to evaluate the most probable radioactivity distribution.

However, the value of the source intensity λ _j obtained in step Sa7 is based on the assumption that the density ρ of the radioactive waste in the storage container 100 is constant. In the storage container 100, the density of the radioactive waste varies depending on the shape and size of the radioactive waste, and the accuracy of the radioactivity distribution is not sufficient. Therefore, next, the density distribution of each mesh M is corrected. FIG. 8 is a diagram for explaining a procedure for correcting the density distribution in the storage container. In FIG. 8, the four meshes M arranged in the upper part of the figure will be described as an example.

In correcting the density distribution of each mesh M, first, the source centroid (radioactivity centroid) between the opposing detectors 2a and 2a is calculated (step Sa8). Here, the source strengths λ _j of the four meshes M are respectively expressed as a, b, c, and d for convenience. These source strengths a, b, c, and d are all values calculated by the ML-EM method described above. In FIG. 8, the distance between the radiation source having a left detector 2a and the source strength a and r _a, the distance between the radiation source having a right detector 2a and the source strength a and _r'a . Similarly, the distance between the radiation source having a left detector 2a and the source intensity b and r _b, the distance between the radiation source having a right detector 2a and the source intensity b and _r'b, left the distance between the radiation source with a detector 2a and the source intensity c and r _c, the distance between the radiation source having a right detector 2a and the source intensity c and _r'c, left detector 2a and the line the distance between the radiation source having a source strength d and r _d, the distance between the radiation source having a right detector 2a and the source intensity d and _r'd.

If Gs is the source gravity center, the source gravity center Gs can be obtained by the sum of the source intensities a, b, c, and d (Gs = a + b + c + d). Further, when the distance between the source center of gravity Gs and left detectors 2a and _{r G,} the distance between the source center of gravity Gs and right detectors 2a and _r'G, the distance _r ^{G 2} is the source centroid Gs Can be expressed by Equation 3.

Next, the density distribution of each mesh is corrected from the difference between the count rates of the opposing detectors 2a, 2a (step Sa9). The count rate of the left detector 2a is N ₁ , and the count rate of the right detector 2a is N ₂ . In this case, the count rate N ₁ can be expressed by Equation 4.

Here, ρ ₁ is the density of the mesh M located between the source gravity center Gs and the left detector 2a. Further, l ₀ (El Zero) is a distance between the detector 2a and the target mesh M at the time of measuring the reference efficiency ε ₀ described above. When Formula 4 is solved for density ρ _{1 and} Formula for count rate N _{2 is} solved for density ρ ₂ , Formula 5 is obtained.

From Equation 5, the tendency of density distribution of a plurality (four) of meshes M located between the opposing detectors 2a and 2a can be seen. Such density ρ ₁ , ρ ₂ ... Ρ _i is calculated for the mesh M located between all the opposing detectors 2a, 2a, and the density distribution of the mesh M in the storage container 100 is expressed by a mathematical formula. 6 is corrected by normalization calculation.

Next, the radioactivity distribution evaluation unit 13 evaluates the radioactivity distribution most likely after the density correction of the radioactive waste in the storage container 100 (step Sa10; radioactivity distribution evaluation step). In evaluating the radioactivity distribution, first, a new detection probability is obtained from the corrected density distribution. FIG. 9 is a diagram for explaining a procedure for obtaining a new detection probability based on the corrected density distribution. In FIG. 9, S is a radiation source arranged on an arbitrary mesh M, and 1 (el) is a distance between the radiation source S and the detector 2a. Further, the density of the mesh M existing between the radiation source S and the detector 2a is set to ρ ₁ , ρ ₂ , and ρ ₃ , and the distances are set to r ₁ , r ₂ , and r ₃ , respectively. In this case, the detection probability C _ij in the mesh M in which the radiation source S described above is arranged can be expressed by Equation 7.

In this way, the density distribution is taken into consideration, the detection probability C _ij of each mesh M is calculated, and the new detection probability C _ij and the count rate y _i are used for each of the plurality of detectors 2a according to Equation 2. The source intensity λ _j is obtained, and these are combined to evaluate the most probable radioactivity distribution. In this configuration, since the radioactivity distribution is evaluated in consideration of the density distribution in the storage container 100, the evaluation accuracy is improved.

Next, the radioactivity distribution evaluation unit 13 determines whether or not the radioactivity distribution satisfies a predetermined convergence condition (step Sa11). As the convergence condition, for example, it can be considered that the convergence has occurred when the calculated change rate (difference from the previous calculation value) of the source intensity λ _j of each mesh M is equal to or less than a predetermined threshold value. In this determination, if the convergence condition is not satisfied (step Sa11; No), the process is returned to step Sa8 and recalculated. In this case, after correcting the density distribution of each mesh M, a radiation source having the same radiation source intensity is set again for each mesh M, and recalculation is performed. Thereby, the evaluation accuracy of the radioactivity distribution is improved.

  On the other hand, if the convergence condition is satisfied (step Sa11; Yes), the radioactivity distribution with high evaluation accuracy can be acquired, and thus the process is terminated. In addition, since the radioactivity amount (Bq) of the radioactive waste in the storage container 100 can be accurately determined by the above-described evaluation of the radioactivity distribution, the radioactivity in the storage container 100 can be calculated by converting this radioactivity amount per unit weight. It can be easily and accurately determined whether or not the radioactive level of the waste is below the clearance level.

As described above, the radioactivity evaluation method according to the present embodiment sets the radiation source S in each of the mesh division step Sa1 for dividing the inside of the storage container 100 in which radioactive waste is stored into a plurality of meshes M, and the plurality of meshes M. Then, from the initial condition setting steps Sa2 and Sa5 for setting the density ρ of the radioactive waste and each of a plurality of positions around the storage container 100, a count rate y _i indicating the number of detected radiations per unit time is detected. Radioactivity distribution of the storage container 100 by statistical calculation based on the counting rate detection step Sa6 and the source intensity λ _j and density ρ of the radiation source S set in the initial condition setting steps Sa2 and Sa5 and the counting rate y _i Radioactivity distribution evaluation steps Sa7 and Sa10 for evaluating the radioactivity evaluation, regardless of the shape and size of the radioactive waste in the storage container 100. It is possible to improve the accuracy of. Moreover, since the process which arranges radioactive waste into the same shape and magnitude | size becomes unnecessary, the burden of the pre-process in radioactivity evaluation can be aimed at.

  Moreover, according to this embodiment, based on the radiation source S set in each of the plurality of meshes M, the center-of-gravity position determination step Sa8 that determines the center-of-gravity position of the plurality of radiation sources S, and the center-of-gravity position of the radiation source S Therefore, since the density correction step Sa9 for correcting the density ρ set in the initial condition setting step is included, the evaluation accuracy of the radioactive distribution of the radioactive waste in the storage container 100 can be further increased. In particular, an object that is difficult to directly confirm the inside visually, such as radioactive waste arbitrarily stored in the storage container 100, has a higher effect.

  Further, according to the present embodiment, the intensity of the plurality of radiation sources S set in the initial condition setting step is the same, and the density ρ of the radioactive waste is uniform. Therefore, the statistical calculation in the radioactivity distribution evaluation step Sa7 Can be performed more easily.

  In addition, according to the present embodiment, the initial condition setting step includes steps Sa3 and Sa4 for measuring the weight of the radioactive waste in the storage container 100 and the height of the storage container 100, and therefore, from the measured weight and height. The density ρ of the radioactive waste can be easily calculated.

  In addition, according to the present embodiment, since the evaluation object is the radioactive waste generated by the nuclear facility or the decommissioning of the nuclear facility, the generated radioactive waste can be easily classified and disposed according to the radioactivity level. For example, whether the radioactive level of the radioactive waste (measuring object) is equal to or less than the clearance level can be determined accurately, rationally and efficiently.

  Although one embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. The present embodiment can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. This embodiment and its modifications are included in the scope of the present invention and the gist thereof, and are also included in the invention described in the claims and the equivalents thereof.

  For example, in the above-described embodiment, the storage container 100 has a rectangular shape (rectangular shape). However, the storage container 100 is not limited to the rectangular shape, and may be a cylindrical shape. In the above-described embodiment, the example using the ML-EM method has been described as a method for evaluating the most likely radioactivity distribution. However, other mathematical methods such as the least square method may be used. it can. In the present embodiment, the radiation source S is a point source arranged at the center of the mesh M. However, a radiation source dispersed inside the mesh M may be used. In this embodiment, it is assumed that the radiation source and the dose are uniformly dispersed in each mesh M as an initial condition. However, the arrangement of the radioactive waste inside the storage container 100 is measured, and each of the meshes M is determined based on the arrangement. A density and a dose may be assigned to the mesh M. In this case, for example, it is preferable to provide a step of measuring the internal state of the storage container 100 using the height detector 5 or the like. In the present embodiment, the space in the storage container 100 is virtually divided into a total of 32 meshes M, but the number of divisions of the meshes M is not limited to this.

DESCRIPTION OF SYMBOLS 1 Radioactivity measuring apparatus 2 Radiation detection part 2a Detector 3 Slide movement part 3a Rail 3b Weight measurement part 4 Rotation movement part 4a Rotation table 5 Height detection part 6 Calculation control part 10 Count rate detection part 11 Mesh division | segmentation process part 12 Initial stage Condition setting unit 13 Radioactivity distribution evaluation unit 14 Storage unit 50 Radioactivity evaluation apparatus 100 Storage container M Mesh S Radiation source

Claims (6)

A mesh division step for dividing the measurement space where the measurement object exists into a plurality of meshes;
An initial condition setting step of setting a radiation source for each of the plurality of meshes and setting a density of the measurement object;
A count rate detection step of detecting a count rate indicating the number of detected radiation per unit time from each of a plurality of positions around the measurement space;
A radioactivity distribution evaluation step for evaluating the radioactivity distribution in the measurement space by statistically calculating based on the intensity and density of the radiation source set in the initial condition setting step and the counting rate;
Radioactivity evaluation method including A center-of-gravity position determining step for determining a center-of-gravity position of the plurality of radiation sources based on the radiation source set in each of the plurality of meshes;
The radioactivity evaluation method according to claim 1, further comprising: a density correction step of correcting the density set in the initial condition setting step based on a barycentric position of the radiation source.   The radioactivity evaluation method according to claim 1 or 2, wherein the intensity of the plurality of radiation sources set in the initial condition setting step is the same, and the density is uniform.   The radioactivity evaluation method according to any one of claims 1 to 3, wherein the initial condition setting step includes a step of measuring a weight and a height of the measurement object.   The radioactivity evaluation method according to any one of claims 1 to 4, wherein the measurement object is a radioactive waste generated by a nuclear facility or decommissioning of the nuclear facility. A mesh division processing unit that divides a measurement space in which a measurement object exists into a plurality of meshes;
An initial condition setting unit for setting a radiation source for each of the plurality of meshes and setting a density of the measurement object;
A count rate detector that is arranged at a plurality of positions around the measurement space and detects a count rate indicating the number of detected radiation per unit time from each of the plurality of positions;
A radioactivity distribution evaluation unit that evaluates the radioactivity distribution in the measurement space by statistically calculating based on the intensity and density of the radiation source set by the initial condition setting unit and the counting rate;
Including radioactivity evaluation apparatus.

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