The present invention relates to a method and an apparatus for measuring radioactivity, radioactivity concentration and radioactivity surface density. More particularly, the present invention relates to a radioactivity measuring method for measuring radioactivity by obtaining a conversion factor without using an actual calibration radiation source and a measuring apparatus for carrying out this method and a measuring method and a measuring apparatus for measuring radioactivity concentration and a method and an apparatus for measuring radioactivity surface density based on the radioactivity measured by the former method and apparatus.
In order to obtain the radioactivity concentration or the radioactivity surface density of a measurement subject such as demolition wastes and the like of nuclear facilities, the radioactivity of the measurement subject must be first obtained. In a conventional radioactivity measurement method, a conversion factor for calculating the radioactivity from a counting rate obtained from a count of a radioactivity detector is acquired by comparison with a conversion factor obtained by separately preparing a calibration radiation source whose radioactivity is already known and actually performing measurement.
Further, although the radioactivity of the surface of the measurement subject must be first measured in order to measure the radioactivity surface density of the measurement subject, it is general to use a xcex2 ray having the weaker penetrability than that of a xcex3 ray in order to measure the radioactivity of the surface. That is, when the xcex3 ray having the stronger penetrability than that of the xe2x8ax96 ray is a target of measurement, it is difficult to determine whether the measured xcex3 ray is emitted from the surface of the measurement subject or emitted from the inside, and hence the xcex3 ray is inappropriate for measurement of the radioactivity of the surface of the measurement subject. Therefore, the radioactivity of the surface of the measurement subject with the xcex2 ray as a measurement target is measured by using a portable type radioactivity detector called a survey meter specified in JIS (Japanese Industrial Standards) Z 4329 xe2x80x9cPortable radioactive surface contamination meterxe2x80x9d or an article carrying out monitor specified in JIS Z 4337 xe2x80x9cInstalled articles surface contamination monitoring assemblies for beta emittersxe2x80x9d.
However, in the method for measuring the surface radioactivity for obtaining the radioactivity surface density mentioned above, since the surface radioactivity is measured by measuring the xcex2 ray by using the survey meter, the survey meter can not be inserted into a narrow tube when the measurement subject is, for example, a metal narrow tube. Accordingly, measurement of the radioactivity of the inner peripheral surface of the narrow tube is difficult.
Furthermore, in the radioactivity measurement method with the xcex3 ray having the stronger penetrability than that of the xcex2 ray being a target of measurement, a conversion factor for calculating the radioactivity is obtained by comparison with the conversion factor acquired by performing actual measurement by using a calibration radiation source. Therefore, in order to accurately obtain the radioactivity of the measurement subject, an optimum calibration radiation source must be produced in accordance with a shape or a dimension of the measurement subject, and this method is not appropriate for the radioactivity measurement of the measurement subjects having different shapes or dimensions. For example, in case of measuring the radioactivity of demolition wastes and the like of nuclear facilities, it is difficult to produce each optimum calibration radiation source for each waste in nature because wastes having various shapes and dimensions are mixed. Thus, this method is hardly applied to the radioactivity measurement of the demolition wastes generated in volume.
It is an object of the present invention to provide a method and an apparatus for measuring the radioactivity, a method and an apparatus for measuring the radioactivity concentration, and a method and an apparatus for measuring the radioactivity surface density, which do not require an actual calibration radiation source, respectively. Further, it is another object of the present invention to provide a method and an apparatus for measuring the radioactivity surface density capable of obtaining the radioactivity surface density by utilizing a xcex3 ray having the stronger penetrability than that of a xcex2 ray.
To achieve this aim, according to the present invention, there is provided a method for measuring the radioactivity comprising: a virtual modeling step for arranging virtual three-dimensional models of a measurement subject and a radiation detector in a virtual three-dimensional space with the same positional relationship as an actual geometric positional relationship; a conversion factor setting step for associating the number of times of gene ration of a virtual radiation ray emitted from the virtual three-dimensional model of the measurement subject with the number of times of incidence of the virtual radiation ray upon the virtual three-dimensional model of the radiation detector and obtaining a conversion factor; an actual counting rate calculating step for calculating a counting rate by actually counting the number of times of incidence of the radiation rays emitted from the measurement subject upon the radiation detector; and a radioactivity calculating step for calculating the radioactivity of the measurement subject from the counting rate and the conversion factor.
Here, the interaction of the radiation ray and a substance is generated with a given probability, and the conversion factor can be obtained by recreating this phenomenon in the pseudo-manner even if actual measurement using the actual calibration radiation source is not used. That is, the conversion factor can be obtained by virtually recreating the three-dimensional shapes and position relationship of the measurement subject and the radiation detector and associating the number of times of generation of the virtual radiation ray emitted from the virtual three-dimensional model of the measurement subject with the number of times of incidence of the virtual radiation ray upon the virtual three-dimensional model of the radiation detector based on the recreation. Further, the actual radioactivity of the measurement subject can be calculated by the obtained conversion factor and the count by the actual radiation detector.
In case of this radioactivity measurement method, it is preferable that the conversion factor setting step includes: a virtual count calculating step for randomly generating a virtual radiation ray from the virtual three-dimensional model of the measurement subject by utilizing the Monte Carlo calculational technique, and for counting the number of times of incidence of the virtual radiation ray upon the virtual three-dimensional model of the radiation detector to obtain a virtual count; and a conversion factor calculating step for obtaining a conversion factor from the number of times of generation of the virtual radiation ray and the virtual count.
By utilizing the Monte Carlo calculational technique, the three-dimensional shapes and positional relationship of the measurement subject and the radiation detector can be virtually recreated, and it is possible to simulate how the virtual radiation ray randomly generated from the virtual three-dimensional model of the measurement subject enters on the virtual three-dimensional model of the radiation detector.
In this case, since a generation rate of the virtual radiation ray from the virtual three-dimensional model of the measurement subject corresponds to the radioactivity of that virtual three-dimensional model, the conversion factor can be obtained from the number of times of generation of the virtual radiation ray and the count in the virtual three-dimensional model of the radiation detector. Furthermore, the actual radioactivity of the measurement subject can be calculated based on the obtained conversion factor and the count by the actual radiation detector.
Moreover, according to the radioactivity measurement method of the present invention, the conversion factor setting step can obtain the conversion factor from the correlation of the number of radiation rays approximately calculated before and after passing through a medium based on a thickness of the medium existing between the measurement subject and the radiation detector, an attenuation coefficient of that medium, a buildup factor of that medium, and a distance between the measurement subject and the radiation detector.
For example, in case of examining shield of the radiation ray, the shield calculation is performed by using a point-kernel ray tracing code. In this point-kernel ray tracing code, the number of radiation rays after passing through a given medium I is approximately obtained from a relational expression based on the number of radiation rays I0 before passing through the medium, a thickness d of the medium, an attenuation coefficient xcexc of the medium, a buildup factor B of the medium, and a distance r between the radiation source and an evaluation point:
I=(1/(4xcfx80r2))I0Bexc2x7xcexcd
That is, the attenuation coefficient xcexc of the medium and the buildup factor B of the medium are determined in accordance with a type of the medium, and the thickness d is determined in accordance with the conformation of the measurement subject, a position of the radioactive contamination and others. If the distance r between the radiation source and the evaluation point is known, the relationship between I and I0 can be approximately derived from the above-described relational expression, and the conversion factor can be obtained from the relationship between I and I0. That is, I0 corresponds to the number of times of generation of the virtual radiation ray emitted from the virtual three-dimensional model of the measurement subject, whilst I corresponds to the number of times of incidence of the virtual radiation ray upon the virtual three-dimensional model of the radiation detector, and I/I0 is thereby the conversion factor. In addition, the actual radioactivity of the measurement subject is calculated by the obtained conversion factor and the count by the actual radiation detector.
According to these radiation measurement methods of the present invention, the conversion factor can be obtained without using the actual calibration radiation source, and the radioactivity of the measurement subject can be accurately and rapidly measured. Therefore, it is possible to liberally and rapidly process the radioactivity measurement of the measurement subjects having various shapes or dimensions. In particular, when obtaining the conversion factor by utilizing the Monte Carlo calculational technique, since the emission phenomenon of the actual radiation ray is simulated, the conversion factor in line with the actual phenomenon can be obtained. Additionally, in case of obtaining the conversion factor based on the relationship between I and I0 calculated by using the point-kernel ray tracing code, since a quantity of calculation is reduced even though the measurement accuracy is lowered as compared with the Monte Carlo calculational technique, the conversion factor can be calculated in a short period of time.
Further, if the radioactivity concentration or the surface density of the measurement subject is obviously even, detection by the radiation detector consisting of one cell can suffice. However, if the radioactivity concentration or the radioactivity surface density is not homogeneous and is unevenly distributed, since an error becomes large with the conversion factor obtained by presupposing the homogenous distribution, it is preferable to obtain the conversion factor taking the maldistribution status (uneven distribution) into consideration. In particular, when a radioactivity level of the measurement subject is low, the radiation detector must be moved to closer to the measurement subject to carry out measurement. In such a case, the maldistribution status of the radioactivity largely affects a value of the conversion factor. In order to be aware of the uneven distribution, it is effective to make the radiation detector as an assembly of a plurality of small detectors (cells), namely, set each one of the radiation detectors as a cell and assemble these cells to obtain one large radiation detector. Furthermore, consideration is given to the measurement subject as if it is divided into parts corresponding to cells of the radiation detector, and the conversion factor is obtained for each cell in accordance with each part. As result, in this conversion factor, the uneven distribution is taken into consideration. With the thus obtained conversion factor, the radioactivity of the measurement subject is obtained in accordance with each part thereof, and the radioactivity of the entire measurement subject based on a sum total of the radioactivity of the respective parts is calculated. Moreover, it is possible to designate a part from which many radiation rays are detected by a large number of cells and estimate the radioactivity concentration and the like in accordance with each part of the designated subject from counting information of each cell.
Therefore, according to the radioactivity measurement method of the present invention, it is preferable that: a radiation detector is made as an assembly of a plurality of cells and a measurement subject is conceptualized as an assembly of parts opposed to the cells; a virtual three-dimensional model of the radiation detector is made as an assembly of a plurality of cells and a virtual three-dimensional model of the measurement subject is made as an assembly of parts in a virtual modeling step; a conversion factor setting step is carried out in accordance with each part of the virtual three-dimensional model of the measurement subject and a conversion factor is obtained for each cell in accordance with each part; an actual counting rate calculating step is carried out in accordance with each cell of the radiation detector and a counting rate is obtained for each cell; and the radioactivity according to each part of the measurement subject is obtained by carrying out a radioactivity calculating step and then the radioactivity of the entire measurement subject is obtained based on the radioactivity of each part. In this case, the accurate evaluation is enabled by obtaining the virtual count for each cell even though the radioactivity is unevenly distributed.
Moreover, according to the present invention, there is provided a radioactivity measurement apparatus comprising: a radiation detector for counting a radiation ray emitted from a measurement subject; three-dimensional modeling means for fetching three-dimensional space coordinates of a surface of a measurement subject and virtually recreating a geometric positional relationship between the measurement subject and the radiation detector by utilizing the fetched coordinate; conversion factor setting means for associating the number of times of generation of a virtual radiation ray emitted from the virtually recreated three-dimensional model of the measurement subject with the number of times of incidence upon the virtually recreated three-dimensional model of the radiation detector and calculating a conversion factor; and radioactivity calculating means for calculating the actual radioactivity of the measurement subject based on an actual counting rate by the radiation detector and the conversion factor.
Therefore, by utilizing the virtual three-dimensional models of the measurement subject and the radiation detector pseudo-recreated by the three-dimensional modeling means, the conversion factor setting means associates the number of times of generation of the virtual radiation rays with the number of times of input to the virtual three-dimensional model of the radiation detector and then obtains the conversion factor. The radioactivity calculating means calculates the actual radioactivity of the measurement subject based on the thus obtained conversion factor and the actually measured counting rate of the radiation detector.
In case of this radioactivity measurement apparatus, it is preferable that the conversion factor setting means includes: simulating means for pseudo-recreating incidence of the virtual radiation ray randomly generated from the three-dimensional model of the measurement subject, upon the virtually recreated three-dimensional model of the radiation detector by utilizing a three-dimensional Monte Carlo calculation code; and conversion factor calculating means for calculating the conversion factor based on the number of times of generation of the virtual radiation ray and a count of incidence of the virtual radiation ray upon the three-dimensional model of the radiation detector.
Additionally, the radioactivity measurement apparatus according to the present invention may calculate the conversion factor from the correlation of the number of radiation rays approximately calculated before and after passing through a medium based on a thickness of the medium existing between the measurement subject and the radiation detector, an attenuation coefficient of the medium, a buildup factor of the medium, and a distance between the measurement subject and the radiation detector.
According to these radioactivity measurement apparatus of the present invention, the radioactivity of the measurement subject can be accurately and rapidly measured by calculating the conversion factor without using the actual calibration radiation source, and especially the measurement apparatus suitable for processing a large quantity of measurement subjects having various shapes or dimensions can be realized. In particular, in case of calculating the conversion factor by utilizing the Monte Carlo calculational code, since the actual emission phenomenon of the radiation ray is simulated, the conversion factor in line with the actual phenomenon can be obtained. Further, in case of calculating the conversion factor from the relationship between I and I0 obtained by using the point-kernel ray tracing code, the conversion factor can be calculated in a short period of time because the calculation amount is small.
Furthermore, in the above measurement apparatus according to the present invention, the radiation detector is an assembly of a plurality of cells, and the three-dimensional modeling means may make a virtual three-dimensional model of the radiation detector as an assembly of a plurality of cells and make a virtual three-dimensional model of the measurement subject as an assembly of parts opposed to the cells. Moreover, the conversion factor setting means may calculate the conversion factor for each cell of the radiation detector in accordance with each part of the measurement subject and obtain the radioactivity of the entire measurement subject based on the radioactivity of each part.
Therefore, the conversion factor can be obtained for each cell of the radiation detector in accordance with each part of the measurement subject, and the radioactivity for each part of the measurement subject can be obtained based on the conversion factors and each counting rate of each cell of the radiation detector actually measured, thereby obtaining the radioactivity of the entire measurement subject.
In addition, the radioactivity concentration measurement method according to the present invention includes: a weight or volume measuring step for measuring a weight or a volume of the measurement subject; and a radioactivity concentration calculating step for calculating the radioactivity concentration by dividing the radioactivity obtained by any of the radioactivity measurement methods mentioned above by the weight or the volume.
The radioactivity concentration can be obtained by dividing he radioactivity of the measurement subject by the weight or the volume. Therefore, the radioactivity concentration can be obtained by dividing the radioactivity of the measurement subject acquired by any of the radioactivity measurement methods mentioned above by the weight or the volume measured by the weight or volume measuring step.
Thus, according to this radioactivity concentration measurement method of this invention, since the radioactivity concentration of the measurement subjects having the various shapes or dimensions can be accurately and rapidly measured, it is possible to rapidly and rationally effect divisional evaluation according to each radioactivity level of the demolition waste which is expected to be generated in high volume by the future nuclear reactor decommissioning.
Additionally, the radioactivity concentration measurement apparatus according to the present invention includes: any of the radioactivity measurement apparatuses mentioned above; weight or volume measuring means for measuring a weight or a volume of a measurement subject; and radioactivity concentration calculating means for calculating the radioactivity concentration by dividing the obtained radioactivity of the measurement subject by the weight or the volume.
The radioactivity concentration can be obtained by dividing the radioactivity of the measurement subject by a weight or a volume of the measurement subject. Therefore, the radioactivity of the measurement subject is obtained by any of the above-described radioactivity measurement apparatuses and a weight or a volume of the measurement subject is obtained by the weight or volume measuring means, respectively. The radioactivity concentration can be calculated by dividing the radioactivity by the weight or the volume by the radioactivity concentration calculating means.
Thus, according to this radioactivity concentration measurement apparatus of the present invention, it is possible to provide the apparatus preferable for carrying out the radioactivity concentration measurement method mentioned above. That is, the radioactivity concentration of the measurement subjects having various shapes or dimensions can be accurately and rapidly measured, and it is possible to rapidly and rationally effect divisional evaluation according to each radioactivity level of the demolition waste which is expected to be generated in high volume by the future nuclear reactor decommissioning.
Furthermore, the radioactivity surface density measurement method according to the present invention includes: a surface area measuring step for measuring a surface area of a measurement subject; and a radioactivity surface density calculating step for calculating the radioactivity surface density by dividing the radioactivity obtained by any of the radioactivity measurement methods mentioned above by the surface area.
The radioactivity surface density can be calculated by dividing the radioactivity of the measurement subject by the surface area. Therefore, the radioactivity surface density can be calculated by dividing the radioactivity of the measurement subject obtained by any of the radioactivity measurement methods mentioned above by the surface area of the measurement subject measured by the surface area measuring step.
Therefore, according to this radioactivity surface density measurement method, since the radioactivity surface density of the measurement subjects having various shapes and dimensions can be accurately and rapidly measured, it is possible to rapidly and rationally effect divisional evaluation according to each radioactivity level of the demolition waste which is expected to be generated in high volume by the future nuclear reactor decommissioning. Moreover, the radioactivity surface density of, for example, the inside of a pipe having a small bore diameter such that the survey meter can not be inserted therein can be accurately and rapidly measured.
In addition, the radioactivity surface density measurement apparatus according to the present invention includes: any of the radioactivity measurement apparatus mentioned above; surface area measuring means for measuring a surface area of a measurement subject; and radioactivity surface density calculating means for calculating the radioactivity surface density by dividing the obtained radioactivity of the measurement subject by the surface area.
The radioactivity surface density can,be calculated by dividing the radioactivity of the measurement subject by the surface area. Therefore, the radioactivity of the measurement subject is obtained by any of the radioactivity measurement apparatuses mentioned above and the surface of the measurement subject is obtained by the surface area measuring means, respectively. The radioactivity surface density can be then calculated by dividing the radioactivity by the surface area by using the radioactivity surface density calculating means.
Thus, according to this radioactivity surface density measurement apparatus of this invention, it is possible to provide an apparatus preferable for carrying out the radioactivity surface density measurement method. That is, the radioactivity surface density of the measurement subjects having various shapes and dimensions can be accurately and rapidly measured, and it is possible to rapidly and rationally effect divisional evaluation according to each radioactivity level of the demolition waste which is expected to be generated in high volume by the future nuclear reactor decommissioning. Additionally, the radioactivity surface density of, e.g., the inside of a pipe having a small bore diameter such that the survey meter can not be inserted therein can be accurately and rapidly measured.