JPH0511063A - Method for measuring radioactivity of crushed body - Google Patents

Abstract

PURPOSE:To make it possible to correct the measured value of the radioactivity of the determined nuclide accurately by performing spectrum stripping correction by using the measured radiation spectrum of crushed body and the response function of a natural nuclide. CONSTITUTION:The amount of gamma rays in a crushed body 1 in concrete, which is generated by a large amount, is detected, and the spectrum of the gamma rays is obtained. The gamma-ray nuclide in the crushed body is identified by using the spectrum. The gamma rays are cast on the crushed body, and the permeability of the gamma rays in the crushed body 8 is measured. The gamma-ray attenuation factor and the density can be computed 7 based on the permeability. Then, the natural nuclide is identified. The response function of a detector 6 is measured based on the characteristics of the detector and the density of the crushed body. Based on the spectrum after spectrum stripping 3 by which the function is subtracted from the measured gamma-ray spectrum, the photoelectric peak area of the determined nuclide is computed 4. The area is divided by the gamma-ray attenuation factor of the photoelectric peak of the determined nuclide in the crushed body, and the net peak area is obtained. In this way, a Compton scattering line is identified and subtracted, and the highly accurate, quick measurement can be performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the radioactivity of solid waste in the particle size state, and more particularly, to the radioactivity intensity of the solid waste in the particle size state which exists in a large amount with high accuracy and The present invention relates to a radioactivity measuring method suitable for rapid measurement.

[0002]

2. Description of the Related Art With the demolition of a nuclear power plant, a large amount of extremely low levels of concrete and the like are generated. It is necessary to quantify the radioactivity intensity of this concrete in a short time.

Conventionally, Japanese Patent Laid-Open No. 2-263188 discloses an apparatus for rapidly measuring extremely low level radioactivity. In this radioactivity measurement system, concrete is crushed into concrete in a particle size state (crushed concrete). Then, a radiation detector is arranged in a cylindrical container, and the crushed concrete is poured around the radiation detector to measure the radioactivity of the crushed concrete body. In addition, the particle diameter of the crushed concrete is photographed by a TV camera, and the radiation attenuation amount of the crushed concrete is quantified by image processing to correct the radiation attenuation.

[0004]

[Patent Document 1] Japanese Patent Application Laid-Open No. 2-263
The conventional technique disclosed in Japanese Patent No. 188 does not take into consideration the influence of the distortion of the radiation spectrum due to the natural radionuclide existing in the crushed concrete itself. ⁴⁰ K, ²³⁸ U decay series nuclides and
There are natural radionuclides of the ²³² Th decay series. That is, the radiation spectrum measured by the radiation detector is that the above-mentioned natural nuclides are also simultaneously measured in addition to the quantitative nuclides, and the photoelectric peak shape and area of the quantitative nuclides are changed, and the error of the radioactivity quantitative value of the quantitative nuclide becomes large. There was a problem.

An object of the present invention is to provide a radioactivity measuring method capable of highly accurately and promptly measuring the radioactivity intensity of a very large amount of concrete having a very low radioactivity level, which is produced in large quantities in accordance with the demolition of a decommissioned nuclear power plant. Especially.

[0006]

In order to achieve the above-mentioned object, in the present invention, a means for correcting the quantitative photoelectric peak area of a measured radiation spectrum with a photoelectric peak area having higher energy than the quantitative photoelectric peak, and a quantitative photoelectric peak. Means is provided for correcting the quantitative photoelectric peak area from the amount of radiation attenuation in the crushed body corresponding to the radiation energy of.

[0007]

[Function] In addition to quantitative nuclides, ⁴⁰ K, ²³⁸ U decay series nuclides and ²³² Th decay series nuclide natural radionuclides are present in the crushed concrete to be measured. In the case of crushed concrete, only gamma rays can be measured by the radiation detector. This is because α rays and β rays are almost self-absorbed in crushed concrete. Further, in the range of the gamma ray energy of about 0.3 to 5 MeV, the interaction of gamma rays with the density of crushed concrete is Compton scattering. Therefore, in order to obtain an accurate radioactivity value of crushed concrete, it is necessary to measure the γ-ray spectrum of concrete and grasp the influence of Compton scattering. Compton scattering includes Compton scattering that occurs in the radiation detector and the subject,
That is, there is Compton scattering that occurs in concrete.
In the former case, it can be treated as a characteristic of the detector and can be obtained and set in advance. In the latter case, it is necessary to measure the density distribution of the crushed concrete because it changes depending on the density distribution of the crushed concrete. In the γ-ray spectrum, if a natural nuclide having an energy higher than the photon peak of the quantitative nuclide (corresponding to the γ-ray energy of the quantitative nuclide) exists, the Compton scattered line of the natural nuclide enters the photon peak area of the quantitative nuclide. Shape and area change. In order to correct this effect, the radioactivity intensity of the above-mentioned natural nuclide is obtained, and by using the density distribution information of the crushed concrete, the Compton scattered line that enters the photoelectric peak area of the quantitative nuclide is identified and subtracted from the photoelectric peak of the quantitative nuclide. By using the method, it is possible to accurately determine the photopeak area of the quantitative nuclide.

Also, γ-rays are self-absorbed in the crushed concrete. In order to correct this self-absorption, the γ-ray attenuation amount (self-absorption amount) in the crushed concrete corresponding to the radiation energy of the photoelectric peak of the quantitative nuclide is measured, and the photoelectric peak area of the quantitative nuclide is calculated by the γ-ray attenuation amount. By dividing, the photoelectric peak area of the quantitative nuclide can be accurately quantified.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. First, the configuration and operation of the radioactivity measuring apparatus of the present invention will be described with reference to FIGS. 2 and 3, and then the radioactivity quantification method of the present invention will be described with reference to FIGS. 1, 4 and 5. .

FIG. 2 is a system configuration diagram of the radioactivity measuring apparatus of the present invention. With the dismantling of the decommissioned nuclear power plant, a large amount of concrete is crushed by a crusher 22 to form a crushed body (crushed concrete) 9. The crushed body 9 to be measured is transferred by the belt conveyor 23 and dropped into the radioactivity measuring container 24. A radiation detector 10 for measuring the radiation (γ-rays) of the crushed body 9 is arranged on the central axis of the radiation measuring container 24. The radiation detection signal of the radiation detector 10 is input to the radiation signal processor 18, and the radiation signal processor 18 creates the γ-ray spectrum 1. In addition, in the radiation detector 10, a shield 13 for shielding the radiation other than the crushed body 9 is arranged on the outer peripheral portion of the radioactivity measuring container 24. Further, the crushed body 9 is irradiated with the irradiation radiation source (γ-ray source) 12 arranged on the outer peripheral portion of the radiation measurement container 24,
A radiation detector 11 for measuring the radiation transmitted through the crushed body 9 is arranged on the central axis of the radiation measuring container 24. The irradiation radiation source 12 is installed in the radiation source container 14 so that radiation does not leak in directions other than the irradiation direction, and a radiation source shutter 15 that stops radiation when irradiation is not performed has a radiation source container 1.
4 is arranged in front of the irradiation direction. The detection signal of the radiation detector 11 is also input to the radiation signal processor 18, and the radiation signal processor 18 creates the γ-ray spectrum 2. That is, the radiation signal processor 18 has a dual radiation processing function. The arithmetic controller 19 uses the γ-ray spectrum 1 and the γ-ray spectrum 2 to obtain the radioactivity intensity in the crushed body 9 by the method described later. Further, the arithmetic controller 19 determines the presence or absence of radioactivity of the crushed body 9 based on the radioactivity intensity, and outputs a command to the distributor 20. The presence / absence of this radioactivity is determined with a constant radioactivity intensity value as the boundary. Sorting machine 20
According to this command, the crushed body 9 is selectively discharged, and the crushed body 9 having radioactivity and the crushed body 9 having no radioactivity are separately inserted into the transport container 21. Further, the arithmetic and control unit 19 issues a command to the drive unit 16 for driving the inner cylinder 17 provided with the rotary blade so that the transfer speed of the crushed body 9 in the radioactivity measuring container 24 becomes appropriate. Based on this command, the driving machine 16 adjusts the rotation speed of the inner cylinder.

FIG. 3 is a basic configuration diagram relating to radiation measurement. The crushed body 9 is placed on the outer circumference on the simultaneous circumference centered on the radiation detector 10 and the radiation detector 11 and transferred. In this configuration, the crushing body 9 itself has an effect of shielding external radiation such as cosmic rays, so that the thickness of the shield 13 can be reduced. The nuclide contained in the crushed body 9 is
Natural nuclides ( ⁴⁰ K, ²³⁸ U decay series nuclide and ²³² Th decay series nuclide) and quantitative nuclides such as ⁶⁰ Co. The respective γ-ray energies are ⁴⁰ K: 1.46 MeV and ²¹⁴ Bi: 0.61, which is a representative γ-ray emitting nuclide of the ²³⁸ U decay series nuclide.
It is 1.12, 1.76 MeV, ²²⁸ Ac: 0.91 and 0.97 MeV, which are representative γ-ray emitting nuclides of the ²³² Th decay series nuclide, and ⁶⁰ Co: 1.17, 1.33 MeV. Therefore, as the radiation detector 10, a φ3 ″ × 3 ″ NaI (Tl) scintillation detector can be sufficiently used. Also, the irradiation radiation source 1
2 may be a γ-ray emitting nuclide that can pass through the crushed body 9 having a thickness of about 20 cm, so ¹³⁷ Cs (0.66 MeV) can be used. Radiation detector 11 for detecting the transmitted γ-rays
Is a φ2 ″ × 2 ″ NaI (Tl) scintillation detector. When a mechanism for sequentially transferring the crushed bodies 9 around the radiation detector as in this configuration is used, it is possible to measure the radioactivity and the γ-ray attenuation amount of the crushed bodies 9 without being affected by external radiation. .

Next, the radioactivity quantification method of the present invention will be described with reference to FIGS. 1, 4 and 5.

FIG. 1 is a block diagram showing a method for quantifying radioactivity according to the present invention. First, the γ-ray dose 1 in the crushed body is detected by the radiation detector 10, and the radiation signal processor 18 obtains the γ-ray spectrum of the crushed body. One measurement example of the γ-ray spectrum is shown in FIG. Here, 25 is a photopeak of ⁶⁰ Co which is a quantitative nuclide, and 26 is a photopeak of ⁴⁰ K which is a natural nuclide. This γ-ray spectrum is used to identify the γ-ray emitting nuclide in the crushed body 9. For example, if the γ-ray spectrum is measured with a radiation source whose γ-ray energy E is known in advance, a photoelectric peak of the γ-ray energy E can be detected in a channel C on the γ-ray spectrum. That is, E = f (C)
Can be expressed as The function f here is the energy characteristic of the radiation detector. Therefore, nuclide identification 2 can be performed by searching the channel of the photopeak. Also,
In the range of gamma ray energy of about 0.3 to 5 MeV,
The interaction of γ rays with the density of the crushed body 9 (about concrete) is Compton scattering. That is, in the measured γ-ray spectrum of the crushed body 9, not only the photoelectric peak but also the Compton scattering component is measured. Because the Compton scattered ray energy becomes lower energy than the photoelectric peak of the scattering species, Compton scattered radiation natural radionuclides of higher energy than a quantitative nuclide ⁶⁰ Co (1.33 MeV) enters the photoelectric peak of ⁶⁰ Co, photopeak The shape of the area and the area change. That is, the error of the quantitative value of radioactivity becomes large unless the influence of Compton scattering is subtracted. The Compton scattering includes Compton scattering that occurs in the radiation detector and Compton scattering that occurs in the crushed body 9. In the former case, it can be treated as a characteristic of the detector and can be obtained and set in advance. In the latter case, it changes depending on the density distribution of the crushed body 9. This density distribution can be measured by irradiating the crushed body 9 with γ rays. γ-ray mass absorption coefficient is 0.1-2.0M
Since there is almost no change for γ rays of about eV and the linear attenuation coefficient is proportional to the density, the radiation attenuation rate can be expressed by the following equation.

EXP (−μp _A t _A + μρ _B t _B )) (Equation 1) where μ is a mass absorption coefficient, ρ _A is the density of the crushed bodies 9, and ρ _B
Is the density of air, t _A is the distance from the outer diameter to the inner diameter of the radioactivity measuring container 24, and t _B is the distance from the radiation source 12 to the outer diameter of the radioactivity measuring container 24. Since ρ _B is a value close to zero, the above equation can be simplified as follows.

[0015] _{EXP (-μρ A t A) ...} ( Equation 2) Thus, to place the irradiation ray source 12 to the outer peripheral portion of the crushing member 9,
The irradiation source 12 irradiates the crushed body 9 with radiation, and the radiation transmitted through the crushed body 9 is measured by the radiation detector 11 (γ-ray transmittance of the crushed body is measured by 8). Since the value obtained by multiplying the irradiation dose by the number 2 is the transmitted radiation dose, it is possible to calculate 7 of the γ-ray attenuation rate and density in the crushed body 9. From the above, a natural nuclide having a photopeak higher than that of ⁶⁰ Co, for example, ⁴⁰ K is identified, and the response function 6 of the detector is obtained from the characteristics of the detector and the density of the crushed bodies 9. This response function 6 is obtained by the following method. The measurement system of the response function 6 is made equal to that of the present invention, and a solid whose density value is known is obtained.
Add ⁴⁰ K and measure the γ-ray spectrum, that is, the response function 6. This is performed for several types of densities, and the densities correspond to the response function 6. This correspondence table is used as the arithmetic controller 1
If it is provided inside 9, the response function 6 of the measured crushed body 9 can be obtained in a short time. An example of this response function 6 is shown in FIG. This shows the response function 6 of ⁴⁰ K. The photoelectric peak value 27 of this response function 6 is set to the same value as the photoelectric peak value 26 of the measured γ-ray spectrum, and other energy regions are multiplied by the ratio of (photoelectric peak 26) / (photoelectric peak 27) to obtain a crushed body of ⁴⁰ K. The response function 6 in 9 is obtained. By subtracting this response function 6 from the measured γ-ray spectrum (corresponding to FIG. 4), the spectrum of only ⁶⁰ Co is obtained. This spectrum subtraction operation is called spectrum stripping 3. The ⁶⁰ Co photopeak area 4 is determined from the spectrum after spectrum stripping 3. The method for calculating this peak area 4 will be the method discussed in the analysis manual presented by the Science and Technology Agency. next,
^{The 60} Co photoelectric peak area 4 is divided by the ⁶⁰ Co photoelectric peak radiation reduction rate in the crushed body to obtain the net peak area. Since the net peak area has a one-to-one correspondence with the radioactivity intensity of the crushed body 9, the ⁶⁰ Co radioactivity intensity of the crushed body 9 can be determined.

FIG. 6 shows a modification of the radioactivity measuring section. When the radioactivity of the natural nuclide becomes low, it becomes difficult to detect the clear photopeak 26 as shown in FIG. Even if the radioactivity of the natural nuclide becomes low, the photoelectric peak of ⁶⁰ Co of the quantitative nuclide is affected by Compton scattering of the natural nuclide. A third radiation detector 28 is arranged in order to accurately correct this Compton scattered ray. Radiation detector 28
Uses a Ge semiconductor detector with high γ-ray energy resolution. Since the Ge semiconductor detector 28 has high γ-ray energy resolution, it can always detect the photopeak of a natural nuclide. The response function 6 shown in FIG. 5 is obtained based on the photopeak of this natural nuclide, and the radioactivity intensity of the crushed body 9 can be measured by the method described above.

[0017]

INDUSTRIAL APPLICABILITY According to the present invention, since the measured radiation spectrum of the crushed body is subjected to spectrum stripping correction using the response function (radiation spectrum) of the natural nuclide, the crushed body capable of accurately correcting the radioactivity measurement value of the quantified nuclide. A device for measuring radioactivity of the body can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram showing a method for quantifying radioactivity according to the present invention.

FIG. 2 is a system diagram of a radioactivity measuring apparatus of the present invention.

FIG. 3 is an explanatory diagram of a radiation measuring unit of the present invention.

FIG. 4 is a characteristic diagram showing an example of a γ-ray spectrum of a crushed body.

FIG. 5 is a characteristic diagram showing an example of a response function.

FIG. 6 is an explanatory diagram showing a modified example of the radioactivity measuring unit of the present invention.

[Explanation of symbols]

9 ... Crushed body, 10 ... Radiation detector, 11 ... Radiation detector, 12 ... Irradiation source.

Claims (2)

[Claims] 1. A crushed body radioactivity measuring apparatus provided with a radioactivity measuring unit for a crushed body obtained by crushing a solid, wherein a spectrum of radiation emitted by a nuclide existing in the crushed body is measured by a radiation detector, A method for measuring the radioactivity of a fragmented body, which comprises quantifying the radioactivity intensity of the quantitative nuclide by correcting the quantitative photopeak area in the spectrum with a photopeak area having energy higher than that of the quantitative photopeak. 2. The method according to claim 1, wherein the radiation attenuation amount in the crushed body corresponding to the radiation energy of the photoelectric peak area is obtained, and the photoelectric peak area is divided by the radiation attenuation amount to obtain the photoelectric A method for measuring the radioactivity of crushed bodies that accurately determines the peak area.