Radioactive strontium measuring device, radioactive strontium measuring method, and program

Abstract

To provide a radiostrontium measurement device that measures an amount of radioactivity of radioactive Sr within two to three days, and can reduce an environment load when throwing away an analyzed sample.SOLUTION: A radiostrontium measurement device comprises: a measurement unit that has a photomultiplier performing a first measurement of light emission of a plastic scintillation based on radiation to be radiated from a first sample, and performing a second measurement of Cherenkov light based on radiation to be radiated from a second sample; and a computation unit that calculates a light emission spectrum of the plastic scintillation on the basis of a distribution of intensity of the light emission of the plastic scintillation subjected to the first measurement, performs a first calculation of an amount of Sr-90 radioactivity on the basis of the light emission spectrum of the plastic scintillation, performs a second calculation of an amount of Sr-89 radioactivity on the basis of a distribution of intensity of the Cherenkov light measured by the measurement unit, and calculates an amount of Sr-90 radioactivity of a strontium sample and an amount of Sr-89 radioactivity thereof on the basis of a first calculation result and second calculation result.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a radioactive strontium measuring device, a radioactive strontium measuring method, and a program that can accurately measure the radioactivity of strontium-89, which is a type of radioactive strontium. [Background technology]

[0002] At nuclear power plants and other nuclear facilities, various radioactive materials (waste, treated water, etc.) are discharged. These radioactive materials are managed according to the prescribed standards according to the radioactivity of the nuclide. Radioactive materials include, for example, radioactive strontium (Sr) such as strontium-90 (hereinafter referred to as Sr-90) derived from spent fuel, its radioisotope strontium-89 (hereinafter referred to as Sr-89), and yttrium-90 (hereinafter referred to as Y-90) generated by the decay of Sr-90, and it may be necessary to measure the radioactivity of each radioactive nuclide individually. Sr-90 has a half-life of about 29 years, while Sr-89 has a half-life of about 51 days.

[0003] The radioactivity of radioactive strontium is obtained by measuring the beta rays emitted from the sample. However, the measured value of this beta ray includes the radioactivity of Sr-89, Sr-90, and Y-90. Conventionally, the following methods have been proposed to individually measure the radioactivity of radioactive Sr, including Sr-90 and Sr-89.

[0004] For example, Non-Patent Document 1 proposes a method of measuring the radioactivity of the strontium fraction and the radioactivity of Y-90 separated from the fraction by milking using a GM counter or the like, determining the radioactivity of Sr-90 from the measured amount, and determining the radioactivity of Sr-89 by subtracting it from the radioactivity of the strontium fraction. According to this method, it is necessary to wait for Y-90 to be produced from Sr-90 in the strontium fraction and for it to reach a state of radioactive equilibrium, and it takes about three weeks from the occurrence of nuclear fission to the end of measurement in order to obtain a measured value.

[0005] Also, Non-Patent Document 2 proposes a method of measuring the radioactivity of a separated strontium fraction at two different times using a GM counter or the like, and calculating the radioactivity amounts of Sr-89 and Sr-90 from the decay or increase. According to this method, it is necessary to wait for a change in the decay curve due to the production of Y-90 from Sr-90 in the strontium fraction, and it takes about one week from the occurrence of nuclear fission to obtain a measured value.

[0006] In addition, Non-Patent Document 3 proposes a method of measuring the radioactivity of Sr-89 and Sr-90 directly by beta-ray spectrometry, a beta-ray spectrum analysis method, using a device capable of measuring the spectrum of beta rays, and then calculating the radioactivity of each by using the least squares method based on the measured values. With this method, it is possible to obtain the measured values ​​in about two days after the occurrence of nuclear fission.

[0007] Furthermore, Non-Patent Document 4 proposes a method of using a photomultiplier tube (abbreviated as PMT) in a liquid scintillation counter (LSC) to measure the spectrum of Cherenkov light emitted by beta rays, mainly Sr-89 and Y-90, emitted from a strontium fraction, and directly separating and measuring Sr-89 and Y-90 based on the measured value. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2016-024133 A [Non-patent literature]

[0009] [Non-Patent Document 1] Radioactive Strontium Analysis Method (Ministry of Education, Culture, Sports, Science and Technology, revised in 2003) [Non-Patent Document 2] Guidelines for the measurement of radioactive materials discharged from light water nuclear power reactor facilities (Nuclear Safety Commission, partially revised on March 29, 2001) [Non-Patent Document 3] Rapid determination of 89Sr and 90Sr in radioactive waste using Sr extraction disk and beta-ray spectrometer, JAEA Y. Kameo et al. [Non-Patent Document 4] Radionuclide Analysis Method Using Liquid Scintillation Counter (Ministry of Education, Culture, Sports, Science and Technology, revised in 1996) Summary of the Invention [Problem to be solved by the invention]

[0010] When it is necessary to obtain the Sr radioactivity measurement results within a few days, the most effective method among the above methods is the measurement using β-ray spectrometry described in Non-Patent Document 3. However, with this method, when the amount of Sr-89 is, for example, about 1 / 5 (Sr-89 / Sr-90≦0.2) or less compared to the amount of Sr-90, the concentration error (2σ) of Sr-89 may become almost as large as the concentration of Sr-89, and the lower detection limit may become larger than that of Sr-90.

[0011] On the other hand, when it is necessary to obtain the Sr radioactivity measurement result within about one week, a measurement using Cherenkov light described in Non-Patent Document 4 has been proposed. According to this measurement method, the beta ray energy of Sr-90 is smaller than the beta ray energy of Sr-89 and Y-90, and the Cherenkov light is weak. Therefore, the beta ray energy of Sr-90 needs to be calculated from the measured value of the beta ray of Y-90. Then, according to this method, after Y-90 is generated from Sr-90, the radioactivity of Sr-90 is calculated based on the measured value of the Cherenkov light of Y-90. According to this method, it takes about one week to obtain the calculation result of Sr-90.

[0012] Therefore, although the radioactivity of Sr-90 can be determined in a short period of time using the beta-ray spectrometry method described in Non-Patent Document 3, there is a problem that the measurement of the radioactivity of Sr-89 has a large error. Therefore, there is a need for a radioactive strontium measurement method that is less susceptible to the influence of the amount of coexisting Sr-90 on the radioactivity of Sr-89, and that does not require the use of a liquid scintillator containing an organic solvent when disposing of analyzed samples as in the method of Non-Patent Document 4, thereby reducing the burden on the environment.

[0013] The present invention aims to provide a radioactive strontium measuring device, a radioactive strontium measuring method, and a program that can measure the radioactivity of radioactive Sr by nuclide within 2 to 3 days and reduce the environmental burden when disposing of analyzed samples. [Means for solving the problem]

[0014] One aspect of the present invention is a measurement unit having a photomultiplier tube that performs a first measurement of plastic scintillation luminescence based on radiation emitted from a first sample having a plastic scintillator pellet with particles of an isolated strontium sample attached to the surface, and a second measurement of Cherenkov light based on radiation emitted from a second sample, which is an aqueous solution containing the same amount of the strontium sample as the first sample, and calculates an emission spectrum of the plastic scintillation based on the distribution of the intensity of the luminescence of the plastic scintillation measured in the first measurement, and and a calculation unit that calculates an Sr-90 count based on the emission spectrum, performs a first calculation of an amount of Sr-90 radioactivity contained in the first sample based on a calculation result, calculates a Cherenkov light spectrum based on a distribution of intensity of the Cherenkov light measured by the measurement unit, calculates an Sr-89 count based on the Cherenkov light spectrum, performs a second calculation of an amount of Sr-89 radioactivity contained in the second sample based on the calculation result, and calculates the amounts of Sr-90 radioactivity and Sr-89 radioactivity of the strontium sample based on the first and second calculation results.

[0015] According to the present invention, the amount of Sr-90 radioactivity can be calculated by measuring the plastic scintillation luminescence of the first sample S1, and the amount of Sr-89 radioactivity can be calculated by measuring the luminescence of Cherenkov light of the second sample S2. According to the present invention, the amount of radioactivity of a sample can be measured in about 2 to 3 days without waiting for the generation of Y-90. According to the present invention, even when Sr-89 is about 1 / 5 or less smaller than Sr-90, the detection accuracy of the amount of Sr-89 radioactivity can be improved. Furthermore, according to the present invention, by using plastic scintillator pellets in the measurement, the environmental load can be reduced without waste liquid treatment of the liquid scintillator.

[0016] The present invention may also include a freeze-dryer that freeze-dries an aqueous solution containing the strontium sample to adsorb particles of the strontium sample onto the surface of the plastic scintillator pellet to produce the first sample.

[0017] According to the present invention, a first sample having plastic scintillator pellets can be formed, and waste liquid treatment of the liquid scintillator during measurement is not required, thereby reducing the environmental load.

[0018] The present invention may also include a washing device for washing the plastic scintillator pellets to remove particles of the strontium sample adsorbed on the surface.

[0019] According to the present invention, by cleaning the plastic scintillator pellet of the first sample using a cleaning device, the plastic scintillator pellet can be reused repeatedly, thereby reducing the cost of measurement and the environmental load.

[0020] One aspect of the present invention includes a step of isolating a strontium sample, a step of producing an aqueous solution containing the strontium sample, a step of injecting a predetermined amount of the aqueous solution into a first container having a plastic scintillator pellet, a step of freeze-drying the aqueous solution contained in the first container to form a first sample having the plastic scintillator pellet with particles of the strontium sample attached to a surface thereof, a step of injecting an amount of the aqueous solution equal to the predetermined amount into a second container to produce a second sample, a step of performing a first measurement of plastic scintillation luminescence based on radiation emitted from the first sample by a measuring unit having a photomultiplier tube, a step of calculating an emission spectrum of the plastic scintillation based on a distribution of intensities of the luminescence of the plastic scintillation measured first, and a step of measuring the intensity of the first measured intensity of the first measured intensity of the first measured intensity of the first measured intensity of the first measured intensity of the second ... a step of calculating an Sr-90 count based on an emission spectrum of a radioactive scintillation of the sample, a first step of calculating an amount of Sr-90 radioactivity contained in the first sample based on the calculation result, a step of secondly measuring Cherenkov light based on radiation emitted from the second sample by the measuring unit having the photomultiplier tube, a step of calculating a Cherenkov light spectrum based on a distribution of intensities of the second measured Cherenkov light, a step of calculating an Sr-89 count based on the Cherenkov light spectrum, a second step of calculating an amount of Sr-89 radioactivity contained in the second sample based on the calculation result, and a step of calculating an amount of Sr-90 radioactivity and an amount of Sr-89 radioactivity of the strontium sample by a calculation unit based on the first and second calculation results.

[0021] According to the present invention, this can be realized by changing the calculation method without changing the configuration of the conventional radioactive strontium measuring device.

[0022] One aspect of the present invention is a program executed in a radioactive strontium measuring device for calculating the amount of radioactivity of a strontium sample, the program instructing a measuring unit having a photomultiplier tube to perform a first measurement of plastic scintillation luminescence based on radiation emitted from a first sample having a plastic scintillator pellet with particles of the isolated strontium sample attached to its surface, and to perform a second measurement of Cherenkov light based on radiation emitted from a second sample, which is an aqueous solution containing the strontium sample in the same amount as the first sample, and to cause a calculation unit to perform a first measurement of the intensity distribution of the plastic scintillation luminescence measured in the first measurement. an emission spectrum of stick scintillation is calculated, an Sr-90 count is calculated based on the emission spectrum of the plastic scintillation, a first calculation is made of the amount of Sr-90 radioactivity contained in the first sample based on the calculation results, a Cherenkov light spectrum is calculated based on the distribution of the intensity of the second measured Cherenkov light, an Sr-89 count is calculated based on the Cherenkov light spectrum, a second calculation is made of the amount of Sr-89 radioactivity contained in the second sample based on the calculation results, and the amounts of Sr-90 radioactivity and Sr-89 radioactivity of the strontium sample are calculated based on the first and second calculation results.

[0023] According to the present invention, it is possible to realize the above by changing the program without changing the configuration of a conventional liquid scintillation counter. Effect of the Invention

[0024] According to the present invention, in measuring the amount of radioactive Sr, the amount of Sr-90 radioactivity can be measured in a short period of time of within 2 to 3 days by measuring plastic scintillation luminescence, and the amount of Sr-89 radioactivity can be measured without being affected by the amount of Sr-90 radioactivity by measuring the luminescence of Cherenkov light from the measurement sample. Furthermore, according to the present invention, the environmental load can be reduced when disposing of analyzed samples. [Brief description of the drawings]

[0025] [Figure 1]FIG. 2 is a diagram showing a first sample to be measured by the radioactive strontium measuring device according to the embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing the configuration of a freeze-dryer for removing water to produce a first sample. [Diagram 3] 1 is a schematic diagram showing a configuration of a radioactive strontium measuring device according to an embodiment of the present invention. [Figure 4] FIG. 2 is a diagram showing the energy spectrum of β rays emitted from a sample. [Diagram 5] FIG. 1 is a diagram showing a conventional spectrum analysis process. [Figure 6] 1 is a flowchart showing the process of a conventional method for measuring radioactive strontium. [Figure 7] 1 is a flowchart showing the process of a radioactive strontium measuring method. [Figure 8] FIG. 2 is a block diagram showing a configuration of a cleaning device that cleans a first sample. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Hereinafter, a radioactive strontium measuring device, a radioactive strontium measuring method, and a program according to an embodiment of the present invention will be described with reference to the drawings.

[0027] The radioactive strontium measuring device according to the embodiment of the present invention is a device for measuring radiation emitted from a sample of strontium (hereinafter, appropriately abbreviated as Sr) isolated from spent nuclear fuel, etc. The radioactive strontium measuring device calculates the amount of radioactivity for each nuclide contained in the sample based on the measurement results. The schematic configuration of the radioactive strontium measuring device will be described below.

[0028] In nuclear power facilities, after spent nuclear fuel is collected, strontium samples are isolated using methods such as wet and dry methods, and an aqueous solution containing the strontium sample is produced. Sr contains a radioactive nuclide that undergoes beta decay. The sample contains the radioactive nuclide Sr-90 and the radioisotope Sr-89. Of the isolated Sr, yttrium-90 (Y-90) is produced from Sr-90. In order to ensure accuracy in radioactivity measurements, a scavenging operation is performed to completely remove the Y-90 contained in the sample. The time is recorded when the scavenging operation is completed.

[0029] In the aqueous solution containing the sample, after the scavenging operation, Sr-90 undergoes beta decay over time to produce Y-90. Thus, the sample contains radioactive Sr-90 and radioisotopes Sr-89 and Y-90. Sr-90, Y-90, and Sr-89 in the sample each emit beta rays. The aqueous solution containing the sample is divided into two vials B1 and B2 in equal amounts. A predetermined amount of the aqueous solution is injected into the first container B1, and a first sample S1 is formed as described below. The aqueous solution is injected into the second container B2 in the same amount as that injected into the first container B1, and a second sample S2 is formed.

[0030] As shown in Fig. 1, the first container B1 is filled with plastic scintillator pellets P up to the bottle mouth. The plastic scintillator pellets P emit plastic scintillation light when radiation penetrates them. The plastic scintillator pellets P are formed into spherical or cylindrical pellets made of resin. An aqueous solution containing a strontium sample after a scavenging operation is poured into the first container B1.

[0031] 2, the first container B1 into which the aqueous solution has been poured is placed in a freeze-dryer 100, and the aqueous solution contained in the first container B1 is freeze-dried. The freeze-dryer 100 includes, for example, a vacuum dryer 101 and a freezing device 110 provided inside the vacuum dryer 101. The vacuum dryer 101 includes, for example, a vacuum container body 102 having an internal space S formed therein, a lid 103 that hermetically closes an opening at the top of the vacuum container body 102 to seal the internal space S, and a vacuum pump 105 that sucks in the air in the internal space S to create a substantially vacuum state in the internal space S.

[0032] The freezing device 110 accommodates an object to be frozen, such as a first container B1, and freezes the contents by cooling. The freezing device 110 has a storage section T that accommodates the first container B1, and cools the inside of the storage section T using a cooling device such as a Peltier element.

[0033] The first sample S1 is placed in the storage section T of the freezing device 110. Then, the lid section 103 is attached to the vacuum vessel body 102, and the internal space S is sealed. Then, the freezing device 110 is operated to cool the first sample S1 and freeze the aqueous solution containing the sample. After the first sample S1 is frozen, the vacuum pump 105 is operated, whereby the water in the frozen aqueous solution sublimes and the strontium in the aqueous solution precipitates and adheres to the surface of the plastic scintillator pellet P.

[0034] This produces a first sample S1 having plastic scintillator pellets P with particles of the strontium sample adhering to the surface. The freeze-dryer 100 freeze-dries the aqueous solution containing the strontium sample, and produces the first sample S1 by adsorbing the particles of the strontium sample onto the surfaces of the plastic scintillator pellets P. In the first sample S1, plastic scintillation light emission is observed every time the particles of the sample emit radiation.

[0035] In the second sample S2, the aqueous solution emits light due to Cherenkov light every time the sample emits radiation. In the second sample S2, Sr-90 has lower beta ray energy than Sr-89 and Y-90, so the Cherenkov light emission is weak. In the second sample S2, Cherenkov light is observed mainly due to radiation emitted from Sr-89 and Y-90.

[0036] 3, the radioactivity of a first sample S1 and a second sample S2 is measured by a radioactive strontium measuring device 1. The radioactive strontium measuring device 1 includes, for example, a measuring unit 2 that measures β rays from a sample S containing a radioactive nuclide, and a calculation device 10 that calculates the amount of radioactivity based on the measurement results.

[0037] The radioactive strontium measuring device 1 is basically configured in the same manner as the general liquid scintillation counter described in, for example, Patent Document 1. The radioactive strontium measuring device 1 is different from the device described in Patent Document 1 in the calculation processing in the calculation device 10, as described later.

[0038] The measurement unit 2 measures the amount of radioactivity of radionuclides contained in measurement targets such as the first sample S1 and the second sample S2. The measurement unit 2 is provided with a measurement chamber 3 that is shielded against radiation with lead or the like. A sample is placed inside the measurement chamber 3. When the measurement target is the first sample S1, the measurement unit 2 performs a first measurement of the luminescence of plastic scintillation based on the radiation emitted from the first sample S1. When the measurement target is the second sample S2, the measurement unit 2 performs a second measurement of Cherenkov light based on the radiation emitted from the second sample S2, which is an aqueous solution containing the same amount of strontium sample as the first sample S1.

[0039] A pair of photomultiplier tubes 4 (PMT) for detecting the light emission from the sample are arranged facing each other on both sides of the measurement chamber 3. The pair of photomultiplier tubes 4 outputs a pulsed electric signal obtained by electrically multiplying the light emission from the sample S to the calculation device 10 side.

[0040] The calculation device 10 includes, for example, a coincidence circuit 11 connected to the pair of photomultiplier tubes 4, a pulse summing circuit 12 connected to the pair of photomultiplier tubes 4, a gate circuit 13 connected to the coincidence circuit 11 and the pulse summing circuit 12, an MCA 14 connected to the gate circuit 13, a calculation unit 15 connected to the MCA 14, a display unit 16 that outputs the calculation results of the calculation unit 15, and an input unit 17 that inputs information to the calculation unit 15.

[0041] The pair of photomultiplier tubes 4 simultaneously output electrical signals to a coincidence circuit 11 and a pulse adding circuit 12. When the electrical signals output from the pair of photomultiplier tubes 4 are input simultaneously, the coincidence circuit 11 outputs a gate signal to a gate circuit 13 described below. The pulse adding circuit 12 outputs a single pulse signal obtained by adding the pair of electrical signals output from the pair of photomultiplier tubes 4 to the gate circuit 13 described below.

[0042] When the gate circuit 13 receives a gate signal output from the coincidence circuit 11, it passes an addition pulse signal and outputs it to the MCA 14 described below. This makes it possible to eliminate counts due to cosmic radiation and natural radiation. The MCA 14 (multi-channel analyzer) adds up the input addition pulse signals for each magnitude (energy value), records the distribution of the energy spectrum (see FIG. 4), and outputs it to the calculation unit 15. The calculation unit 15, input unit 16, and display unit 17 are composed of a device capable of calculation, such as a personal computer.

[0043] The calculation unit 15 calculates the energy spectrum of β rays emitted from the sample by β-ray spectrometry based on the distribution of the energy spectrum recorded by the MCA 14. Based on the distribution of the recorded energy spectrum, the calculation unit 15 decomposes it into individual spectra of Sr-90, Sr-89, and Y-90 by least squares approximation as described below, and calculates the amount of Sr-90 radioactivity based on the measurement result of the plastic scintillation light emission of the first sample S1, and calculates the amount of Sr-89 radioactivity based on the measurement result of the Cherenkov light of the second sample S2. The details of the calculation process of the β-ray spectrometry by the calculation unit 15 will be described later. The calculation unit 15 displays the calculation results on the display unit 14.

[0044] The display unit 16 is a display device to which the calculation results of the calculation unit 15 are output. The display unit 16 is a display device such as a liquid crystal display, an organic EL display, etc. The display unit 16 may be configured separately from a personal computer, a tablet terminal, a smartphone, etc.

[0045] The arithmetic device 10 also includes a storage unit (not shown) that stores data necessary for the calculations. The storage unit stores programs and various data necessary for the calculations of the arithmetic unit 15. The storage unit 14 is realized by a storage medium such as a RAM, a ROM, a HDD, or a flash memory.

[0046] Next, a method for calculating the amount of radioactivity of a nuclide based on β-ray spectrometry, which is executed in the calculation unit 15, will be described.

[0047] Figure 4 shows the spectrum of beta ray energy recorded by MCA14 based on the measured beta ray count. The spectrum of beta ray energy emitted from sample S is the sum of beta rays emitted from multiple nuclides Sr-90, Sr-89, and Y-90. Beta ray spectrometry is used to calculate the radioactivity of multiple nuclides Sr-90, Sr-89, and Y-90 individually.

[0048] Figure 5 shows the spectrum of beta-ray energy divided into five fractions. Each fraction shows the ratio of counts of three nuclides, Sr-90, Sr-89, and Y-90. In the conventional method of measuring radioactive strontium, the ratio of the three nuclides in each of fractions 1 to 5 is calculated by the least squares method based on the following formula (1). In formula (1), Xi that minimizes S is found.

number

number

[0049] In the plastic scintillation emission in the first sample S1, radiation emitted from Sr-90, Sr-89, and Y-90 is measured. The calculation unit 15 calculates the count of the first sample S1 based on the output value of the measurement unit 2. The calculation unit 15 calculates the energy spectrum of β rays. The calculation unit 15 individually calculates the radioactivity (concentration) of Sr-90 in the first sample S1 based on the analysis results. At this time, the calculation unit 15 analyzes the energy spectrum of β rays by least squares approximation based on the measurement value, breaks it down into a single spectrum of Sr-90, and then calculates the single radioactivity of Sr-90.

[0050] The calculation unit 15 calculates the radioactivity of Y-90 as a function of Sr-90 based on the calculation result and the time elapsed after purification of the strontium sample. The calculation unit 15 converts the total value of the calculated radioactivity of Sr-90 alone and the radioactivity of Y-90 alone calculated as a function of Sr-90 into a first count, which is the total value of the counts of β rays emitted from Sr-90 and Y-90 in the sample S. The calculation unit 15 subtracts the Y-90 count from the first count to calculate the Sr-90 count. The calculation unit 15 calculates the radioactivity of Sr-90 alone based on the Sr-90 count.

[0051] In the measurement of Cherenkov light in the second sample S2, since the beta rays of Sr-90 are weak, the radiation emitted from Sr-89 and Y-90 is measured. The calculation unit 15 calculates the second count of the second sample S2 based on the output value of the measurement unit 2. The calculation unit 15 calculates the energy spectrum of the beta rays. The calculation unit 15 individually calculates the radioactivity (concentration) of Sr-89 in the second sample S2 based on the analysis result. At this time, the calculation unit 15 analyzes the energy spectrum of the beta rays by least squares approximation based on the measurement value, and resolves it into a single spectrum of Sr-89. The calculation unit 15 subtracts the Sr-90 count calculated based on the elapsed time after purification of the strontium sample from the second count to calculate the Sr-89 count. The calculation unit 15 calculates the single radioactivity of Sr-89 based on the Sr-89 count.

[0052] 6 shows each step of a radioactive strontium measurement method using a conventional radioactive strontium measurement device. Using the radioactive strontium measurement device, a first measurement is performed based on scintillation emission of Cherenkov light based on β-rays emitted from the aqueous solution sample after the scavenging operation (step S100). Based on the first measurement result of the Cherenkov light of the aqueous solution sample, the energy spectrum of β-rays of the aqueous solution sample is analyzed (step S102). Based on the energy spectrum of β-rays, the amount of Sr-89 and the amount of Y-90 are calculated using β-ray spectrometry (step S104).

[0053] The aqueous solution sample is left for about one week (step S106). A radioactive strontium measuring device is used to perform a second measurement based on scintillation emission of Cherenkov light based on beta rays emitted from the aqueous solution sample after about one week has passed (step S108). Based on the second measurement result of the Cherenkov light of the aqueous solution sample, the beta ray energy spectrum of the aqueous solution sample is analyzed (step S110). Based on the beta ray energy spectrum of Sr-89 and Y-90, the amount of Y-90 is calculated using beta ray spectrometry (step S112).

[0054] The amount of Sr-90 is calculated based on the increase in the amount of Y-90 in the second measurement relative to the amount of Y-90 in the first measurement (step S114). Based on the calculation results, the radioactivity amounts of Sr-89, Sr-90, and Y-90 contained in the sample are calculated individually (step S116). According to this method, when the amount of Sr-89 is, for example, about 1 / 5 or less compared to the amount of Sr-90 as described above, the detection limit may be larger than that of Sr-90. Also, according to this method, it takes several days for Y-90 to be produced.

[0055] 7 shows each step of the radioactive strontium measurement method using the radioactive strontium measurement device 1 according to the embodiment. First, as a preparation step before measurement, a strontium sample is extracted from spent nuclear fuel. Then, an aqueous solution containing the strontium sample is generated. A scavenging operation is performed on the strontium sample in the aqueous solution to isolate the strontium sample (step S200). An aqueous solution containing the isolated strontium sample is generated (step S201). A predetermined amount of the aqueous solution is injected into a vial bottle B1 (first container) filled with plastic scintillator pellets (step S202).

[0056] The aqueous solution contained in the first container is freeze-dried using the freeze-dryer 100 to form a first sample S1 having plastic scintillator pellets with strontium sample particles attached to the surface (step S203). The measurement unit 2 performs a first measurement of the plastic scintillation luminescence of the first sample S1 (step S204). The calculation device 10 calculates the emission spectrum of the plastic scintillation based on the distribution of the intensity of the luminescence of the plastic scintillation that has been first measured (step S205). The calculation unit 15 performs beta ray spectrometry based on the emission spectrum of the plastic scintillation to calculate the Sr-90 count (amount) (step S206). The calculation unit 15 performs a first calculation of the amount of Sr-90 radioactivity contained in the first sample S1 based on the calculation result (step S207).

[0057] The same amount of aqueous solution as that injected into vial B1 is injected into vial B2 (second container) to generate second sample S2 (step S208). Measurement unit 2 performs a second measurement of the Cherenkov light emission of second sample S2 (step S209). Calculation unit 15 calculates the Cherenkov light emission spectrum based on the distribution of the second measured Cherenkov light emission intensity (step S210). Calculation unit 15 performs beta ray spectrometry based on the Cherenkov light emission spectrum and calculates the Sr-89 count (amount) (step S211).

[0058] The calculation unit 15 performs a second calculation of the amount of Sr-89 radioactivity contained in the second sample S2 based on the calculation result (step S212). The calculation unit 15 calculates the amount of Sr-90 radioactivity and the amount of Sr-89 radioactivity of the strontium sample based on the first calculation result and the second calculation result (step S213).

[0059] After the measurement is completed, the vial B1 and the plastic scintillator pellets P of the first sample S1 are cleaned using the cleaning device 200. After cleaning, the vial B1 and the plastic scintillator pellets P are reused.

[0060] 8, plastic scintillator pellets P of the first sample S are collected in a collection container 201 provided in a cleaning device 200. A cleaning section 203 is provided in the collection container 201. The cleaning section 203 supplies cleaning water to clean the plastic scintillator pellets P in the collection container 201. The cleaning water passes through a drain pipe (not shown) of the collection container 201 and is collected in a waste liquid collection section 202 provided below. During the cleaning process, particles of the strontium sample adsorbed on the surfaces of the plastic scintillator pellets P are removed.

[0061] The washed plastic scintillator pellets P are dried by a drying section 204 provided integrally with or separately from the recovery container 201. Using the same process as above, the vial B1 of the first sample S is also washed and dried using the cleaning device 200. The dried plastic scintillator pellets P and vial B1 are reused to produce the first sample S1.

[0062] The reused plastic scintillator pellets P are discarded after a predetermined number of uses. As described above, the measurement of radioactivity using the plastic scintillator pellets P does not use a liquid scintillator containing an organic solvent, so that the burden on the environment can be reduced when disposing of analyzed samples. In addition, the plastic scintillator pellets P can be reused using the cleaning device 200, so that the cost related to the measurement can be reduced.

[0063] As described above, the radioactive strontium measuring device 1 can be realized by changing the calculation method without changing the configuration of the conventional radioactive strontium measuring device. Compared with the conventional measurement using a liquid scintillator, the radioactive strontium measuring device 1 can measure the amount of Sr-90 radioactivity and the amount of Sr-89 radioactivity in a short period of about 2 to 3 days without waiting for the generation of Y-90, a daughter nuclide of Sr-90. When measuring the amount of radioactive Sr contained in the first sample S1, the radioactive strontium measuring device 1 can measure the amount of Sr-90 radioactivity within 2 to 3 days by utilizing the emission spectrum from the plastic scintillator pellet P.

[0064] Moreover, the radioactive strontium measuring device 1 can measure the amount of Sr-89 radioactivity independently without being affected by the amount of Sr-90 radioactivity by utilizing the Cherenkov light spectrum of the second sample S2. In measuring the spectrum of the Cherenkov light of the second sample S2, the radioactive strontium measuring device 1 can reduce the influence of Sr-90 radioactivity by avoiding the Cherenkov emission region due to the maximum β-ray energy of Sr-90 radioactivity, and can measure the amount of Sr-89 radioactivity by measuring the Cherenkov light due to Sr-89. The radioactive strontium measuring device 1 can improve the detection accuracy of the amount of Sr-89 radioactivity even when Sr-89 is smaller than Sr-90.

[0065] According to the radioactive strontium measuring device 1, in measuring the first sample S1, the plastic scintillator pellets P are washed and reused, so the environmental burden when analyzing samples is reduced. According to the radioactive strontium measuring device 1, in measuring the second sample S2, a liquid scintillator containing an organic solvent used in a normal liquid scintillation counter is not used, so the environmental burden when analyzing samples is disposed of is reduced.

[0066] The above-mentioned calculation unit 15 is realized, for example, by a processor such as a CPU executing a program (software) stored in a storage unit. Each step of the calculation process in the calculation unit 15 is processed by executing a program installed in the calculation device 10. In addition, some or all of the functions of these components may be realized by hardware (including circuitry) such as an LSI, ASIC, FPGA, or GPU, or may be realized by a combination of software and hardware. The program may be stored in a storage device such as an HDD or flash memory in advance, or may be stored in a removable storage medium such as a DVD or CD-ROM, and installed by mounting the storage medium in a drive device.

[0067] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and spirit of the invention. [Explanation of symbols]

[0068] 1. Radioactive strontium measuring device 2. Measurement section 4 Photomultiplier tube 15 Arithmetic section 100 freeze dryer 200 Cleaning Equipment B1 1st container B2 2nd container P Plastic scintillator pellets S First sample S1 First sample S2 Second sample

Claims

1. a measurement unit having a photomultiplier tube for performing a first measurement of plastic scintillation luminescence based on radiation emitted from a first sample having a plastic scintillator pellet with particles of the isolated strontium sample attached to its surface, and for performing a second measurement of Cherenkov light based on radiation emitted from a second sample which is an aqueous solution containing the same amount of the strontium sample as the first sample; Calculating an emission spectrum of the plastic scintillation based on the distribution of the intensity of the light emitted by the first measured plastic scintillation, calculating an Sr-90 count based on the emission spectrum of the plastic scintillation, and first calculating an amount of Sr-90 radioactivity contained in the first sample based on the calculation result; calculating a Cherenkov light spectrum based on the distribution of the Cherenkov light intensity measured by the measurement unit, calculating an Sr-89 count based on the Cherenkov light spectrum, and performing a second calculation of the amount of Sr-89 radioactivity contained in the second sample based on the calculation result; A calculation unit that calculates the amount of Sr-90 radioactivity and the amount of Sr-89 radioactivity of the strontium sample based on the first calculation result and the second calculation result. Radioactive strontium measuring device.

2. a freeze-drying machine for freeze-drying an aqueous solution containing the strontium sample and adsorbing particles of the strontium sample onto the surface of the plastic scintillator pellet to produce the first sample; 2. The radioactive strontium measuring device according to claim 1.

3. a cleaning device for cleaning the plastic scintillator pellets to remove particles of the strontium sample adsorbed on the surface; 3. The radioactive strontium measuring device according to claim 1 or 2.

4. isolating the strontium sample; forming an aqueous solution containing the strontium sample; pouring a volume of the aqueous solution into a first container having a plastic scintillator pellet; freeze-drying the aqueous solution contained in the first container to form a first sample having the plastic scintillator pellet with particles of the strontium sample attached thereto; pouring the aqueous solution into a second container in an amount equal to the predetermined amount to form a second sample; a step of performing a first measurement of luminescence of plastic scintillation based on radiation emitted from the first sample by a measurement unit having a photomultiplier tube; A step of calculating an emission spectrum of the plastic scintillation based on a distribution of the intensity of the light emitted by the plastic scintillation measured first; calculating the Sr-90 count based on the emission spectrum of the plastic scintillation; A step of performing a first calculation of the amount of Sr-90 radioactivity contained in the first sample based on the calculation result; a step of performing a second measurement of Cherenkov light based on radiation emitted from the second sample by the measurement unit having the photomultiplier tube; calculating a Cherenkov light spectrum based on the second measured distribution of the Cherenkov light intensity; calculating the Sr-89 count based on the Cerenkov light spectrum; A step of performing a second calculation of the amount of Sr-89 radioactivity contained in the second sample based on the calculation result; and calculating, by a calculation unit, the amount of Sr-90 radioactivity and the amount of Sr-89 radioactivity of the strontium sample based on the first calculation result and the second calculation result. Method for measuring radioactive strontium.

5. A program executed in a radioactive strontium measuring device for calculating the amount of radioactivity of a strontium sample, The measurement section has a photomultiplier tube. a first measurement of plastic scintillation luminescence based on radiation emitted from a first sample having a plastic scintillator pellet with particles of the isolated strontium sample attached to its surface; performing a second measurement of Cherenkov light based on radiation emitted from a second sample, which is an aqueous solution containing the strontium sample in the same amount as the first sample; The calculation unit has Calculating an emission spectrum of the plastic scintillation based on the distribution of the intensity of the first measured light emission of the plastic scintillation; Calculating the Sr-90 count based on the emission spectrum of the plastic scintillation; A first calculation is made of the amount of Sr-90 radioactivity contained in the first sample based on the calculation result; calculating a Cherenkov light spectrum based on the second measured distribution of the Cherenkov light intensity; Calculating the Sr-89 count based on the Cherenkov light spectrum; A second calculation is made of the amount of Sr-89 radioactivity contained in the second sample based on the calculation result; A program for calculating the amount of Sr-90 radioactivity and the amount of Sr-89 radioactivity of the strontium sample based on the first calculation result and the second calculation result.

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