JP7061300B1 - Inspection method for contamination of powdery waste with radioactive substances - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for inspecting surface contamination (radiation measurement just in case) by a radioactive substance of powdery waste which is a target of NR waste generated from a nuclear facility or the like. The present invention is an inspection method for inspecting contamination of powdery waste, which is a target of non-radioactive waste, with radioactive substances, and (a) a detection surface of a radiation measuring instrument. A step of preparing a measuring tray having at least one opening having a width equal to or less than the width, and (b) a step of spreading powdery waste in the measuring tray to a substantially uniform thickness. (C) Includes a step of determining the surface radioactivity density of the powdered waste by β-ray measurement while scanning the surface of the powdered waste spread close to the detection surface of the radiation measuring instrument. The substantially uniform thickness is characterized by being less than or equal to the maximum flight distance that does not completely block β-rays emitted from powdery waste. [Selection diagram] Fig. 1

Description

The present invention generally relates to a method for inspecting powdery waste for contamination by radioactive substances, and more specifically, for powdery waste that is a target of non-radioactive waste generated from nuclear facilities and the like. Regarding the inspection method of contamination by radioactive substances in waste.

In addition to radioactive waste that requires special management from the viewpoint of radiation protection at facilities that handle nuclear fuel materials or nuclear raw materials (hereinafter, may be abbreviated as nuclear fuel materials, etc.) such as nuclear facilities, radioactive waste A large amount of non-material waste (hereinafter, also referred to as “NR waste”) is generated. Regarding this NR waste, the Ministry of Economy, Trade and Industry has issued guidelines (instructions) on its handling (Non-Patent Document 1), and it is necessary to judge it as waste and manage it according to the contents. For example, materials installed in a controlled area where there is a risk of contamination and articles used in a controlled area where there is a risk of contamination, which are judged to be NR waste, should be subjected to radiation measurement evaluation just in case. Etc. are required.

In addition, regarding articles (including waste) taken out of the controlled area, it should be said that there is no contamination in accordance with other regulations, such as the Ionizing Radiation Hazard Prevention Regulations (September 30, 1972, Ministry of Labor Ordinance No. 41). It is necessary to carry out after confirming. Therefore, when treating NR waste generated in nuclear facilities, etc., it is required to carry out treatment and management including appropriate radiation measurement in accordance with such various regulations.

In order to inspect surface radioactive contamination (hereinafter, also referred to as surface contamination density, surface radioactive density, or simply surface contamination) by radioactive substances such as nuclear fuel material of NR waste in the above-mentioned radiation measurement just in case. , The α-ray measurement by the direct survey method or the indirect survey method (hereinafter, also referred to as the Sumiya method) is often performed. In such α-ray measurement, it is necessary to apply an α-ray detector near the surface of the object to be measured, or to apply an α-ray detector to a smear filter paper from which the surface of the object to be measured has been wiped off.

Even when the NR waste is a powdery waste, α-ray measurement by a direct or indirect survey method can be basically used to inspect the surface contamination by radioactive substances. However, it is difficult to inspect contamination by radioactive substances that have entered the inside of the powder (especially deep in the lower part) by α-ray measurement by the direct or indirect survey method. It is also possible to measure the average concentration of radioactive substances inside the powder by measuring γ-rays from the outside of the container containing the powdered NR waste, but in that case, the local powdered NR waste is locally measured. It is difficult to confirm the presence or absence of residual radioactive materials.

Patent Document 1 discloses a method for separating and treating non-radioactive waste from waste containing radioactive substances and non-radioactive substances discharged from nuclear equipment. The treatment method includes a step of collecting waste and a step of measuring β-rays at the first radiation detection station (18) in order to confirm the presence or absence of surface contamination by radioactive substances in the collected waste. A step of measuring γ-rays at the second radiation detection station (22) and a step of storing the uncontaminated waste in a container (36). The third radiation detection station (40) includes a step of measuring γ-rays from the outside of the container to confirm the presence or absence of contamination.

In Patent Document 2, pre-clearance measurement (measurement of surface contamination density, γ-ray measurement and β-ray measurement) and clearance measurement (average radioactivity concentration,) of the clearance target separated from the dismantled waste associated with the dismantling and removal of the nuclear facility Disclose a separation / clearance processing system including γ-ray measurement). In the pre-clearance measurement, it is disclosed that β-ray measurement is performed from above and below the measurement tray while the measurement object is placed on the measurement tray and moved (paragraph 0046 of Patent Document 2).

However, in Patent Documents 1 and 2, neither of them is intended to measure powdery NR waste, and is a method for inspecting radioactive surface contamination (surface radioactivity density) including contamination by radioactive substances including the inside of powder. Is not disclosed.

Japanese Unexamined Patent Publication No. 61-68577 Japanese Unexamined Patent Publication No. 2007-248066

Regarding the handling of "non-radioactive waste" in nuclear facilities (instruction), NISA, NISA-111a-08-1, Ministry of Economy, Trade and Industry, May 27, 2008

The present invention has been made in view of the above-mentioned problems of the prior art, and is an inspection of surface contamination of powdery waste, which is a target of NR waste generated from nuclear facilities, etc., by radioactive substances (just in case). Radiation measurement) is intended to provide a method.

One aspect of the present invention provides an inspection method for inspecting the presence or absence of contamination of powdery waste, which is a target of non-radioactive waste, with radioactive substances. The inspection methods include (a) a step of preparing a measuring tray having at least one opening having a width equal to or less than the width of the detection surface of the radiation measuring instrument, and (b) a powdery state in the measuring tray. While scanning the process of laying the waste in a substantially uniform thickness and (c) the detection surface of the radiation measuring instrument close to the surface of the laid powdery waste, the powdery waste Including the step of determining the surface radioactivity density of the It is a feature.

In another aspect of the invention, the substantially uniform thickness is set so that the surface radioactivity density obtained by β-ray measurement is less than 0.4 Bq / cm ² . In another aspect of the invention, the substantially uniform thickness is set to a thickness of 1.5 g / cm ² or less.

It is a figure which shows the flow of the inspection method of one Embodiment of this invention. It is a figure which shows the measuring tray of one Embodiment of this invention. It is a figure which shows the β ray measuring instrument (survey meter) of one Embodiment of this invention.

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a flow of an inspection method according to an embodiment of the present invention. In step S1, a measuring tray is prepared. FIG. 2 is a diagram showing a measuring tray according to an embodiment of the present invention. FIG. 2A is a top view of the measuring tray 1, and FIG. 2B is a cross-sectional view taken at the position BB'. The measuring tray 1 is composed of a frame 2 surrounding four sides, a central partition portion 4, and a bottom portion 5. The frame 2, the partition 4, and the bottom 5 are made of a material such as wood, metal, or plastic that is relatively easy to process. The bottom 5 may be made of a wire mesh having a pitch (opening) capable of holding powdery waste, a thin polypropylene (PP) film, or the like. In that case, if necessary, additionally, the detection surface 18 of the radiation measuring instrument is applied from the lower side of the bottom portion 5, and radiation (β-ray) measurement can be performed just in case.

Between the frame 2 and the partition 4, there are openings 3A and 3B on which powdery waste can be spread. The width W of one opening has a width substantially equal to or less than the width of the detection surface 18 of the radiation measuring instrument. As a result, the detection surface 18 of the radiation measuring instrument is scanned once along the length direction (arrow A direction) of the openings 3A and 3B, so that the measurement of the entire opening can be performed without leaving an unmeasured area. It will be possible to do it reliably. The frame 2 (four edges) has a height T of about 10 to 20 mm from the surface (bottom surface) of the bottom portion 5. In FIG. 2, there is one partition 4 and two openings, but the frame 2 can be expanded and the partition can be expanded according to the amount of powdery waste to be measured, measurement work efficiency, and the like. Two or more of 4 can be provided to make three or more openings.

In step S2 of FIG. 1, the powdery waste is spread on the measuring tray. The powdery waste referred to here refers to powdery waste (hereinafter, also referred to as “powdered NR waste”) that is a target of NR waste generated in facilities that handle nuclear fuel materials and the like. means. Nuclear fuel materials and the like include uranium and thorium, which are α-ray sources and β-ray sources, and their compounds. Facilities that handle nuclear fuel materials include, in addition to nuclear facilities, facilities that handle refined uranium, manufacturing facilities such as chemical factories that use uranium as a catalyst, and processing facilities that are not directly related to nuclear power. Is done. Nuclear facilities include, for example, refining facilities, nuclear reactor facilities, reprocessing facilities and the like described in Non-Patent Document 1. Further, NR waste is basically determined in accordance with the guidelines of Non-Patent Document 1, but in the facility where the target waste is generated, in addition to the nuclear facility, the above-mentioned nuclear power is directly referred to. It shall include facilities that are not related to the subject.

In step S2, for example, when the measurement tray 1 of FIG. 2 is used, powdery NR waste is spread over the openings 3A and 3B so as to have a substantially uniform thickness. Here, the powdery NR waste is not only fine powder such as sand or soil, but also fine granules having a size of about 1 mm to several mm (<about 10 mm). And may include cuts. The waste can include any material such as paint or rusted iron strips, metal, pebbles, wood, paper, plastic, vinyl and the like.

The substantially uniform thickness of the powdery NR waste is such that β-rays emitted from the entire powdery NR waste including at least the vicinity of the surface (bottom surface) of the bottom 5 of the measuring tray 1 are the waste. Thickness that can be measured from the top surface, in other words, β released from powdered NR waste, assuming that the entire powdered NR waste is almost uniformly contaminated with radioactive material. The thickness must be less than or equal to the maximum range that does not completely shield the wire. As a result, even if a hot spot of radioactive material exists on the bottom surface of the measuring tray and is covered with powdery NR waste, it is possible to prevent the hot spot from being overlooked.

In addition, the detection limit of the surface contamination density (surface radioactivity density) when the thickness is set to the maximum range or less is the contamination density 0.4Bq by α-radionuclide (for example, uranium), which is the standard for carrying out to the outside of the controlled area. Must be less than / ^cm2 . As a result, in the determination of the presence or absence of surface contamination by radioactive substances, it can be determined that there is contamination when the surface contamination density is 0.4 Bq / cm ² or more, and it is determined that there is no contamination when the surface contamination density is less than 0.4 Bq / cm 2. can. This judgment standard value of 0.4 Bq / cm ² is the "radiation isotope that emits α rays" specified in Appendix 3 of the Ionizing Radiation Hazard Prevention Regulations (September 30, 1972, Ministry of Labor Ordinance No. 41). It corresponds to 1/10 of the limit of surface contamination by "elements" (4Bq / cm ² ), and it is said that the value (1/10 of the limit) must not be exceeded when the article is taken out of the facility. be.

Judgment of the presence or absence of surface contamination in places where measurement by the α-ray detector is difficult is performed by judging from the measurement results by the β-ray detector that there is contamination when the surface contamination density is 0.4 Bq / cm ² or more. If it is less than, it is judged that there is no contamination. This judgment standard value of 0.4 Bq / cm ² is equivalent to the above-mentioned standard value for taking out the facility with α rays, and is the Ionizing Radiation Hazard Prevention Regulations (September 30, 1972, Ministry of Labor Ordinance No. 41). ) Corresponds to 1/100 of the limit of surface contamination (40Bq / cm ² ) by "radioactive isotopes that do not emit α rays" specified in Attached Table 3 of), and the value when the article is taken out of the facility. It corresponds to 1/10 of the limit of (4.0 Bq / cm ² ).

Therefore, the present inventors can actually perform an experiment while changing the thickness of the powdered NR waste using a standard β-ray source, and the β-ray can be measured from the upper surface of the NR waste. The thickness (thickness below the maximum range of β rays) and the detection sensitivity at the time of measurement were confirmed to be below the detection limit density by the method described below.

The method for measuring the surface radioactivity density is specified in JIS Z 4504, and in the present invention, the measurement of the surface radioactivity density of the powdery NR waste is created according to the method. The following formula (1) is a formula for calculating the surface radioactivity density (Bq / cm ² ) specified in JIS Z 4504.

According to the equation (1), it is necessary to obtain the equipment efficiency ε _i and the radiation source efficiency ε _s . Therefore, using a standard radioactive source, the two efficiencies were calculated as follows. At that time, the radioactive substance is uranium ( ²³⁸ U), and assuming β-rays with a maximum energy of 2.273 MeV emitted by Pa-234 m generated in the decay process, Sr- has almost the same maximum energy value of 2.280 MeV. A 90-Y-90 calibration standard radiation source (emission surface area 10 × 10 cm) was used. As the β-ray measuring instrument (survey meter), BDBP-03 (window area 300 cm ² , equipment efficiency 0.486) manufactured by ATOMTEX was used.

For 5 units of β-ray detector BDBP-03, a secondary radiation source (thorium radiation source with a emission surface of 30 × 30 cm) priced using a standard radiation source for Sr-90-Y-90 calibration was used. As a result of obtaining the equipment efficiency ε _i , the average value of the five units was 0.532.

In order to obtain the radiation source efficiency ε _s , first, the absorption rate (transmittance) of β rays is obtained by changing the thickness of the powdery NR waste, and the powdery NR waste is placed on the tray at a uniform concentration. The source efficiency ε _s was calculated on the assumption that it was spread. Specifically, it was obtained as follows.

(I) Method for measuring β-ray absorption rate by powdery NR waste A β-ray detector (BDBP-03) is set on the lower surface of the measuring tray 1, and the structure (0) at the bottom 5 of the measuring tray 1 is set. A weighed powdery NR waste was spread almost uniformly on a PP film having a thickness of 2 mm), and an Sr-90-Y-90 standard radiation source was placed on the NR waste to measure the counting rate. The counting rate was recorded while gradually increasing the thickness (g / cm ² ) of the powdery NR waste. The thickness of the absorption layer is 0.2 mm of the PP film plus the thickness of NR waste (g / cm ² ), and the absorption rate is (Net cps with NR waste) / (without NR waste). Net cps).

(Ii) Measurement of transmittance of powdery waste Five samples of powdery NR waste extracted arbitrarily were used to test the thickness and transmittance of NR waste. Although there are some variations depending on the NR waste, almost the same transmittance approximation formula was obtained. Table 1 below shows the measurement results of the thickness (shielding thickness) and transmittance of powdered NR waste (average value of 5 samples). The maximum range of aluminum metal at the β-ray energy is 1.097 g / cm ² , but for powdered NR waste, the shielding thickness (absorption thickness) is 1.174 g / cm, which is slightly thicker than that. It can be seen that even with ² , the transmission is about 0.3%.

(Iii) Source efficiency ε _s
Regarding the radiation source efficiency ε _s , JIS indicates 0.5 for β rays with a maximum energy of 0.4 MeV or more, and 0.25 for α rays or β rays with a maximum energy of 0.15 MeV or more and less than 0.4 MeV. If the radiation source efficiency of the radioactivity density is known in advance, that value can be used. Therefore, using the measured transmittance value for the thickness of the powdery NR waste, assuming that the radioactivity concentration in the powdery NR waste is uniform, the transmittance is integrated in the thickness direction. The source efficiency ε _s was obtained from the above. At that time, since it was measured in the 2π direction, it was standardized so that it would be 0.5 without an absorber. The calculation results are shown in Table 2 below.

The surface radioactivity density (Bq / cm ² ) is determined using the calculated equipment efficiency ε _i (mean value 0.532) and the source efficiency ε _s according to the thickness of the powdery waste in Table 2. , Can be calculated by the calculation formula (1) specified in JIS Z 4504 described above. Next, the limit value (detection limit density) of the surface radioactivity density is obtained. The following formula (2) in the "Confirmation Procedure for Abolition of Radiation Facilities and Radioactivity Measurement Manual" of the Japan Society for Radiation Safety Management was used to calculate the detection limit density by the direct method.

The calculation results are shown in Table 3 below. From Table 3, the detection limit density (Bq / cm ² ) increases as the thickness of the powdery NR waste (g / cm ² ) increases, but the detection limit at a thickness of 1.46 g / cm ² . The density was 0.36 Bq / cm ² , which was found to be lower than the contamination density of 0.4 Bq / cm ² , which is the standard for carrying out of the controlled area described above. Therefore, considering that it is better for work efficiency to measure as much NR waste as possible at one time, the thickness (thickness density) that can be measured within the maximum range of β rays is about 1.5 g / g. It was found that the thickness (thickness density) of cm ² or less is sufficient.

Next, in step S3 of FIG. 1, β-ray measurement from the surface of the powdery NR waste is performed. FIG. 3 is a schematic diagram of β-ray measurement according to an embodiment of the present invention. The measurement surface (detection surface) 18 of the β-ray measuring instrument (survey meter) 16 is brought into contact with the surface of the powdery NR waste spread in the measuring tray 1 in step S2. As mentioned in the description of FIG. 2, the width of the detection surface 18 is set to have a width substantially equal to or wider than the width W of one opening of the measuring tray 1. As a result, by scanning the detection surface 18 once along the length direction (arrow A direction) of the openings 3A and 3B of the measurement tray 1, β-rays in the entire opening without leaving an unmeasured region. It is possible to perform the measurement reliably.

As the β-ray measuring device 16, for example, a plastic scintillation detector can be used. The β-ray measuring instrument 16 sends a detection signal to the display 20 via the communication cable 19. The display 20 can display the β-ray measurement result, and can also enable the measurement data to be transmitted to the personal computer 40 by wire or wirelessly. When the surface contamination density obtained from the measurement result by the β-ray detector is 0.4 Bq / cm ² or more, it can be determined that there is contamination, and when it is less than that, it can be determined that there is no contamination. As described above in the description of FIG. 2, the bottom 5 of the measuring tray 1 is made of a wire mesh having a pitch (opening) capable of holding powdery waste, a thin polypropylene (PP) film, or the like. If this is the case, the detection surface 18 of the β-ray measuring instrument 16 can be selectively added from the lower side of the bottom 5 to perform radiation (β-ray) measurement just in case.

An embodiment of the present invention has been described with reference to the drawings. However, the present invention is not limited to these embodiments. The present invention can be carried out in a mode in which various improvements, modifications and modifications are added based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

1 Measurement tray 2 Measurement tray outer frame 3A, 3B Inside the measurement tray (powder-like waste storage)
4 Partition plate in the measuring tray 5 Bottom of the measuring tray 16 β-ray measuring instrument (survey meter)
18 Measurement surface (detection surface)
19 Communication cable 30 Display unit 40 Personal computer (PC)

Claims (6)

It is an inspection method for inspecting the contamination of powdery waste that is the target of non-radioactive waste with radioactive substances.
(A) A step of preparing a measuring tray having at least one opening having a width equal to or less than the width of the detection surface of the radiation measuring instrument, and a step of preparing a measuring tray.
(B) A step of spreading the powdery waste in the opening of the measuring tray to a substantially uniform thickness.
(C) A step of determining the surface radioactivity density of the powdery waste by β-ray measurement while scanning the surface of the powdery waste spread close to the detection surface of the radiation measuring instrument. And, including
The inspection method, wherein the substantially uniform thickness is equal to or less than the maximum range of β rays emitted from the powdery waste. The inspection method according to claim 1, wherein the substantially uniform thickness is set so that the surface radioactivity density obtained by the β-ray measurement is less than 0.4 Bq / cm ² . The inspection method according to any one of claims 1 or 2, wherein the substantially uniform thickness is set to a thickness of 1.5 g / cm ² or less. The inspection method according to any one of claims 1 to 3, wherein the radiation measuring instrument is a plastic scintillation detector (survey meter) capable of measuring β rays. The measuring tray is square, and the four frame portions have a height of 10 to 20 mm from the bottom surface, and further, the widths substantially equal to the width of the detection surface of the radiation measuring instrument face each other from the frame portion. The inspection method according to any one of claims 1 to 4, which has at least one partition extending to another frame. The inspection method according to any one of claims 1 to 5, which is an inspection method performed in a facility that handles purified uranium.

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