JP2014228530A - Design method of radiation shield structure, and dosimeter shield body using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of easily designing a shield structure for creating a radiation shield space, and to provide a dosimeter shield body used for the method.SOLUTION: A radiation quantity (complete shield radiation quantity) is measured in a state in which a probe P of a dosimeter is completely shielded by a shield body 100. A radiation quantity (environmental radiation quantity) is measured in a state in which the probe P is extracted from the shield body 100. The shield body 100 is mainly formed of lead 8 of a thickness of 10 mm. The radiation transmittance is determined on the basis of the complete shield radiation quantity, environmental radiation quantity, and the thickness of the lead. When a shield space of a target dose is created, the shield material and thickness of the shield structure of the target dose are set using the radiation transmittance.

Description

  The present invention relates generally to radioactive contamination, and to a design method for a structure that shields radiation and a dosimeter shield used therefor.

  Patent document 1 is disclosing the structure which shields the upper direction and the side of a reactor pressure vessel. Patent Document 2 discloses the treatment of radioactive waste accompanying the dismantling of a building when a room where a radiation source is installed in a PET facility, a medical radiation irradiation facility, an accelerator facility, etc., is constructed with a neutron beam shielding structure. We have proposed a neutron shielding structure that can reduce costs.

  Due to the Fukushima Daiichi Nuclear Power Plant accident that occurred on March 11, 2011, the Fukushima Daiichi Nuclear Power Plant accident caused radioactive contamination not only in the vicinity of the Fukushima Daiichi Nuclear Power Plant but also in a wide area. In this current situation, the inventor of the present application determines the radiation dose emitted by the measurement object using an available radiation dose measuring device (hereinafter referred to as “dosimeter”) while reducing the influence of radiation from the surroundings as much as possible. A dosimeter shield was proposed as an auxiliary tool for measurement (Patent Documents 3 to 6). According to this, the radioactive contamination site and the contamination degree of the contaminant can be measured almost accurately using a commercially available portable dosimeter.

JP 2005-214658 A JP 2011-58922 A JP 2012-159517 A Design registration No. 1455233 Design registration No. 1455269 Design registration No. 1455271

  Artificial decontamination activities are being carried out extensively and hard as a countermeasure against radioactive contamination. However, even if decontamination is performed, it has already been recognized that there is a place where the dose reduction effect is limited or the decontamination effect disappears immediately. This fact can be clearly understood by measuring the radiation dose using the dosimeter shield.

  Examples of such places include private houses located in satoyama, places adjacent to parks and forests, reservoirs and reservoirs in urban areas, places adjacent to fallow fields and fields where agriculture has been abandoned, and three-dimensional The location adjacent to the high-rise building that is the radiation source and its courtyard can be listed.

  In addition, even on roofs that are likely to be decontaminated, tile roofs and slate roofs that are fragile are likely to cause accidents during the decontamination work. It is practically difficult. In addition, the area adjacent to the Fukushima Daiichi Nuclear Power Station generally has a high amount of environmental radiation, and decontamination work itself is difficult.

  Areas where it is difficult to perform decontamination itself, even if decontamination is limited, the dose reduction effect is limited, or even if the decontamination effect disappears immediately, people who must live and work there The fact is that they are always exposed to radiation. Under these circumstances, it is required to create a radiation shielding space in order for humans to live safely.

  The shielding structures disclosed in Patent Documents 1 and 2 are characterized in that the radiation source is clear and the nuclides and concentrations present in the space to be confined by the shielding structure are clear.

  On the other hand, the problem at the Fukushima Daiichi NPS is that both the nuclide and the concentration of the scattered radioactive material are unclear. In addition to this, portable radiation measuring instruments or dosimeters used in the field generally display only the dose equivalence rate and counting rate, so it is necessary to design shielding structures based on this. There is no situation.

  However, as described above, the fact that radiation is emitted from an unidentified nuclide and the amount of radiation energy is unclear is based on any data as a basis for designing a shielding structure. A fundamental design problem arises.

  It is an object of the present invention to provide a method capable of facilitating the design of a shielding structure for creating a radiation shielding space and a dosimeter shielding body used for the method.

  The present inventor creates an actually shielded state in the space to be shielded, measures the radiation dose in the shielded state, and uses the obtained value as basic data (benchmark). The present invention is proposed based on the viewpoint that the shielding rate to be included in the shielding structure to be obtained can be easily and accurately obtained.

The above technical problem is, according to the first aspect of the present invention,
Prepare a portable dosimeter shield that can shield the detection part of the portable dosimeter,
In a place to be shielded, measure the amount of environmental radiation using a dosimeter,
In the place to be shielded, measure the completely shielded radiation dose with the detection part of the dosimeter surrounded by the dosimeter shield,
Obtain the radiation transmittance of the dosimeter shield from the environmental radiation dose and the complete shielding radiation dose,
A radiation shielding structure design method, wherein a material and a thickness dimension of a shielding structure to be constructed are set based on the radiation transmittance and a target dose at the place to be shielded.

The above technical problem is, according to the second aspect of the present invention,
It has a shape that surrounds the radiation detection part of the portable dosimeter,
This is accomplished by providing a portable dosimeter shield that can shield the detection portion of the portable dosimeter from ambient radiation with a predetermined shielding capability.

It is a perspective view of the dosimeter shield of an Example. It is a longitudinal cross-sectional view of the shielding body of FIG. It is a front view of the shielding body of an Example. It is a top view of the shielding body of an Example. It is a right view of the shielding body of an Example. It is a left view of the shielding body of an Example. It is a rear view of the shielding body of an Example. It is a bottom view of the shielding body of an Example. It is sectional drawing along the AA line of FIG. It is sectional drawing along the BB line of FIG. It is sectional drawing along CC line of FIG.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

  First, NaI scintillation type survey meter TCS-172B manufactured by Hitachi Aloka Medical Co., Ltd. is widely used as a portable dosimeter. This dosimeter has a main body, and a cylindrical probe P (see FIG. 2) connected to the main body via a cable (reference numeral 14 in FIG. 2 attached). (Radiation detection unit) S is arranged. The radiation amount detected by the probe is displayed on the display unit of the main body. This dosimeter has a function of selectively displaying a dose equivalence rate (μSv / h) and a counting rate (CPS).

  Based on the above, FIGS. 1 and 2 show a dosimeter shield 100 of an embodiment for completely shielding the detection unit S of the probe P. FIG. FIG. 3 is a front view of the dosimeter shield 100, and FIG. 4 is a plan view. FIG. 5 is a right side view of the dosimeter shield 100, and FIG. 6 is a left side view. FIG. 7 is a rear view of the dosimeter shield 100, and FIG. 8 is a bottom view. 9 is a cross-sectional view of the dosimeter shield 100 taken along line AA in FIG. 3, FIG. 10 is a cross-sectional view taken along line BB in FIG. 4, and FIG. It is sectional drawing along the -C line.

  The dosimeter shield 100 is portable and is conveniently equipped with a handle H to carry the dosimeter shield 100. 1 is a perspective view, and FIG. 2 is a longitudinal sectional view. The shield 100 has a cylindrical side wall 2 with a bottom 4 and optionally a lid 6. The lid 6 is not necessarily essential. The cylindrical side wall 2 is preferably longer than the length dimension of the probe P, but may be shorter than this, and has at least a length that can surround the detection portion S of the probe P.

  In the illustrated example, the cylindrical side wall 2 has a cylindrical outer shape, but this outer shape is arbitrary. It may be a prismatic cylinder.

  The tubular side wall 2, the bottom portion 4, and the lid 6 employ lead 8 as a main shielding material, and the lead 8 is surrounded by an outer skin 10 such as a stainless steel plate. Lead 8 is known to have a high radiation shielding ability.

  The thickness dimension W of the lead 8 on the cylindrical side wall 2 and the bottom part 4 is designed to be a predetermined dimension. Specifically, the thickness dimension of lead 8 was 10 mm (W = 10 mm). The thickness dimension of the lead 8 is applied to at least the detection portion S of the probe P.

  The lid 6 is formed with a notch 12 through which a cable 14 connecting the probe P and a main body (not shown) can pass. Of course, the lid 6 is provided when the length dimension of the cylindrical side wall 2 is larger than the length dimension of the probe P.

  When the probe P is inserted into the shield 100 from its tip, the detection portion S of the probe P is surrounded by the lower end portion and the bottom portion 4 of the cylindrical side wall 2. When the radiation dose is measured in this state, this measurement value means the radiation dose inside the shield 100. In other words, by completely surrounding the radiation detection part of the dosimeter and shielding the radiation detection part from environmental radiation, and defining this shielding condition, the measured values are used as basic data as described below. Thus, it is possible to determine the shielding rate, the shielding material, the thickness dimension, and the like to be included in the shielding structure to be constructed. By adopting such a design method, it is possible to easily design an appropriate shielding structure that matches the situation in the field.

  That is, in the design method of the radiation shielding structure according to the embodiment, first, a state where the probe P is inserted into the shielding body 100 at a place where the shielding structure is to be formed (place to be shielded), that is, a complete shielding state. Measure the radiation dose with. Here, it is assumed that the measured value is 0.02 μSv / h.

  Further, the radiation dose is measured at the same place in the state where the probe P is taken out from the shield 100 (non-shielding state: environmental radiation dose). Here, it is assumed that the measured value is 0.09 μSv / h.

  The difference value 0.07μSv / h between the dose (0.09μSv / h) in the unshielded state (environmental radiation dose) and the dose in the completely shielded state (0.02μSv / h) is the radiation dose that has entered the shield 100. is there. The thickness dimension of the lead 8 of the shield 100 is 10 mm. From this numerical value, the radiation transmittance in the said field can be calculated | required based on the following formula 1.

(Formula 1) Radiation transmittance = (0.09−0.02) /10=7.0×10 ⁻³ (μSv / h) / mm

Knowing this radiation transmittance (7.0 × 10 ⁻³ ), if a shielding space of 0.4 μSv / h is set as the target dose T at the site, the environmental radiation dose is now 0.09 μSv / h Therefore, assuming that the wall of the shielding structure is made of lead, the lead thickness dimension W can be obtained by the following equation (2).

(Formula 2) W (mm) = (0.09−0.04) /7.0×10 ⁻³ = 7 (mm)

  That is, in this environment, if the lead is enclosed with a thickness of 7 mm, the dose in this shielded space becomes the target dose of 0.04 μSv / h. Further, if lead is not selected as the shielding material, a thickness dimension may be set such that the selected shielding material exhibits a shielding ability corresponding to the shielding ability of lead having a thickness dimension of 7 mm.

By the way, thickness dimensions of various shielding materials and radiation attenuation values (unit: cm) are known. For example, for nuclide ⁸⁰ Co, the lead half-value layer is 1.2, the 1 / 10-value layer is 4.0, the iron half-value layer is 2.0, the 1 / 10-value layer is 6.7, the concrete half-value layer is 6.1, The 1/10 valence layer is 20.3. For nuclide ¹³⁷ Cs, lead half-value layer is 0.7, 1 / 10-value layer is 2.2, iron half-value layer is 1.5, 1 / 10-value layer is 5.0, and concrete half-value layer is 4.9, 1 The / 10 valence layer is 16.3. For nuclide ²²⁶ Ra, the half-value layer of lead is 1.3 and the 1 / 10-value layer is 4.4, the half-value layer of iron is 2.1, the 1 / 10-value layer is 7.1, and the half-value layer of concrete is 7.0, 1 The / 10-valent layer is 23.3. For nuclide ¹⁹² Ir, lead half-value layer is 0.6, 1 / 10-value layer is 1.9, iron half-value layer is 1.3, 1 / 10-value layer is 4.3, and concrete half-value layer is 4.1, 1 The / 10-valent layer is 13.5.

  Using such thickness dimension and attenuation value, a thickness dimension may be set at which the selected shielding material exhibits a shielding ability corresponding to the shielding ability of lead having a thickness dimension of 7 mm.

Also known is the effective dose transmission of gamma rays from radioisotopes. For example, specific examples of nuclide ¹³⁷ Cs are as follows.

(1) Iron:
Thickness (cm) Transmittance
0 1.00
0.5 9.34E-01
1 8.58E-01
2 6.78E-01
5 2.48E-01
8 7.29E-02

(2) Lead:
Thickness (cm) Transmittance
0 1.00
0.2 8.46E-01
0.3 7.77E-01
0.4 7.12E-01
0.5 6.50E-01
1 4.05E-01
2 1.46E-01
3 5.03E-02
4 1.69E-02

(3) Concrete:
Thickness (cm) Transmittance
0 1.00
5 8.69E-01
10 6.36E-01
15 4.17E-01
20 2.55E-01
25 1.49E-01
30 8.40E-02
35 4.62E-02
40 2.48E-02

  Of course, it is also possible to set the shielding material and thickness dimension of the wall that defines the shielding space based on the already known attenuation value and transmittance as described above. This is used as basic data based on the above-mentioned dose in the unshielded state (environmental radiation dose) measured in the field and the dose in the completely shielded state, and the design value of the shielding material of the shielding structure to be made and its thickness Therefore, the shielding structure suitable for the site can be designed easily and appropriately.

  In the present invention, the specific configuration of the dosimeter shield 100 described with reference to FIGS. 1 and 2 is merely an example. Portable dosimeters are available in various forms. For example, a dosimeter having a GM pancake type probe is known. In addition, a personal dosimeter made by, for example, US RADeCO, whose detection part and main body are integrated is known. It goes without saying that the present invention can also be applied to these by taking into account the shape, the position of the detection unit, and the like so as to shield at least the detection unit.

100 Radiation Shield of Example 2 Cylindrical Side Wall 4 Bottom 6 Lid P Probe

Claims (7)

Prepare a portable dosimeter shield that can shield the detection part of the portable dosimeter,
In a place to be shielded, measure the amount of environmental radiation using a dosimeter,
In the place to be shielded, measure the completely shielded radiation dose with the detection part of the dosimeter surrounded by the dosimeter shield,
Obtain the radiation transmittance of the dosimeter shield from the environmental radiation dose and the complete shielding radiation dose,
A radiation shielding structure design method, wherein a material and a thickness dimension of a shielding structure to be constructed are set based on the radiation transmittance and a target dose at the place to be shielded.   The method for designing a radiation shielding structure according to claim 1, wherein shielding ability of the portable dosimeter shielding body is defined in advance. The main material of the portable dosimeter shielding material is lead,
The method for designing a radiation shielding structure according to claim 2, wherein the lead thickness is predetermined. It has a shape that surrounds the radiation detection part of the portable dosimeter,
A portable dosimeter shield that can shield the detection part of a portable dosimeter from environmental radiation with a predetermined shielding ability. A portable dosimeter shield used in a dosimeter having a main body and a cylindrical probe connected to the main body via a cable,
A bottom with a shielding function;
A cylindrical side wall having a shielding function and capable of receiving the probe;
A dosimeter shield that shields the radiation detection part of the probe with a predetermined shielding ability by the bottom and side walls.   The dosimeter shield according to claim 5, wherein a height dimension of the cylindrical side wall is larger than a length dimension of the probe.   The dosimeter shield according to claim 6, further comprising an openable / closable lid at an upper end of the cylindrical side wall.

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