Patent Application
20210181360
This invention relates to a device for detecting a radiation to inspect the presence/absence of radioactive contamination of an object, and more particularly, to a radioactive contamination inspection device that takes a case where a transport vehicle evacuates from a controlled area, such as decommissioning sites and interim storage facilities, into account.
The Ordinance on Prevention of Ionizing Radiation Hazards regulates that a controlled area is set when radiation exceeding a given value is treated, and contamination by radiation is inspected when articles are carried out of the controlled area. Thus, article inspection monitors are used in nuclear-related facilities.
Also in decommissioning sites and interim storage facilities, the inspection of radioactive contamination is needed when transport vehicles evacuate from a controlled area. In this case, a measurement target is large and cannot be inspected by an article inspection monitor, and hence a method of measuring radiation while scanning a vehicle body radiation by a β-ray survey meter is employed.
However, this method takes a long time to accurately inspect a large vehicle. Thus, in a facility through which a large number of large vehicles enter and exit, such as interim storage facilities, contamination inspection of such vehicles is a bottleneck in the entire work.
To reduce the work time, radioactive contamination inspection of a vehicle body surface using a gamma camera or an automatic inspection device for automatically scanning a large radiation detector has been proposed (see, for example, PTL 1 to 4).
PTL 1 discloses a method of measuring a radioactive concentration (surface contamination density) of a surface of an object to be measured by using a detector sensitive to α-rays or β-rays. PTL 2 discloses a method in which in order to inspect a measurement target having a complicated shape, the measurement is performed by placing the measurement target in a container formed of a deformable scintillation detector.
PTL 3 discloses a method of detecting characteristic X-rays emitted in response to decay of radioactive cesium and measuring a radioactive concentration of a surface to be measured or the inside of a measurement target. PTL 4 discloses a method of using a γ-ray (X-ray) camera in which elements sensitive to γ-rays or X-rays are arranged in an array to measure a radioactive concentration (surface contamination density) of an object surface.
Other conventional technologies are a plastic scintillator capable of measuring a radiation uniformly with high sensitivity in the entire detection unit (large area) and an optical fiber larger area radiation monitor (see, for example, PTL 5). PTL 5 employs a method in which light emitted from a plastic scintillator is condensed by an optical fiber, and the plastic scintillator and the optical fiber are formed so as to conform to the shape of an object to be measured, thereby measuring a radiation from the object to be measured having a curved shape.
[PTL 1] Japanese Patent Application Publication No. 2005-172771
[PTL 2] Japanese Patent No. 5669782
[PTL 3] Japanese Patent Application Publication No. 2015-180872
[PTL 4] Japanese Patent No. 5400988
[PTL 5] Japanese Patent Application Publication No. H09-243752
However, the conventional technology has the following problems.
Various methods as described above have been proposed for the detection of surface contamination by radiation. However, most of the methods are methods for grasping a radiation amount by condensing scintillation light emitted by reaction between a radiation and a substance.
In this method, a detector, a condensing device, and a photoelectron converter are increased in size and are difficult to be made physically close to a narrow portion, and accurate radiation measurement of an object to be measured having a complicated shape is difficult. Thus, for example, when a measurement target is a narrow site, such as a vehicle wheel well and a vehicle bottom surface, or a surface having a complicated shape, such as a mixer truck, radioactive contamination cannot be inspected by the above-mentioned conventional methods disclosed in PTL 1 to 4.
As described in PTL 5, a laminated plastic scintillator or a radiation detection optical fiber in which the shape of a sensitive region can be deformed in order to perform surface contamination inspection of a narrow portion or a curved surface has been proposed. However, the measurement target is γ-rays, and is affected by an environmental radiation, and hence there is a problem in that a detection limit cannot be set to be sufficiently lower than a reference value.
In particular, at a location where an environmental radiation amount is high, inspecting radioactive contamination of a surface to be measured requires a shield body for blocking γ-rays entering a detector from other than the measurement target. Thus, surface contamination in a narrow site or a complicated, labyrinthine shape is difficult.
For a radiation detector using laminated plastic scintillators or optical fibers, light needs to be reliably blocked, and condensing efficiency is low, and hence it is difficult to achieve a practically available level.
Also in the case where a GM survey meter for surface contamination inspection is used, accurate radiation measurement requires sufficient shielding in order to distinguish between a radiation from a contaminated matter in a region to be measured and an environmental radiation derived from a contaminated matter existing in environments. Thus, surface contamination inspection in a narrow site or a complicated, labyrinthine shape is difficult.
This invention has been made in order to solve the problems as described above, and it is an object thereof to provide, for example, a radioactive contamination inspection device capable of efficiently performing contamination inspection of a narrow portion which has a complicated shape and for which it is difficult for a general radiation detector to be made close, such as a wheel well of a vehicle and a vehicle bottom surface.
A radioactive contamination inspection device according to this invention includes: a detection unit having a sensitive surface that has a shape conforming to a shape of an object surface, which is a measurement target and a radioactive contamination amount of which is to be measured; a mechanism unit for holding the detection unit in a state in which a distance from the sensitive surface to the object surface falls within a desired range set in advance; and a measurement unit for calculating the radioactive contamination amount of the object surface on the basis of a measurement result from the detection unit.
According to this invention, the radioactive contamination inspection device has a configuration in which the detection unit having the sensitive surface that has the shape conforming to the shape of the object surface of which the radioactive contamination amount is to be measured can be held in the state in which the distance from the sensitive surface to the object surface falls within the desired range. Such a configuration enables the thickness of a measurer to be thinner than hitherto, and enables the area and shape of a semiconductor detector necessary for securing detection efficiency to be selected correspondingly to detection efficiency of a measurement target. Consequently, for example, a radioactive contamination inspection device capable of efficiently performing contamination inspection of a narrow portion which has a complicated shape and for which it is difficult for a general radiation detector to be made close, such as a wheel well of a vehicle and a vehicle bottom surface, can be provided.
Preferred embodiments of a radioactive contamination inspection device of this invention are described below with reference to the drawings.
In Embodiment 1, description is given on the assumption that measurement targets are transport vehicles (hereinafter simply referred to as “vehicles”) that evacuate from a controlled area, such as decommissioning sites and interim storage facilities.
First, specific regions to be inspected in this invention related to a vehicle for which radioactive contamination is to be measured are described with reference to the drawings. The regions to be inspected of the vehicle in Embodiment 1 are roughly divided into the following three regions.
Region A: wheel vertical surface A1, wheel top surface A2, top surface A3 of wheel well, and inner vertical surface A4 of wheel well for front wheel
Region B: wheel vertical surface B1, wheel top surface B2, top surface B3 of wheel well, and inner vertical surface B4 of wheel well for rear wheel
Region C: vehicle bottom surface C1
The wheel vertical surface A1 corresponds to an outer vertical surface of the front wheel. By using a circular detector 11, radioactive contamination of the wheel vertical surface A1 is quantitatively measured. The wheel top surface A2 corresponds to a surface of the front wheel that comes contact with a traveling surface. By using a semi-circular detector 12, radioactive contamination of the wheel top surface A2 is quantitatively measured.
Radioactive contamination of the top surface A3 of the wheel well and the inner vertical surface A4 of the wheel well is quantitatively measured by using the same semi-circular detector 12 as that used to measure the wheel top surface A2. The semi-circular detector 12 is inclined to measure the inner vertical surface A4 of the wheel well as illustrated in
The detector lengths of the detectors 11 and 12 are roughly as follows in the case of the vehicle illustrated in
Detector length of detector 11: 75 cm (diameter)×π=236 cm
Detector length of detector 12: 110 cm (diameter)×π/2=173 cm
Next,
As illustrated in
The wheel vertical surface B1 corresponds to an outer vertical surface of the rear wheel. Similarly to the front wheel, the circular detector 11 is used to quantitatively measure radioactive contamination of the wheel vertical surface B1. The wheel top surface B2 corresponds to a part of the rear wheel that comes into contact with a traveling surface. Similarly to the case of the front wheel, the semi-circular detector 12 is used to quantitatively measure radioactive contamination of the wheel top surface B2.
Radioactive contamination of the top surface B3 of the wheel well and the inner vertical surface B4 of the wheel well is quantitatively measured by using a detector 13 that can be manually changed into a shape turned down at the corner as illustrated in
As illustrated in
The detectors 11, 12, and 13 are set to the rear wheel by manually pushing an arm portion as illustrated in
The detector lengths of the detectors 11 and 12 are the same as in the case of the front wheels, and the detector length of the detector 13 is roughly as follows.
Detector length of detector 13: 150 cm
Next,
The front side of the vehicle bottom surface is inspected by using the detector 14 illustrated in
Detector length of detector 14: 150 cm
Detector length of detector 15: 100 cm
For the inspection using the detectors 14 and 15, a distance sensor is mounted for each detector block, and an average distance between the detector and the vehicle bottom surface is calculated by the distance sensor for each given traveling distance. Then, distance correction of the count rate is performed to calculate a surface contamination location.
For the inspection of the vehicle bottom surface, the following second method may be employed. In the second method, instead of using the detectors 14 and 15, detectors are disposed at equal intervals on the ground at a vehicle stop position to measure the count rate. In this case, a distance sensor is mounted for each detector block, and an average distance between the detector and the vehicle bottom surface is calculated by the distance sensor. Then, distance correction of the count rate is performed to calculate a surface contamination value.
In the above-mentioned detectors 11 to 15, in order to support inspection targets having various shapes, a plurality of compact and thin radiation detectors (for example, semiconductor detectors) can be arranged to secure detection efficiency. At the same time, in the detectors 11 to 15, a plurality of compact and thin radiation detectors are mounted to a flexible base or a base molded in advance so as to conform to the shape of an inspection target, thereby efficiently inspecting a narrow inspection target having a complicated shape.
As described above, according to Embodiment 1, a thin radiation detector that can be deformed to conform to the shape of an inspection target can be used to obtain a radioactive contamination inspection device capable of efficiently inspecting a narrow site or an object having a complicated shape.
Embodiment 1 has been described above on the assumption that the detector is manually set. In Embodiment 2, a radioactive contamination inspection device including a control mechanism for specifying an insertion position of the detector is described.
The detector unit 10 is a detector for quantitatively measure a radioactive contamination amount of an inspection target. The detectors 11 to 15 in Embodiment 1 described above correspond to the detector unit 10.
The arm 20 has one end 20a to which the detector unit 10 is mounted. On the other hand, the arm 20 has the other end 20b mounted so as to pass through the driving control device 40.
The laser distance meter 30 is a sensor that is mounted to an upper part of the driving control device 40 to measure the shape of measurement target.
The signal processing unit 41 in the driving control device 40 has a function of reconstructing a 3D shape of the measurement target on the basis of the measurement result from the laser distance meter 30. The driving control unit 42 in the driving control device 40 has a configuration and a function capable of driving and controlling the positions of the detector unit 10 and the arm 20 in the vertical direction, the horizontal direction, and the depth direction of the measurement target on the basis of the reconstructed 3D shape of the measurement target.
In addition, the measurement unit 43 has a function of processing a signal from the detector unit 10 to quantitatively evaluate the radiation amount.
As illustrated in
Means for measuring the distance to a surface to be measured is not limited to the laser distance meter 30. Any means capable of measuring the distance to a surface to be measured with necessary accuracy, such as a stereo camera, a contact sensor, and an ultrasound sensor, can be used.
Thus, in
Even when the position of the detector unit 10 is fixed, by calculating the surface contamination concentration on the basis of detection sensitivity at the farthest distance in a viewing angle of the detector unit 10, contamination can be found without fail. Thus, even when a method of fixing the detector is used, the presence/absence of contamination can be determined.
The semiconductor elements 101 are arranged on a circuit board 102 and covered with an electromagnetic shield casing 103. Depending on the number of the semiconductor elements 101, for example, a sensitive surface having a size of about 10 cm×10 cm can be formed. By increasing the number of the semiconductor elements 101, a larger sensitive surface can be formed.
The size of the sensitive surface may be determined such that the fluctuation in distance to a surface to be measured does not affect the detection sensitivity with respect to the shape of an assumed measurement target, for example, the radius of curvature of the tire for the wheel well.
As a signal processing method for the semiconductor array, the respective semiconductor elements 101 can be connected to individual preamplifiers. A plurality of semiconductor elements 101 can be collectively connected to one preamplifier. In the case where a plurality of semiconductor elements 101 are simply connected in parallel, a DC component of dark current is added, and hence noise increases.
As the semiconductor element 101, not only a silicon diode but also a chemical semiconductor such as CdTe can be used. The semiconductor element 101 may have either of an ohmic junction and a Schottky barrier junction. The thickness of the semiconductor element 101 is determined by an air dose at a measurement location and energy of a radiation from a measurement target.
When a radioactive substance of the measurement target is radioactive cesium, not only γ-rays of 662 keV but also characteristic X-rays of 32 keV are radiations to be measured.
As illustrated in
Pulses output from the detector unit 10 are input to the subsequent signal processing unit 41. The signal processing unit 41 selectively counts only a radiation in an energy region of the measurement target.
The energy region is selected by a method of using an energy window capable of selecting only pulses having a pulse height in a predetermined range or a method of using a filter based on difference in pulse waveform.
When a radiation is made incident from the cathode, most of characteristic X-rays of 32 keV are reacted near the cathode, and hence electrons and holes both have shorter arrival times at the electrodes. On the other hand, γ-rays of 662 keV are reacted even at a very deep position near the anode, and hence in particular, it takes a longer time for holes to reach the electrode.
As illustrated in
As described above, by using the bandpass filter to adjust the frequency characteristics to a frequency bandwidth corresponding to a particular energy region, the sensitivity for regions other than the particular energy region can be reduced. This means that a general energy window as illustrated in
In either of the case where a bandpass filter is used and the case where an energy window is used, only a radiation in an energy region to be measured can be selectively counted. Thus, the signal processing unit 41 selectively counts only a radiation in an energy region to be measured, and transfers a coefficient value to the measurement unit.
The measurement unit 43 calculates, on the basis of the counted value of only a radiation in the energy region to be measured, a radioactive concentration of the surface to be measured from detection sensitivity determined by the distance from the detector unit 10 to the surface to be measured and a background count rate.
For the detection sensitivity, sensitivity at a distance in a predetermined range is determined in advance by physical calculation, and is stored in the measurement unit 43 as a data table. The background can be measured before the measurement of the measurement target, and can be determined by arithmetic operation from an air dose rate.
By inputting a reference value of radioactive concentration (surface contamination density), the measurement unit 43 can generate a warning when the radioactive concentration of the surface to be measured is equal to or larger than the reference value. The measurement unit 43 can record a contaminated position and displays the contaminated position on a display unit.
As described above, according to Embodiment 2, the thin detector unit can be inserted to an appropriate position on the basis of a 3D shape of a measurement target that is reconstructed on the basis of measurement results of distances to the measurement target. As a result, a narrow site and an object having a complicated shape can be efficiently inspected.
Embodiment 3 is different from Embodiment 2 described above only in that a scintillator is used as the detector unit 10 to detect a radiation instead of using a semiconductor element sensitive to a radiation.
The scintillator may be an organic scintillator such as a plastic scintillator, and may be an inorganic scintillator such as sodium iodide and cesium iodide.
In the case where a scintillator is used as the detector unit 10, by thinning an incident window to such a degree that can transmit α-rays and β-rays, α-rays and β-rays emitted from radioactive substances adhering on the surface to be measured can be directly measured.
As described above, Embodiment 3 can provide the same effects as in Embodiments 1 and 2 described above even when a scintillator instead of a semiconductor element is used as a detector.
Embodiment 4 is different from Embodiment 3 described above only in that the detector unit 10 is formed of a flexible scintillator. The detector unit itself can be bent, and hence it is unnecessary to connect a plurality of detector units 10. As a result, the detector unit itself can be made conform to the shape of a surface to be measured.
As specific examples, flexible light guides such as jelly and oil can be used. Even when the light guide is not flexible, the scintillator and the light guide can be optically coupled with a flexible material so that the detector unit can be made conform to a complicated shape without degrading collection efficiency of scintillation light. This method can further reduce the fluctuation in distance.
Embodiments 1 to 4 have been described for the case where a physical amount to be measured is surface contamination density. However, the use of a semiconductor element or a scintillator as the detector unit 10 enables radioactive concentration (Bq/kg) including the inside of a measurement target to be measured.
Thus, the measurement target is not limited to a vehicle whose surface can be contaminated, and may be a measurement target for which internal radioactive concentration needs to be measured because the measurement target absorbs radioactive substances inside, such as agricultural crops and trees.
Detection sensitivity in this case is determined by physical calculation in consideration of not only the geometric shape and the distance to a detector but also the size and density of a measurement target. Also by using a radiation source that simulates a measurement target, the radioactive concentration inside the measurement target can be determined.
When a radioactive substance to be measured is radioactive cesium, the contaminated state of a surface to be measured can be detected with high accuracy by measuring characteristic X-rays. To detect the characteristic X-rays, a shielding structure, which is required for detecting γ-rays, is not needed. Thus, by specializing in detecting characteristic X-rays, radioactive contamination of the surface of a transport vehicle can be detected with high accuracy with a simple structure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-061907 | Mar 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/068963 | 6/27/2016 | WO | 00 |
