Orbital-Rail-Type Automatic Inspection Device for Pipeline Welds Using Radiation Dose Prediction Results from FLUKA Simulation

Abstract

Pipeline welds typically do not have secondary reinforcement, rendering welds highly vulnerable to leakage accidents caused by the movement of gases or liquids. Therefore, identifying internal defects in welds through radiographic testing (RT) is critical for a visual and quantitative evaluation of weld defects. In this study, we developed a device that can automatically inspect the circumferential connection between pipes by applying a digital radiography testing (DRT) technique that can convert radiation signals into real-time electrical signals by using a digital detector array (DDA). Gamma rays were used to minimize spatial constraints in the inspection environment and optimization was performed to satisfy quality requirements set by international standards. Furthermore, FLUKA simulation was performed to predict radiation intensity for accurate radiation leakage identification to enable the shielding design to be supplemented with lead rubber. This measure considerably reduces the safe distance for radiation leakage during field testing. The results confirmed the feasibility of a novel automated inspection technique that integrates automatic inspection devices and ensures safety using radiation, the byproduct of which is a hazardous material.

1. Introduction

After pipe welding, the final quality of the welded joints is typically assessed through nondestructive testing to identify defects. The methods for evaluating defects in welded joints are categorized into inspection techniques that inspect the interiors of welds and heat-affected zones (HAZs). Typical methods for evaluating the internal structures of welded joints include ultrasonic testing (UT), which provides the information on the approximate shapes and presence of defects by evaluating the feedback time consumed by sending and receiving ultrasonic signals and their wavelength bands, and radiographic testing (RT), which can quantitatively determine the size and shape of the defect in grayscale through radiation penetrating the photosensitive film.

Although RT always involves a radiation exposure risk, this method is the preferred nondestructive testing technique for welded joints because of its advantages, such as the ability to visually inspect the shapes, sizes, and presence of weld defects.

Typically, the quality of conventional RT results depends on the type and intensity of radiation and the film specifications. However, inspection methods differ considerably depending on the type of radiation available and the restrictions placed on the use of radiation energy according to the laws on nuclear safety of each country.

In the Republic of Korea, the focus area of this research, radiation testing is highly regulated (see

Table 1. Contents of radiation safety management standards of the Republic of Korea.

Items	Main Content
Restrictions on the use of radiation sources	(1) Ir-192, Se-75    - In radiological facilities, less than 150 Ci    - In other areas, less than 20 Ci (Se-75: 47 Ci) (2) Co-60, X-ray generator    - For use only in radiological facilities
Control of public access during radiation work	(1) Restricted access area accessible to the general public    - Access is restricted when more than 1 µSv/h occurs.

). In particular, determining the safe distance of radiation that is accessible to the atomic regulatory authority has many restrictions. Although shielding equipment for safety has been developed, limited studies have focused on automatic inspection devices and testing techniques that can replace these devices [1,2].

Therefore, it is necessary to verify the welds that require mandatory radiographic inspection. Generally, structures made from steel plates are mostly based on carbon steel. For structural components, welding positions and the application of automated welding devices are very smooth, allowing for consistent and sound weld quality. However, in the case of piping, welding around the circumference of the pipe requires welding in all positions and the likelihood of defects in the welds increases significantly with changes in position.

In the heavy industry sector, which currently focuses on the construction of large structures, most weld inspections for structures are performed using ultrasonic testing. For piping welds, radiographic testing is used to ensure quality. However, the conventional radiographic testing method using film is highly repetitive and labor-intensive, resulting in low inspection efficiency.

In this paper, before designing and manufacturing the developed device, the actual radiation exposure environment and inspection conditions were modeled in order for us to simulate them accurately. This allowed for the visual verification of the movement path of the radiation source and the leakage dose. Based on these predicted values, we were able to achieve an efficient shielding design and ensure a safe radiation distance. The purpose of minimizing the shielding device is to significantly reduce the weight of the automation device and provide greater assistance to the inspectors. The ultimate goal was to develop portable inspection equipment that was easy for inspectors to carry and move rather than simple automated inspection equipment.

By reducing the safety distance through a minimal shielding design and optimal shielding, safety issues regarding passage and access for the general public are resolved.

In addition, if this safety distance can be reduced, a work environment in which other production processes such as welding and painting work, etc. can be performed together during radiation inspection is possible, thereby improving productivity.

These constraints and challenges associated with using radiation require the development of an optimal automatic inspection device that can satisfy radiation exposure restrictions and reduce the testing speed.

As part of our study on automatic inspection devices using radiation, we developed an automatic device for inspecting pipe circumference welds by using industrial digital radiographic testing (DRT) technology. This technology can electrically convert radiation signals into digital images by using a digital detector array to replace conventional RT techniques using films. This method allows the real-time inspection results to be viewed on a monitor [3,4,5].

The structure and overall research content of this paper are as follows.

Firstly, as described in Section 2, we utilized a digital detector array, a key equipment in digital radiography testing, to verify the quality differences among radiation sources and determine the radiation sources to be applied in the inspection device. Furthermore, we established inspection techniques and sequences for automated testing and assembled the equipment.

As described in Section 3, conservative theoretical calculations regarding radiation leakage were implemented, and these were modeled to predict radiation leakage values and radiation values according to distance using FLUKA simulation. Subsequently, based on these predictions, we compared and verified experimental values against simulation values by using radiation on the actual equipment at the location of leakage.

Through verification, additional shielding designs were incorporated, and through two times of simulation and experiment comparisons, radiation leakage was mitigated, satisfying the target radiation distribution values in theory.

2. Experimental Details

2.1. Digital Detector Array Quality Assessment Using Radioactive Isotopes (Se-75 and Ir-192)

Typically, nondestructive testing of welds using radiation is performed using radioactive isotopes and X-ray generators. Naturally occurring radioactive isotopes are advantageous for inspecting thick objects because their energy decreases in intensity and dissipates over time. Furthermore, they have a wider energy range than that of X-rays.

Various X-ray generators are available depending on the principle of generating X-rays. The most common type is the vacuum-tube X-ray generator, which can be used continuously until the occurrence of malfunction. Users can select and use the desired tube voltage (kV) and tube current (mA) depending on the specifications of the device, which shortens the inspection time and ensures optimal results [6,7,8].

However, most vacuum-tube X-ray generators are large and not portable. Therefore, using such devices in narrow spaces between pipes to inspect pipe welds in the field is not feasible. In such cases, radioactive isotopes are practical for inspecting installed pipes. To address this problem, this study developed an inspection device in which radioactive isotopes are used instead of X-rays to inspect pipe welds when automatically rotating the device in narrow spaces between installed pipes. Therefore, as explained in this section, the performance of a digital detector array was evaluated using Se-75 and Ir-192, which are radioactive isotopes typically used to inspect welds. Tests were conducted to satisfy the quality specifications of the inspection according to international quality standards and the results were used to confirm the image characteristics produced by the gamma rays.

Because this study was focused on developing a device for inspecting pipes installed in the field, digital detector array (DDA) quality assessment was conducted using gamma rays because this approach is easy to use. Furthermore, a bendable DDA bent according to the pipe diameter was used in the experiment.

Table 2. Specifications of the bendable digital detector array used in this study.

	▪ Scintillator type	Gadox
	▪ Pixel size	140 μm
	▪ Pixel matrix	768 × 2304 mm
	▪ Active area	327 × 107 mm
	▪ Internal shieling	300 kV
	▪ Wireless	Battery and WLAN
	▪ Bendable	8″ (O.D = 202 mm)

presents the geometries and specifications of the bendable DDA used in this study.

Table 3. Digital detector array (DDA) quality assessment results using gamma rays (Se-75).

▪ Radioactive isotopes: Se-75 (72 Ci) ▪ IQI: ASTM 1A, 1B EN 10 FE (ISO 19232-1) [9] ▪ Related standards: (1) ASME BPVC Section V: Nondestructive Examination (2) ISO 17636-2: Nondestructive testing of welds—Radiographic testing—Part 2: X- and gamma-ray techniques with digital detectors [10]
No.	Size/Type	Thickness (mm)	Material	IQI Requirement (mm)	Acquired IQI (mm)
1	2″ Pipe	3	Stainless steel	0.20 (DWDI)	0.20
2	2″ Pipe	5.5	Stainless steel	0.20 (DWDI)	0.20
3	4″ Pipe	13	Carbon steel	0.33	0.33
4	10″ Pipe	4	Stainless steel	0.16	0.16
5	12″ Pipe	15	Carbon steel	0.33	0.33

and

Table 4. DDA quality assessment results using gamma rays (Ir-192).

▪ Radioactive isotopes: Ir-192 (46 Ci) ▪ IQI: ASTM 1A, 1B EN 10 FE (ISO 19232-1) [9] ▪ Related standards: (1) ASME BPVC Section V: Nondestructive Examination (2) ISO 17636-2: Nondestructive testing of welds—Radiographic testing—Part 2: X- and gamma-ray techniques with digital detectors [10]
No.	Size/ Type	Thickness (mm)	Material	IQI Requirement (mm)	Acquired IQI (mm)
1	400 mm Plate	10	Carbon steel	0.25	Not detected
2	400 mm Plate	15	Carbon steel	0.25	0.20
3	400 mm Plate	20	Carbon steel	0.33	0.33
4	400 mm Plate	30	Carbon steel	0.40	0.51

summarize the DDA quality assessment results using gamma rays (Se-75 and Ir-192) and evaluate whether the image quality indicator (IQI) satisfies the quality standards through tests of varying thicknesses according to the types of pipe and plate.

The experimental results revealed that the image quality obtained with DDA was highly satisfactory when using Se-75. However, with Ir-192, the IQI requirements were met only at a thickness of 20 mm, making it unsuitable for other thicknesses.

This issue arises because the higher radiation dose from Ir-192 causes the DDA pixels to pass through without producing a dense image. The image particles from Ir-192 are also tougher than those from Se-75, affecting the DRT image quality.

Regarding these gamma ray characteristics, the International Organization for Standardization (ISO) provides an exception that slightly lowers the quality standard for certain thicknesses when using gamma rays and DDA, which aligns with our experimental results. Therefore, we used Se-75 to obtain stable image quality.

2.2. Selection of the Radiographic Examination Technique

RT testing of pipes using radiation is categorized into double-wall single images (DWSIs) and double-wall double images (DWDIs) based on the pipe’s outer diameter. As shown in Figure 1, the DWSI technique is used for medium and large pipes with an outer diameter of 80 A (3.5 inches or larger). The radiation source is placed close to the welded area, penetrates the first pipe wall, and images the weld on the opposite side [11].

DWDI is used for small pipes. Due to the small outer diameter, inspection is performed at a long source-to-film distance (SFD) on a flat elevated state without wrapping the film or DDA. This study focused on developing an inspection device for large pipes in the field and selected a technique that can automatically perform DWSI inspection.

2.3. Configuration of the Orbital-Rail-Type Digital Radiography Inspection Device

An inspection device that can automatically perform DWSI inspection using orbital rotation was developed based on the experimental results. This device consists of three parts: the mechanical part, the detector part, and the controller part (for powering and operating the device).

The mechanical part involves configuring the rails for easy installation and secure attachment to the pipes. The rails are adjusted to the pipe diameter and the fixing method minimizes the need for wrenches or spanners in the field.

Next, the optimal angle was set using a tungsten collimator to ensure accurate imaging of the radiation source in the digital detector array’s active area. The collimator directs the radiation source during inspection and blocks it elsewhere for safety. Details on the angle and design of the tungsten collimator are in Section 3.

The tungsten collimator, which is connected to the radiation source, is attached to a 10 m long source guide tube to prevent kinking and deviation during rotational inspection. The angle was determined to allow one collimator to cover multiple pipe diameters.

The inspection device was designed to be flat to fit the narrow spaces (100–150 mm wide) between installed pipes. The detector part includes a 10 mm thick solid lead-plate shield to prevent radiation leakage. The lead-plate shield covers the entire bendable DDA area and is designed as a slot type for easy attachment and detachment in radiation-permitted areas.

The controller, which operates the mechanical part, is configured to use a smart pad with wireless communication available in the DDA. The system determines the number of inspections needed based on the pipe’s thickness and diameter, memorizes each section’s location after inspection, and moves to that location for re-inspection.

Inspection results are sent in real time to a desktop or mobile device via wireless communication from the controller.

Figure 2 shows the device configuration and Figure 3 presents the inspection flowchart.

3. Development and Verification of the Inspection Device for Field Application

Handling radiation in open spaces without fixed facilities has many risks. The proposed inspection device, which automatically rotates and can be mounted on pipes, requires safety verification for radiation leakage and public accessibility.

To ensure radiation safety, we conducted an ALARA (as low as reasonably achievable) evaluation through simulation, after the design was completed, based on a theoretical framework [12,13,14,15].

$$H_{irradiating}(\raisebox{1ex}{$\mu Sv$}\!\left/ \!\raisebox{-1ex}{$h$}\right.)=\Gamma ·B·\frac{S}{D^2}{e}^{-ln2·{\sum }_i\frac{T_i}{{HVL}_i}}$$

(a)

Formula for calculating the theoretical radiation dose rate.

$${\mathit{D}}_{general area}(m)=\sqrt{\frac{\Gamma }{1\mu Sv/h}·B·S{e}^{-ln2·{\sum }_i\frac{T_i}{{HVL}_i}}}$$

(b)

Formula for calculating 1 μSv/h.

$${\mathit{D}}_{general area}\left(m\right)=\sqrt{\frac{2030\frac{\mu Sv·m^2}{Ci·h}}{\frac{1\mu Sv}{h}}·5·47Ci·e^{-ln2·\frac{10}{1.0}}}=21.60 \mathrm{m}$$

(c)

Ambient dose equivalent formula.

Table 5. Formula for predicting the half-value layer and theoretical radiation dose for each material using a Se-75 radiation source.

HVL (mm)	Concrete	Lead	Tungsten	Steel
Se-75	30	1.0	0.8	8

, which includes the equation for calculating the radiation dose rate from Se-75, can be interpreted as follows:

(a) The equation calculates the radiation dose rate using the gamma constant (Γ, rhm factor) and the source intensity (S) for Se-75 at a distance D (Distance) from the location of the dotted line source. This equation follows a negative exponential function (Exp) attenuated by the shielding material thickness (T) and the half-value layer (HVL) of the shielding material. The HVL value is provided in the Operating and Maintenance Manual from the manufacturer of the radiation source. (b) According to domestic law, the target dose rate is 1 µSv/h. This is the minimum dose rate requiring monitoring for the general public. (c) The verification of the radiation shielding performance of the inspection device developed in this study should include cross-validation through leakage radiation dose measurements by regulatory agencies [16].

Figure 4 presents the formula for determining the shield size based on the irradiation angle of the collimator. The source-to-detector distance (SDD) was 650 A (26 inches), the largest in actual piping applications, allowing shielding based on an outer diameter of 660 mm. Theoretical calculations determined that a tungsten collimator with a 30° × 15° angle required a minimum shield length of approximately 381 mm in the longitudinal direction of the weld line and 177 mm in the DDA width direction [17,18,19]. A shield measuring 400 mm × 200 mm was used. This study modeled and simulated the current inspection and device geometry to evaluate radiation leakage, ensuring that the penetration area of the radiation source and the lead-plate shield arrangement were within acceptable limits.

The verification of the radiation shielding performance of the developed inspection device should include cross-validation through leakage radiation dose measurements by regulatory agencies [16]. Figure 4 illustrates the formula for determining shield size based on the collimator’s irradiation angle. The source-to-detector distance (SDD) in this study was 650 A (26 inches), suitable for shielding based on an outer diameter of 660 mm in actual piping applications.

Theoretical calculations indicated that a tungsten collimator with a 30° × 15° angle required a minimum shield length of approximately 381 mm in the longitudinal direction of the weld line and 177 mm in the DDA width direction [17,18,19]. A shield measuring 400 mm × 200 mm was used.

In this study, we modeled and simulated the current inspection and device geometry to evaluate the leakage of radiation, ensuring that the radiation source penetration area and the lead-plate shield arrangement met acceptable standards.

3.1. Radiation Shielding Performance Comparing FLUKA Code and Field Measurements

Field Measurements Using a Survey Meter

The shielding area was measured at a designed collimator irradiation angle of 30° (along the weld length) × 15° (across the DDA width), as illustrated in Figure 5. After horizontally fixing the inspection device, radiation was clamped to directly measure leakage direction and intensity. Measurements were taken at intervals of 20° from 0° (position ①) to 180° at a distance of 12 m from the radiation source.

Figure 6 shows dose values exceeding the acceptable range in Sections ④ to ⑤ and Sections ⑤ to ⑥. Under conditions where the pipe is horizontal, the weld bead was positioned perpendicular to the ground, with the collimator’s irradiation angle at 15° across the DDA width at the measurement location.

3.2. Cross-Assessment of Measurements Data through MCNP Simulation

3.2.1. Computer Simulation Evaluation Methods

FLUKA (FLUtuierende KAskade) code, a particle transport simulation tool, was used to assess radiation leakage.

Table 6. Input parameters of FLUKA modeling and radiation energy calculation formula.

Division	Details
Input Parameter	Input Card	Input Condition
Physical setting	Default	Precisio
Source term	Beam	Type: Isotope/Momentum
	Hi-Prope	Type: Region Part: DOSE-EQ
Scoring	Usrbin	Type: Region Part: DOSE-EQ
	Auxscore	Set: AMB74
Transport	Emf-On	Type: Transport γ: 50 keV
NPS *	Start	No: 3.0 × 1010

* Number of particle history.

outlines the setup wherein “Precisio” was adjusted to validate simulation results against measured radiation leakage values. The ISOTOPE parameter reflected Se-75′s radiation emission characteristics.

The energy radiated per decay was absorbed by the measuring instrument. Effective dose rates were calculated using AMB74 and AUXSCORE (FLUKA Software, Joint development by CERN and INFN, http://www.fluka.org) to compute dose equivalents. Secondary radiation, emitted from primary gamma rays, was measured up to 50 keV. The simulation involved 3.0 × 10¹⁰ particle histories to achieve a dose rate of 1 µSv/h. Dose distribution values were obtained using a dose map scored by Region Type [20,21,22].

$$\mathrm{E}=\frac{GeV·Volume}{g·decay}$$

Depending on the distance and survey location

(a)

Energy per decay in the ion chamber (E).

$$\mathrm{D}(\mathrm{G}\mathrm{y}/\mathrm{h})=\frac{GeV·Volume}{g·decay}\times \frac{1.6\times {10}^{-10}\mathrm{J}}{GeV}\times \frac{{10}^3\mathrm{g}}{\mathrm{k}\mathrm{g}}\times \mathrm{S}(\mathrm{C}\mathrm{i})\times \frac{3.7\times {10}^{10}decay}{Ci·s}÷\frac{3600\mathrm{s}}{\mathrm{h}}\times \frac{1}{ionChamberVolume}\times \frac{\mathrm{G}\mathrm{y}}{\mathrm{J}/\mathrm{k}\mathrm{g}}$$

S = Source Intensity

(b)

Absorbed dose in the ion chamber (D).

$$\mathrm{H}=\mathrm{D}(\mathrm{G}\mathrm{y}/\mathrm{h})\times \frac{1.2Sv}{Gy}$$

(c)

Ambient dose equivalent (H) formula.

3.2.2. Geometry Modeling

Table 7. Modeling geometry to perform simulations.

Type	Details
Blackhole area	Size	100 m × 100 m × 20 m
Evaluated area	Size	50 m × 50 m × 20 m
Shielded for Radiography Testing	Size	200 mm × 380 mm
	Thickness	10 mm (Lead)
Pipe (for inspecting)	Dia.	500 A (out. 558.8 mm)
	Leng.	10 m
	Thickness of shielding material	5.5 mm (Iron)
Source	Intensity	47 Ci
	Active Source Size (Dia.)	3.0 × 3.0 mm
	Energy Spectrum (peak)	97, 121, 136, 264, 279, 401 keV
Collimator	Angle range of irradiation	30° × 15°
	Size	Leng.	72 mm
		Depth.	41 mm
	Thickness of shielding material	21 mm (Tungsten)

outlines the material characteristics simulated for radiation safety assessment. The actual tungsten collimator was modeled based on manufacturer-provided drawings and physical measurements, detailed in Figure 7.

The inspection device geometry and radiation shielding were modeled in an open environment to simulate real-world radiographic testing conditions. Measurements were conducted using Se-75 with a maximum intensity of 47 Ci, applicable for field inspection scenarios [16,23,24,25,26].

3.2.3. Verification and Comparison with Field Measurements Using FLUKA Simulation

The simulated geometry closely resembled the actual device, revealing a similar dose distribution trend at a 15° angle from the tungsten collimator when leaking from a horizontal pipe position, as depicted in Figure 8.

Analysis of the dose map along the X–Y and Y–Z axes showed scattered irradiation extended approximately ±11 m in the X direction and over 52 m in the Y direction from the viewpoint. The radiation scattering angle confirmed by simulations was approximately 26.3° from the center. These findings suggested that the initially designed shielding range should be expanded. Considering the 26.3° angle distribution around the center, the minimum required shielding width was determined to be 274 mm.

3.3. Complementation of the Shielding Area of Inspection Equipment

Geography Re-Modeling and Design

Figure 9 illustrates the redesigned shielding area to accommodate a 15° wide irradiation angle from the tungsten collimator. Efforts were made to minimize the space between the pipe, DDA, and shielding device.

Additionally, lead rubber, with a thickness of 0.35 mmPb per sheet, was used instead of lead plates to enhance flexibility for rotation and adherence in areas needing improvement. The shielding design was adjusted to account for asymmetrical heights near the motor. Flexible lead rubber ensured smooth rotation without interference. An additional 70 mm was allocated for the straight section and 53 mm was allocated for the folded section due to the rotating motor.

3.4. Comparison of Simulation Results and Field Measurements in Supplemented Shielding Conditions

After adjusting the shielding conditions in the simulation, radiation dose values leaking in the width direction were reduced as shown in Figure 10. The target dose of 1 µSv/h could now reach a maximum distance of 26 m.

This was about 5 m less than the value initially conservatively estimated by the theoretical equation, indicating a close approximation considering the analysis error rate and theoretical calculation.

Using an improved inspection device depicted in Figure 11, radiation leakage doses from a field-installed pipe were examined. The method involved dividing the area into 20° sections around the radiation source and using a portable survey meter for measurements.

Results showed significantly reduced radiation leakage doses compared to initial levels, with the distance reduced from 55 to 26 m. Consistent radiation doses were measured across the section, aligning closely with theoretical predictions and simulations indicating similar or lower doses than the target.

In this study, we developed a high-quality automatic radiography inspection device with stable and effective shielding, validated through comparison of actual leakage measurements with simulations based on conservative theoretical formulas [27,28,29,30].

4. Conclusions

In this study, we developed an automated device for real-time internal defect detection in pipeline welds by combining passive RT techniques using conventional films with digital RT technology.

Furthermore, this study developed an automated inspection device using DRT technology, facilitating seamless integration with automation for real-time feedback and the automatic detection of internal defects in welded pipe sections. Conservative theoretical models and FLUKA simulations were employed to predict the path and intensity of radiation scattering, enabling the development of a device capable of automatically inspecting hazardous radiation sources on the circumferential welds of pipes.

The findings of this study were as follows:

• Using gamma rays instead of X-rays offers advantages for placing inspection equipment in narrow spaces between pipes. However, gamma ray radiography quality is generally inferior to that of X-rays. Specifically, the Se-75 radiation source can produce high-contrast images for thinner materials compared to Ir-192 and meets DRT quality standards.
• The theoretical calculation results from the conservative approach and the FLUKA simulation results differed by approximately 5 m from the safety distance and the error rate was small. However, concerning the leakage angle of the tungsten collimator through micro processing and the theoretical shielding area, the actual measured radiation dose and safety distance differed considerably. This phenomenon could be attributed to differences in the amount of scattering in the transmission path of radiation and different variables such as absorption and reflection.
• When applying the atomic regulation authority’s conservative safety constant in the theoretical formula, the corresponding distance was 21.6 m if the target dose of 1 μSv/h was satisfied. Because of using the inspection device with the supplemented shielding conditions through FLUKA simulation to test pipes installed in the field, a value of approximately 2.15 μSv/h was measured at 12 m. When converting this value using the inverse square law, the target dose of 1 μSv/h could be satisfied at approximately 17.5 m. This phenomenon could lower dose values measured in the field when compared to those in the simulation results because of the additional shielding effect of the surrounding structures.
• Through this study, we developed equipment capable of automatically inspecting welds by rotating them using radioactive sources. To further develop the equipment in the future, the following research directions and needs were identified:

  (1)

  Prolonged exposure to radiation in Digital Detector Arrays (DDAs) can introduce noise that may adversely affect image quality. Although this study confirmed satisfactory performance for pipe thicknesses around 10 mm, it is necessary to assess potential degradation in image quality with pipes of maximum thickness.

  (2)

  X-rays are essential as radiation sources that can produce the optimal quality of digital radiography tests. It is expected that there will be a need to develop inspection devices using small-sized X-rays that can be integrated into the equipment.

  (3)

  Lastly, it is expected that the development of an automatic inspection device using a video-type inspection method will be necessary to improve inspection efficiency. There will still be difficulties due to the image distortion of test results but research to overcome these and establish optimal conditions is expected to continue to be necessary.