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CHAPTER 8 INDUSTRIAL RADIOGRAPHY SUMMARY Gamma-ray radiography is one of a number of technologies used in industry for safety assessment and quality control purposes. In particular, it is widely used in the chemical, petrochemical, and building industries for radiographic inspection of pipes, boilers, and structures where the economic and safety consequences of failure can be severe. With ongoing developments in ultrasonic inspection technologies as well as in x-ray radiography, satisfactory alternatives for many of the gamma-ray radiography applications exist. There are, however, a number of specific applications, such as pipeline inspection in remote locations, underwater, and in chemical plants with wrapped or closely spaced piping, where the advantages of mobility, ease of use, and low power requirements of radionuclide-based radiography may make replacement difficult and uneconomic, at least with current alternative technologies. Although gamma-ray radiography is generally performed with Category 2 iridium-192 and cobalt-60 sources, their portability and use in remote locations poses higher exposure risks than other, equivalent-activity radionuclide sources. Improved detector technology could enable the use of lower activity sources with consequently decreased risk. The committee judges that except in some specialized applications, alternative inspection technologies are already increasingly replacing gamma-ray radiography in industry. In some instances, such as ultrasonic inspection, the replacement rate is currently limited by the availability of trained personnel. NONDESTRUCTIVE INSPECTION Radiography, using radionuclide sources and x-ray sources, is one of a number of techniques developed for the nondestructive inspection (NDI) of structures, such as pipes, and components manufactured by industry today. Radiography and other NDI techniques have become indispensable in many industrial sectors for safety assurance purposes because they have the capability to detect the presence of manufacturing defects, such as incomplete welds and porosity in the walls of tubes, as well as cracks and other flaws that develop during service. Because defects can compromise safety and lead to failure, NDI is governed by standard codes of practice in many industrial settings and may be mandated by law. For instance, pipelines, such as the Alaska pipeline, are constructed from welded sections, and the welds joining them are inspected during construction and periodically during their operating lives to check for cracks and corrosion. Similarly, pipes in chemical plants and refineries are routinely inspected during the construction of the plant and periodically during service. The construction industry also relies on NDI when building with steel-reinforced concrete: The integrity of rebar reinforcements during building construction is usually inspected to ensure that they are continuous and intact. In fact, NDI by radiography is practiced in an enormous range of applications and industries and has grown to be a major industry in its own right. Radiation sources are used widely for examining gas and oil pipelines, pipes and pressure vessels in chemical plants, vehicles, and aircraft, and although the radionuclide sources provide some undeniable benefits, they also pose some risks. These are applications in which the 135
136 RADIATION SOURCE USE AND REPLACEMENT convenience, small size, and mobility of radionuclide radiography sources make them particularly attractive to the service industry. The small size and mobility of the sources also make them more vulnerable to seizure than many other types of radioactive sources considered by the committee. Industrial radiography, whether by x-ray radiography or gamma-ray radiography, is used to detect defects, for instance, in much the same way as dentists use x-ray radiography to check for decay in teeth. Transmission of radiation through a component produces an image on film that is readily interpretable, revealing spatial and density variations. These images are traditionally recorded on sheets of film, although increasingly film is being replaced by solid- state detectors. The process is illustrated in Figure 8-1. When there is a discontinuity in the material, for example, a void or crack, the radiation absorption through that section is reduced and more radiation passes through the material, giving rise to contrast on a film or a variation in signal on a detector. Indeed, radiography is often referred to as a volumetric inspection because it gives a projected picture of the internal structure of a component. The thickness of a component that can be inspected and through which an image can be recorded depends on the shielding properties of the component material and the energy and intensity of the radiation. The shielding properties depend on the atomic mass of the component material and the energy of the radiation. Lead shields radiation very strongly (it has a high atomic number), allowing inspection of only thin components. Concrete is a much less effective shield for gamma and x-rays because it is composed of lower-atomic-number elements and so can be examined in thick sections. Similarly, plastics are even less effective as shielding materials and so still thicker sections can be inspected. Higher energy gamma and x-rays have greater penetrating capabilities. Many components and structures are made of materials such as concrete, steel, and other metals, and quite often composites of these materials. If one uses too much energy for a given application, the film or detector can become saturated or contrast can be lost. Industrial radiographers would like to use sufficiently high energy to penetrate the part and form an image in a reasonable amount of time but not so much to lose contrast (because low energies and high absorption increase contrast) or saturate the detector. The most commonly used sources are iridium-192 and cobalt- 60. Iridium-192 emits gamma rays with a range of energies up to 820 keV (averaging 380 keV) and cobalt-60 emits two gamma rays, one at 1.173 MeV and one at 1.333 MeV. The peak energy is important because it corresponds to the most penetrating gamma rays. The typical activity of these sources makes them Category 2. For certain specialized applications, selenium-75 and ytterbium-169 are also used. These radionuclides are listed along with their half-lives and the average energy of their gamma rays in Table 8-1. FIGURE 8-1 Schematic of the internals of a portable, hand-held radiography camera. Such a system typically uses iridium-192 as the gamma source. SOURCE: Image provided by the committee.
INDUSTRIAL RADIOGRAPHY 137 TABLE 8-1 Properties of Gamma-Emitting Radioactive Isotopes Most Commonly Used in Industrial Radiography Average Gamma Energy Radionuclide (keV) Half-Life Cobalt-60 1,250 5.3 yr Iridium-192 380 75 d Selenium-75 217 120 d Ytterbium-169 145 32 d SOURCE: Table provided by the committee. One parameter widely used in the industry to measure the penetration of gamma- and x- ray radiation having a given energy is the half-value thickness, which is the thickness at which the radiation intensity has decreased to one-half through absorption and scattering processes. The half value is dependent on the energy of the incident irradiation and the density of the material. Figure 8-2 shows the half-value thickness for several common shielding materials. Radionuclide Radiography Sources The majority of mobile gamma-ray radiography devices use iridium-192 sources, although there are a significant number of radiography devices using cobalt-60 sources. These latter sources require more shielding. Consequently, they are heavier and are not usually hand- held devices but nevertheless are mobile when mounted on a trolley. A photograph of a hand-held device is shown in Figure 8-3 and the internal structure is shown schematically in Figure 8-1. When the device is not in use, including during transport, the radiation source is contained within the camera in a depleted uranium shield so that external radiation is reduced to safe levels. In operation, to record a radiographic image, the source is moved from the shielded region through a tube to the collimator/end stop, allowing gamma rays to pass through the test item, such as a pipe, exposing the film to form a radiograph. When the radiograph is complete, the source is retracted into the shield and the device moved to make another inspection either in the same location or driven to another site. Nature of the Industrial Gamma-ray Radiography Industry The industrial gamma-ray radiography industry is highly diverse and consists of a large number of individual companies. These range from relatively large companies offering a wide variety of NDI tools and services to small operators specializing, for instance, in gamma-ray radiography for in-field pipeline inspection. Furthermore, the relatively low cost of gamma sources, their portability, straightforward image interpretation, and simple radiological safety measures makes entry into the market relatively inexpensive for small companies. Also, the use of portable gamma sources allows radiography to be carried out in remote locations where electric power may not be readily available.
138 RADIATION SOURCE USE AND REPLACEMENT 102 Half-Thickness (cm) 10 1 cobat-60 10-1 Legend Lead selenium-75 10-2 Iron Cadmium ytterbium-169 iridium-192 Copper Aluminum 10-3 LUCITE 10-4 10 30 60 150 400 800 2000 5000 10000 Energy (keV) FIGURE 8-2 The half-value thickness (thickness that shields one-half of the incident photons) is shown for several common shielding materials. Each dashed line indicates the approximate location of the average energy for the gamma rays emitted by the radionuclide with which it is labeled. Note that the actual half-value is slightly higher for radionuclides that emit some higher-energy gamma rays, such as iridium-192. SOURCE: Adapted from ORTEC (2007). FIGURE 8-3 Radiography cameras. Note that the hand-held device is an iridium-192 radiography camera, as is the sealed source mounted on the end of the cable shown to the right. The device on wheels is a cobalt-60 radiography camera. SOURCE: Images provided by the committee.
INDUSTRIAL RADIOGRAPHY 139 Safety Issues During use, when the source is outside its storage shielding, it can create radiation fields in which permissible occupational dose standards can be exceeded in a short period of time. Data indicate that occupational exposures received by gamma radiography workers are among the highest of all radiation workers in the United States. The average total effective dose equivalent (TEDE) for personnel working in gamma radiography in 2005 was 5.2 mSv (520 mrem), compared to 1 mSv (100 mrem) for workers in commercial nuclear power reactors. If the devices are not handled properly, the radiation sources can cause radiation overexposures and radiation burns. Accidents occur primarily when the radiographer does not return the source to the fully shielded position and fails to perform a survey to confirm the source is in a fully shielded position. Those performing radionuclide radiography are subject to much greater radiation exposures than those in the medical irradiator sectors, where exposures are not routine parts of the job. Occupational doses to workers using radiation generators are much lower. Radiation generators are typically placed in shielded rooms or bunkers and can be turned off in an emergency situation. Further, portable radiation generators can be turned off when they are not in use. The safety concerns associated with industrial radiography also have economic ramifications. For instance, the immediate vicinity of locations where gamma radiography is being performed has to be cleared of other personnel. At sites where there are other workers, it is not unusual for other workers to have to stop work while the radiography is carried out. This is a cost and one of the factors that can influence the choice of nonradioactive methods of NDI because for some alternatives the area does not need to be cleared and other work can continue uninterrupted. Regulatory Framework There are two regulatory frameworks that govern industrial radionuclide-based radiography. One governs the use (through licensing) and the other governs the transportation of the radiography machines containing radionuclide sources in addition to those described earlier. In addition, transportation of portable radionuclide sources that are used in the field is regulated by Department of Transportation regulations contained in 49 CFR Parts 100-185 and U.S. NRC transportation regulations contained in 10 CFR Parts 20 and 71. Technicians using radioactive sources are certified by either state programs or the American Society for Non- Destructive Testing. These certification programs comply with 10 CFR Part 34âLicenses for Industrial Radiography and Radiation Safety Requirements for Industrial Radiographic Operations. Because of safety concerns, industrial gamma-ray radiography sources are considered to be a âspecial formâ and must meet âspecial form requirementsâ: (1) the radionuclide source must be contained in a solid piece or a sealed capsule that can only be opened by destroying the capsule, (2) at least one dimension of the capsule must not be less than 5 mm, and (3) the source must satisfy the specific requirements of 49 CFR Â§ 173.469 that the source capsule does not break, melt, or leak after being subjected to a variety of prescribed impact, deformation, leaching, and high-temperature exposure tests. The other regulatory framework is much more complex and concerns the accepted use and methodologies of NDI techniques for specific applications. These are embodied in codes that, as in many other industries in the United States, and internationally, cover the design, manufacture, inspection, insurance coverage and qualification, and operation of its systems. For
140 RADIATION SOURCE USE AND REPLACEMENT some applications, for instance in assuring that welds are defect-free, permanent retention of the radiographic images is sometimes also a legal requirement. Many of the codes covering industrial radiography, using radionuclides and x-ray sources, pertinent to the committeeâs charge have been promulgated by the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code and by the American Petroleum Institute (API). The former establishes the rules of safety governing the design, fabrication, and inspection of boilers and pressure vessels and nuclear power plant components during construction to provide a margin for deterioration during service. Radiographic examination has been permitted by the ASME Boiler and Pressure Vessel Code since 1931. The API codes govern the manufacture of oil and gas steel pipelines, how they are assembled by welding, and their inspection. The codes of practice not only prescribe the use of specific inspection techniques but can also affect the choices made in selecting alternative inspection techniques. In some cases, radiography using radionuclide sources is specified, whereas in others radiography is described without specifying whether the source is a radionuclide or an x-ray machine. In the opinion of the committee, changes could be made by the governing bodies of the codes after appropriate representation of the merits and viability of alternative inspection modalities to radionuclide radiography. However, there is also a structural inertia in the adoption of alternative methods associated with the way in which codes are used. This can be illustrated by the hypothetical example in Figure 8-4 in which a refinery plant operator contracts to have its plantâs pipes inspected. In this example, companies that offer inspection services respond with a quotation for the service. Unless specified by the plant operator, the service company typically selects a technique for the inspection based on its expertise and the practices that its insurance carrier will accept. In turn, the methods that the inspection company can employ must comply with the technical codes of practice. The inertia in replacing radionuclide radiography sources with a technically viable alternative largely comes from the time it takes for the technical committees of the code-making organizations to consider and change the code if it decides it is appropriate. Alternative Technologies There are many NDI techniques in use today, ranging from eddy current testing, acoustic emission, and radiography to magnetic induction and ultrasonics. Table 8-2 lists commonly used methods of nondestructive testing and their effectiveness in detecting particular kinds of defects, relevant to the inspection of pressure vessels and pipes where radiography has traditionally been a principal tool. Refinery owner issues RFQ for inspection of pipes Insurance carrier stipulates Service company responds acceptable methods with quote based on technical codes Service company selects NDI tools Codes based on discussions In technical community FIGURE 8-4 Process for selecting nondestructive testing method on the left, and factors that influence or dictate the process on the right. NOTE: RFQ = Request for quote. SOURCE: Provided by committee.
INDUSTRIAL RADIOGRAPHY 141 TABLE 8-2 Common Imperfections and the Nondestructive Examination Methods That Are Generally Capable of Detecting Them SOURCE: Reprinted from ASME 2004 Boiler and Pressure Vessel Code, Section V, by permission of ASME. All rights reserved.
142 RADIATION SOURCE USE AND REPLACEMENT The most direct substitute for radionuclide radiography is the use of x-ray radiography because x-ray sources can produce radiation over a range of different energies, including those generated by the radionuclides, to perform the same inspection. In many industrial applications, such as inspection of high-value components, electronic circuits, and devices (spark plugs, for instance), x-ray radiography is already becoming more widely used as a quality control tool in manufacturing, especially with the development of compact, microfocus x-ray tubes. These industrial applications typically occur in factory settings where components can be inspected in a fixed location and the higher cost of x-ray facilities is outweighed by their increased productivity, improved imaging resolution, or higher energy for greater penetration. Advances in compact, high-energy x-ray sources as well as recent developments in computerized x-ray tomography, which can provide three-dimensional images of a component, promise to extend the use of x-ray radiography to applications outside factory settings. Already a number of companies around the world offer x-ray radiography inspection services in place of gamma-ray radiography for pipe inspection. A number of companies in the United States and elsewhere provide compact linacs and betatrons to replace the radionuclide sources (see Figure 8-5 and Chapter 4). For inspection of larger components and structures, such as boilers, pipelines, and bridges, other techniques (principally ultrasonic inspection) have been gaining ground on radiography. One of the principal reasons is that radiography only provides a projection image and is relatively insensitive to features such as thin cracks aligned perpendicular to the beam, which give poor contrast. Figure 8-6 illustrates a radiography source in use examining a component that has a thin crack perpendicular to the radiation tracks and a thin crack parallel to the radiation tracks. A cartoon of the exposed film can be found in the lower right-hand corner of the figure, showing that the defect aligned parallel to the gamma-ray paths is detectable and the defect perpendicular to the radiation is not. This is especially important in the inspection of complex-shaped components where defect identification is difficult in the presence of variations in thickness associated with their shape. In contrast, the use of ultrasonics enables images to be formed in different conditions to increase the likelihood of detecting flaws. FIGURE 8-5 Photograph of an Inspecta betatron particle accelerator hanging in a harness and used for nondestructive examination. The x-ray energy can be varied from 2 MeV up to 6 MeV, which allows the radiographer to examine thick sections of steel or concrete. SOURCE: Copyright Inspecta.
INDUSTRIAL RADIOGRAPHY 143 Gamma rays Defect parallel to radiation beam Film Developed Radiograph Radiation source Defect parallel to Defect perpendicular radiation beam to radiation beam Defect perpendicular to radiation beam FIGURE 8-6 Cartoon of radiographic examination of a component with a parallel defect. SOURCE: Image provided by committee. Ultrasonic testing dates back to the work of S. Y. Sokolov, a Russian scientist, in 1929. Sokolov conceived of using high-frequency, highly directional sound waves propagated through a component to detect the presence of defects by their interaction with the sound waves. Pulses of ultrasound are generated by a piezoelectric transducer which is placed on the component. The transmitted and reflected waves propagating through a component are detected with one or more sensors also attached to the component. By placing the transmitter and detector(s) at different locations, a series of images can be recorded. Features and discontinuities that are in poor contrast in one image can be observed in another image. In this respect, the technique is identical to the formation of medical sonograms used to visualize the developing fetus in utero, among many other applications. The ultrasonic images of components are generally more complex than medical sonograms because of elastic scattering (bouncing or reflection of sound waves off interfaces with significant density differences). However, depending on the techniques used, the output from ultrasonic sensors may not be a direct visual representation of the volume of the component, and so may be less readily interpreted than projection images such as radiographs (see Figure 8-7). Consequently, ultrasonic testing requires more skilled, specially trained personnel to interpret its results. There are several characteristics of ultrasonic waves that can be utilized in addition to simple absorption used in radiography, including frequency, polarization, and phase. Major breakthroughs in the development of ultrasonic testing have included transducers that generate only shear waves, enabling other types of discontinuities to be detected; the development of time-of-flight diffraction which allows the top and bottom edges of discontinuities to be displayed and provides better accuracy in through-thickness measurement; and phased-array testing, which produces images of internal structures similar to that of medical ultrasound.
144 RADIATION SOURCE USE AND REPLACEMENT (a) (b) FIGURE 8-7 (a) Radiograph of a metal tube inside another larger metal tube. (b) Typical output from ultrasonic testing (of a different component) illustrating six panels of information obtained from the same component. SOURCE: (a) Blettner, A., et al. (2000), (b) Creech, M. (2006). Over the past two decades, these and other advances in computational imaging and electromagnetic acoustic transducers have made ultrasonic techniques a viable alternative for many weld and structural inspection purposes. Already they are competing with and replacing radiography in many applications, especially in the manufacturing and quality control sectors. For instance, ultrasonic techniques are used along with radiography for inspection during manufacture of large steel pipelines used in the petroleum industry. Ultrasonic techniques are used for the inspection of critical components such as castings that are much too thick for radiography. Indeed, large metal castings or forgings, such as gas turbine rotors, can presently be examined only by ultrasonic techniques. There is also an increasing trend toward the use of ultrasonics for in- service inspection. In the railroad industry, for example, ultrasonic techniques are preferred because they are less expensive than radiography. There are also many cases in which in-service inspection by radiography is impossible, because it requires that the radiographic plate (film or
INDUSTRIAL RADIOGRAPHY 145 detector) be placed inside an operating component. In many of these applications, ultrasonic techniques are used instead. The costs of devices that can replace current gamma radiography in some applications vary a great deal, reflecting differences in the specifics of the inspection system, pricing for the customer, and how the system is used. Some suppliers are reluctant to offer prices for quotation in a public report, and so the figures listed here are anecdotal prices cited by customers. Portable pulsed x-ray radiography systems begin at approximately $50,000 and go up in price from there. Moveable accelerator-based radiography systems begin more in the range of $200,000. Ultrasonic systems typically range from $50,000 to $100,000. All of these cost more in maintenance and operation than gamma radiography systems. FINDINGS There are important and high-value applications where the simplicity, ease of inspection, and portability still favor radiography or where radiography cannot easily be replaced. These include the inspection of insulation-wrapped pipes at high temperatures, where contact methods such as ultrasonics are either not feasible or not viable economically because it would entail stripping the wrapping and cooling the pipes. Another example is in the underwater inspection of oil and gas pipelines where remote operation is essential and electrical power severely limited. Similar considerations also apply to the inspection of oil and gas pipelines in unshielded and remote locations on land. Nevertheless, by some estimates provided to the committee (Creech, 2006), about 50 percent of industrial radiography performed today could be performed by ultrasonic methods, and a further 25 percent could be inspected using x-ray radiography as opposed to gamma radiography. With the prospect of continuing advances in computational resources, low-cost sophisticated analysis software, and image-matching algorithms, it is likely that ultrasonics and x-ray radiography (as well as other inspection technologies not discussed in this report) will further reduce the fraction of component inspections by gamma radiography. Phased-array detector technology and the other technologies mentioned are being developed for other applications. However, despite these trends, the rate at which gamma radiography is replaced may well be paced by the present, limited availability of personnel skilled in other methods, especially ultrasonics, as well as regulatory factors discussed above.