6

Radioactive Sources and Alternative Technologies in Industrial Applications

This chapter concerns industrial applications that use radioactive sources or alternative technologies other than for sterilization applications, which are described in Chapter 5. These are industrial radiography, industrial gauges, well logging, calibration systems, and radioisotope thermoelectric generators (RTGs). Notably, this chapter does not discuss material modification, which uses radiation from radioactive sources, x-rays, or electron beams (e-beams) to alter the properties of materials for various commercial applications with the main application being cross-linking of polymer chains for products such as wire insulation, automobile tires, and latex gloves.¹ This is because only a small fraction (estimated to be less than 10 percent) of the global supply of cobalt-60 is used for this application² and therefore is not a driver in decisions for adoption of alternative technologies.

6.1 INDUSTRIAL RADIOGRAPHY

Industrial radiography has been used for more than 50 years and is an essential tool of nondestructive testing (NDT) for safety assessment and quality controls in many industries. According to an industry representative who briefed the committee, there are more than 10,000 radiography sources sold globally per year, with about 4,000 of those sold into the U.S. market. There are more than 1,000 licensees of radiography cameras in the United States.³

Industrial radiography relies on transmission and absorption/attenuation of short-wavelength electromagnetic energy (gamma-ray photons and x-rays) to visualize structures such as welds and castings for internal defects or porosity; gas and oil pipes to detect blockage, corrosion, and pipe wall thickness; industrial structures to ensure that there are no cracks or blockages; and aircraft and automobile parts for defects. A gamma radiography camera or x-ray tube directs a beam of gamma rays or x-rays at the item being tested, and a detector (film or electronic) that is lined up with the beam on the other side of the item records the gamma rays or x-rays that pass through the material. The number of photons that pass through the material is proportional to its thickness and density. Because the material is thinner or less dense where there is a crack or a flaw, more photons pass through that area. The detector creates an image from the rays that pass through, called a radiograph, which shows cracks or flaws. Radiography also shows differences in material density. The presence of a metal inclusion in a plastic sample, for instance, is identified with radiography whether or not the material has different thickness at that point. Also, a crack

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¹ See International Atomic Energy Agency, Material Modification, at https://www.iaea.org/topics/material-modification.

² Ian Downie, Nordion, presentation to the committee on October 13, 2020.

³ Mike Fuller and Mark Shilton, QSA Global, Inc., presentation to the committee on October 13, 2020.

or defect within a material may not result in a measurable thickness difference in the material but may produce a difference in photon absorbance and scattering relative to the intact material that is revealed with radiography. It is essential for the source to emit energy that can penetrate the examined material and result in an image that has adequate contrast and definition for either processed film or a digital image.

Some radiography is performed in shielded enclosures or vaults to protect the operator and the public from radiation exposure. Most frequently, radiography is performed at remote field locations, requiring transport of the source and detector/film to the location, typically in a mobile darkroom truck. For example, to inspect new oil or gas pipelines, a sensitive film is taped over the weld around the outside of the pipe. The radiography camera is positioned either externally or internally (via a pipe crawler) to the pipe, and a radioactive source travels to the position of the weld. When in position, the radioactive source is remotely exposed and a radiographic image of the weld is produced on the film, which is later developed and examined for signs of flaws. Other examples where industrial radiography is performed in the field include oil refineries, chemical plants, offshore platforms, lay barges, storage tanks, pressure vessels, pipelines, bridges, and buildings.

6.1.1 Radioisotope Technologies

Most radiography cameras use iridium-192, but a significant number of radiography devices use cobalt-60 or selenium-75. The chosen radioisotope depends on the material to be radiographed and its thickness. The higher-energy gamma rays of cobalt-60 are normally used for thicker sections of steel, from one to several inches. Iridium-192 is used for steel up to 2.5 cm in thickness, and selenium-75 is used for lighter and thinner metals. Radiography sources are typically Category 2 sources in the International Atomic Energy Agency (IAEA) system. Table 1.2 shows the half-life and radioactive emissions and energies of these radioisotopes, and Table 6.1 summarizes the basic characteristics of industrial radiography cameras based on the particular radioisotope.

Gamma radiography cameras are constructed of a steel casing, typically welded closed, that encases a depleted uranium, tungsten, or lead shield. Depleted uranium (DU) is normally used as shielding for iridium-192 due to DU’s high density and ability to safely shield the high-activity sources with the external dose rates meeting regulatory requirements. Tungsten is typically used for shielding selenium-75 sources. The source is attached to a short wire or a short flexible cable often called a “pig tail” (see Figure 6.1) that positions the source in the shielded position and allows for the source to be securely locked in the stored position.

A gamma radiography camera can be either projector style or directional style (see Figure 6.2a–c). In a projector-style camera, the source is projected out from the shielded position of the camera and travels in a guide tube to the desired position and then is returned to the shielded position at the conclusion of the exposure time. In a directional-style camera, the source does not leave the device, but is moved out of its shielded storage position to a point where the source can expose an object in a limited direction.

TABLE 6.1 Basic Characteristics of Industrial Radiography Cameras Based on Radioisotope

^a From ASNT, 2019.

Conventional radiography cameras have a compact physical envelope for iridium-192 and selenium-75 sources that require less shielding and are thus lighter than cobalt-60 source cameras. The radiography cameras also do not require any electrical power, which makes them appealing to several industries because most inspections happen in outdoor remote field locations. Gamma radiography cameras can operate under challenging climatic and physical conditions. Most industrial radiography cameras are designed to withstand normal and accident conditions in use in accordance with the applicable standard (ISO 3999) and during transport based on guidance by the IAEA (2018c). Under this guidance, the cameras must meet stringent test requirements such as 9-m drop test and an 800°C thermal test. These requirements make the gamma radiography camera robust and well suited for use in field applications.

The small size of conventional gamma radiography cameras (notably those using iridium-192 and selenium-75 sources) makes them easy to transport and to handle at job sites (see Figure 6.3a–c) that have physical challenges such as working at height, muddy or dusty conditions, and extreme temperatures. They can be easily maneuvered around facilities, and they can move through small-diameter pipes to make radiographs without difficulty. However, they have safety and security disadvantages because the cameras house a high-activity Category 2 radionuclide, and they are frequently transported and often used in remote sites with no special security measures in some countries. When the amount of radioactive material is substantial, as with industrial radiography sources, accidents could have severe or even fatal consequences (Coeytaux et al., 2015; IAEA, 1998). Thousands of these cameras are in use or in transport around the world at any time.

Little has changed in gamma radiography over the past 10 years; however, some changes in equipment design have improved safety of operation. Small Controlled Area Radiography (SCAR) is a directional gamma radiography system that uses a lower-activity radioactive source (selenium-75) within a compact exposure device with built-in collimation to enhance the ability for radiographic inspection with far less potential whole-body dose and to reduce the radiation safety exclusion zone to a few meters in diameter instead of up to 100 m (see Figure 6.4a–b). It also allows other work to take place on the site because the radiation scatter is lower. SCAR is used in locations where the area to perform radiography is relatively small, such as on oil platforms. The SCAR technique is more popular in other countries than in the United States. This is likely because historically, other countries have used lower-activity sources for performing industrial radiography than the United States because the annual dose limit to the worker is 2 rem (20 millisievert [mSv]) as opposed to the U.S. limit of 5 rem (50 mSv). The activities of sources commonly used in the United States are 100–150 curie (Ci) (3.7–5.6 terabecquerel [TBq]) of iridium-192, whereas the activities of sources used in most other countries are in the range of 20 to 50 Ci (740 gigabecquerel [GBq] to 1.85 TBq) to limit personnel exposure.

6.1.2 Alternative Technologies

While both x-ray and gamma-ray photons can produce a similar image quality on a radiograph that is required by the industrial code, the equipment itself is vastly different. In contrast to radionuclides, x-ray devices generate a continuous range of photon energies up to a certain maximum depending on the operating voltage. Conventional x-ray devices generally require 220-V power, which can be challenging to supply in the field. They also require a cooling system, and they are too large to move through and around pipes and other infrastructure. Most x-ray systems are more suitable for permanent installation work and cannot be used at most temporary job sites economically because of their size, weight, and accessibility and electrical power requirements, and they usually require expensive scaffolding. In comparison, the source guide tubes used in gamma radiography cameras can be introduced into extremely small, confined areas to produce radiographic images.

X-ray systems are typically not as robust in challenging physical environments as the gamma industrial radiography cameras. The x-ray tube is more likely to sustain damage in the field environment. Therefore, while x-ray sources are suitable for use in a fixed facility, they have not been the preferred option for radiography in field locations. However, advancements in x-ray design have led to the development of pulsed x-ray sources that operate using battery power and have physical size close to that of a radioactive source housing (Light, 2008). Pulsed x-ray units have greatly improved in portability and ruggedness over the years and could be a viable alter-

TABLE 6.2 Comparison of Performance Between Automated Ultrasonic Testing (AUT) and Gamma Radiography for Different Types of Defect

SOURCE: Modified from Mike Fuller and Mark Shilton, QSA Global, Inc., presentation to the committee on October 13, 2020, to include x-ray.

native in some remote locations (Golden, 2014). They have some advantages over traditional gamma radiography, such as reduced exposure times and the need for a smaller controlled areas. However, the need for frequent costly replacement of the tubes and for frequent use still make gamma radiography the more reliable and preferred option.

As noted in Chapter 4, RadiaBeam, a small business recipient of the National Nuclear Security Administration’s Small Business Innovation Research (SBIR) program, developed a Micro-Linac that the company investigated as a potential replacement for iridium-192 sources for industrial radiography. These machines were deemed to be too expensive to manufacture and could not compete against the relatively low cost of iridium-192 sources. RadiaBeam is at initial research stages of developing a battery-operated 1-MeV compact linac that, if successful, would produce higher-energy x-rays than an x-ray tube and thus would have energies similar to those of iridium-192. Because of the low power usage, this technology would not require water cooling. This proposed concept requires considerable additional work to create a working model that could be field tested. The RadiaBeam representative who briefed the committee acknowledged that it is unlikely that this new technology could match the low price of using readily available and proven radioisotopes, despite the need for routine replacement of iridium-192 sources.⁴

Automated ultrasonic testing (AUT), an alternative to industrial radiography, emits ultrasonic waves into an inspected material. These waves reflect or scatter off flaws or defects in the material as well as the outer surfaces of the material. The detector measures the time differences in the return of these acoustic waves from the flaws and the outer surfaces to determine the shapes and locations of the flaws. Scans need skillful examination and interpretation by trained and certified technicians, and interpretation of the images can be subjective based on operator experience (Moran et al., 2015). Although radioisotope and x-ray radiography methods can detect a wide spectrum of flaws that may result from welding, differences in the physics make each method sensitive to a particular flaw type—radiography is suited to detect volumetric flaws such as slag and porosity, whereas ultrasound is more suited to detect planar flaws such as cracks and lack of fusion. A comparison of the measurement response between industrial radiography using gamma and AUT is shown in Table 6.2.

In 2009, the U.S. Nuclear Regulatory Commission (U.S. NRC) funded Pacific Northwest National Laboratory (PNNL) to perform a literature review (Moran et al., 2010) to help understand issues related to the interchangeability of AUT with industrial radiography. The PNNL review targeted the replacement of radiography with AUT specifically during the construction of nuclear power reactors; however, it is likely that the results can be extrapolated to other current uses of radiography and AUT. The review concluded that AUT was feasible for radiography in some cases; however, the techniques for the use of AUT are not currently adequately defined and would need to be specified or a performance standard would need to be defined.

6.1.3 Alternative Technology Adoption Considerations

Radioactive sources continue to be the preferred method for industrial NDT in remote field operations because the alternatives are not yet equal or superior in terms of their ability to detect faults, they provide images that are

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⁴ Salime Boucher, RadiaBeam Technologies, LLC, presentation to the committee on December 17, 2020.

not readily interpretable, and they are more expensive and not as robust. For these reasons, adopting alternatives such as x-rays and AUT for industrial NDT has been slow.

As in medical applications, with the advances being made in computational resources and sophisticated analytical software, industrial radiography is increasing its use of digital radiography to replace conventional film images in some applications. The image is captured on phosphor plates and digitized, which can then be easily interpreted and stored. The digitized image can be enhanced using software, which then makes the image easier to interpret. Because the phosphor plates require less energy to produce an image, the exposure time and the radiation exclusion area can be reduced. In addition, it allows for the use of lower-activity radioactive sources.

There are currently some disadvantages of using digital radiography versus film, such as higher upfront capital costs, limitations on the use of the phosphor plates in field locations, phosphor plates that are flat and currently cannot be bent around pipes, and additional training requirements. However, as these are addressed, the use of digital radiography is expected to increase. A 2017 white paper issued by SGS, a leading testing inspection services company, shows estimates that the use of digital radiography is approximately 10 percent of all radiography and will likely increase in the future (Montes and Taylor, 2017).

As with radiography versus AUT, accelerator technologies will most likely complement radiography, rather than substitute for it. Newer technologies will likely be used together with radioactive sources unless regulations prohibit radioisotopes or there are challenges with radioisotope availability. Different physical principles apply to the various types and qualities of ion and e-beams, so they all measure different parameters and are all of value. The 2019 workshop on accelerators organized by the Department of Energy (DOE) (2019) concluded that additional fundamental technology improvements are needed to miniaturize, stabilize (i.e., increase operational life and ruggedness), and lower the cost of generating high-energy beams of ions and electrons.

Developing nations are also using nonradioisotope techniques for NDT. Notably, the IAEA has been providing technical assistance on a variety of radiation techniques for NDT to Member States requesting such training and guidance. For example, in 2009 with IAEA assistance, Vietnam’s Atomic Energy Institute’s Center for Nondestructive Testing began to adopt digital radiography to replace traditional x-ray film, and 10 years later had placed into operation 15 digital radiography machines (Marais, 2019).

Industrial radiography equipment is very robust and can be used for decades with minimal required maintenance. This results in companies’ not replacing working equipment and deferral of capital costs of buying more sophisticated and expensive systems. Operating an industrial radiography camera is not difficult to learn or to perform. Although only about 160 hours of training are required to receive certification to operate this device, the mandatory requirements include the principles of radiation and radiography, safety training, radiographic film interpretation and processing, as well as on-the-job training as a radiographer’s assistant.⁵ With its ease and low cost of operation and ability to be used in remote locations without external power, many developing countries will continue to use the gamma equipment in lieu of x-ray systems.

The committee is aware of significant work being done in France to identify replacement technologies for gamma radiography cameras. A collaborative working group in France, coordinated by the Confederation France Pour les Essais Non Destructif and the French Society for Radioprotection, has been investigating improving the safety performance of industrial radiography and possible alternatives to using gamma sources. At conferences organized by the IAEA (Martin, 2013) and others,⁶ presenters acknowledged that replacement of gamma radiography with AUT or other alternatives is still some years away. This is due to technical constraints of the new technology and the time needed to conduct validations and standards development. However, it is recognized that these new technologies will continue to be primarily complementary to gamma radiography.

One reason that industrial radiography is likely to remain widely used in the immediate future is that no alternatives have been developed for some applications, for example, obtaining a profile radiograph of a valve body to determine whether the valve is fully closed. Advanced ultrasonic equipment and techniques will continue to be developed; however, most users are still unwilling to accept or interpret results, and many users want to

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⁵ See https://atslab.com/training/rt-certification.

⁶ Ad Hoc Meeting of Stakeholder States Involved with Technological Alternatives to High-Activity Radioactive Sources, IAEA. Vienna, Austria, May 23–24, 2019.

see a direct image that does not require interpretation.⁷ Higher level of equipment and higher level of technical qualifications mean higher cost. Users are unwilling to pay higher costs for alternative technologies when basic proven methods work at lower cost. Overall, radioisotope radiography offers considerably lower operating costs to achieve good inspection results. In addition, some NDT methods may be irrelevant as a valid inspection method. Most NDT engineers weigh inspection methods for detectability of defects, type of defect in the subject material, accessibility, and economics of the method being utilized.

6.2 INDUSTRIAL GAUGES

Fixed radioactive industrial gauges have been used throughout industry for 70 years for measuring the thickness, density, or fill level of a product while it is being manufactured or processed without contacting the material itself. Thickness gauges are used in processing to ensure that an entire product or material is the same thickness throughout, or to make sure the coating on a material is even. Density gauges are used in cement, petroleum, and road production to make sure that the density of a material is consistent. Fill-level gauges verify the amount of material within a vessel, provide continuous monitoring during production, and can be found in many industries including bottling plants.

Fixed radioactive industrial gauges are permanently mounted in a specific location. Typically, objects under scrutiny pass by the fixed gauge containing the radioactive source on a conveyor belt, pipeline, or in a vessel for inspection. A radiation detector is placed on the opposite side of the object from the source. When the radioactive source is exposed, radiation is emitted from the gauge and some of the radiation will pass through the solid or liquid being tested. The rest of the radiation is absorbed by the object. Any radiation that can pass completely through the object will be measured at the detector and turned into an electrical signal allowing for analysis.

Spinning pipe gauges evaluate the lengths of used steel pipes that are intended to be reused. This allows the identification of corrosion, erosion, or other defects in the pipe before it can be placed into a system that, should even a single pipe fail, might cause devastating economic, environmental, and health problems.

In conveyor gauges, flow-rate measuring instruments measure the amount of substance that flows through a pipe cross-section per time unit. The measured quantity is outputted as mass or volume. Typical flow-rate applications measure gases, vapors, and reactive or viscous liquids in pipelines. They also measure bulk solids on conveyor belts in many industries, such as mining, building materials, power generation, and the pulp and paper industry.

Blast furnace gauges are used in steel making, and cobalt-60 sources are used to gauge the wear of the refractory lining by monitoring the lining thickness of these sealed vessels. These gauges can provide data on gas and material flows, information useful for precision control of the blast furnace.

In mining, minerals such as gold, copper, and zinc are released from the mining slurry using an autoclave of large diameter with thick tank walls. The gauge is used within the autoclave to measure the level and point level of the slurry and operates under abrasive and high-temperature and -pressure conditions.

In the oil and gas industry, a process to remove the coke from heavy crude occurs under extreme temperatures, reaching up to 930°F. Industrial gauges are used to measure the coke level in the tank. Other measuring technologies tend to fail or are extremely unreliable.

The petrochemical industry uses level gauges in various processes to separate out products in large tanks with thick vessel walls. Typically, the separation processes are exposed to highly abrasive or corrosive material. Using an invasive measurement technique inside the tank is not possible due to these conditions.

Dredging uses a gauge to measure the density and flow rate of the dredged material and is a well-established method in the dredging industry. Mounted on the outer wall of the pipeline, the density measurement system provides stable readings under extreme vibration. Density measurement systems in dredging applications are used for continuous process control on pipelines. During measurement, the flow properties of the material are not affected by this noncontact method.

In all these applications, the density or level gauge is a well-established technology that provides accurate and repeatable results, real-time measurement during operation, and high measurement stability. It also has no impact

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⁷ David Tebo, Team Inc., presentation to the committee on June 12, 2020.

on the material being measured, because measurement is contactless, with straightforward installation outside of the vessel.

6.2.1 Radioactive Sources

The primary radionuclides used in fixed industrial gauges are cesium-137 and cobalt-60. The activities range from 0.05 Ci to 5 Ci (1.85–185 GBq) for cesium-137 and 0.25 Ci to 10 Ci (9.75–370 GBq) for cobalt-60 and are categorized within Category 3 and Category 4 sources based on the IAEA’s categorization system.

The gauges have a simple rugged construction consisting of a steel casing filled with lead, tungsten, or steel as shielding (see Figure 6.5). The radioactive sources used in the device meet special form requirements and the high-performance standards in ISO 2919 for gauging sources. The gauges are operated by rotating the source into the exposed position to take the measurement and then back to the stored, shielded position at the conclusion of the measurement. The devices require little maintenance and operate reliably for years with minimal support.

The gauges are typically used in harsh environments such as in high temperatures and pressures, in and around corrosive and abrasive materials, and under excessive vibration. The facilities requiring testing are typically large, thick-walled vessels and often in hard-to-reach locations. The gauge sources are permanently mounted externally from the vessel and require high energy to penetrate the walls.

6.2.2 Alternative Technologies

As mentioned above, radiometric measurement systems, that is, those using radioactive sources, are typically used when there are extreme process conditions, such as high temperatures, high pressures, and corrosive industrial environments, because radiometric measurements do not require contact with the material being measured. When these types of conditions are not present, industry has often preferred to use alternative nonradioisotope techniques. These techniques (see Figure 6.6) use radar, guided radar, ultrasonics, and differential pressure, and they require access to the material inside the tank or pipe.

Radar is used for level measurements. These radio-wave systems are normally mounted at the top of a tank filled with a liquid or solid. The system sends a radar signal into the product and receives back a reflection of the signal. Based on the time taken for the signal to return, the system analyzes the current fill level of the tank. Radar has high measurement accuracy, and it is not affected by temperature or pressure.

Ultrasonic-level measurement works by mounting an ultrasonic transducer at the top of a container containing liquid. The transducer sends out an ultrasonic pulse that gets reflected back from the surface of the liquid. The sensor then calculates the fill level based on the time between the transmitted and received signals.

Differential pressure measurement can measure level, density, and flow rate. It operates by measuring the different pressures on the inside and the outside of the vessel. The differential gets converted into an electronic signal and analyzed. This technique can cover a large range of different applications and can detect differences of a few millibars.

6.2.3 Alternative Technology Adoption Considerations

The major consideration for adopting alternative technologies to replace gauges using radioactive sources is the capability to operate in harsh process environments where the measurement system is not in direct contact with the material being measured, as mentioned previously. Radiation measurements are useful for these noncontact conditions. An x-ray system would appear to offer that capability, and most gauge manufacturers make some x-ray systems. However, these x-ray systems are not yet as rugged as the radioactive source measurement systems.

Moreover, many gauges with Category 3 sources are used in extreme process control systems such as high temperatures and/or pressures, media that are abrasive or sticky, locations where they are subject to excessive vibration or agitation, and other harsh operating environments that currently other technologies cannot withstand. For these reasons, it is unlikely that there will be an alternative using any other technology in the near future for gauges using Category 3 sources.

Although differential pressure (DP) is used quite frequently, where possible, the limitations here can be temperature, buildup, sticky media, abrasive media, or changing medium density. Very often users ask for two independent physical principal measurements so that differential pressure and radiometric measurements can be combined for redundancy in the results. In the mining and petrochemical applications, DP could be used but has not proven to be as reliable as radiometric gauges due to inaccurate readings when the media density changes and the short sensor lifetime. Therefore, it has not been a feasible alternative.

For dredging operations, ultrasonic systems have been used as a replacement for the radiometric gauges, but they require a high installation effort, are difficult to calibrate, and are very sensitive to vibration.

In the oil and gas industry and especially in the “cokers,” there is no feasible alternative due to the extremely high temperatures needed in these vessels.

With all these nonradioisotope technologies and applications, there may be some improvement in the near future, but the physical limits are the constraining factors, and these would have to be overcome to have viable alternatives.

Overall, with increase in the sensitivity of the detectors, there is a trend to use lower-activity sources, and in many cases the Category 3 sources currently used can be replaced with Category 4 sources. With the increased sensitivity, the Category 3 sources can be used for longer periods of time even as the source decays to lower activity, so they do not have to be replaced as often, thereby delaying the need for disposal. As a result, over the near future, many of these applications will be using Category 4 sources and subsequently decrease the security risks.

For some applications there is no viable alternative technology that can currently replace the use of gauges with radioactive sources. As with industrial radiography, the use of alternative technologies is now complementary to the use of gauges with radioactive sources, and this condition will probably continue into the near future.

6.3 WELL LOGGING

Well logging has been used for more than 90 years to explore the structure and composition of rocks and fluids in the subsurface, to measure fundamental petrophysical properties of reservoirs, and to estimate resource potential. The most common application of well logging is in the search for recoverable hydrocarbon reserves by the petroleum industry. Well logging is also an important technique used in the search for mineral, geothermal, and groundwater resources.

There are more than 900,000 active oil and gas wells in the United States and millions more around the world. From 2014 to 2018, more than 19,000 oil and gas wells were drilled each year in the United States. Prior to the COVID-19 pandemic, approximately 21,500 petroleum wells were forecast to be completed⁸ per year from 2020 to 2022 (Garside, 2019). The pandemic-induced downturn in economic activity and petroleum consumption led to a marked decrease in oil and gas prices and a concomitant decrease in drilling and exploration. (See additional discussion in Section 6.3.3.)

Modern well logging is done either simultaneously while drilling (logging while drilling [LWD]) or after the well is drilled by lowering a wireline incorporating dedicated instruments into an open or cased borehole. Each approach has specific advantages. Briefly, LWD provides rapid subsurface information that can help guide drilling in near real time, but the extreme pressure, temperature, and mechanical conditions of the drilling environment, together with the need to power a relatively small logging tool and recover log data, limit both the type of device that can be deployed and the amount of data that can be reliably transmitted to the surface while drilling. In contrast, wireline logging permits use of a wider range of logging tools, but because the data become available only after the borehole has been drilled, this information cannot be used to make decisions while drilling. Drilling a petroleum exploration well is an expensive and risky activity that costs the operator on average $200,000–$300,000 per day in the case of an offshore drilling rig (IHSMarkit, 2020). Drilling for mineral or groundwater exploration and production is less expensive, but still costly.

Well logging is a very specialized activity. An operator, typically a major international oil company operating alone or as the leader of a syndicate of companies, contracts with a service provider to design and deploy a suite of appropriate logging tools, collect the log data, and provide interpreted well logs. These logs are then used by the operator to infer subsurface petrophysical parameters that can be used to estimate resource potential, cost of production, and concomitant project risk. There are more than 200 well logging service providers in the United States. Almost all are small- to medium-size companies and provide an estimated 60 to 70 percent of the logging units in the United States. However, the number of logging units of a logging company are not equated

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⁸ Well completion incorporates the steps taken to transform a drilled well into a producing one. These steps include casing, cementing, perforating, packing with gravel, and installing a production tree, all of which are generally done after the well has been logged.

with volume of logging business that a company provides. The major international integrated logging companies have a worldwide business volume that is much larger than that of the 200 small- to medium-size logging companies combined. Of the four major international logging companies, two (Halliburton and Baker-Hughes) are U.S.-origin, and Schlumberger, a French-origin company, has headquarters in the United States and France. The fourth, Weatherford, was formed from a conglomeration of several UK- and U.S.-based companies and has exited the U.S. market. It is uncertain how the competition will play out in the coming years to decades given various factors affecting demand for well logging (see discussion in Section 6.3.3). The major logging companies have the technological and financial resources for development of alternatives to radioisotope-based well logging. They have invested in research on alternatives, and some have launched either commercial or experimental nuclear-based alternative techniques.

A wide range of logging techniques have been developed and deployed, which for the purpose of this report can be classified into three broad categories (see Table 6.3):

1. Nonnuclear techniques that do not incorporate a source of ionizing radiation (see brief discussion below about aspects of some of these techniques);
2. Conventional radioisotope techniques that incorporate a sealed source of ionizing radiation (see Section 6.3.1 for detailed discussion); and
3. Alternative neutron source techniques that use deuterium and/or tritium accelerators (see Section 6.3.2 for detailed discussion).

Nonnuclear techniques include acoustic arrays, electrical and electromagnetic sensors, magnetometers, nuclear magnetic resonance tools, and temperature, pressure, and dimensional sensors. These techniques tend to complement the radioisotope-based techniques, as discussed in more detail in Section 6.3.3. Technical aspects of these nonnuclear techniques include the following concepts about what is being measured, how the measurements are made, and what logs are recorded (see Sidebar 6.1 about importance of well logs in petroleum exploration).

Self-potential, also called spontaneous potential (SP), measures the voltage difference between electrodes in a downhole tool. By adding a source of electrical current in the sonde, resistivity can be measured between the

TABLE 6.3 Well Logging Techniques

NOTE: AmBe = americium-241/beryllium; D-T = deuterium-tritium; LWD = logging while drilling; RF = radio frequency.

SOURCE: NRC, 2008.

electrodes or the transmitter and receiver coils. The material composition of the rocks in the inspected geological layer affects the measured resistivity. A log of the resistivity measurements (see Figure 6.7) can be used to estimate the layer’s porosity and fluid content of the pores. Electromagnetic induction coils are typically used for measuring resistivity. Specifically, the transmitter coil sends the electromagnetic signal into the geological formation, and the induced signals from the formation are collected by the receiver coil to measure the apparent resistivity. In addition, a dielectric constant tool is sometimes used to assist in measuring water content and rock types in the formation. The dielectric constant measures the capacity of a material to store electrical energy when an electric field is applied, and in this particular case, the tool uses microwave frequencies in the range of a few Megahertz to 1.1 Gigahertz.

Directly measuring formation pressure is useful for calibrating other measurements and for cross-comparing results directly or indirectly linked to formation pressure. Such calibrations and comparisons include thermistor sondes measuring borehole temperature, borehole calipers measuring diameters, and downhole televiewers measuring shapes.

Sonic logging is accomplished by measuring the speed of acoustic waves in the formation between a source and a receiver in the sonde. Acoustic wave speed is an indicator of porosity and fluid content for a given type of rock. More sophisticated information can be gathered from arrays that measure the speeds of compressional (P), shear (S), and even guided (Stoneley) waves.

Magnetic logs are made by lowering various types of magnetometers in the borehole, most commonly in mineral exploration to determine variations in magnetic properties of the formation such as susceptibility and remanence (the magnetic induction remaining in a substance after removing the external applied magnetic field). Depending on the application, variations in the total magnetic field, its vector components, and/or its tensor gradients are recorded.

Porosity and permeability of geological formations depend on the fluid content and the pore spaces containing fluids. By applying a strong magnetic field, the hydrogen atoms in the fluid (typically water and hydrocarbons) will align their magnetic moments. This technique is called nuclear magnetic resonance (NMR), which is a well-known medical imaging method (although in medicine, it is known as magnetic resonance imaging [MRI]). When the magnetic field is decreased, the hydrogen nuclei relax to their original state and emit signals that can be detected.

The measured signals can be used to measure the location, concentration, and density of the hydrogen atoms and thus infer the porosity and permeability of the formation. Because the magnetic field strength decreases precipitously with distance and geological media such as mud on the borehole walls attenuate the magnetic field, NMR is useful for measuring porosity and water content in rocks in close proximity to wellbore.

6.3.1 Radioisotope Technologies

The two most common radioisotope technologies used in petroleum well logging employ cesium-137 sources and americium-241-beryllium (AmBe) neutron sources, often packaged in the same downhole tool. Both technologies are characterized by stable radiation emissions that, unlike many candidate alternative technologies, are not affected by extreme downhole environmental conditions such as continuous and intense mechanical shock and vibration, high temperatures, and high pressures. Both source types contain radioisotopes held in solid refractory oxides or glass ceramics that are securely sealed using double- or triple-walled metal containments.

Radioisotope sources have advantages that are well suited to downhole applications in extreme conditions: small size; stable radiation output over the course of a well logging job; simple operation; relatively low cost; no power requirements; and isotropic radiation, which is optimal for a borehole application. In addition, cesium-137 sources have a relatively long service life of about 15 years. Moreover, the activity range for these sources is typically at 1–3 Ci (37–111 GBq) and are in Category 4 amounts (CISA, 2019). The disadvantages of sealed sources are that they cannot be switched off or pulsed, their gamma or neutron energies cannot be changed, and they represent a potential security and radiation safety risk if lost, stolen, or misused (CISA, 2019).

The 2008 National Academies report noted that, while a Category 3 cesium-137 radioactive source could potentially be replaced by an alternative x-ray machine such as a linac, there were significant obstacles in developing a practical tool (NRC, 2008). These problems included the size of the machine, its broad energy spectrum, stability, and anisotropic radiation. Given these issues, little development work has been done since 1987. The 2008 committee judged that replacement of these sources was not a priority, and there appears to have been no material advance in cesium-137 logging technologies since that time.

AmBe neutron sources are an admixture of ²⁴¹AmO₂ oxide and ⁹Be metal powders. These are tightly compressed into a cylindrical form to maximize the likelihood of an alpha-particle reaction with beryllium and are usually encapsulated in a welded or triple-walled stainless steel container (see Figure 6.8).

Radioactive decay of americium-241 yields alpha particles, which interact with beryllium-9 atoms to yield neptunium-237 atoms, carbon-12 atoms, free neutrons, and 4.4-MeV gamma rays. The neutron source strength

is governed by the americium-241 activity, which for logging sources can be up to 16 Ci, the maximum activity permitted for a Category 3 source. Neptunium-237 decays to protactinium-233 (half-life 2.1 million years); thus, the principal alpha activity is from americium-241 decay.

There are at least three categories of security risk associated with active (i.e., in regular use) or disused AmBe neutron sources: (1) loss of control of an active source during logging operations; (2) loss of control of an active source during transport to/from the logging site or during temporary storage; and (3) loss of control of a disused source during permanent storage. Any of these loss-of-control incidents could potentially result in radiological injury, site contamination, and/or area denial through accidental dispersion or deliberate use of a radiological dispersal device (RDD).

6.3.2 Alternative Technologies

The 2008 National Academies report (NRC, 2008) contains an overview of alternative neutron source technologies that could potentially be used in well logging because at that time many AmBe sources were in Category 2 and thus within scope of the 2008 report but, as noted in Table 6.4, all new sources are in Category 3. These alternatives included accelerator-based neutron sources incorporating deuterium and/or tritium, and sealed californium-252 neutron sources. This section focuses on the alternative technologies using deuterium and/or tritium due to californium-252 being a radioisotopic source. Californium-252 produces neutrons via spontaneous fission and can be a reliable source of neutrons; however, because of its relatively short half-life of about 2.6 years, it would have to be replenished more frequently than AmBe sources. But californium sources have a higher neutron yield than AmBe sources, and an equivalent neutron generation between the two types would be 27 mCi (1 GBq) for californium-252 versus 16 Ci (592 GBq) for the typical AmBe source (CISA, 2019). Some benefits and limitations of alternative technologies using deuterium and/or tritium versus AmBe for neutron sources are summarized in Table 6.5.

Since publication of the 2008 NAS report, considerable work has been done in D-D, D-T, and T-T (tritium-tritium) accelerator research. For example, with support from the National Nuclear Security Administration’s (NNSA’s) SBIR program, Starfire Industries developed the nGen® portable D-D neutron generator and deployed it in its QL-40 Compensated Neutron Logger tool, demonstrating formation responses similar (but not identical) to conventional AmBe sources. Accelerator-sourced pulsed neutron logging is now available from an increasingly wider range of service providers. But while the availability of switchable accelerator-based neutron logging tools is increasing, formation evaluation using sealed AmBe neutron sources remains a preferred option. The reasons for this preference are discussed in the next section.

TABLE 6.4 Sealed-Source Technologies and Logging Parameters

SOURCE: CISA, 2019.

TABLE 6.5 Advantages and Disadvantages of Radioisotope and Alternative Well Logging Technologies

NOTE: D-D = deuterium-deuturium; D-T = dueturium-tritium.

SOURCE: Modified from presentation by Mike Fuller and Mark Shilton, QSA Global, Inc., to the committee on October 13, 2020, to include x-ray.

6.3.3 Alternative Technology Adoption Considerations

As described in a previous section, alternative replacement of Category 3 cesium-137 radioactive sources such as those used in well logging has not been a priority because of the lower activity, and no progress with adopting alternatives has been made. This section focuses on alternative technology adoption considerations associated with AmBe neutron sources

Driven by safety and security concerns, the petroleum industry has been investigating the use of alternative neutron sources for well logging for a number of years (Bond et al., 2011). Large multinational logging companies have multimillion-dollar research and development (R&D) budgets for alternative technologies. New technology development is not carried out in smaller companies that lack sufficient capital, R&D budgets, and market incentives. As a consequence, smaller service providers must rely on extant proven logging tools and can be expected to resist technological changes that have the potential to substantially affect their business (Badruzzaman et al., 2015).

Overall, efforts to replace current radioactive sources face a number of technical, logistical, and financial challenges. One challenge is that alternative (accelerator-based) tools are thought to be less accurate than radioactive source devices in porosity determination (see, e.g., Badruzzaman, 2014; Badruzzaman et al., 2019). Also, NMR and acoustic measurements are complementary to radioactive source technologies and not a replacement for them (Badruzzaman et al., 2015). In particular, the NMR measurement can classify fluids and can indicate permeability, but NMR cannot provide mineralogy. In addition, acoustic measurements can indicate rock anisotropy but not provide mineralogy (CISA, 2019). As for mineralogy, AmBe source spectroscopy tools are already being replaced by D-T neutron spectroscopy tools (Pemper et al., 2006; Radtke et al., 2012). However, some accelerator-based neutron sources incorporate security-sensitive dual-use technologies⁹ that can complicate construction and use, especially in politically unstable regions.

Any user of an alternative tool would need to develop new calibration protocols and possibly develop new correlation methods to compare an alternative tool’s responses to that of a conventional radioactive source using standard reference formations. This is because modern well log analysis is based on, and referenced with respect to, large volumes of legacy data measured from reservoirs over many decades using conventional logging methods (especially those that used traditional sealed cesium-137 and AmBe neutron sources). Because the spectral energy characteristics of AmBe sources differ from those of D-T and T-T accelerator sources, and because those differ-

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⁹ A dual-use technology has applications in commercial products and in weapon systems. A D-T neutron generator is a dual-use technology, and these generators are subject to export regulations.

ences could potentially lead to additional (but not insurmountable) uncertainty in exploration decisions, adoption of alternative accelerator-based technologies has been gradual and has been critically dependent on technique validation with respect to legacy data. This poses an opportunity that could potentially be addressed by collective actions of industry associations and possibly government agencies.

The 2008 National Academies report noted that the AmBe sources used in elemental analysis logs could be replaced by switchable D-T accelerators (NRC, 2008). Notably, T-T accelerators are not as useful as D-T accelerators because of the approximately 100-fold lower neutron yield and because tritium (a radionuclide) is used for both the accelerating and target nuclei. Furthermore, while replacing the AmBe porosity tool would be more difficult, one of the major service providers had by 2008 marketed two D-T accelerator tools, one for nuclear porosity logging and one for LWD wireline logging. As a consequence, the report recommended that an industry working group be tasked with solving the technical obstacles to implementing accelerator-based replacements for AmBe well logging sources (NRC, 2008). The Nuclear Special Interest Group of the Society of Petrophysicists and Well Log Analysts has made considerable progress in addressing this recommendation, and now most major and some small- and medium-size service providers offer pulsed accelerator-based sources for neutron-based well logging as part of their product suites.

Participants of the 2019 DOE (DOE, 2019) workshop discussed that:

• There have been some notable recent developments: (1) a low-energy (> 300 keV) x-ray device and (2) neutron generators other than D-T accelerators (Badruzzaman et al., 2019; Bondarenko and Kulyk, 2017; Jurczyk, 2018; Simon et al., 2018).
• For D-T generator-based neutron porosity, although one company has marketed tools for both wireline logging and logging while drilling, other companies have not brought these tools to market due to economic factors. In addition, the LWD tool has shown good performance, but the wireline tool has encountered poor performance due to borehole environmental conditions.
• Although the recent x-ray density tool development is based on promising field tests of the 1980s-era R&D on the 3.5-MeV linac x-ray density tool (King, 1987), the new tool is smaller, simpler, and promising in field tests. But it still confronts challenges in application to more severe LWD environmental conditions.

In addition, a density technique, known as inelastic neutron-gamma density (INGD), uses gamma rays produced during inelastic scattering of high-energy neutrons. INGD was first reported in the mid-1990s for cased-hole applications; it was incorporated in a D-T generator–based LWD tool in 2000 (Evans et al., 2000) and marketed in 2012 (Reichel et al., 2012). However, because of the mixed neutron-photon physics, the INGD technique is not as accurate as the gamma-gamma density, but it can be used in special circumstances. Moreover, for an alternative density measurement based on photon physics, use of bremsstrahlung x-rays would provide a much closer analog to cesium-137-based gamma rays. The fundamental physical mechanism for both x-rays and gamma rays is Compton scattering. Thus, an x-ray generator–based mechanism could potentially replace the cesium-137-based method. In contrast, the neutron-gamma method offers “a pseudo-density,” useful only when gamma-gamma density “is either unavailable or unobtainable” and depending on circumstances detailed by Badruzzaman et al. (2014).

Another accelerator-based technique is a dense plasma focus (DPF) alpha-particle accelerator. Using the (alpha-Be) reaction, the DPF accelerator can generate a neutron spectrum that very closely matches an AmBe source’s spectrum. This technique thus has been shown to duplicate the neutron porosity response almost exactly. However, the DPF (alpha-Be) accelerator will require long-term R&D before being incorporated into a commercial logging tool (Badruzzaman et al., 2019).

The petroleum industry is a notably cyclic enterprise. At present, with low oil and gas prices, well logging companies are experiencing extreme and sustained revenue and equipment utilization challenges. Logging infrastructure utilization is down 50 to 60 percent and price pressure is driving deep discounting and concomitant diminished revenue. Staff layoffs of 50 to 60 percent, due to COVID-19 and reduced oil prices, have had severe impacts, and bankruptcies are pervasive in the industry at this time. As a result, major service providers are reevaluating market needs. Large capital purchases or changes in technology would not be feasible for many companies in the current market environment.¹⁰ As a consequence of reduced utilization, many cesium-137 and AmBe neutron sources

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¹⁰ Kenny Jordan, Association of Energy Service Companies, presentation to the committee on October 13, 2020.

may be “stranded” in large and small well logging companies who have no incentive to switch to an alternative technology, because these sources are a significant capital asset that could be costly to replace. Furthermore, for a logging company in financial distress, the costs required to securely dispose of an unused source may be prohibitive. These stranded sources therefore pose a significant security risk, especially if reduced overall company budgets have led to reduced spending and emphasis on source storage security.

The future demand for well logging in petroleum exploration and production and other applications is uncertain. Most nations now recognize the importance of taking action to immediately and sharply reduce greenhouse gas (GHG) emissions to slow and even reverse the effects of climate change. These actions will have sweeping effects on energy generation and distribution. For example, global efforts to limit GHG emissions have already led to decreased demand for coal¹¹ for electricity generation, with a concomitant increase in the deployment of renewable energy technologies including solar, wind, and geothermal.

The global petroleum industry will almost certainly be significantly affected by international actions taken to reduce GHG emissions. Some of those changes are already under way (Krauss, 2020). One of the most likely changes will be a reduction in demand for liquid fuels for transportation as automobile and truck fleets become increasingly electrified.¹² Decreased demand leads to a lower petroleum price, which leads to decreased spending on exploration, which in turn would be expected to lead to reduced expenditure on well logging services.

Two points are relevant to this study in this low-price scenario. First, decreased expenditure on well logging services would mean that many extant conventional cesium-137 and AmBe sources would become surplus and would either be stored by the logging contractor, disposed of, or possibly abandoned. Depending on the cost and availability of a safe disposal pathway, this could either lead to increased security risk, or decreased security risk.

Second, a possible consequence of decreased spending on well logging services would be a reduction in R&D efforts to build alternative well logging sources. As noted above, only a few of the major service providers have the R&D capability to develop alternatives to radioactive sources. Without a significant future market, these companies would not have a financial incentive to make such investments.

While overall demand for oil and gas is likely to decrease in the coming decades, there will still be a need for petroleum fuels in key applications, such as liquid fuels for aviation and natural gas as a transition fuel for electricity generation. Exploration for these resources will continue to require accurate and reliable well logging services, using either conventional or alternative sources.

Areas where the logging market might be expected to grow include mineral exploration and production, groundwater exploration, and geothermal exploration. For example, demand for copper is estimated to grow by a factor of 4 to 5 from 2015 to 2100, driven by population growth and renewable energy systems (Schipper et al., 2018). But perhaps the most important future market sector for future growth in well logging services will be subsurface carbon capture and storage, where many of the same methods used in conventional oil and gas exploration and production will be used to select, verify, and monitor carbon dioxide storage reservoirs (NETL, 2017a,b, n.d.).

6.4 CALIBRATION SYSTEMS

Calibration systems produce radiation fields of known energy and intensity for calibration of radiation monitoring equipment, dosimeters to ensure their accurate operation, and industrial and teletherapy devices that use cobalt-60. Calibration systems use high-activity radioactive sources (approximately 400–2,200 Ci [15–82 TBq]). These are Category 2 sources based on the IAEA categorization system. Calibration facilities make use of cesium-137 and cobalt-60 sources. The workhorse irradiator used for cobalt-60 calibrations was the Nordion-produced Gammacell 220, which was discontinued in 2008,¹³ but many of these irradiators are still in facilities worldwide (IAEA, 2019c); Hopewell Designs has created a replacement irradiator (Rushton et al., 2016). The remainder of

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¹¹ In many instances, coal-fired power plants have been replaced with natural gas plants. The coal-to-gas switch has saved around 500 million tonnes of carbon dioxide since 2010 (IEA, 2019).

¹² For example, General Motors recently announced that it will introduce 30 new global electric vehicles by 2025 and will phase out gasoline- and diesel-engine vehicles by 2035. Also, electric vehicles now represent 54 percent of market share in Norway (Klesty, 2021), overtaking those powered by petrol, diesel, and hybrid engines (which will be phased out completely by 2025).

¹³ See https://www.nordion.com/products/irradiation-systems.

this section focuses entirely on cesium-137 calibration systems due to the security risk posed by powdered cesium chloride, which is very dispersible compared to solid, metallic cobalt-60 sources.

Radioactive calibration sources produce precisely measured dose rates that are traceable to accepted standards. In the United States, the National Institute of Standards and Technology (NIST) serves as the primary radiation dosimetry laboratory, and as such, it maintains the national measurement standards and calibrates the instruments for secondary laboratories. A network of secondary and tertiary facilities ensures that every radiation detection instrument measures accurately and is traceable to the NIST standard. Internationally, metrology demonstrates the equivalence of measurements in different countries and facilitates accurate trade measures. Independent of the source of radiation for medical and other applications, the standard and the calibrators are still needed.

Calibration is needed for multiple radioactive source applications. To give a sense of the scale of calibration needed in the United States, there are more than 19,000 radionuclide-specific licenses. These include hospitals and cancer treatment facilities, nuclear medicine clinics, research facilities, universities, training facilities, and industrial facilities including oil exploration companies. Instruments are used to ensure compliance with safety and health regulation associated with the license. Other users consist of federal government agencies, namely the Environmental Protection Agency and DOE, which involves the National Laboratories, the Radiological Assistance Program, the Federal Radiological Monitoring and Assessment Center, the Radiation Emergency Assistance Center/Training Site, the Nuclear Emergency Search Team, and the Accident Response Group, as well as and state and local entities. All of these agencies and programs rely on calibrated instruments, making it even more important to ensure that the United States retains a national capability to calibrate radiation instruments properly. These instruments require periodic calibration to ensure that they provide accurate information to the wide variety of stakeholders that use them.

6.4.1 Radioisotope Technologies

Cesium-137 was selected more than 50 years ago as the basis of national and international calibration because of its optimal single energy spectrum (661.7 keV), long half-life, and moderate shielding requirements relative to other radionuclides. In addition, cesium-137 provides a photon energy in the middle of the region spanning x-ray tubes, cobalt-60, and linear accelerators, and therefore covers energies ranging from 10 keV to 10 MeV. At NIST, a cesium-137 calibrator is used to determine the standard for radiation dose in air, or air kerma. The cesium-137 source used is in the form of cesium chloride. Because of its ease of dispersibility, cesium chloride raises security concerns.

Millions of radiation detectors calibrated annually with cesium-137 are being used in the United States and globally, including at ports of entry to measure the radioactivity of cargo, at nuclear power plants to monitor surroundings, at medical facilities to ensure safety of patients and medical staff, and wherever there is release or suspected release of radiation.

Cesium-137 calibrators (see Figure 6.9) have outstanding reproducibility (approximately 0.1 percent over periods of months to years) and enable low uncertainty measurements required for standardization at NIST and subsequently in transferring standards to calibration facilities and end users.

An expert who briefed the committee noted that significant knowledge and procedures are built on the assumption of the availability of cesium-137 radiation fields.¹⁴ Many national and international regulations, recommendations, and document standards, including those issued by the American National Standards Institute, the National Council on Radiation Protection and Measurements, the International Organization for Standardization, and the IAEA, rely on cesium-137 calibrators. In addition, calibration facilities using cesium-137 calibrators need to demonstrate that they are capable of transferring the national standard in order to be certified by accreditation programs and other regulatory programs.

The current number of cesium-137 irradiators used for ionizing radiation metrological applications is estimated to constitute only 1 to 2 percent of the total number of cesium-137 irradiators used in the United States (CIRMS, 2019).

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¹⁴ Malcolm McEwen, National Research Council of Canada, presentation to the committee on January 28, 2021.

6.4.2 Alternative Technologies

There are no obvious replacements for cesium-137 irradiators as a reference radiation field for ionizing radiation metrology. The uniqueness of using cesium-137 for metrology is the precision that its characteristics offer (see Table 6.6). Moreover, cesium-137’s 661.7-keV emission energy is close in value to the energies of beta and gamma emissions of the radioactive materials that researchers and public health officials commonly need to measure.

The gamma rays from cesium-137 and cobalt-60 are monoenergetic, which makes their penetration and delivered dose predictable and easy to calculate. Artificially produced x-rays, from electron beams striking a metal target (bremsstrahlung), have a broad spectrum ranging from the energy of the electron beam down into the kilo-electron volt or hundreds of electron volt range. It is conceivable that the broad spectrum of x-rays could be accounted for

TABLE 6.6 Consideration of Technology Alternatives to Cesium-137 for Dose Standardization, Calibration, and Testing of Instruments

SOURCE: Ronaldo Minniti, National Institute of Standards and Technology, presentation to the committee on January 28, 2021.

in developing new calibration standards. However, to match the precision of the present standards, the voltage and current of the accelerating source would require extremely fine regulation. Additionally, the bremsstrahlung target would need to be precisely machined, and the electron beam would need to be accurately directed onto that target. These requirements make matching the natural parameters of these radioisotopes practically unachievable at this time.

Replacing radioactive cesium chloride with other well-established, less dispersible forms of cesium-137, such as vitrified and pollucite sources, which have been used for cesium-137 gauging and well logging sources since the 1980s, could be acceptable for calibration applications because the cesium-137 spectrum would be maintained. The source size would need to increase to accommodate the lower specific activity of glass formulations. As noted in Section 1.3, such less-soluble and less-dispersible forms of cesium-137 have been developed in India and are in use in blood irradiators. After trying different methods, researchers at the Bhabha Atomic Research Center (BARC) in 2015 developed the technique of accurately controlled pouring of vitrified cesium-137 into stainless steel pencils that are then loaded into blood irradiators. To the committee’s knowledge, there is no such ongoing research into the suitability of other forms of cesium for calibration applications.

Since 2015, the French company ATRON Metrology has partnered with the French national metrological laboratory to develop an alternative method to calibrate radiation meters. This method uses an electrostatic accelerator that directs an electron beam onto a tantalum target to produce x-rays. The accelerator is tuned to produce x-rays in the energy domain between cesium-137 and cobalt-60 gamma energies (Bordy et al., 2019). While ATRON advertises this method to be realistic of the type of spectrum encountered at nuclear power plants, it is not the type from discrete radiation sources. Also, the drift of the device is 0.3 percent over 11 months, and the uncertainty in the calibration of radiation survey meters is less than 7 percent, which are both considerably larger than the accuracy of cesium-137 calibrators. In addition, the working life of the tube is indicated at about 4,000 hours, a significantly shorter service life than in a cesium-137 calibrator, potentially resulting in higher maintenance costs for the ATRON technology in comparison. Furthermore, calibration is still referenced back to the French national metrological facility, which still uses cesium-137 (Chapon et al., 2016).

6.4.3 Alternative Technology Adoption Considerations

NIST’s position is that elimination of cesium chloride from use in calibration instrumentation could be detrimental to the nation’s emergency response capabilities. However, the U.S. and other governments have considered policy changes to eliminate cesium chloride from use in radioactive sources, and that policy could be revisited in the near future. The committee found that NIST is not taking steps to prepare for a possible policy change by exploring alternative technologies and performing equivalency tests to ensure that there are no adverse impacts to current calibration and testing capabilities.

If a substitute for cesium-137 calibrators became available in the future that could meet all of the metrological requirements, then all documentary standards published to date, and regulations from the various regulatory bodies (such as the U.S. NRC) and accreditation agencies at national and international levels, would have to be redeveloped such that the security and safety of radiation workers and the public are not affected. Until cesium irradiators based on a new form of cesium (other than cesium chloride) or another type of source become available, calibration facilities will need to rely on existing cesium-137 irradiators. Without a suitable substitute, the elimination of low- to mid-range Category 2 cesium-137 sources in calibration facilities would have a negative effect on the calibration infrastructure in the United States and worldwide, directly affecting the safety and security of the public.¹⁵

6.5 RADIOISOTOPE THERMOELECTRIC GENERATORS

RTGs are a type of nuclear battery that uses thermocouples to convert the heat released from decay of the radioisotope into electricity. RTGs are rather simple in design and have no moving parts. They have been used as power sources in situations where the systems that use them cannot be easily accessed, need to remain operat-

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¹⁵ Letter from Ronaldo Minniti and Michael Mitch, NIST, to Charles Ferguson, National Academies, on May 21, 2020; Malcolm McEwen, National Research Council of Canada, presentation to the committee on January 28, 2021.

ing without human interference for long periods of time, and are incapable of generating solar energy efficiently. Based on these circumstances, RTGs have been used as power sources in satellites, space probes, and uncrewed remote structures such as lighthouses built by the former Soviet Union inside the Arctic Circle, Russian stations in Antarctica, and U.S.-commissioned arctic monitoring sites.

6.5.1 Radioisotope Technologies

The radioisotopes used for RTGs need to fit three main criteria: have a relatively long half-life so that they can produce sustained levels of energy; have a high power density (power per unit mass of radioisotope), and emit radiation that can be shielded. Plutonium-238¹⁶ and strontium-90 are the most commonly used radioisotopes for RTG fuel. Both radioisotopes have long half-lives, 87.7 years and 28.8 years, respectively. The power densities for both radioisotopes are also relatively large, with 0.57 W/g for plutonium-238 and 0.46 W/g for strontium-90. A significant difference between the two radioisotopes involves the radiation produced: alpha for plutonium-238 and beta for strontium-90. Because of its limited penetrating capability, the alpha radiation emitted from plutonium-238 gives this radioisotope an advantage over the more penetrative beta radiation from strontium-90 because it minimizes the shielding required.

Plutonium-238 has been the preferred radioisotope for RTGs for space missions because of the lower shielding requirement and therefore lighter weight. The longer half-life of plutonium-238 is also an advantage for space missions because refueling is not possible. Starting in the 1960s, plutonium-238–based RTGs have powered more than two dozen U.S. space missions. Most recently, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) has been the power source for the Perseverance rover, which landed on Mars on February 18, 2021. It contains about 4.2 kg of plutonium-238 (NASA, 2020), or about 73,000 Ci (2.7 PBq) of initial activity, making it a Category 1 source. MMRTG provides approximately 110 W of electrical power when freshly fueled. Just 2 days prior to the landing, Idaho National Laboratory researchers announced that they are working on the next-generation power system that is designed to be three times more efficient than Perseverance’s power system by using dynamic power conversion with a Stirling or Brayton thermal cycle (ANS, 2021).

The greatest limitation of plutonium-238 is the difficulty of manufacturing it in sufficient quantities. Following the 1988 shutdown of the last domestic plutonium production reactor at the Savannah River Site (Smith et al., 2019), stockpiles of the fuel were projected to be depleted in 2018. In 2015, DOE addressed the forthcoming shortage by reestablishing production of plutonium-238 at the Oak Ridge National Laboratory (ORNL) for future National Aeronautics and Space Administration (NASA) missions (Walli, 2015). As of February 2021, the High Flux Isotope Reactor at ORNL has produced almost 1 kg of plutonium-238, and it has the capacity to produce up to 700 g annually. To help meet NASA’s goal of 1.5 kg per year by 2026, DOE announced on February 16, 2021, that Idaho National Laboratory (INL) will ramp up use of its Advanced Test Reactor (ATR) to produce plutonium-238; the first two irradiation campaigns in the ATR are expected to yield 30 g in spring 2021 (DOE, 2021).

For ground-based applications, RTG use also started in the 1960s. In the former Soviet Union, more than 1,000 RTGs powered by high-activity Category 1 strontium-90 were deployed. The largest RTG was the IEU-1 with an initial activity of 465,000 Ci (17.2 PBq), and the smallest RTG, designated Beta-M/S, had an initial activity of 35,700 Ci (1.32 PBq) (Porter, 2015). By the early 2000s, almost all Soviet and Russian-made RTGs had exceeded their original service lives. With coordination from the IAEA, several countries have provided financial and technical assistance that has helped Russia eliminate use of almost all of its RTGs. For example, Norway dedicated €20 million (about $24 million) to assist Russia in removing and securing 180 RTGs in the coastal areas of northwest Russia along the Barents, White, and Kara seas. Norway also had provided solar installations as alternative power sources to replace the RTGs that powered lighthouses (Digges, 2015). By the close of 2019, nearly 1,000 RTGs that had been deployed in northern Russia and 4 that had been deployed in Antarctica had been decommissioned and disassembled with the strontium-90 sources stored at the Mayak Production Association, located in the Chelyabinsk Oblast in Russia. Only 12 RTGs remain in Kamchatka, but these are also scheduled to be removed in the near future (NASEM, 2020).

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¹⁶ Unlike plutonium-239, plutonium-238 is nonfissile, so it cannot be used in nuclear power plants or nuclear weapons.

The United States deployed far fewer RTGs compared to Russia, and by August 2015, the remaining 10 RTGs in use were removed from Burnt Mountain, Alaska, by the U.S. Air Force. The strontium-90 sources were removed and disposed of at the Nevada National Security Site, which is authorized to take and secure U.S. government-owned disused sources (Romano, 2015). These concerted national and international removal and replacement efforts for land-based RTGs have practically eliminated a whole class of high-activity radioactive sources.

New devices are being designed in the United States and perhaps elsewhere. For example, Zeno Power Systems, based in the United States, is developing a next-generation radioisotope power system that converts the heat from decaying strontium-90 into electricity intended primarily for government and commercial space applications. This new-generation design by Zeno aims to increase the specific power of the system and be more lightweight making it suitable for space exploration. According to the developers, the current Technology Readiness Level (TRL) for the RTG is TRL 3, with plans to progress it to TRL 4 in the coming months. They are estimating deployment of the technology by 2025. The primary technological challenges remaining in the development of the technology are demonstrating cost-effective, repeatable, and high-quality fuel capsule fabrication and conducting the rigorous testing required to qualify any RTG for spaceflight. The developers also recognized the market risk, given the level of uncertainty of the nuclear and space industries.¹⁷

6.5.2 Alternative Technologies

As noted above, ground-based use of RTGs has been eliminated. The biggest challenges for a replacement technology were for the alternative power source operating in the Arctic region to withstand temperatures below 0°C and to provide reliable power during the very limited sunlight available in the autumn and winter months. A notable alternative technology development program took place from the early 2000s to 2007 and involved a cooperative effort among NNSA’s Global Threat Reduction Initiative, the Russian Federation Navy, Sandia National Laboratories, the Southwest Technology Development Institute, the New Mexico State University Institute for Energy and Environment, the Kurchatov Institute, and the Norwegian Kystverket Lighthouse Commission. This collaboration tested a photovoltaic (PV) and battery system in two locations, Honnigsvag installation in Norway and the Cape Shavor lighthouse in the Russian Federation. The Kurchatov Institute installed a system at a third location in Karbas, Russian Federation—a small wind turbine system in addition to the PV and battery system. These electrical power systems are what is needed to power a 10-W light-emitting diode signal beacon, produced by the Nav-Dals company of St. Petersburg, Russia. The PV system consisted of five 40 peak-watt modules and a 950 ampere-hour nickel cadmium battery bank operating at 12 V.

During the sunny summer period, the battery bank maintained a full charge. From the autumnal equinox through the early winter period, the battery entered the mode of continuous discharge, but enough power was available for operating the light signal. The maximum discharge of the battery was about 65 percent of the total. In comparison, the hybrid system with the small wind turbine but the same type of PV modules and battery had a total discharge of 45 percent. The researchers concluded that either system was sufficient, but the wind turbine provided additional reliability (Hauser et al., 2007).

6.5.3 Alternative Technology Adoption Considerations

For space missions at and beyond the orbit of Mars, the available solar energy is not sufficient for powering space probes and rovers. RTGs will thus continue to be used by the United States and other space-faring nations in a safe and secure manner as discussed below. NASA’s RTGs have provided safe and reliable power for more than 50 years and more than 25 missions. The safety features involve layered defense with a robust fuel, a modular design, and multiple physical barriers. In particular, the fuel consists of plutonium dioxide in fire-resistant ceramic pellets to reduce the likelihood of dispersion in an accident. If the ceramic fractured, it would break into relatively large pieces instead of into breathable microscopic particles. In addition, iridium encases each fuel pellet and provides a corrosion-resistant, very high melting temperature layer of protection. Moreover, heat-resistant graphite forms

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¹⁷ Tyler Bernstein, Zeno Power Systems, letter to Ourania Kosti, National Academies, on January 31, 2021.

the impact shells for additional protection around the fuel. DOE is responsible for RTG production and delivery to NASA and conducts a safety analysis before each mission (NASA, 2005).

In 2009, because of concerns about reliability of supplies of plutonium-238, the European Space Agency decided to investigate alternative radioisotopes for powering its space RTGs. The agency selected americium-241 because it was economically affordable at high-isotopic purity and readily available from European reprocessing of commercial spent nuclear fuel, although its power density is about one-fifth of plutonium-238’s. In 2013, a European technical consortium announced that a prototype americium-241–fueled RTG had been tested and that the United Kingdom’s National Nuclear Laboratory had produced the americium-241 (Ambrosi et al., 2013). In 2019, the European consortium projected that by the second half of the 2020s, its RTG program would be ready to deliver a power system for space missions (Ambrosi et al., 2019).

6.6 CHAPTER 6 FINDINGS AND RECOMMENDATIONS

Finding 14: Little progress has been made domestically with adopting alternative technologies for some other commercial applications, particularly in some nondestructive testing applications and well logging. That is because there are currently no viable or cost-effective alternatives, the alternatives either compromise or do not offer enhancements in performance, or they produce data on material and structures that are not directly comparable to those produced by radioactive sources.

NDT often involves inspections of materials at remote outdoor locations where there can be harsh environmental and industrial conditions. In deciding on the NDT method to use, NDT technicians consider the capability for detectability of defects, types of defects in the subject material, accessibility to the inspected material, and economics of the method. Although x-ray methods can provide radiographic results similar to those from gamma-emitting radionuclides, x-ray systems require reliable electricity and cooling systems and tend to be larger and less robust than gamma radiography cameras. However, advancements in x-ray design have led to the development of pulsed x-ray sources that operate using battery power and have physical size close to that of a gamma radiography camera. However, the need for frequent costly replacement of the x-ray tubes and the frequent inability to operate still make gamma radiography the more reliable and preferred option. Micro-linacs have been explored but are more expensive than radiography cameras. Ultrasonic scans require skillful examination and interpretation by trained and certified technicians. While radioisotope and x-ray radiography methods can detect a wide spectrum of flaws, the differences in the physics between radiographic and ultrasonic techniques make each method sensitive to a particular flaw type—radiography is suited to detect volumetric flaws such as slag and porosity, whereas ultrasound is more suited to detect planar flaws such as cracks and lack of fusion.

For well logging, replacement technologies for radioactive sources confront several technical, logistical, and financial challenges. Although a cesium-137 source could potentially be replaced by an alternative x-ray source such as a linac, significant obstacles include the size of the x-ray source, its broad energy spectrum, source stability, and anisotropic radiation. Little development work has been done since 1987. For replacement of AmBe neutron sources, alternative (accelerator-based) tools are considered to be less accurate than the AmBe source in porosity determination. Because of differences in the physics, replacing current methods could create interpretation issues, including changed porosity and lithology sensitivity. Furthermore, some accelerator-based neutron sources incorporate security-sensitive, dual-use technologies that can potentially impede use, especially in politically sensitive regions.

Recommendation H: The National Nuclear Security Administration should engage with other offices within the Department of Energy, the National Science Foundation, and professional societies, to support equivalency studies for well logging and industrial radiography service providers that are considering replacing their radioactive sources and adopting an alternative technology. The findings of these studies should be made broadly available.

The lack of adoption of accelerator-based open-hole logging tools has been slow for three main reasons. First, the proposed alternatives mostly have not replicated exactly what the radioactive tools provide. Second, there

are no current strong business drivers for making the transition even for the major logging companies, although these large companies have the technological and financial wherewithal and have done research on alternatives. Third, the small- to medium-size logging companies would not have the technological capability or funding to develop, test, and deploy accelerator-based technology. Consequently, even if the technology were a perfect match in response characteristics, the small- to medium-size logging companies will not be able to readily transition. If pushed to make a transition, most claim they likely would go out of business.

There is an opportunity that could potentially be addressed by collective actions of industry associations with the partnership and support of government agencies.

As to further development and adoption of alternative technologies for gamma radiography, experts who presented at IAEA conferences in 2013 and 2019 have recognized that replacement with ultrasonic testing is at least several years away. They have underscored the technical constraints of the alternative technology and called attention to the need to conduct validations and standards development. Even with such validations, ultrasonic testing will likely continue to complement gamma radiography. Nonetheless, equivalency studies could help pave the way for further consideration and adoption of alternatives. It would also help to develop techniques for showing ultrasonic images that are comparable to what users are accustomed to seeing with radiographs and that would not require interpretation as is the case with current ultrasonic techniques.

Finding 15: No progress has been made domestically and internationally with adopting alternative technologies for calibration systems to replace cesium-137 and cobalt-60 sources. No obvious nonradioisotope alternatives exist for replacing the cesium chloride sources used in these applications, and no research and development is currently dedicated to exploring alternatives. The lack of alternatives poses an obstacle in global efforts to eliminate cesium-137 in the form of cesium chloride.

More than 50 years ago, cesium-137 was chosen as the basis for national and international calibration due to its monoenergetic gamma radiation in the middle of the measured spectrum of energies and its high precision and reproducibility in calibration facilities. The cesium-137 standard is being used to calibrate millions of radiation detectors annually in the United States and globally. The detectors’ deployment includes nuclear power plants to monitor surroundings, ports of entry to measure the radioactivity of cargo, medical facilities to ensure the safety of patients and medical personnel, and wherever there is a release of suspected release of radiation. The security concern is that calibration systems use cesium-137 in the form of cesium chloride, which poses potential dispersal hazards. However, there have been no domestic or international efforts toward developing alternative technologies.

Recommendation I: The National Institute of Standards and Technology should engage with the research community as well as federal, industry, and international partners to initiate research on alternatives to cesium chloride for calibration applications. This engagement should start immediately to prepare for the possible future elimination of the use of cesium-137 in the form of cesium chloride.

NIST’s position is that eliminating cesium chloride in calibration instrumentation would have adverse effects such as on the nation’s emergency response capabilities. The U.S. and other governments have considered policy changes that would eliminate cesium chloride in radioactive sources such as blood and research irradiators. To prepare for possible policy changes that would seek to eliminate cesium chloride from all high-activity radioactive sources, NIST should begin to explore other options for alternatives to cesium-137 such as high-energy x-ray technologies or different chemical forms of this radionuclide such as pollucite or vitrified cesium. The vitrified form of cesium is used in India in blood irradiators. Replacing radioactive cesium chloride with these other less-dispersible forms of cesium-137 could be acceptable for calibration applications because the cesium-137 spectrum would be maintained. Useful steps for NIST would include consulting and working with the research community and partners in federal and state agencies and industry as well as international partners and performing equivalency tests to ensure that there are no adverse impacts on current calibration and testing capabilities.