Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
CHAPTER 1 INTRODUCTION On September 28, 2006, a man identifying himself as Abu Hamza al-Muhajir, thought to be the leader of al-Qaeda in Iraq, appealed in an internet recording to experts in the sciences, âespecially nuclear scientists and explosives expertsâ to join the field of jihad or holy war by using unconventional weapons, including dirty bombs, on American targets (Rising, 2006). Since the attacks on the United States on September 11, 2001, such threats are taken much more seriously. A recent report on preventing radiological terrorism states that âThe likelihood of stolen Russian [ionizing radiation sources] being smuggled into the United States seems relatively low since a terrorist group would probably try to obtain a [radiation source] that is already located in the United States rather than risk detection at a point of entry into the countryâ (National Research Council, 2007). Whether terrorists would be able to obtain or gain access to a significant quantity of radioactive material and carry out an attack is a matter for analysis and debate, but the availability of high-intensity radiation sources is an important element of the risk (see Chapter 3 for a more detailed discussion of risk).1 Radiation sources come in many different types, forms, and intensities. The International Atomic Energy Agency (IAEA) issued a revised categorization system for radiation sources in 2003, ranking them according to the hazards they pose in descending order from Category 1 to Category 5 (IAEA, 2005a). (The definition of each category is provided in this chapter.) According to a survey by the U.S. Nuclear Regulatory Commission (U.S. NRC, 2007a), as of 2006 there were approximately 28,200 civilian Category 1 radiation sources in approximately 1,000 devices2 in the United States, sources that, if not safely managed or securely protected, would be likely to cause permanent injury to a person who came in contact with them for more than a few minutes, and would cause fatal exposures in a few minutes to an hour, if not shielded. Another roughly 25,500 sources in approximately 4,000 devices could cause permanent injury to someone in contact with them for a short time (minutes to hours), and without shielding could be fatal to a person exposed for a period of hours to days (Category 2 sources). Historically, the U.S. NRC and the Agreement States have issued and kept track of materials licenses (who is licensed to have radiation sources, what the sources are used for, and how much the licensee is permitted to hold). In the period immediately following the September 11 attacks, the U.S. NRC was unable to tell decision makers in the administration and Congress the number and locations of radiation sources in the United States. This was because neither the U.S. NRC nor most of the Agreement States maintained their own inventories of the actual radiation sources held by licensees and the locations of those sources, although the licensees themselves were required to maintain records on their own sources. The U.S. NRC took steps to remedy that situation, conducting voluntary surveys in 2004, 2005, and 2006 to learn the size and scope of the challenge entailed by source tracking and to establish 1 The terms radiation source and sealed source refer to encapsulated radioactive material. Devices such as x-ray tubes and linear accelerators can generate radiation and are referred to in this report as radiation generators, when not specifically identified by name. 2 The distinction between devices and sources is important. Many sources may be contained in a single device. For example, a Gamma KnifeÂ® used for treatment of brain lesions contains 201 cobalt-60 sources, each of which is a Category 2 source. The loaded device is a Category 1 device because of the aggregation of sources. This study examines possible replacements of radiation sources based on their applications, so it is focused mostly on devices, looking at replacing all of the radiation sources in a device or replacing the device with one that does not use radioactive material. 13
14 RADIATION SOURCE USE AND REPLACEMENT communication channels. Each year, the U.S. NRC has improved its reporting and identified additional licensees that should be surveyed, and each new survey has accounted for several thousand sources not included in the previous yearâs tally. A comprehensive radiation source tracking system for all Category 1 and 2 sources is to be in place by the fall of 2008 (U.S. NRC, 2007b).3 Concern about the potential for a radiological attack, the lack of information about the quantities and character of radiation sources in civilian use, and the modest level of security evidently afforded to many radiation sources at the time prompted Representative Edward J. Markey of Massachusetts and Senator Hillary Rodham Clinton of New York to sponsor a bill called the Dirty Bomb Prevention Act. The bill was not enacted into law, but language from the bill was included in the 2005 Energy Policy Act (commonly abbreviated as EPAct). A section of the EPAct is devoted to security of radiation sources, defined as high-hazard radioactive material. The section requires several actions by the federal government, including requirements that the U.S. NRC establish the national source tracking system mentioned earlier, lead an interagency task force to report to Congress on reducing the risk of radiological attacks (often referred to as dirty bomb attacks or radiological dispersal device [RDD] attacks), and request this study by the National Research Council of the National Academies on radiation source use and replacement. This study identifies the uses of radiation sources and how feasible it is to reduce the hazard from these sources by finding alternative means to accomplish their tasks, or by using less hazardous radioactive sources (see the full statement of task in Sidebar 1-1). The interagency task force issued its first report to Congress on radionuclide radiation source protection and security (U.S. NRC, 2006a), and the Department of Energy issued its report on alternatives to industrial radioactive sources in 2006 (DOE, 2006). There is a growing body of research and reports on radiological security and consequence management related to incidents with radionuclide radiation sources (see, e.g., Harper et al., 2007; IAEA, 2006a, 2003a,b, 2001; Musolino and Harper, 2006; CRCPD, 2006; ANL, 2005; Ferguson and Potter, 2004; Medalia, 2004; Zimmerman and Loeb, 2004; CDC, 2003; DHS, 2003; DOE/U.S. NRC, 2003; Ferguson and Lubenau, 2003; Ferguson et al., 2003; GAO, 2003; Van Tuyle et al., 2003; NCRP, 2001). The U.S. NRC and Agreement States develop, implement, and enforce regulations that protect against radiation exposures, both accidental and malicious. In addition, radiological safety is the central mission of the radiation protection systems recommended by the International Commission on Radiological Protection and the National Council on Radiation Protection and Measurements. Consideration of technological alternatives to radionuclide radiation sources also has been part of the radiation protection efforts (for an overview see Lubenau and Strom, 2002), and is recommended by the Health Physics Society (2006). This study builds on the work cited here to address the statement of task. SCOPE OF THE STUDY AND OVERVIEW OF THE REPORT STRUCTURE This study is not meant to be a review of all radiation sources. The U.S. NRC study request explicitly confines the study to Category 1 and 2 sources. Category 3 sources are mentioned as illustrations of particular points in the report (see, e.g., gamma-ray tools in Chapter 9) and as possible future concerns to the U.S. NRC. Further, both the U.S. NRC and Congress are chiefly interested in radiation source uses and possible replacements in the United States. Some radiation sources have uses outside the United States. These have only been included in the report if there is a reasonable expectation that the use might be adopted in the United States or to provide context. 3 The U.S. NRC has directed its staff to evaluate extending reporting and additional security requirements to Category 3 sources. The IAEA system suggests that a sliding scale of additional security measures be applied according to category.
INTRODUCTION 15 SIDEBAR 1-1 STATEMENT OF TASK The principal task of this study is to review the current industrial, research, and commercial (including medical) uses of radiation sources to identify uses for which: 1. the radiation source can be replaced with an equivalent (or improved) process that does not require the use of radioisotopes; or 2. the radiation source can be replaced with another radiation source that poses a lower risk to public health and safety if it is involved in an accident or used in a terrorist attack. The study should explicitly consider technical and economic feasibility and risks to workers from such replacements. The National Academies will issue a public report at the conclusion of the study. The report will contain a review of radiation source use, potential replacements for sources that pose a high risk to public health or safety, and findings and recommendations on options for implementing the identified replacements. The committee has focused primarily on identifying the radiation source applications and evaluating the technological options for replacing the radiation sources currently used in those applications. Evaluating the radiation source security and risks, and more specifically the probabilities and consequences of a radiological attack using radiation sources, is outside the committee's charge. The committee does, however, consider these factors in general terms because they help to prioritize among the radiation sources that should be considered for replacement and because they affect costs and perceived costs of using radiation sources. The statement of task states that the report should âexplicitly consider technical and economic feasibility.â The committee interprets âeconomic feasibilityâ to mean that a possible replacement should not be economically prohibitive, not that a replacement may be more (or less) expensive if it would provide the similar technical outputs. This element of the task statement does not require cost-benefit analyses, although such analyses may be valuable and necessary as part of implementing replacements (see Chapter 10). DEVELOPMENT OF A RADIATION SOURCE CATEGORIZATION SYSTEM Many of the radiation sources used throughout the world in medicine, industry, agriculture, research, and education are in the form of sealed sources, which are radioactive materials contained or bound in a solid material and encapsulated, typically in one or two welded stainless steel containers. The many applications of sealed sources, involving a variety of radioactive materials in a wide range of quantities, require varying levels of control and security according to the hazards the sources pose. Because they are sealed, intact sources typically present a risk of external radiation exposure only; that is, they irradiate tissue from outside the body. Sources that are leaking or have been punctured or broken can also cause internal radiation exposureâirradiation from inside the body, if ingested or inhaledâand contamination of the environment. However, as is described in Chapter 3, economic and social effects may be the most significant consequences of malicious use of radioactive material. The safety and security mechanisms in place for radioactive sources vary widely from country to country based in part on the differences in regulatory infrastructure for controlling radiation safety and security. This has led to concern among national and international agencies responsible for the safe use and transport of radiation sources.
16 RADIATION SOURCE USE AND REPLACEMENT Building on efforts begun in the 1990s and motivated by the September 11 attacks, the IAEA4 Board of Governors approved a revised Code of Conduct on the Safety and Security of Radioactive Sources in 2003 (IAEA, 2004a) using a system of source categories described in Categorization of Radioactive Sources (IAEA, 2005a). The Code of Conduct provides guidelines for countries in the development and harmonization of policies, laws, and regulations on the safety and security of radioactive sources. The categorization system was developed to provide for a logical international risk-based ranking of radioactive sources based on their potential for harm to human health and for grouping of source use practices into discrete categories. It was also intended to address the need for governments and regulatory authorities to make risk- informed decisions in establishing regulatory infrastructures, improving control over radioactive sources (including regulatory measures, registries, and import/export controls), prioritization of regulatory resources, preparing for and responding to emergencies, optimizing security measures for radioactive sources, including potential malicious use, and addressing other issues. Basis for the IAEA Categorization System Radiation sources pose low risks to radiation workers and the public when they are managed safely and securely. When mismanaged, however, they can cause an array of deterministic effects5 that can lead to acute radiation sickness, permanent damage to limbs and organs, and death, depending on the source and how it is mishandled. The IAEA chose human health and safety as the primary attribute of importance in the development of the categorization system, and focused on the potential to cause deterministic health effects as the basis for the system. The potential for harm involves not only the physical properties of the source (the radionuclide, type of radiation emission, and activity of the radiation source) but also the way in which the source is used. The actual practice in which the source is used and the shielding provided by devices containing the sources were also considered. Factors that were not considered include socioeconomic consequences of accidents or malicious acts, stochastic effects, such as the increased risk of cancer, and use of sources for medical reasons. The structure of the categorization system is based on a threshold level of risk associated with deterministic effects, above which a source is considered âdangerousâ6 because it could cause a fatal or life-threatening exposure or result in a permanent injury that decreases the quality of life. The IAEA threshold dose levels corresponding to these risks differ based on 4 One of the primary missions of the IAEA is to help countries upgrade nuclear safety and security, and prepare for and respond to nuclear and radiological emergencies. To fulfill this mission, IAEA plays an instrumental role in developing international conventions, standards, and expert guidance, with input from the agencyâs member countries. The IAEA focuses its efforts regarding radiation sources on assisting countries to protect people and the environment from harmful radiation exposure. 5 Effects associated with radiation are described as either deterministic or stochastic. Deterministic effects manifest themselves in a relatively short time after a high-intensity exposure to radiation (e.g., 1 or more sieverts) and can range from erythema (skin redness) to disruption of body-organ functions. Stochastic effects are increased risks of various maladies that manifest themselves over a longer time period. Most notable among the stochastic effects is induction of cancer, but heart disease and other conditions can also result, depending on the exposures. 6 As defined in the IAEA Safety Standards Series No. GS-R-2, Preparedness and Response for Nuclear or Radiological Emergency, a âdangerous source â¦ [is] a source that could, if not under control, give rise to exposure sufficient to cause severe deterministic effects.â A severe deterministic effect is one that is fatal or life-threatening or results in a permanent injury that decreases the quality of life. The dose considerations included not only external exposures to a bare (unshielded) source, but also dispersal of a source, for example, by fire, explosion, or human action, resulting in a dose from inhalation, ingestion, or skin contamination. See IAEA (2005a).
INTRODUCTION 17 the organ affected, such as 1 Gy delivered to the whole body (see Sidebar 1-2),7 1 Gy delivered to the bone marrow in 2 days, 6 Gy delivered to the lung in 2 days, 5 Gy delivered to the thyroid in 2 days, or 25 Gy absorbed in skin or surface tissue at a depth of 2 cm for most parts of body or 1 cm for the hand for a period of 10 hours (IAEA, 2005a, 2006). The authors of the system calculated the activity, referred to as the âDâ value, of each relevant radionuclide corresponding to that threshold risk level. This provides a relative ranking of radioactive sources and the practices in which they are used. Devices, including single- source devices, are classified into five categories, according to their potential for causing harmful health effects if not managed safely and securely. There is flexibility in the system, in that, although common practices (such as high-dose-rate brachytherapy) are grouped in one category, particular sources may be assigned to a category based on their activity (A) alone, by dividing their activity by the D value, resulting in an A/D ratio. Also, aggregations of sources in one location can be categorized by summing their A/D ratios. Although the categorization system was focused on sealed sources, it may also be applied to unsealed radioactive material in some situations. The definitions of the five categories, provided in plain language in IAEA (2005a), are listed below, and Table 1-1 lists the A/D ratios and examples of practices for each of the five categories in the categorization system. If not managed safely or securely, â¢ Category 1 sources could lead to the death or permanent injury of individuals in a short period of time. â¢ Category 2 sources could lead to the death or permanent injury of individuals who may be in close proximity to the radioactive source for a longer period of time than for Category 1 sources. â¢ Category 3 sources could lead to the permanent injury of individuals who may be in close proximity to the source for a longer period of time than Category 2 sources. Sources in Category 3 sources could but are unlikely to lead to fatalities. â¢ Category 4 sources could lead to the temporary injury of individuals who may be in close proximity to the source for a longer period of time than Category 3 sources. â¢ Category 5 sources could cause minor temporary injury of individuals, but are unlikely to do so. 7 Throughout this report, quantities are reported in SI units, which are explained in Sidebar 1-2. A glossary of terms can be found in Appendix C.
18 RADIATION SOURCE USE AND REPLACEMENT SIDEBAR 1-2 RADIATION QUANTITIES AND UNITS The metric system of units known as the SystÃ¨me International dâUnitÃ©s (International System of Units, or SI units) is based on units for seven basic physical quantities; all other quantities and units are derived from the basic quantities and units. The basic physical quantities are listed in the first table below. The radiation and radiological units cited in this report are derived units, defined in the second table. SI Base Units Name Symbol Quantity Meter m Length Kilogram kg Mass Second s Time Ampere A Electric current Kelvin K Thermodynamic temperature Mole mol Amount of substance Candela cd Luminous intensity SI Derived Units Relevant to This Report Expressed in Expressed Name Symbol Other SI Units in SI Base Units Energy, amount of heat Joule J Nm m2 kg sâ2 Power, radiant flux Watt W J/s m2 kg sâ3 Electric charge Coulomb C sA 2 â3 â1 Electric potential difference Volt V W/A m kg s A Celsius temperature degree Celsius Â°C K a Activity becquerel Bq sâ1 â1 Specific activity becquerel per gram Bq/g s gâ1 Absorbed doseb Gray Gy J/kg m2 sâ2 Dose equivalentc Sievert Sv J/kg m2 sâ2 Exposure (x- and Î³ -rays)d Coulomb per kilogram C/kg kgâ1s A a Activity of a radioactive substance is defined as the number of decays per time. Its SI unit is the becquerel (Bq) corresponding to one radioactive decay (disintegration) per second; its old unit, the curie (Ci), was originally defined as the activity of 1 gram of radium-226 and later as 3.7 x 1010 Bq. b Absorbed dose or dose is defined as the energy absorbed per unit mass of medium. Its SI unit, gray (Gy), is defined as 1 joule (J) of energy absorbed per kilogram of absorbing medium; its old unit is the rad, defined as 100 erg of energy absorbed per gram (g) of absorbing medium, which is 0.01 Gy. c Dose equivalent is defined as the dose multiplied by a radiation-weighting factor to account for the differences in biological harm to human organs that result from differences in radiation type and energy for the same physical dose received by the organ. The SI unit of equivalent dose is the sievert (Sv); the old unit is the rem, which is equal to 0.01 Sv. For x rays, gamma rays (Î³ rays), and electrons the weighting factor is 1; for protons it is 5; for alpha particles it is 20; and for neutrons it ranges from 5 to 20 depending on neutron energy. d Exposure is related to the ability of photons to ionize air. Its old unit, roentgen (R), is defined as charge of 2.58 x 10-4 C produced per kilogram of air. SOURCE: Adapted from Tables 3 and 4 from International System of Units (SI) (2006).
INTRODUCTION 19 TABLE 1-1 IAEA Radiation Source Categories Activity Examples of Practices Examples of Threshold Category Ratio and Devices Activity Levels (TBq) 1 A/D > 1,000 Radioisotope thermoelectric generators (RTGs), americium-241 60 panoramic irradiators, large self-shielded cobalt-60 30 irradiators, teletherapy, fixed multibeam cesium-137 100 teletherapy (Gamma KnifeÂ®) iridium-192 80 2 1,000 > A/D > Smaller self-shielded irradiators, industrial americium-241 0.6 10 gamma radiography, well logging devices californium-252 0.2 cobalt-60 0.3 cesium-137 1.0 iridium-192 0.8 3 10 > A/D > 1 High- and medium-dose-rate brachytherapy, americium-241 0.06 fixed industrial gauges (level gauges, dredger cobalt-60 0.03 gauges, high-activity conveyor gauges, spinning cesium-137 0.1 pipe gauges), well logging devices iridium-192 0.08 4 1 > A/D > Low-dose-rate brachytherapy (except strontium- 0.01 90 eye plaques and implant sources), thickness gauges, portable gauges, bone densitometers 5 0.01 > A/D > X-ray fluorescence devices, static eliminators, Exempt electron-capture devices quantity/D NOTE: 1 TBq = 27 Ci. SOURCE: Adapted from Table 1 of IAEA (2004a). PREVIEW OF TRADE-OFFS AND METHODOLOGICAL APPROACH Radiation sources have many valuable uses in medicine and industry. They contribute to the production of such final goods as sterile medical equipment, blood products that do not promote graft versus host disease in transplant recipients, cancer treatment, oil and gas development, and product quality assurance. A business or other organization chooses to use radiation sources over other alternatives because it perceives the benefits from using the sources to be greater than the costs it bears to use them. The perceived benefits arise because a business believes that the radiation sources allow it either to accomplish tasks that may not be feasible with other available technologies or to accomplish these tasks less expensively than with other technologies. The costs borne by the business include both direct financial costs of use (capital and operating) as well as indirect financial costs related to the safe and legal requirements of use (security, insurance, regulatory compliance, safety training). If all of the costs borne by society were included in the costs seen by radiation source users, then an efficient, and one could argue, socially desirable pattern of use would result. However, if the costs borne by society are substantially higher than those borne by users, then an inefficient and socially undesirable pattern of use may result. The commissioning of the study was motivated primarily by concern about one potentially large social cost: diversion of radiation sources for use in terrorism. Examining the study task in terms of costs applies a useful structure to the enquiry, pushing for quantification of costs where possible, and describing costs where they cannot readily be quantified. For the purposes of this study, the total social costs (TSC) of use of a radiation source technology can be divided into three components: the private costs of use (PUC), the costs associated with terrorism risks (TRC), and other social costs (OSC) for costs not borne by the user aside from those related to terrorism: TSC â¡ PUC + TRC + OSC (1-1).
20 RADIATION SOURCE USE AND REPLACEMENT In common economics terminology, PUC is the internal cost of use, and the sum of TRC and OSC is the external cost of use. As explained in Chapter 3, TRC primarily reflects the increased risk of the use of an RDD that would be borne by those persons exposed, the owners of the affected property, and the governments responsible for responding and mitigating the effects. OSC reflects other external costs, such as those related to safe disposal of spent sources or risks to employees not fully borne by firms through health and safety liability. If it were possible to monetize the external costs confidently, then one could imagine imposing user fees or regulatory burdens such that the private costs seen by users (PUC) equaled the TSC. Internalizing the external cost would create incentives for users to seek out the most desirable technology from the social perspective, perhaps even abandoning some uses altogether. The responses of users to the TSCs in turn would induce suppliers of radiation sources to search for alternative technologies with lower total social costs so that efficiency would increase over time. Implementing such an approach, however, is not practical because the external costs cannot be confidently monetized. Most importantly, it is not possible to monetize the terrorism risks (TRC) because we do not have a firm basis for predicting the relationship between particular radiation source uses and the expected costs of terrorism. While it may be possible to identify representative scenarios of RDD deployment or other acts of terrorism involving radiation sources, it is not possible to quantify the probabilities of these scenarios or how any particular type of radiation source contributes to them. Consequently, the committee cannot recommend a simple algorithm in seeking to identify desirable radiation source replacements. The methodological approach of the committee can be described in terms of a number of discrete steps. However, these steps were not and could not be followed in strict order. Rather, they convey the overall logic of the committeeâs efforts. Step 1: Identify radiation source technologies with relatively large TSCs The divergence between private and social costs of the use of radiation source technologies is primarily a function of their contribution to risk of the use of radiation sources in a terrorist attack and the actual cost of disposal of the used source, for sources that cannot now be disposed. The size of TSC is a function of a number of factors: the quantity of radioactive material used in the technology, the ease with which the material can be diverted from the technology, the likelihood of timely discovery of the diversion, the ease with which the material can be dispersed, and the costs of responding to a dispersion (including both the cost of cleanup and the consequences of the exposures). Step 2: Identify alternative technologies offering potentially large reductions in TSCs Technologies with large TSCs offer the greatest potential for overall reductions in the risk of radiological terrorism. There are five general types of alternative technologies to consider. 1. It may be possible to replace the radionuclide radiation sourceâbased technology with a technology not involving radiation. For example, ultrasonics technology can substitute for some types of radiography. 2. An alternative technology may replace the use of radioactive sources with x-rays or an electron beam from an x-ray tube or electron accelerator. Similarly, americium- beryllium sources may be replaced by neutron generators (particle accelerators that generate neutrons from targets).
INTRODUCTION 21 3. An alternative technology may replace one radionuclide with another that poses less risk. For example, americium-241 beryllium sources, which are of concern in part because of americium-241âs long half-life, might be replaced in some applications by californium-252, which has a much shorter half-life and higher specific activity (requiring a smaller quantity overall). 4. The chemical and mechanical form of the radiation source may be changed to make it less valuable in terrorist diversion. For example, cesium-137 might be incorporated into a mineral (pollucite) or polycrystalline ceramic to make it harder to disperse than the more common form, cesium chloride. 5. Looking over the entire life cycle of use, alternative approaches to security, transportation, and disposal may be introduced (see, e.g., Van Tuyle et al., 2003). The committee searched for technically feasible alternatives that offer reductions in TSC, ÎTSC. The delta symbol, Î, is used here to indicate change, so ÎTSC is a reduction or increase in TSC. Step 3: Assess the implications for changes in private costs, ÎPUCs, of alternatives Technically feasible alternatives may involve substantial increases in the private costs of use (ÎPUC). Alternative technologies that are currently available commercially, but are not widely used, will generally involve higher overall private costs than the technology in use or else they would already be in use. The higher costs may involve more costly capital, higher operating costs, or costly adjustments in other aspects of operations, such as irradiation of smaller batch sizes or multiple measurements with less effective measuring devices. When the magnitudes of both ÎTSC and ÎPUC are large, it may be socially worthwhile to consider public policies to make the technological transition more financially attractive or at least feasible for users. Step 4: Assess the implications for changes in other social costs, ÎOSC, of alternatives Alternative technologies may increase or decrease the other social costs involved in current use. For example, replacing radioactive sources with radiation generators may reduce disposal costs that are not currently borne by users (those that are not part of PUC). Moving from one radionuclide to another might change the exposure risks to employees or the transportation risks. It is also important to consider the fate of capital equipment and radiation sources used in the replaced technology. Viewing these steps comprehensively, the approach of the committee can be thought of as seeking alternatives to current uses for which the difference between all of the costs of the radiation sources and the alternative technologies has the largest negative value: ÎTSC â¡ ÎPUC + ÎTRC + ÎOSC (1-2). That is, the committee sought to identify opportunities where alternative technologies appear to offer reductions in TSC. It is worth noting several limitations of this methodological approach. First, the committee tried to use actual costs wherever possible, although some of these costs are approximate at best, and some costs simply could not be assessed. Second, the committee does not have sufficient resources or time to look far beyond existing technology. In particular, the costs of realizing alternative technologies that are physically feasible but not yet commercially available can only be assessed roughly. Finally, external components of TSCs can also only be assessed qualitatively. Still, even with these limitations, the committee chose this approach because its
22 RADIATION SOURCE USE AND REPLACEMENT structure makes explicit the key considerations in evaluating implementation of alternatives, and enables the committee to discuss costs, even if they are not readily expressed in terms of dollars.