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Radiation Source Use and Replacement: Abbreviated Version CHAPTER 2 RADIATION SOURCES IN THE UNITED STATES AND THEIR USES AND ORIGINS SUMMARY As a first step in understanding the uses of radiation sources in the United States, and to lay the groundwork for finding possible replacements, the committee examined the physical, chemical, and radiation characteristics of the radiation sources in use or available for manufacture; who uses the sources; and how they are applied. To carry out this aspect of its charge, the committee examined information collected by the U.S. Nuclear Regulatory Commission (U.S. NRC) and the U.S. Department of Energy (DOE), solicited input from representatives of the source manufacturers and distributors and from end users in several sectors, and carried out background scientific literature surveys. This chapter presents the results of those investigations. Each of the major applications is described in greater detail with an examination of possible replacements in Chapters 5 through 9. RADIONUCLIDES: THEIR FORMS IN CATEGORY 1 AND 2 RADIATION SOURCES AND THEIR DISPOSITION PATHS Radionuclides are types of unstable atomic nuclei. Each radionuclide can be identified by its chemical element (e.g., cesium), atomic mass (different isotopes of an element have different masses, e.g., cesium-135 and cesium-137), and its energy state in the case of metastable radionuclides (barium-137m). Critical characteristics of a radionuclide are its half-life, its mode of decay (alpha [α], beta [β], electron capture, spontaneous fission, and gamma emission [γ]), and the energies associated with any radiation emitted. For radionuclides whose decay products are also radioactive, one must also consider the same characteristics of the decay product. For this study, the chemical form and structure of the bulk that incorporates the radionuclide (e.g., cesium chloride in granular salt form) is also important. Before examining each important radionuclide and the major radiation source devices, it is useful to understand the disposition or disposal paths available for these sources. The waste classification system for disposal of civilian low-level radioactive waste in the United States is based on the harm the waste might cause 100 to 500 years in the future (see Title 10 of the Code of Federal Regulations, Part 61). U.S. NRC-regulated low-level waste is categorized as Class A, B, C, or Greater-than-Class C (in order of ascending hazard) based on the concentration of the radionuclide within its waste form.1 Classes A, B, and C waste can be disposed in near-surface disposal facilities, such as the Barnwell Waste Management Facility in South Carolina and the US Ecology facility near Richland, Washington.2 The federal 1 The concentration in the waste form may be quite different from the concentration when in use because of radioactive decay and because of packaging: In some cases, the concentration of the waste form for a source the size of a thimble may be averaged over a 200-liter drum in which it is packaged for transportation and disposal. See, e.g., GAO (2005). 2 South Carolina law will close the Barnwell facility to waste from all states except New Jersey, Connecticut, and South Carolina after June 30, 2008. The Richland facility is open only to Colorado, Idaho, Montana, New Mexico, Nevada, Alaska, Hawaii, Oregon, Utah, Washington, and Wyoming.
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Radiation Source Use and Replacement: Abbreviated Version government (DOE) is responsible for the disposal of Greater-than-Class-C waste (P.L. 99-240), which the U.S. NRC has determined is not generally suitable for near-surface disposal. DOE is in the process of planning to develop an environmental impact statement on options for Greater-than-Class-C waste,3 but no civilian disposal facility is now available for this type of waste. Relatively short-lived radionuclides, such as cobalt-60, have no upper concentration limit for Class C (or even Class B, in the case of cobalt-60) because the radioactive material decays sufficiently over the centuries for the hazard to diminish below regulatory limits. As a practical matter for disposal, however, there are also waste acceptance criteria at the disposal sites and these criteria preclude some high-activity sources because of worker exposure limits. DOE has disposal facilities for and regulatory authority over its own radioactive waste. DOE facilities operate under the same safety requirements (i.e., dose rate limits to workers, the public, and inadvertent intruders), but they do not use the Class A, B, and C system and can dispose of radiation sources with fewer impediments. Those facilities, however, only accept DOE waste. The National Nuclear Security Administration’s Offsite Source Recovery Project (OSRP) is responsible for recovery of sealed sources that represent threats to public health and safety and security. The OSRP recovers unwanted and abandoned sources, particularly sources that have no disposal path available.4 The OSRP maintains or contracts for short- or long-term storage, recycles or reuses radioactive material when appropriate, and disposes of recovered sources if an appropriate disposal site is available (DOE or commercial; U.S. NRC, 2006a). Most of the recovered sources are stored onsite at Los Alamos National Laboratory or at a contractor’s facility. The OSRP will take and store the sources free of charge, so licensees are not stuck with their sources forever, and abandoned sources and sources held by bankrupt licensees are secured. A licensee can pay for delivery of its sources to the OSRP so that it can rid itself of them on its own schedule rather than on the OSRP’s schedule. The OSRP is discussed further in Chapters 9 and 10. The Conference of Radiation Control Program Directors (CRCPD) has a program partially funded by DOE for assisting licensees and agencies that are in possession of unwanted radioactive material.5 The CRCPD “offers assistance in finding affordable, legal disposition for radioactive material through: Storage for decay, adoption by an individual, reuse by a device manufacturer, reprocessing of the material, acceptance by state or federal government, and commercial storage” (see CRCPD, 2007). The program provides education, guidance, and assistance in arranging for appropriate disposition of the radionuclide radiation sources, and funds for state and local governments to dispose of the radioactive material. One of these disposition options is to connect licensed parties seeking radiation sources with those who possess unwanted sources. (See U.S. NRC, 2006a, for a full description of options for 3 DOE recently issued a Notice of Intent to Prepare an Environmental Impact Statement for the Disposal of Greater-Than-Class-C Low-Level Radioactive Waste (Federal Register, 72, No. 140, (July 23, 2007):40135). 4 The OSRP is not the first program for recovering unwanted radionuclide radiation sources. When cesium-137 brachytherapy sources began to replace radium sources (and to encourage the switch) the U.S. Public Health Service funded a program to recover and dispose of radium sources that were disused, unwanted, or orphaned. The Environmental Protection Agency (EPA) took over the program, which ended in 1983. Subsequently, the Conference of Radiation Control Program Directors (CRCPD), with EPA support, mounted another radium recovery program. Together, these programs recovered 440 grams of radium (Lubenau, 1999). The CRCPD continues to assist states in retrieving and disposing of radioactive sources through its Orphan Sources Initiative. “In certain limited cases, the EPA and DOE, through CRCPD, provide funds to state radiation control programs for the disposition of radioactive sources when the owner cannot afford the costs of disposition or should not be held liable for those costs” (National Research Council, 2006a). 5 A member of the authoring committee for this report is executive director of the CRCPD.
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Radiation Source Use and Replacement: Abbreviated Version disposal of radiation sources.) The radionuclides in Category 1 and 2 sources in the United States are summarized in Table 2-1 and described in some detail along with their production and disposition options in the text that follows. TABLE 2-1 Summary of Radionuclides in Category 1 and 2 Radiation Sources in the United Statesa Radionuclide Half-life Radioactive Emissions and Energies Typical Specific Activity (TBq/g) [Ci/g] Total Activity in U.S. Inventory (TBq) [Ci] Major Applications Typical Activity (TBq) [Ci] Physical or Chemical Form Americium-241 432.2 yr α–5.64 MeV, γ-60 keV, principal 0.13 [3.5] 240 [6,482] Well logging 0.50.8  Pressed powder (americiu m oxide) Californium-252 2.645 yr α–6.22 MeV, fission fragments, neutrons, and gamma rays 20  0.26  Well logging 0.0004 [0.011] Metal oxide Cesium-137 (Ba-137m) 30.17 yr β-518 keV max with γ-662 keV (94.4% of decays) or β-1.18 MeV max 0.75  104,100 [2.8 million] Self-contained irradiators 75 [2,000] Pressed powder (cesium chloride) Teletherapy 50 [1,400] Calibrators 15  Cobalt-60 5.27 yr γ-1.173 and 1.333 MeV 3.7  7.32 million [198 million] Panoramic irradiators 150,000 [4 million] Metal slugs Self-contained irradiators 900 [24,000] 11  Teletherapy 500 [14,000] Metal pellets Industrial radiography 4  Iridium-192 74 d β-1.46 MeV max with 2.3 γ-380 keV average, 1.378 MeV max (0.04% of decays) 18.5  5,436 [146,922] Industrial radiography 4  Metal Plutonium-238 87.7 yr α–5.59 MeV, and γ-43 keV (30% of decays) 2.6  34.7  RTG 10  Metal oxide Pacemakers (obsolete) 0.1  Fixed gauges 0.75  Selenium-75 119.8 d γ-280 keV average, 800 keV max 20–45 [530–1200] 9.7  Industrial radiography 3  Elemental or metal compound Strontium-90 (Yttrium-90) 28.9 yr β-546 keV 5.2  64,000 [1.73 million] RTG 750 [20,000] Metal oxide aNuclear decay data for this table and throughout the report are from Firestone and Shirley (1996).
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Radiation Source Use and Replacement: Abbreviated Version Cobalt-60 Cobalt is a metal element with only one stable isotope: cobalt-59. When natural cobalt slugs are placed in a nuclear reactor, the nuclei absorb thermal neutrons to make cobalt-60, a radionuclide with a 5.27-year half-life. Cobalt-60 undergoes beta decay (emits an electron and a neutrino) and emits two gamma rays with each decay; one at 1.173 MeV and one at 1.333 MeV.6 Cobalt-60 sources are produced as high-specific-activity sources for teletherapy and industrial radiography and industrial sources for irradiators and other applications. High-specific-activity sources are small pellets (typically cylinders 1 mm in diameter and height) of metal produced in specialized high-flux nuclear reactors (e.g., Atomic Energy of Canada Ltd.’s NRU reactor in Chalk River, Canada; the Advanced Test Reactor in Idaho7; and the Research Institute of Atomic Reactors’ SM reactor in Dimitrovgrad, Russia), several thousand of which might be put into one source capsule to make a teletherapy source. Canadian CANDU power reactors produce the vast majority of the industrial cobalt-60 used in the United States (U.S. NRC, 2006a). The industrial targets are in the form of "pencils," which are sealed zircaloy tubes that house a stack of small, cylindrical cobalt slugs. A cobalt pencil is typically irradiated in the reactor core for approximately two or more years (Slack et al., 2003). Figure 2-1 shows a typical cobalt-59 pencil, slug, and pile of pellets before irradiation. The price of cobalt-60 sources varies based on the specific activity, total activity, and design of the source, ranging from about $40 to $53/TBq ($1.5 to $2/Ci) for industrial sources (see, e.g., Smith, 2006) to $215/TBq ($8/Ci) for a teletherapy source and $4,300/TBq ($160/Ci) for Gamma Knife® and radiography sources (S. Laflin, International Isotopes, Inc., personal communication with M. D. Lowenthal, July 19, 2007; G. Moran, Source Products Equipment Corporation, personal communication with M. D. Lowenthal, July 18, 2007).8 FIGURE 2-1 Typical cobalt-59 pencil and slug, which is irradiated to make industrial cobalt-60 sources. Also shown is a pile of cobalt-59 pellets, which can be irradiated in a high-flux reactor to make cobalt-60 teletherapy and industrial radiography sources. The ruler uses English units (1 inch = 2.54 cm). SOURCE: Image provided by committee. 6 There are four other gamma rays from cobalt-60 with lower emission probabilities. Any of these may be emitted during a particular decay, but fewer than one in every 6,000 decays is accompanied by one of these rays, so they are of lesser importance. 7 International Isotopes, Inc., contracts with DOE to have high-activity cobalt-60 produced in the Advanced Test Reactor. 8 These costs, like the costs for radiation sources and devices cited throughout this report, reflect the figures quoted to the committee by manufacturers and or customers. They are not necessarily representative of all prices paid by customers, which vary based on the size of the order, special requests, and other business relationships or agreements.
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Radiation Source Use and Replacement: Abbreviated Version Disposal of cobalt-60 sources in low-level waste disposal facilities is allowed under federal regulations; however, as noted above, high-activity sources are generally precluded by the waste acceptance criteria at the disposal sites because of worker exposure limits. The U.S. NRC (2006a) reports that the disposal costs at the Barnwell Disposal Facility in South Carolina are $1,870/ft3 of waste, plus surcharges including $11.32/GBq ($0.419/mCi), which totals about $130,000 for disposal of just one 11.1-TBq (300-Ci) cobalt-60 source. In practice, high-activity cobalt sources typically are returned to the manufacturer and distributor or sometimes taken by the federal government under the OSRP. MDS Nordion and International Isotopes, Inc., for example, mix pellets of cobalt from moderately decayed sources with cobalt pellets fresh from the reactor in some newly fabricated sources to balance overall activity and achieve the specified activity level for a new source. A few other facilities will store cobalt-60 and other relatively short-lived radionuclides for decay. There is a cost associated with returning used cobalt-60 sources to a manufacturer and distributor. That cost varies according to the quantity and age of the material and the cost of transportation, but is typically in the tens of thousands of U.S. dollars. The OSRP had recovered 606 cobalt-60 sources comprising 2,340 TBq (63,197 Ci) and registered another 442 sources totaling 16,759 TBq (452,481 Ci) as excess or unwanted, as of January 2007. Cesium-137 Cesium is a highly reactive alkali metal element with one stable isotope: cesium-133. The radionuclide cesium-137, which is produced by fission in a nuclear reactor, has a 30.17-year half-life and decays by beta decay to barium-137, which is stable, in 15 percent of the decays and to become barium-137m, a metastable radionuclide, in 85 percent of the decays. Barium-137m decays to stable barium-137 with a half-life of 2.55 minutes, emitting a 661.7-keV gamma ray (see Figure B-2 in Appendix B). Radioactive cesium sources are mostly used in self-shielded irradiators, which take advantage of its moderate gamma energy (requiring moderate shield thicknesses) and its 30-year half-life (enabling a source to last for the lifetime of the device). Cesium-137 is produced by fissioning uranium nuclei and then chemically separating the cesium from the irradiated nuclear fuel or targets.9 Most facilities that chemically process (reprocess) spent nuclear fuel to recover uranium and plutonium leave cesium in the waste stream. The cesium actually is made up of four isotopes: cesium-133 (stable), cesium-134 (2-year half-life), cesium-135 (2.3 million years), and cesium-137. Cesium-134 is produced in very small concentrations, which are reduced further by decay. Cesium-137 constitutes about 25 to 32 percent of the cesium atoms. All cesium atoms share in any cesium chemical reaction or compound so the cesium-137 concentration is diluted by the presence of other cesium isotopes. Separated radioactive cesium sold internationally is produced only by the Production Association Mayak (PA Mayak), in the Chelyabinsk region of Russia and sold through the U.K.-based company, REVISS. It is supplied as cesium chloride, a crystalline salt (it is chemically and structurally related to table salt, sodium chloride) that can be made in a range of particle sizes, from centimeter-scale blocks to powder, as is used in the manufacture of radioactive cesium chloride sources. After cold-pressing to form a pellet inside a stainless steel thimble-shaped receptacle, the receptacle is loaded in a protective stainless steel capsule that is welded to form the inner containment, and a second stainless steel jacket is welded over the first to 9 Because there are four atomic mass units between cesium-133 and cesium-137, and because xenon-136 (a material that would decay to cesium-137 after absorbing a neutron) is a noble gas that cannot be formed into a dense target, it is impractical to produce cesium-137 by irradiating low-atomic-number targets with neutrons (neutron activation).
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Radiation Source Use and Replacement: Abbreviated Version form the actual sealed radioactive cesium chloride source. The production of radioactive cesium chloride sources is carried out at around 200°C because cesium chloride is hygroscopic. Cesium chloride undergoes a change in crystal structure above 469°C with an accompanying 19 percent density reduction. It melts at 645°C with a further increase in volume (Zull, 1996). Because of the volumetric changes that occur on heating and cooling, which could distort or rupture the stainless steel container if the radioactive cesium chloride were packed into the capsule near its maximum density, the cesium chloride is emplaced in the capsule in the form of a porous, pressed pellet with a density of about 2.5 to 2.7 grams per cubic centimeter (g/cm3). (The density of pure cesium chloride crystals is 3.99 g/cm3.) Cesium chloride is soluble in water at room temperature and so, if it is intentionally or accidentally removed from its container, it can readily be dispersed. If a leak in the stainless steel container were to occur, the cesium chloride could dissolve in water and contaminate the nearby environment, as happened in the water tank of a panoramic irradiator facility used for sterilizing medical devices in Decatur, Georgia, in 1988 when a radioactive cesium chloride source containment failed due to thermal cycling and stress corrosion cracking (Setser, 1990; see Chapter 3 for a description of this incident). Cesium chloride is highly reactive in the environment; binding to surfaces and even migrating into concrete (see Chapter 3). If it enters the body, it disperses wherever water goes and delivers a whole-body dose. One approach to decreasing the problems posed by the very high solubility of cesium chloride in water is to use another compound containing cesium-137 as a direct replacement for the cesium chloride powder. Alternative, lower specific activity forms of cesium-137 sources are currently used for lower Category 3 sources used in some industrial process control and in well logging devices. Two vitrified forms (glasses) containing more dilute concentrations of cesium-137 than in radioactive cesium chloride have been used. One is prepared by absorption of the cesium-137 isotope into a silicate zeolite which is then heated to form a glass typically containing about 0.22 TBq/cm3 (6 Ci/cm3) of cesium-137, according to QSA Global (H. Evans, QSA Global, verbal presentation to the Committee on Radiation Source Use and Replacement, Irvine, California, February 1, 2007), a factor of 8 lower than radioactive cesium chloride, which contains 50 atomic percent (79 weight percent, which is 0.64 to 0.82 TBq/g [17 to 22 Ci/g] depending on isotopic mix or 1.7 to 2.1 TBq/cm3 [45 to 58 Ci/cm3] for a cesium chloride density of 2.6 g/cm3). The other is formed by a sol-gel process from a cesium-137 isotope salt. In this process, the cesium is attached through ion exchange to solid particles of glass formative materials in a suspension. These particles are gelled through chemical reactions and then heated to form a glass pellet. According to QSA Global, this latter process typically produces glass with 0.65 TBq/cm3 (18 Ci/cm3) (H. Evans, February 1, 2007), which is a factor of 2.6 lower than radioactive cesium chloride. The glasses are substantially less soluble in water and consequently less easily dispersed than the cesium chloride pellets. Being brittle, they can, like cesium chloride, be dispersed in an explosion. The glass forms of cesium-137 sources rather than cesium chloride are used in well logging because there are currently well logging standards (10 CFR § 39.2) that stipulate that the form of the cesium source must withstand impact loading (forces applied suddenly) during service and resist long-term chemical attack in the event that they are lost belowground. Both of these glass processes are commonly used to make sources in the range from 0.0075 to 0.11 TBq (200 mCi to 3 Ci), although larger sources can be made, especially with a process line tailored to making larger sources. In the past, another, lower specific activity form of cesium-137, but with higher cesium loading than the glass, was available. It is a compound commonly referred to by its mineral name, pollucite. Pollucite tumor irradiation sources were manufactured in France by ORIS-Bio International in the early 1990s but were apparently discontinued because the maximum specific activity attainable with pollucite is lower than that of cesium chloride. Pollucite, (Cs,Na)2.[Al2Si4O12], has a density of 3.3 g/cm3 and melts above 1900°C. Because it can contain up to 10 atomic percent cesium (43 weight percent, which is 0.35 to 0.45 TBq/g [9.5 to 12 Ci/g],
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Radiation Source Use and Replacement: Abbreviated Version or 1.2 to 1.5 TBq/cm3 [31 to 40 Ci/cm3]), its specific activity is lower by a factor of 1.5 than that of radioactive cesium chloride at a density of 2.6 g/cm3. Pollucite is known to be substantially less soluble in water than cesium chloride; the naturally occurring mineral is the primary ore for cesium and requires a series of acid-base reactions to extract the cesium. Further studies have shown that pollucite resists attack by high-temperature water (Komareni and Roy, 1983) and under hydrothermal conditions (Minura et al., 1997). Indeed, based on its resistance to water attack, pollucite has been identified in several studies as the preferred host to tie up cesium-137 in the long-term immobilization of nuclear waste (Clarke, 1983; Komareni and Roy, 1983; Clarke et al., 1981). Radioactive cesium pollucite is much more difficult to produce than radioactive cesium chloride, which makes the pollucite form not only more expensive but also more difficult to research. The main difficulty in producing pollucite, as well as other alternative compounds including the glass forms of cesium, is associated with the volatility of cesium at high temperatures. This volatility leads to the creation of more contamination and process waste than is created in production of the other material forms of radioactive cesium mentioned here. To minimize volatility requires low-temperature chemical processing together with the capture of any cesium-137 vapor. A number of chemical reactions have been demonstrated to produce pollucite powders, including a hydrothermal reaction from cesium hydroxide and solid aluminum metal and silica (MacLaren et al., 1999): (2-1). Alternatively, cesium-137-substituted pollucite can be synthesized by mixing colloidal silica solution with solutions of aluminum and cesium nitrates in stoichiometric proportions, followed by evaporation and calcining. This is the method used by ORIS-Bio International, which also sintered the pellets to 1200°C (Hess et al., 2000). A cesium-137-substituted pollucite has also been synthesized using a zeolite route, similar to the method used for creating the glass form of cesium. Irrespective of the method of forming pollucite powders, they could be pelletized by warm pressing prior to sealing in stainless steel capsules. Production of cesium-137 pollucite at a facility that currently makes cesium chloride radiation sources would require establishment of a new process line, although the new line might be able to use some of the preexisting equipment. This would entail an investment that the committee is not equipped to estimate. An alternative approach to reducing solubility and dispersibility is to make cement incorporating the cesium-137 by the addition of cement paste and fillers. This approach has the advantage of low-temperature processing and, with judicious choice of cement phase, low aqueous solubility (Mimura et al., 1997). However, the dilution associated with making cement limits the attainable specific activity. Also, the product remains a brittle solid that could degrade due to radiation effects, so it does not lower the cesium’s potential dispersibility in an explosion. The cementitious approach has advantages for large-scale immobilization of wastes containing cesium-137. It has been difficult to gather information on the details of production processes for cesium-137 sources at PA Mayak. A source distributor told the committee that there are difficulties in making high-activity vitrified sources, including self-shielding effects, although self-shielding is not a significant factor in these sources. Another expert alluded to radiation damage to the glass, which is more plausible (V. Pet’kov, Nizhni Novgorod State University, personal communication to M. D. Lowenthal, May 28, 2007). Researchers in Russia who work with PA Mayak informed the committee that although PA Mayak mainly produces cesium-137 sources in the chloride form, it also makes smaller quantities of both the glass and pollucite cesium sources. They are also working actively on developing phosphate ceramic forms (i.e.,
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Radiation Source Use and Replacement: Abbreviated Version CsMgPO4) that they say can potentially be made with specific and total activity more comparable to radioactive cesium chloride (B. F. Myasoedov, Russian Academy of Sciences, personal correspondence to M. D. Lowenthal, May 28, 2007; Pet’kov, personal communication, 2007). The committee’s ability to learn about the Russian products, production methods, and research and development was somewhat limited—the committee is neither a potential customer nor a potential sponsor of work on these topics. An entity that can act in one of those roles could make more progress in gaining information and promoting the development of the more robust matrixes for cesium-137 (see Chapter 10 concerning research and development). The price of cesium-137 chloride ranges from about $250/TBq ($9.30/Ci) for very large purchases (over 4,500 TBq [120,000 Ci]) to $10,500/TBq ($390/Ci) for a single relatively small (0.75-TBq [20-Ci]) source. A single 81.5-TBq (2,200-Ci) source costs a little over $34,000 (REVISS, 2003). Many cesium-137 sources cannot be disposed in existing low-level radioactive waste disposal facilities. The Class C limit for cesium-137 is about 170 TBq/m3 (4,600 Ci/m3). The radiation sources and their packaging (over which volume the activity can be averaged) may be substantially smaller than a cubic meter, so the activity limit for disposal of a source is lower than 170 TBq. Further, just as with cobalt-60, waste-acceptance criteria restrict what sources may be disposed at a particular facility with much lower limits than concentration limits listed in the federal regulations. The U.S. NRC (2006a) reports that the Barnwell Disposal Facility in South Carolina does not accept cesium-137 sources containing more than 0.37 TBq (10 Ci). The U.S. Ecology low-level waste disposal facility in Richland, Washington, can dispose of a cesium-137 source of activity up to 1.1 TBq (30 Ci) in a 200-liter (55-gallon) waste drum, because that is the maximum allowed in the U.S. NRC Branch Technical Position on Concentration Averaging and Encapsulation (U.S. NRC, 1995)10 Some manufacturers that use cesium-137 will take back used cesium-137 sources for a fee. The cesium-137 may be repackaged for reuse or, if a Canadian company handles the decommissioning of the cesium source, the source may be disposed of in Canada. The OSRP had recovered 393 cesium-137 sources totaling about 310 TBq (8,393 Ci) as of January 2007, and another 600 sources totaling 905 TBq (24,446 Ci) were registered as excess or unwanted sources (Pearson, 2007). Iridium-192 Iridium, one of the two densest metals (22.42 g/cm3, same as osmium), is very hard and brittle, and is difficult to machine. It is also very resistant to chemical reaction and has a high melting point (over 2400°C). Natural iridium, which is found alloyed with platinum and in nickel ores, is 37 percent iridium-191 and 63 percent iridium-193. Iridium-192 radiation sources are used in gamma radiography (e.g., nondestructive inspection of pipes) and in brachytherapy, although brachytherapy sources in the United States are below the Category 2 threshold. Iridium-192 radiation sources are made by irradiating natural iridium in a nuclear reactor. The iridium-191 can capture a neutron to create iridium-192, which has a 73.83-day half-life and has a 95 percent probability of decaying by beta decay to platinum-192 and emitting gamma rays and a 5 percent probability of decaying by electron capture to form osmium-192. In the 10 According to the facility’s 2007 rate sheet (US Ecology Washington, Inc., 2007), the marginal cost of disposal of a single drum could be as low as $20,630 if the radiation source is placed in a lead pig prior to solidification, reducing the dose rate at the surface of the drum below 10 mSv/hr (1 rem/hr). If a pig is not used, the cost is more like $64,310, based on a surface dose rate of 64 mSv/hr (6.4 rem/hr).
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Radiation Source Use and Replacement: Abbreviated Version decay to platinum-192, on average 2.33 gamma rays are emitted with energies ranging from 135 keV to 1.378 MeV,11 with an average energy of 380 keV. Iridium sources are usually in the form of wires or stacks of thin foil discs rather than bulk material pellets, slugs, or powders. Used iridium-192 sources typically can be shipped back to their manufacturer and distributor or stored for decay because of their relatively short half-life. So, although disposal of iridium-192 is not a problem, the short half-life forces users to replace the sources frequently, meaning that many sources are in transport and storage at any given time. Americium-241 Americium is an actinide or transuranium element with no stable isotopes. Like the other actinides, americium oxidizes fairly readily. Americium is produced by successive neutron captures in uranium-238, its activation products and decay products, to produce plutonium-241, which decays to americium-241 with a 14.4-year half-life. Americium is recovered from aging plutonium stocks in which it builds up through radioactive decay. Americium-241 decays with a half-life of 432.7 years by emitting an alpha particle. The alpha particle has an average energy of 5.465 MeV and is accompanied by a 13.9-keV x ray in 43 percent of decays and a 59.5-keV x ray in 36 percent of decays, and no x rays in the other decays. The decay product, neptunium-237, is also radioactive, with a 2-million-year half-life. Americium-241 is used both as an alpha source and with beryllium as a neutron source (called an americium-beryllium or Am-Be source). In an Am-Be source, some of the alpha particles from decay of the americium are absorbed in the beryllium, which then emits a neutron with energy ranging from 0 to about 11 MeV with the average energy at about 6 MeV. Am-Be produces about 1 neutron for 20,000 alpha decays. Am-Be sources, such as those used in oil well logging, are typically formed by cold pressing mixtures of americium oxide (AmO2) and beryllium powders to form a pellet which is then either diffusion bonded to a metal strip (for small sources) or sealed in a welded stainless steel container. Once supplied by the DOE Isotopes Program (separated from plutonium at Los Alamos National Laboratory), americium-241 is now only produced by the Russian radionuclide production facility at the PA Mayak. Americium-241 is supplied globally by PA Mayak, but through longstanding business agreements with PA Mayak and REVISS Services, the majority of Russia’s americium-241 is made available to the West via QSA Global (formerly AEA Technologies QSA, Inc., formerly a part of Amersham). REVISS may also manufacture Am-Be well logging sealed sources directly. The Am-Be material is sold by the gram (1 g is about 0.127 TBq [3.4 Ci]), but the price of a radiation source depends on several factors including the manufacturing batch size and particularly the customer’s design specification regarding both physical/mechanical integrity requirements and neutron output. The prices are not public, but anecdotally the price range of a typical logging-while-drilling Am-Be well logging source now is approximately $30,000 to $50,000 and for a wireline Am-Be source it is $60,000 to $80,000. Because of the current shortage of Am-Be, there is a queue to get new sources as they come available. Some customers are willing to pay a premium above the current base prices for expedited delivery and other special services. QSA Global has said that the supply chain is responding to the Am-Be shortage by building a capability for more production. New Am-Be sources can also be obtained from old Am-Be sources. The “recommended working life” of an Am-Be source is 15 years, after which the source manufacturers recommend that the sources be recertified (if it is in good 11 The 1.378-MeV gamma is emitted in one of every thousand decays. Two other, higher energy gamma rays (1.384 MeV and 1.406 MeV) are emitted in 3.2 and 3.9 of every 100,000 decays, respectively.
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Radiation Source Use and Replacement: Abbreviated Version condition), reencapsulated (if the capsule is slightly damaged, but the design is still in use), or recycled (if the design is no longer in use or the damage to the capsule is severe, then the raw Am-Be can be removed and manufactured into a new source). Some companies, such as Gammatron, Inc., offer Am-Be source recycling services; QSA Global does not, although representatives of the company have suggested that it would if interest in the service grows. Americium-241 sources cannot be disposed in commercial low-level waste disposal facilities. The Class C limit for alpha-emitting transuranic nuclides with half-life greater than 5 years is 3,700 Bq/g (100 nCi/g ), a factor of nearly 300 million lower than the concentration in a typical radiation source. The only current disposition path for these sources is through the OSRP, which stores them pending approval to dispose of them. Sources that are determined to have originated in the DOE Isotopes Program (or its predecessors) can be disposed in the Waste Isolation Pilot Plant (WIPP), a deep geologic repository for transuranic waste of “defense origin.” Other Category 2 americium sources have no disposal option available. As of January 2007, the OSRP had recovered or collected 10,154 of these sources, totaling 507 TBq (13,698 Ci). Another 1,012 sources totaling 62.6 TBq (1,689 Ci) have been registered with the OSRP as excess or unwanted sources (Pearson, 2007). Plutonium-238 Plutonium is an actinide or transuranium element with no stable isotopes. It is a silvery-white reactive metal that turns a dull, darker hue when it oxidizes, which it does readily. It has low solubility in pure water, but saltwater and halide acids attack it vigorously. Plutonium-238 is produced by neutron absorption in neptunium-237, which itself is produced by irradiation of uranium in a reactor followed by chemical separations. Plutonium-238 has a half-life of 87.7 years, decaying by alpha decay with an average energy of 5.486 MeV. The decay product, uranium-234, is a naturally occurring radionuclide.12 The heat generated by decay in relatively pure plutonium-238 is such that a solid sphere of the material the size of a golf ball will glow red from thermal radiation if it is not actively cooled. The only current user of high-activity plutonium-238 sources in the United States is the National Aeronautics and Space Administration (NASA), which uses the sources in radioisotope thermoelectric generators (RTGs) for probes that require a significant nonsolar power source (e.g., the Cassini probe that was sent to the planet Saturn). The United States has some hundreds of kilograms of neptunium-237 stored in solution at the Savannah River Site. A facility for production of plutonium-238 and manufacture of the RTGs is being constructed at the Idaho National Laboratory. The heat sources are in the form of an oxide pellet with 1.25 TBq (33.6 Ci) of plutonium-238 constituting 80 percent of the metal in the oxide for a 1-W heat output (NASA, 2006). A source of this activity is estimated to cost $3,600 (NASA, 2004). The sources are loaded into a robust housing that is designed to keep them intact even in case of launch or reentry accidents. Like americium-241 sources, plutonium-238 sources do not have a commercial disposal pathway. The OSRP has recovered 2,169 sources comprising 407 TBq (10,993 Ci) and has registered 112 more (298 TBq or 8,043 Ci) as excess or unwanted as of January 2007 (Pearson, 2007). The OSRP has disposed of many of the recovered sources in the WIPP. 12 Uranium-234 constitutes 0.0055 percent of natural uranium because although it has a half-life of 245,000 years, it is also a decay product of uranium-238.
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Radiation Source Use and Replacement: Abbreviated Version Selenium-75 Selenium is a volatile, reactive, and corrosive element chemically resembling sulfur and forming extremely toxic compounds. It has moderate density (4.3 g/cm3 to 4.8 g/cm3) and melts at 217°C. Selenium has several natural isotopes: selenium-74 (0.89 percent), selenium-76 (9.36 percent), selenium-77 (7.63 percent), selenium-78 (23.78 percent), selenium-80 (49.61 percent), and selenium-82 (8.73 percent). Selenium-75 decays by electron capture with a half-life of 119.8 days to stable arsenic-75, emitting an average of 1.75 gamma rays with an average energy of 215 keV each, and a peak energy of 800 keV. It is used in radiography cameras for thin-walled structures, although it is not commonly used in the United States. Californium-252 Californium is an actinide element with no stable isotopes. It is produced by successive neutron captures in actinide targets. Californium-252 has a 2.645-year half-life and decays by spontaneous fission 3.1 percent of the time and by alpha decay in the other 96.9 percent. The fissions release neutrons, and thus californium-252 is a very intense neutron source (2.3 × 1012 neutrons per second per gram). Because a uranium-238 nucleus must absorb 14 neutrons without undergoing other reactions that reduce the number of nucleons to yield a californium-252 nucleus, californium is produced in very small quantities. Oak Ridge National Laboratory currently produces only about 0.25 grams of californium-252 per year from feedstock at the Savannah River Site. The Research Institute for Atomic Reactors in Dmitrovgrad, Russia, is the only other facility that produces this radionuclide, and its production capacity is estimated at 0.025 grams per year (NRC, 2003). Yet it takes only tiny quantities to make a useful source: A Category 2 californium source contains at least 0.2 TBq, which is only about 10 mg of californium-252. The OSRP has 16 californium-252 sources registered for recovery, totaling less than 37 GBq (1 Ci), and has already recovered 12, also totaling less than 37 GBq (Pearson, 2007). Strontium-90 Strontium is a reactive metal typically found as an oxide or a salt. It has four stable isotopes, strontium-84, -86, -87, and -88, the last of which is the most naturally abundant (82.6 percent). The radionuclide strontium-90 is a fission product produced in 5.8 percent of thermal fissions in uranium-235 and 2 percent of thermal fissions in plutonium-239. Strontium-90 decays by beta decay (0.546 MeV) with a half-life of 28.78 years to yttrium-90, which itself decays by fairly high energy (2.28 MeV) beta decay with a 2.67-day half-life. Category 1 and 2 strontium-90 sources in the United States are only used as power sources in RTGs. Category 3 strontium-90 sources are used in radiotherapy of very superficial lesions. Strontium-90 is generated in nuclear power or isotope production reactors and is found in high-level radioactive waste and fallout from nuclear weapons testing. Strontium-90 does not emit penetrating gamma rays, so when it is a contaminant it is only a concern for external exposure if it is deposited on the skin. The major concerns are internal exposures because of the high-energy beta emissions and because strontium is in the same chemical group as calcium, so the human body concentrates ingested strontium in the bones where it resides essentially permanently rather than being eliminated through common bodily functions. High-activity strontium-90 sources, however, produce significant bremsstrahlung radiation from stopping of the high-energy electrons emitted by nuclear decay. This bremsstrahlung radiation
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Radiation Source Use and Replacement: Abbreviated Version can be enough to cause deterministic health effects if a very high activity source is involved (such an incident occurred with an RTG in the Republic of Georgia in 2002). The committee found no reliable cost estimates for strontium-90, although Malvadkar and Parsons (2002) report a cost found on the internet of $250 per watt thermal for a strontium-90 RTG. With about 5.4 TBq per watt, this translates to approximately $45 per TBq (about $1.70 per Ci) in an RTG that might contain over 1,500 TBq. As with cesium-137, strontium-90 can be acquired from the PA Mayak, in Russia. Also like cesium-137, there is a 259-TBq/m3 (7,000-Ci/m3) limit on the concentration of strontium-90 allowed in near-surface disposal of low-level radioactive waste. Thus, high-activity civilian strontium-90 sources have no disposal option available now. OSRP has recovered 10 strontium-90 sources containing nearly 2,740 TBq (74,000 Ci). Another 100 sources containing nearly 13,500 TBq (364,000 Ci) have been registered with OSRP for recovery (Pearson, 2007). Most of the activity in these registered sources is concentrated in a small number of RTGs. USES OF CATEGORY 1 AND 2 RADIATION SOURCES IN THE UNITED STATES The major applications for the high-activity sources include large panoramic irradiators for bulk sterilization, self-shielded irradiators for research and blood irradiation, teletherapy and Gamma Knife® machines for cancer therapy, RTGs, radiography cameras, and well logging tools. Each of these device applications is described briefly below. Some of the security hazards associated with the applications are discussed in the next chapter, and all but the RTGs are discussed in greater detail in Chapters 5 through 9. Panoramic Irradiators Among devices that use radiation sources, panoramic irradiators have the greatest activity. A typical panoramic irradiator contains 40,000 to 260,000 TBq (1 to 7 million Ci) of cobalt-60. These facilities are used primarily to sterilize single-use medical products and devices, but they are also used to sterilize other products. Panoramic irradiators are contained in large buildings with radiation shielding provided by a maze of concrete walls (the walls around the irradiation chamber are typically around 2 m [6 ft] thick). The cobalt-60 sterilization facilities use standard cobalt-60 pencils mounted in a source array. Each pencil contains 16 slugs of cobalt-60 with each slug approximately 8 mm in diameter and 2.5 cm in length (1/4 in. × 1 in.) having an activity of approximately 37 TBq (1,000 Ci). The pencils are clipped into a large, usually planar array that measures roughly 3 m × 6 m (10 ft × 20 ft) in size. The array is kept shielded in a pool that covers the array with several meters of water to serve as shielding when the array is not in use. Products to be sterilized are placed in containers that are carried by a conveyer (a belt or a hanger system) that passes the containers through the shielding maze and pauses next to the array for a set irradiation dose. In modern facilities, the entire process is automated. Panoramic gamma irradiators and their replacements are discussed in Chapter 6. In 2002, the International Atomic Energy Agency (IAEA) reported that 142 sterilization and food irradiators operated worldwide (IAEA, 2002). This figure is probably now over 160 (see Chmielewski and Haji-Saeid, 2004), and there are 63 in the United States today. The cobalt-60 pencils come from two source suppliers: The Canadian company MDS Nordion has the largest share of the market, and the international marketing consortium REVISS has the rest. REVISS acquires its cobalt-60 from both PA Mayak and the Argentine CNEA (see Production of Category 1 and 2 Radiation Source Material, below).
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Radiation Source Use and Replacement: Abbreviated Version Self-Contained Irradiators Self-contained irradiators, also known as self-shielded irradiators, are used mainly for biomedical and radiation research, and blood irradiation. There are three self-contained irradiator device manufacturers operating today: MDS Nordion of Canada; CIS-US, Inc., a French company that no longer manufactures new machines but is still servicing those in existence here in the United States; and a U.S. company, J. L. Shepherd and Associates. There are 1,341 self-contained irradiators that use radionuclide radiation sources in the United States, approximately 85 percent of which use cesium-137, while nearly all of the remaining devices use cobalt-60 (U.S. NRC, 2007a).13 These include blood irradiators, research irradiators, and calibration irradiators. Self-contained gamma irradiators and their replacements are discussed in Chapter 5. The blood irradiators typically weigh about 1,000 kg and contain from 75 to 260 TBq (2,000 to 7,000 Ci) of cesium-137, with a typical source loading of 110 TBq (3,000 Ci). The U.S. NRC estimates that about 30 of the roughly 550 blood irradiators in the United States now use cobalt-60 (U.S. NRC, 2007a). (The use of cobalt-60 sources in self-contained irradiators is discussed in Chapter 5.) Self-contained irradiators used for research have a much broader spectrum of source sizes and device weights. The approximately 192 cobalt-60 research irradiator devices can weigh up to 4,000 kg and can contain over 1,100 TBq (30,000 Ci). The roughly 490 cesium-137 research irradiators can contain similar activity levels (740 TBq or 20,000 Ci) but weigh no more than 3,000 kg. This shows that cesium-137 dominates the research irradiator field, too, with over 72 percent of the roughly 680 machines using this radionuclide while nearly all of the rest use cobalt-60. Some devices, including some kinds of radiation detectors and dosimeters, require irradiation calibration that is both precise and accurate at high doses. Some calibration sources used for these purposes are also considered self-contained irradiators. Radionuclide sources are typically used for this purpose because the decay energy (and decay rate) is known or readily calculable. These calibration irradiators are, on average, smaller-activity sources than the other self-contained irradiators, but some hold about 80 TBq (2,200 Ci) of cesium-137. The U.S. NRC reports 104 of these in non-fuel-cycle facilities, one of which uses cobalt-60 and all of the others of which are loaded with cesium-137. Radiotherapy: Teletherapy and Gamma Knife® Teletherapy devices are used to treat malignant tumors. Unlike brachytherapy, where small radiation sources are placed in or near the tumor, the teletherapy radiation source is kept at a distance from the patient and a beam of radiation is directed to the tumor. Teletherapy machines contain 37 to 550 TBq (1,000 to 15,000 Ci) and, at least in the United States, virtually all (247 of 248) use cobalt-60 (one uses cesium-137). Most of these are thought to be in storage for decay or converted to nonmedical uses (i.e., for fixed radiography, research irradiation, or teaching and research), because linear accelerators have replaced nearly all cobalt-60 teletherapy devices in medical practice in the United States, even in many veterinary clinics. There are an estimated 3,000 teletherapy machines in use around the world, and some of the older machines (e.g., the abandoned teletherapy device at Goiania Brazil) still contain cesium-137. The Gamma Knife® competes with linear accelerator machines for the treatment of centimeter-sized brain tumors in areas of the brain where conventional surgery generally is not 13 One device in the United States is classified as a self-contained irradiator with a californium-252 source.
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Radiation Source Use and Replacement: Abbreviated Version possible. These machines are licensed to contain up to 245 TBq (6,600 Ci) of cobalt-60, encapsulated in 201 sealed sources each with an activity of up to 1.2 TBq (33 Ci). There are approximately 200 Gamma Knife® devices worldwide, including at least 104 in the United States. Elekta, a Swiss/Swedish company is the sole manufacturer of the Gamma Knife®, while MDS Nordion is the main source for the small cobalt-60 sealed sources. A Chinese company, GammaStar, has begun to market a competing device and Elekta is now selling a new version of the Gamma Knife® with 192 sources, instead of the 201 used in previous models. RTGs RTGs convert the heat produced from radioactive decay into electricity by the method of thermoelectric conversion. RTGs were used to provide electrical power to remote stations and for space missions. The older, legacy RTGs contain strontium-90, although NASA currently uses Pu-238 RTGs for special space missions. Both radionuclides generate relatively significant amounts of heat per decay. Large RTGs can contain several thousand TBq (hundreds of thousands of Ci), with the typical source size being 750 TBq (20,000 Ci) (IAEA, 2005b). The plutonium-238 RTGs made by DOE for NASA are produced, as needed, for specific NASA space missions. There are currently no commercial RTGs in the U.S. inventory, and the committee was told that the remaining U.S. RTGs are well secured. There are now international efforts to improve security and, in some cases, replace Russian RTGs with alternative technologies. Well Logging Neutron Sources The interaction of the neutrons from an Am-Be well logging source with the surrounding environment produces useful information about the geologic features through which the well was bored. The U.S. NRC’s Interim Inventory reports 300 Category 2 well logging devices in the United States (U.S. NRC, 2007a). Many more are Category 3 devices. Americium-241 is now supplied exclusively by REVISS via the Russian radionuclide production facility at PA Mayak. QSA Global, Inc. is currently the sole manufacturer of Am-Be well logging sources. They receive partially fabricated Am-Be sources from REVISS and perform the final encapsulation into their sealed-source product. The three largest oil-field service companies, Schlumberger, Halliburton, and Baker Hughes, manufacture their own well logging tools in-house, incorporating the supplied Am-Be sealed sources. In addition to these “big three,” there are medium-sized companies (e.g., Weatherford) and many smaller well logging oil-field service companies. Thermo-Electron is the largest manufacturer that sells well logging tools to these smaller service companies. Industrial Radiography Radiography devices are used to nondestructively examine the integrity of structures, manufactured components, metal forging, pipes, and welds, as well as fiber composites and composite structures. Many of the devices are portable and use radionuclides such as iridium-192, cobalt-60, selenium-75, ytterbium-169, and tellurium-170, with iridium-192 by far the most commonly used radionuclide. The major manufacturers and distributors for industrial radiography devices include QSA Global, Industrial Nuclear, Source Production and Equipment Company, Agiris, and CIS-US. However, the actual producers of the radioactive material are MDS Nordion, PA Mayak, and the consortium of European reactors.
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Radiation Source Use and Replacement: Abbreviated Version PRODUCTION OF CATEGORY 1 AND 2 RADIATION SOURCE MATERIAL Figure 2-2 shows the method of production of the radionuclides in Category 1 and 2 sources in the United States, all of which are produced in nuclear reactors.14 In reactors, the absorption of thermal neutrons by target material is used to produce radionuclides such as cobalt-60 and iridium-192. Fission products such as cesium-137 and strontium-90, and transuranics such as americium, plutonium, and californium, are generated by the capture of neutrons in a uranium target and are obtained by chemical processing of the irradiated fuel or target. The radionuclides of concern for use in radiological dispersal devices are generally produced in research reactors; however, a few commercial reactors are also used. There are over 250 research reactors currently operating worldwide, and 100 of these are involved in radionuclide production (DOE/U.S. NRC, 2003). However, only a few of these reactors are involved in major production for commercial/industrial/medical use. Note that radionuclide production in accelerators results in the production of small, short-lived radionuclides, none of which poses a significant terrorist threat. COMMONLY USED RADIONUCLIDES IN THE U.S. INVENTORY As noted at the beginning of Chapter 1, the best data on types and quantities of radiation sources in the United States available now are from the U.S. NRC Fiscal Year 2006 Interim Inventory of Radioactive Sources Data Analysis, which reports that there are 28,200 civilian Category 1 radiation sources and 25,532 Category 2 sources licensed by the U.S. NRC and Agreement States in the United States. These are estimates: The U.S. NRC staff estimates that there are 5,036 devices in total, and about 715 of these contain Category 1 sources and perhaps 4,320 devices contain Category 2 sources. The 215 devices that are listed having undetermined application in the summary of the 2006 Interim Inventory are counted here as containing Category 2 sources, but the committee was unable to check these numbers. These numbers do not include sources or devices at nuclear power plants or nuclear fuel cycle facilities, which were not included in the interim inventory (U.S. NRC is including them in the 2007 Interim Inventory). The estimated numbers of Category 1 and 2 devices in each state are presented in Figure 2-3. These data do not include manufacturers and distributors, fixed gauges, or devices for which the type was listed as undetermined. FIGURE 2-2 Origins of reactor-produced radionuclides for radiation sources. SOURCE: Adapted from Connell, et al. (2006). 14 A good reference on the topic of radionuclide radiation source production and use is by Ferguson et al. (2003).
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Radiation Source Use and Replacement: Abbreviated Version FIGURE 2-3 Estimated numbers of Category 1 and 2 devices broken down by state. These data do not include manufacturers and distributors, fixed gauges, or devices for which the type was listed as undetermined. Locations are mostly based on the contact location listed on the license. SOURCE: Constructed with data from U.S. NRC (2007a). The method used to collect the data—a voluntary survey of U.S. NRC and Agreement State15 licensees reporting only what Category 1 and 2 sources were in inventory at the date of their response to the survey—does not ensure that every source is accounted for and does not track where the sources are. When the U.S. NRC imposed increased controls on Category 1 and 2 licensees, the agency examined every materials license, which resulted in identification of 545 additional U.S. NRC licensees who are licensed to hold Category 1 or 2 radiation sources but had not been surveyed in previous years. Only 10 percent of these actually had Category 1 or 2 sources in their possession when surveyed. The Agreement States verified to the U.S. NRC that they checked their own licensee lists. U.S. NRC staff members expressed their concern that, in the absence of a source tracking 15 Section 274 of the Atomic Energy Act of 1954 as amended authorizes the U.S. NRC to “enter into agreements with the Governor of any State providing for discontinuance of the regulatory authority of the Commission … with respect to [explicitly identified] materials within the State …. During the duration of such an agreement it is recognized that the State shall have authority to regulate the materials covered by the agreement for the protection of the public health and safety from radiation hazards.” States that have entered into such an agreement with the U.S. NRC are called Agreement States. The U.S. NRC retains authority in these states over nuclear fuel cycle facilities, export and import of nuclear materials, and radioactive waste disposal (as determined by the U.S. NRC).
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Radiation Source Use and Replacement: Abbreviated Version system, some people are trying to extract more information from the Interim Inventory than the surveys were meant to provide. Fuller and more accurate data are indeed expected from the National Source Tracking System, which is scheduled to come online in the summer of 2008, but the data available now are adequate for the committee’s purposes. To appreciate the prevalence of different radionuclides in commercial use, one can look at the total activity in Category 1 and 2 radiation sources by radionuclide (see Figure 2-4) and the numbers of Category 1 and 2 devices by radionuclide (see Figure 2-5 and Table 2-1). There are over twice as many Category 2 devices as there are Category 1 devices, but the Category 1 devices comprise 99 percent of the total activity. Nearly all of the activity (96 percent of the total for all sources) is in the form of cobalt-60 sources at 62 panoramic irradiators used primarily for sterilization (see Chapter 5). Most of the rest is in self-contained irradiators (1,117 that use cesium-137 and 224 that use cobalt-60; see Chapter 4) and radiotherapy devices (351 that use cobalt-6016 and 1 that uses cesium-137; see Chapter 6). There is only one panoramic irradiator that uses cesium-137, and it is a dry storage panoramic irradiator. U.S. NRC actions on the use of radioactive cesium chloride sources in panoramic irradiators, even dry source irradiators, indicate the commission’s skepticism about the use of such sources in panoramic irradiators. (See the discussions of cesium chloride in panoramic irradiators in Chapters 3 and 10.) The committee has not examined RTGs, and so has excluded the 24 Category-1 and 10 Category-2 strontium-90 devices and any plutonium-238 devices from discussions in later chapters. RTGs are a concern in the republics of the former Soviet Union, where over a thousand of the Category 1 devices were produced, many of which are stored with little or no security in place. The Category 1 RTGs in inventory in the United States are not a major concern in the committee’s view because there are few of them and they are stored in secure government facilities only for military applications. The U.S. government also produces plutonium-238 RTGs for deep-space missions (probes to the outer planets). None of these is listed in the inventory (most are Category 1 devices and can contain several thousand TBq), but they are produced by DOE and delivered to NASA just prior to launch of the probes. FIGURE 2-4 Share of total activity in radiation sources in the United States by radionuclide. SOURCE: Constructed with data from U.S. NRC (2007a). 16 This number includes 104 Gamma Knife® devices and 247 radiotherapy devices. The latter number reported in the U.S. NRC 2006 Interim Inventory (U.S. NRC, 2007c) is much higher than the number of devices currently used for teletherapy in the United States, perhaps by a factor of 10. Many of these are in the process of storage for decay at a centralized facility, and others are being used for other irradiation applications.
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Radiation Source Use and Replacement: Abbreviated Version FIGURE 2-5 Numbers of Category 1 and 2 devices in the United States. Each identifier lists the radionuclide, the number of devices, and the percentage of the total number of devices in that category. The areas on these pie charts are approximately normalized to the same scale, so that equal areas represent equal numbers. NOTE: Several assumptions have been made about the IAEA categories of devices that are not explicitly categorized in the Interim Inventory Summary. SOURCE: Constructed with data from U.S. NRC (2007a). RADIATION SOURCE PRODUCTION AND DISTRIBUTION One can visualize the international supply chain for radionuclides as a pyramid system. At the top are a small number of organizations that have access to nuclear reactors and produce radionuclides in significant quantities. A summary of the main producers of radioactive material in the world is shown in Figure 2-6. The two largest producers of the commercial radionuclides are MDS Nordion of Canada
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Radiation Source Use and Replacement: Abbreviated Version (with the largest share of the cobalt-60 market) and REVISS, an Anglo-Russian consortium. Argentina, through its National Atomic Energy Commission (CNEA) also produces some cobalt-60 which it is on contract to sell to REVISS. MDS Nordion contracts with the Canadian government’s Chalk River Nuclear Laboratory and with nuclear power utilities to have radionuclides produced in their reactors. REVISS acquires all of its supplies of some radionuclides from the PA Mayak. Other radionuclides are acquired from several sources. The old U.S. Atomic Energy Commission (AEC) was once very active in producing and distributing radionuclides for commercial, medical, and research applications initiated as part of the Atoms for Peace Program in the 1960s. The U.S. government, through the DOE Isotope Program (which inherited the older AEC program) ceased supplying cesium-137 many years ago and just recently ceased supplying americium-241, leaving REVISS as the sole supplier of these two radionuclides. The DOE Isotope Program now mainly produces relatively short-lived radionuclides for research and medical applications, although it does also produce californium-252. International Isotopes, Inc., contracts with DOE to produce cobalt-60 sources. A small number of European research reactors are used to produce iridium-192. There are other, regional, suppliers of radionuclides, such as the Eastern European, South African, Indian, and Chinese producers (Van Tuyle et al., 2003). To the committee’s knowledge, they do not supply outside their regions and do not yet serve in the United States, although they might in the future. At the next level of the pyramid are the manufacturers and distributors that manufacture sealed sources from the radionuclide product (unsealed radiation sources) or repackage encapsulated sources and place them into devices for purchase by licensed users. There are approximately two dozen manufacturers and distributors in the United States that receive or hold Category 1 and 2 source materials. In some cases the radionuclide producer is vertically integrated with the device manufacturer. That is, the same company produces sources and manufactures devices, as is the case with MDS Nordion. FIGURE 2-6 Major international radionuclide producers. SOURCE: Adapted from Van Tuyle, G., et al. (2003).
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Radiation Source Use and Replacement: Abbreviated Version At the bottom of this supply pyramid are the licensed users. According to the IAEA, there are more than 20,000 users of what the IAEA considers high-risk radioactive sources and devices (IAEA, 2002) across the world. An examination of the U.S. inventory of radiation sources shows that the top four radionuclides (in decreasing order based on total activity) are cobalt-60, cesium-137, americium-241, and iridium-192. However, simply examining the inventory quantity alone is not sufficient when assessing the risk posed by these radionuclides if used for malevolent purposes or for evaluating options for implementing replacements of high-risk sources. These risk considerations are discussed in Chapter 3. FINDING AND RECOMMENDATION Finding: The radionuclide radiation sources examined in this study are used in applications that are important to the nation’s health, safety, and economic strength. High-activity radiation sources are used in the United States and her modern societies in a variety of ways: they are used in devices that improve the success of medical procedures—ensuring that medical devices and implants are sterile, preventing fatal complications from bone marrow transplants, and providing noninvasive techniques for treating brain lesions; they are used in devices for inspecting the integrity of buildings, bridges, and industrial equipment; and they are used to seek out oil and gas resources deep in the ground. These applications are immensely valuable to the United States. The question is not whether these activities should continue, but whether lower risk replacements for the radiation sources are feasible and practical, and what steps should be taken to implement replacements for the sources that pose a high risk to public health and safety. Recommendation: Replacement of some radionuclide radiation sources with alternatives should be implemented with caution, ensuring that the essential functions that the radionuclide radiation sources perform are preserved. As the nation seeks to improve safety and security, the value and benefits of current practices should be recognized and, where possible, the services the devices provide should not be compromised. Some replacements do entail trade-offs with respect to safety, security, costs, convenience, and performance, as discussed in Chapters 3 through 9. These trade-offs should be considered carefully. A reduction in the performance of a device may be acceptable if it provides sufficient benefits in safety, for example. Replacement should preserve acceptable performance of these applications to preserve the benefits that these applications provide, on many of which the United States has come to rely.