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Major Sources of Technologically Enhanced Naturally-Occurring Radioactive Materials TENORM spans a wide spectrum of raw materials and products destined for use, recycling, or disposal. This chapter summarizes TENORM concerns associated with various industrial activities and notes unique characteristics of possible importance in dose assessment. Because of the diversity of sues, materials, and processes (table 3.1), it is difficult to summarize radionuclide concentrations and waste volumes here. For that kind of information, the reader is referred to an Environmental Protection Agency (EPA) review (EPA 1993b). URANIUM MINING Uranium production from surface mining operations generates large volumes of overburden with either ambient or elevated, but below-ore-grade, concentrations of uranium and its decay products. Smaller amounts of waste rock are produced by underground uranium mines. The ratio of overburden to ore has increased as less-accessible and lower-grade ores have been exploited. In the 1950s, the ratio was about 10:1; by the 1980s, it had increased to about 60:1. Most of the mines in question are in the western states: Arizona, Colorado, New Mexico, South Dakota, Texas, Utah, and Wyoming. A 1989 survey showed the average radium-226 concentration in uranium-mine overburden to be about 0.9 kBq/l~g (25 pCi/g). Those mining wastes are distinct from uranium mill tailings (UMT), which are the ore residues discharged to a waste pond after extraction of the uranium, typically by sulfuric acid leaching. Although 90-95% of the uranium in the ore is extracted, most of the uranium-decay-product activity remains with 61
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64 GUIDELINES FOR EXPOSURE TO TENORM the UMT. UMT are regulated under EPA's standards for uranium and thorium mill tailings (40 CFR Part 192) and are therefore not a focus of this report. Some of the TENORM considered in this report has had processing and disposal histories similar to those of UMT, and UMT have been the focus of more research than most other TENORM. However, such UMT properties as radon emanation coefficients and leachability of radionuclides should not be generalized to the entire TENORM spectrum of materials without due consideration of material similarities and differences. PHOSPHATE-FERTILIZER AND ELE1\¢IENTAL-PHOSPHORUS PRODUCTION Up to about 0.02% uranium can substitute for positions typically occupied by atoms of calcium in the structure of the mineral carbonate fluorapatite (Durrance 1986~. This mineral commonly occurs in phosphate rock, the ore for the production of phosphoric acid and elemental phosphorus. Commercial extraction of uranium has occurred at several phosphoric acid plants in Florida and Louisiana (DOE 1996~. The transfer of uranium-series radionuclides to both the waste materials and the fertilizer product makes each of these a diffuse source of TENORM. Phosphate operations in Florida amount to about 80% of domestic production; other major mining and processing plants are in Idaho, Louisiana, Mississippi, North Carolina, and Wyoming. Two distinct manufacturing processes, a wet process and a thermal process, are involved. The wet process treats the ore with sulfuric acid to yield phosphoric acid and hydrated calcium sulfate (gypsum). This "phosphogypsum" (PG) waste contains trace amounts of 226RaSO4 coprecipitated with the CaSO4~n H2O. About 80% of the 226Ra in the ore follows the PG, which results in an average 226Ra concentration of about 1.1 kBq/kg (30 pCi/g). The volumes of the waste are large: the process yields about 5 tons of PG for every ton of phosphoric acid produced. The PG waste has been disposed of both on land and in rivers. Land disposal generally results in large piles (called "gyp stacks"~. Hull and Burnett (1996) found process waters contained in these stacks to have high ionic strength, low pH (1.4-2.5), and low concentrations of 226Ra (0.08-0.30 Bq/L), because of high sulfate concentrations, but high concentrations of uranium and lead-210 (up to 65 Bq/L). Studies on the Mississippi River and the Rhine estuary have shown that where PG is discharged to such fresh or brackish water, the gypsum dissolves rapidly, releasing the radium, which remains in solution or sorbs onto suspended sediment (Pennders and others 1992; Kraemer and Curwick 1991~. Lead-210 and polonium-210 in the PG occur as an insoluble residue that settles in the estuarine discharge zone, whereas the gypsum dissolves. The mineralogic occurrence of the 2~0Pb and typo in the PG
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MAJOR SOURCES OF TENORM 65 is probably similar to that observed by Landa and others (1994) in acid-leach UMT effluent lead sulfate crystallites in a gypsum matrix. Sediment enrichment in uranium has also been seen in a tidal marsh in Spain where phosphate-fertilizer manufacturing plants discharge liquid and solid wastes to a river (Martinez-Aguirre and others 1996~. About 85% of the uranium partitions to the phosphoric acid, which is further processed to produce a variety of fertilizers with uranium-238 concentrations of about 740-2200 Bq/kg (20-60 pCi/g). Gypsum is often applied to soils as a fertilizer source of calcium and sulfur. PG is an inexpensive and readily available byproduct source of gypsum for agricultural uses. To limit radiation exposure (principally by direct gamma radiation and indoor-radon inhalation exposure), EPA (1992d) issued a ruling that bans the agricultural use of PG that contains 226Ra at over 370 Bq/kg (10 pCi/g). Pipe scales that contain 226Ra at up to 3.7 x 103 kBq/kg (1 x 105 pCi/g) are a low-volume, discrete TENORM waste at wet-process plants. The thermal process involves heating the phosphate rock to about 1300 °C to yield an elemental phosphorous product and a calcium silicate vitreous slag waste containing 226Ra at about 0.7-2 kBq/kg (20-50 pCi/g). The low coefficient of radon emanation from this glassy material (the fraction of the radon formed in a radium-bearing solid that escapes to the atmosphere) limits the radon-exposure pathway. Another atmospheric pathway involves the volatilization of 2~0Pb and typo associated with the heating of the ore (EPA 1989d); such releases are regulated under the Clean Air Act (see chapter 7~. At a thermal plant in the Netherlands, where such stack off-gases are vented through a wet scrubber for emission control, the water from the system is discharged to an estuary. In sharp contrast with the case of the wet-process PG effluent noted above, about 30% of the 2~0Pb and 10% of the typo are dissolved in the thermal- plant effluent; through dilution with seawater by a factor of about 200, the typo figure increases to 100% (Pennders and others 1992~. The bioavailabilit,v of a soluble radionuclide can be expected to be much higher than that of its insoluble form, so the importance of understanding TENORM processing and geochemical forms of radionuclides when doing dose assessments is clear. The application of phosphate fertilizers to soils may increase their uranium and radium content. Over 50-80 y of application, the concentrations of 238U and 226Ra in the plow layer could be increased from a few percent to several times background (NCRP 1987b; Pfister and others 1976~. RESIDERS OF COAL CO1\IBUSTION The reducing conditions under which coals form are conducive to the accumulation of uranium. Typical mucks, peats, [ignites, and coals contain
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66 GUIDELINES FOR EXPOSURE TO TENORM uranium at about 0.05-3 ppm. Thorium is strongly adsorbed by peats, and the typical coal contains thorium at 1-10 ppm (Boyle 1982~. On combustion of coals, most of the uranium, thorium, and decay products remain with the ash. For its evaluation of coal ash, EPA (1993b) considered compos*ed fly ash plus bottom ash with a literature-derived mean 226Ra concentration of about 0.14 kBq/kg (3.7 pCi/g). Although most coals have decay products in secular equilibrium with the parent, a young, postglacial peat deposit in northeastern Washington state with about 0.1% uranium has less than 10% ingrowth of the possible decay-product activity. This deposit has been exploited as a uranium source, and the lack of decay-product activity rendered the UMT here more benign than those at a typical uranium mill a factor that was considered in the licensing decision (Stohr and Erickson 1984~. Disequilibrium can also be seen in combustion products. Data on fly ash presented by Baxter (1996) suggest strong enrichment (with respect to other uranium-series nuclides) in 2~0Pb and Typo, presumably because of volatilization and subsequent condensation on the fly-ash particles. The bottom ash is assumed to show depletion in these radionuclides. Such differential behavior and the resulting concentration differences should be considered in dose assessments. Radon emanation from ash is a possible exposure pathway from both ash disposal piles and use of fly ash as a concrete aggregate. The low coefficients of radon emanation from glassy materials, such as coal ash and phosphate slag, mitigate exposures. OIL AND NATURAL-GAS PRODUCTION AND PROCESSING Oil and natural-gas reservoirs commonly contain large quantities of saline water. These brines come to the surface during pumping operations and require disposal after separation of the water from the oil and gas. The disposal of oilf~eld brines in a manner that does not result in the salinization of soil and water has been a concern since the early days of the petroleum industry. The radiation hazard was recognized later; the brines tend to be low in uranium (because of reducing conditions in the petroleum reservoir) and low in thorium, but they can contain elevated concentrations of 226Ra and 228Ra (Perel'man 1977~. In the United States, more than 90% of such brine is disposed by injection underground, sometimes in enhanced oil-recovery wells, and at other times solely with waste disposal as a goal. The remainder is disposed by surface discharge to earthen evaporation or seepage pits or to wetlands, streams, and so on (Smith 1992~. Some brines have been applied to dirt roads for dust control (Rittiger and Yusko 1996~. At offshore wells on the Outer Continental Shelf of the Gulf of Mexico, overboard disposal of produced well solids (formation sand, TENORM scale, and so on) is banned, but overboard disposal of production
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MAJOR SOURCES OF TENORM 67 water (treated to remove such solids) is allowed (Minerals Management Service 1996a). At some facilities in Texas, radium has been removed from brines by treatment with activated charcoal prepared from walnut hulls. The spent charcoal is thus rendered a solid TENORM waste (Ruth McBurney, Texas Department of Health, personal communication, 1993~. In Pennsylvania, a brine-treatment facility using pH adjustment and flocculation techniques to remove metals yielded a sludge that was dewatered and sent to a landfill. The sludge, which contained 226Ra and 228Ra at about 0.9 kBq/kg (25 pCi/g) each, triggered portal radiation detectors at the landfill, and this initiated an investigation by the Pennsylvania Department of Environmental Protection (Rittiger and Yusko 1996~. As brine flows through pipes at the oil field, temperatures tend to drop and solutes tend to precipitate, forming a scale consisting of sulfates, carbonates, and silicates of calcium, strontium and barium along the interior walls. Radium tends to coprecipitate with these compounds, resulting in a radioactive scale. 226Ra concentrations as high as 15,000 kBq/kg (400,000 pCi/g) have been reported, but typical concentrations are 4-400 kBq/kg (100-10,000 pCi/g). Exposure scenarios associated with these scales include gamma-ray exposure of workers at oil-production platforms and exposure to soil contamination at pipe- reaming facilities. Operations like the latter are conducted to maintain flow at the oil wells. Scale can be removed from pipes and other production equipment by mechanical methods, including cutting, shearing, and high-pressure blasting with water, sand and cryogenic carbon dioxide pellets (Lancee and others 1997~. Chemical decontamination methods that use salts of amino carboxylic acids and proprietary reagents are available for the dissolution of scale and other surficial TENORM materials; radium can be precipitated from the spent solutions and the solid concentrate disposed of (Coil 1997; Lancee and others 1997~. Sludges are related deposits, typically found settled on the bottoms of equipment and storage tanks at various points in the oil-gas-water separation processing stream. The sludges are often oily, and disposal in burn pits used to be common. Large quantities of dewatered TENORM-contaminated scales and sludges have been stored in barrels at production facilities pending development of regulatory guidance. In 1992, an estimated 410,000 barrels of such TENORM waste was stored in Louisiana alone. In 1994, two commercial state-licensed TENORM-waste facilities opened in Louisiana and Texas, TENORM waste is diluted to reduce its specific activity to meet state criteria for reuse or disposal. Limited quantities of TENORM wastes from the oil and gas industry have been disposed of at low-level radioactive-waste sites licensed by the Nuclear Regulatory Commission (Minerals Management Service 1996a). In addition to near-surface burial, TENORM waste can be disposed of by deep subsurface encapsulation in abandoned well bores or injection into
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68 GUIDELINES FOR EXPOSURE TO TENORM permeable formations. Geologic and engineering criteria for such disposal on the Outer Continental Shelf of the Gulf of Mexico have recently been released by the Minerals Management Service (1996b). State regulations on down-hole encapsulation and injection of TENORM oil and gas wastes in onshore wells are in place or pending in Texas, Louisiana, and Mississippi (Minerals Management Service 1996a). Equipment and piping that handle only the natural-gas fraction are not subject to scale and sludge deposits. However, radon-222 is carried from the reservoir with the gas, and its decay products tend to plate out on the interior surfaces of pipes, valves, and equipment in the gas plant (Gesell 1975~. Short- lived, gamma-emitting radon decay products, such as bismuth-214, can pose an exposure hazard to plant workers, but the environmental fate of the longer-lived decay products 2~0Pb and typo is of concern after disposal of scrap metal from such operations, as is occupational exposure during maintenance and repair of disassembled equipment (Summerlin and Prichard 1985~. MUNICIPAL WATER TREATMENT Conventional water-treatment processes designed to remove suspended solids and dissolved chemical contaminants from drinking-water supplies also remove radionuclides. During the period of atmospheric nuclear-weapons testing, the US Public Health Service and others did much work on removal of fission products, such as strontium-89 and strontium-90, from water supplies (Straub 1971~. A variety of treatments, including lime-soda ash softening and phosphate coagulation, that were shown to be effective for radiostrontium removal can also remove substantial quantities of other alkaline earth metals, including radium (Menetrez and Watson 1983~. Lime softening is effective in removing uranium from water. The radionuclide concentrations in the sludges generated by these treatments will be a function of the raw-water radionuclide concentrations and He radionuclide-removal efficiencies. Regional geology is the key determinant of raw-water radionuclide concentrations. Water supplies win elevated concentrations of radium are found with the greatest frequency in the north central and Coastal Plain states. Water supplies with elevated levels of uranium are found most frequently in the western states (Horton 1985~. Groundwater supplies with elevated concentrations of typo have been reported in Florida (Harada and others 1989~. Lime-softening sludges from water supplies in Illinois and Wisconsin that have raw-water 226Ra concentrations of 0.04-0.2 Bq/L (1-5 pCi/L) have 226Ra concentrations of 0.2-1.1 kBq/l~g (6-30 pCi/g) (EPA 1993b). The sludges are most often disposed of in onsite lagoons or at municipal landfills with little regulatory control (for further discussion see chapter 7~.
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MAJOR SOURCES OF TENON 69 Treatments designed specifically to remove radium include coprecipitation with barium sulfate and selective sorbents. The latter include ion-exchange resins, barium sulfate-coated alumina, and manganese dioxide- coated polymers. Some of these can have 226Ra concentrations as high as 3,700 kBq/kg (100,000 pCi/g) and might require disposal in low-level-waste burial grounds; likewise, brines from the regeneration of high-efficiency radium- removal resins might have high radium concentrations that present liquid-waste disposal problems. Indeed, at some municipal wastewater-treatment plants, elevated concentrations of radium in sewage sludge have been attributed to residual materials discharged to the sewer systems by drinking-water treatment plants (Nuclear Regulatory Commission 1997b). Activated-carbon filters, used for removal of the short-lived 222Rn, can be handled with a delay-and-decay method before disposal (Lowry 1983~. METAL MINING AND PROCESSING This category has by far the largest TENORM solid-waste volume an estimated US inventory of about 50 billion tons- most of it with NORM concentrations less than 10 times background. On the basis of geologic reasoning, Bliss (1978) has outlined the types of metallic ores (other than uranium) whose mining and extraction might lead to TENORM problems. The list is broad and includes: · Ores of rare-earth elements, molybdenum, gold, aluminum, lead-zinc, iron, tin, vanadium, copper, and other metals (commercial- scale byproduct recovery of uranium has occurred in connection with the extraction of copper and gold). · Placer deposits of any metal (for thorium and its decay products). · Ores that result from intense weathering, such as bauxite. The remainder of this section focuses on metal resources, but selected non-metal resources might be associated with TENORM (Bliss 1978~. These include organic deposits (such as black shales), fluorspar, granite, and clays. Liquid and solid wastes from metal mining and processing include mine waters, overburden, mill tailings, pipe scales, smelter slags, and spent leachates. The presence of sulfide minerals in overburden and tailings is an important consideration. Oxidation of these materials on exposure to air generates sulfuric acid. As seen in chapter 2, one can expect migration of
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70 GUIDELINES FOR EXPOSURE TO TENORM radium to differ from migration of uranium and thorium under such a weathering regime. Although high sulfate concentrations in processing and disposal environments will limit the mobility of radium, the presence of other anions associated with metal-extraction processing can increase radium mobility. Chlorination is a process in which ores are treated with chlorine gas and then water to recover soluble metallic chloride salts; the process is used extensively with gold ores. At a plant in Oregon, chlorination of zircon-bearing sands was used to extract zirconium, niobium, tantalum, and haLnium. The process rendered radium, as well as these economic metals, water-soluble. The finely ground process tailings contained 226Ra at about 20 kBq/kg (500 pCi/g), much of it occurring presumably as soluble RaCl2. Seepage water at this tailings disposal site contained up to 1.7 kBq/L (45,000 pCi/L) (Boothe and others 1980; Bliss 1978~. GEOTHERMAL ENERGY PRODUCTION TENORM wastes associated with geothermal-energy production are similar to those associated with oil and gas production: temperature changes lead to precipitation of solids from hot formation waters in piping, equipment, and retention ponds at the surface. 226Ra and 228Ra are the radionuclides of concern in the pipe scales and the solids dredged from holding ponds for spent geothermal fluids. The possibility of locally increased atmospheric 222Rn concentrations near geothermal plants exists (Gesell and Adams 1975~. OTHER INDUSTRIES Metal casting: Foundries use refractory sands to create molds for casting steel-alloy parts. The molds are eventually disposed of in landfills. The foundry sands~ined from deposits in Florida, South Africa, Australia, India, and Brazil contain elevated concentrations of uranium and thorium that occur in heavy accessory minerals, such as zircon and monazite. State regulations restricting the disposal of radioactive material and the use of portal radiation monitors at landfills have in some cases made it difficult for the industry to dispose of discarded casting molds (Anonymous 1995~. The radon-emanation coefficients of these accessory minerals tend to be low, about 0.1-5% for zircon and monazite, compared with about 10-40% for soils and UMT (Landa 1987; Barretto and others 1975~. Such differential environmental mobility factors should be considered for the atmospheric pathway in radiologic dose assessments of these wastes.
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MAJOR SOURCES OF TENOR 71 Pulp and paper: Radium-bearing barium sulfate scales have been found deposited at various points in paper mills (Coil 1997~. Such scales probably were responsible for an incident reported by the Pennsylvania Department of Environmental Protection in which a paper-mill digester tank taken to a scrap-metal facility triggered radiation monitors (Yusko 1997~. Soils from former radium-processing or -manufacturing sites: Radium was extracted from uranium ores at a variety of sites in the United States during the early 20th century. The radium was used extensively for medical purposes and for the production of luminous paint until the 1950s. Residual radium contamination of soils at such sites has required cleanup under the Superfund or other programs (Neiheisel 1990; Simon 1990; Landa 1984~. TENORM in Selected Nonnuclear Industries in Other Nations: Most of the categories covered in the discussion above have been the subjects of multiple investigations of TENORM occurrences in the United States. Some additional categories of TENORM in nonnuclear industries have received more attention in other nations and are noted briefly in table 3.2. Many industrial processes use feed materials with NORM or have TENORM as byproducts. In some cases, the existence of TENORM is ignored by industrial managers and workers, and TENORM wastes might yet be discovered. But some nonnuclear industries are aware of TENORM in their processes or wastes. Table 3.2 presents a selected list of nonnuclear industries in which TENORM play a role either in processes or as byproducts. MINIMIZATION OF TENORM Risks posed by TENORM can sometimes be reduced or redirected to other populations by the application of specific technologies. For example, in the case of radionuclide removal from municipal drinking-water supplies, worker exposure might increase because of handling of radionuclide-bearing treatment residues (such as sludges, spent ion-exchange resins, and spent granular activated carbon) or inhalation of emanated radon and its decay products; at the same time, exposure of the water-supply users will decrease. The use of scale inhibitors or in situ removal of radionuclides from oil-field production fluids by the introduction of sorbents downhole can limit the buildup of TENORM in piping and equipment (Lancee and others 1997~. The removal of uranium as an economic product from phosphoric acid production circuits will decrease the exposure of people who obtain foodstuffs from fertilized soils.
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72 GUIDELINES FOR EXPOSURE TO TENORM h~ _ 2 ~ ~ i, ~ , ~ _ ~ ~ _ ~ f ~ . ~ ~ ~ I ' 3 ~ 0~ ~ ~ roll Ha R i LD1~
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MAJOR SOURCES OF TENORM C) ._ o C) C) - is Em 73 ~- ~: `) _ ~ _ ~ ~ ~At, ., ~
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74 NONRADIOLOGIC IMPACTS GUIDELINES FOR EXPOSURE TO TENORM This report focuses on the radionuclides associated with TENORM. But nonradioactive inorganic and organic contaminants are also associated with these materials. For example, metal-mining wastes can contain a wide variety of inorganic contaminants associated with the ores and their processing; sludges with elevated radium concentrations at oil-water separators can also contain appreciable concentrations of oil. In establishing groundwater standards for remedial actions at inactive uranium-processing sites (40 CFR Part 192), EPA provided specific concentration limits for nitrate and molybdenum, as well as for uranium and radium, because these constituents had been found in high concentrations at many UMT sites (EPA 1995~. Risk assessments for TENORM should consider such exposures. CONCLUSIONS TENORM present unique problems because of their large volumes and widespread occurrence in industrial products, byproducts, and wastes. The physical, chemical, and radiologic properties of TENORM vary widely. 226Ra and its decay products are the radionuclides of primary concern, but other uranium- and thorium-series nuclides should also be considered. As discussed further in chapter 4, the leachability, sorption, and biologic availability of these radionuclides can be expected to vary with the processing history and siting environment of the TENORM. We might not know all sources of TENORM.
Representative terms from entire chapter: