Industrial Sectors with TENORM

Introduction

The following sections are largely taken from EPA 1993.

The majority of TENORM issues center around waste from industrial processes. Most of the wastes we will be addressing are produced in very large volumes, but are of low activity. While some wastes are disposed of, others are put to commercial uses. The improper disposal, re-use, and recycling of diffuse TENORM has led to circumstances resulting in contamination events and unnecessary public exposures.

Disposal in piles or stacks can lead to groundwater contamination and to airborne releases of radioactive particulates and radon. Improper use and/or disposal, such as for soil conditioning or fill around homes, can lead to buildup of radon gas in homes, direct exposure to individuals, and contamination of soil and of the crops growing in the soil.

Reuse of TENORM-contaminated materials, such as in concrete aggregate, can lead to increased radiation risks to members of the public in a variety of ways.

This overview is not comprehensive. It is representative of the types of industries that have TENORM. The summaries presented in EPA 1993 were extracted from studies which were conducted to characterize the presence of radioactivity, industry practices, and waste and materials. It should be noted that quality data are not available for many of these industries, and some sectors are not overly willing to share data. Therefore, the data presented in Table 6 are often extrapolated and best estimates.

The main radionuclides investigated in the uranium series in industrial TENORM situations are ²³⁸U, ²³⁴U, ²³⁰Th, ²²⁶Ra, and ²²²Rn (and progeny). In the thorium series, we look at ²³²Th, ²²⁸Ra, and ²²⁰Rn (and progeny). In addition, ⁴⁰K should be characterized. Radium-226 is used here to show the relative activity and volume among the TENORM sectors. In assessing dose and risks, all radionuclides need to be considered. Ten industrial sectors will be examined:

• Uranium Overburden and Mine Spoils
• Phosphate Industry Wastes
• Phosphate Fertilizers and Potash
• Coal Ash
• Oil and Gas Production Scale and Sludge
• Waste Water Treatment Sludge
• Metal Mining and Processing Waste
• Geothermal Energy Production Waste.
• Paper and pulp Industry
• Scrap Metal Release and Recycling

Phosphate fertilizers are not wastes, but are included because of their widespread use. The paper and pulp industry has only recently been identified as being impacted by TENORM, and was not addressed in EPA 1993.

Table 6. Estimated annual production rates and average ²²⁶Ra concentrations

Adapted from EPA 1993

Unlike ore (source material) and mill tailings (byproduct material), uranium overburden is not regulated by the Atomic Energy Act (AEA 1954) or the Uranium Mill Tailings Remedial Action (UMTRA) program (EPA 1978), and therefore is considered TENORM.

The uranium mining industry began in the 1940s primarily for the purpose of producing uranium ore for use in weapons and soon after for nuclear fuel fabrication. The majority of the mines are located in the west, mainly in Utah, Colorado, Wyoming, Arizona, South Dakota, New Mexico, and Texas. Mining of uranium ores by surface and underground methods produces large amounts of bulk material, including overburden, low-grade ore, and mining spoils. Surface mining produces the bulk of the spoils.

Overburden, which is beneath the top soil and overlies the ore deposit, contains limited amounts of natural uranium and its progeny; average ²²⁶Ra concentrations are .92 Bq/g (25 pCi/g) (Table 7).

Mining spoils include low-grade ore and other materials excavated during the mining process. Low-grade ore contains significant amounts of uranium but usually not enough to make milling economically attractive. The concentrations of ²²⁶Ra at the interface of overburden and low-grade ore boundaries vary from about 0.1 to ~10 Bq/g (three to several hundred pCi/g).

The estimated total volume of mine waste produced is about 4 billion metric tons (MT), almost 90% of this within the last 20 years by surface mining. Although demand has fallen off, the deposits that were recently mined are of lower quality and harder to access; therefore, the amount of waste per volume of ore generated has increased significantly. As of 1988, there were 3.1 billion MT of unreclaimed overburden in the United States. Uranium mining and milling in America had virtually ceased due to market forces. However, there is now some renewed interest in mining and milling of uranium because stockpiles are low and the price of uranium is rising.

Most uranium overburden is piled and stabilized where it is mined. Uranium overburden has few uses. It is typically used for backfilling mined out areas and for constructing site roads. Mine reclamation will utilize overburden as the practice is implemented. Only about 4% of the mines have been reclaimed. ORNL has found overburden and mine muck used as road aggregate in Colorado (Rice 1995).

Most areas where uranium mining has occurred are remote and arid. These areas are starting to become more populated, and chances for exposures to populations are increasing. A good example of this phenomena is Moab, Utah. The population of Moab was stagnant and actually decreasing during the 1970s and early 1980s. The population of Moab has increased dramatically in recent years with the advent of recreational activities like mountain biking and river rafting.

Not only is the population increasing, but tourism in the back country is increasing. The possibility of exposure to TENORM is a concern (to me) because numerous uranium mines are located in eastern Utah. These abandoned mines have spoils piles that may not be under any control, and can be accessible. Old mining roads into the back country are used by the recreationists.

Radon concentrations are reduced by escape through diffusion and advection at varying rates. The amount of ²¹⁰Po and ²¹⁰Pb are also reduced by the amount of radon lost. If a radon emanation coefficient of 0.3 is used (sandstone matrix), the ²¹⁰Po and ²¹⁰Pb concentrations are 0.7 times that of ²²⁶Ra or about 0.66 Bq/g (18 pCi/g). Radon rates were not given because its release rate from the surface of the overburden is determined from the ²²⁶Ra concentration.

EPA has started to investigate the hazards and risk from uranium overburden.

Table 7. Radionuclide concentrations of Environmentally Significant Radionuclides in Uranium Mining Overburden

Source: EPA 1993.

Phosphate rock extraction is the fifth largest mining industry in the United States in terms of quantity of material mined. Domestic production from these open pit mines was 38 million MT in 1988. Florida produces about 80% of domestic capacity, with North Carolina and Tennessee generating 10% and Idaho, Utah, Montana and Wyoming the balance. Phosphate rock is processed to produce phosphoric acid and elemental phosphorus. These are then combined with other chemicals to produce phosphate fertilizers, detergents, animal feeds, other food products, and phosphorus chemicals. The production of fertilizers accounts for over 90% of the phosphate rock demand in the United States.

Phosphate ore contains one-third quartz sands, one-third clay minerals, and one-third phosphate particles. Uranium in phosphate ores ranges in concentration from 20 to 300 ppm (0.26 to 3.7 Bq/g) (7 to 100 pCi/g). Thorium is present in background amounts, ~1 to 5 ppm (3.7 to 22.2 mBq) (0.1 to 0.6 pCi/g).

When the phosphate particles are separated from the rest of the ore (beneficiated), two types of wastes are produced: phosphatic clay tailings and sand tailings (Fig.6). The clay slimes contain 48% of the radionuclides in the host ore, the sand tailings contain 10%, and the remaining 42% is carried by the phosphate rock. Florida clay slime contains about 1.67 Bq/g (45 pCi/g) ²²⁶Ra.

Phosphogypsum is the principal waste by-product generated during the phosphoric acid production process (wet process), and phosphate slag is the principal waste by-product generated from the production of elemental phosphorus (thermal process). It is estimated that there have been over 8.2 billion MT (9.1 billion short tons) of phosphogypsum generated between 1910 and 1991. Impurities contained in the phosphogypsum and phosphate slag include uranium and thorium and their progeny (Table 8). These can become concentrated in the waste by-products.

During the wet process, there is selective separation and concentration of radionuclides. About 80% of the ²²⁶Ra follows the phosphogypsum, while about 86 % of the uranium and 70% of the thorium are found in the phosphoric acid. Typical radium concentrations in phosphogypsum stacks fall within a range of 0.41 to 1.3 Bq/g (11 to 35 pCi/g), with progeny also in that range. The radon emanation coefficient for phosphogypsum is estimated at a value of 0.2.

Phosphate waste is stored in large stacks. Each facility may have one or more stacks that range from 2 to 300 hectares and range in height from 3 to 60 meters. Much of the stack is covered with water in ponds and ditches.

Radon flux rates from phosphogypsum stacks vary widely, due to the radium concentration in the parent rock, the emanation fraction, and other factors. Average fluxes have been reported to vary from 0.063 to 0.44 Bq/m²-sec (1.7 to 12 pCi/m²-sec), with a mean value of 0.25 Bq /m²-sec(6.8 pCi/m²-sec).

Gamma radiation exposure rates from phosphogypsum stacks have been measured around ~0.287 uSv/h (33 uR/h). Radiation surveys conducted in areas where large volumes of phosphate ores are stockpiled have yielded gamma exposure rates ranging from ~0.174 to 0.87 uSv/h (20 to 100 uR/h), with an average of ~0.522 uSv/h (60 uR/h).

Some phosphogypsum is used for agricultural and construction purposes. EPA has ruled that "Phosphogypsum intended for agricultural use must have a certified average concentration of ²²⁶Ra of no greater than 10 pCi/g" (EPA 1992).

During the thermal process, vitrification yields slag, a material that contains the non-volatile radionuclides. This slag has been found to contain uranium and thorium concentrations in the range of 0.74 to 1.85 Bq/g (20 to 50 pCi/g) and ²²⁶Ra concentrations in the range of 0.15 to 1.5 Bq/g (4 to 40 pCi/g) (Table 9). Because of the high temperatures involved, some radionuclides are vaporized during the process. As much as 95% of the ²¹⁰Pb and ²¹⁰Po have been measured in stack releases. Eventually, these isotopes decay and ingrow back to equilibrium with the ²²⁶Ra.

Click Here for Flow Chart.

The total slag inventory in the United States in 1991 is estimated at 224 to 424 million MT (247 to 467 million short tons). The radon emanation coefficient for slag is estimated to be very low because of the vitrified matrix. A value of 0.01 was assumed for the referenced report.

Radon flux measurements conducted on Idaho slag indicate that very little radon escapes the vitrified matrix of the slag. An average radon flux rate of 0.02 Bq/m²-sec (0.5 pCi/m²-sec) is estimated for a typical phosphate slag pile. For comparison, measurements taken on two phosphate ore samples revealed radon fluxes of 2.11 and 2.37 Bq/m²-sec (57 and 64 pCi/m²-sec); radon fluxes from native soil samples ranged from 0.063 to 0.63 Bq/m²-sec (1.7 to 17 pCi/m²-sec).

Table 8. Radionuclide Concentrations in Phosphogypsum

Gamma radiation exposure rates of ~0.87 uSv/h (100 uR/h) have been measured on slag piles. Phosphate slag has been used as aggregate in making roads, streets, pavements, residential structures, concrete aggregate, railroad ballast, and buildings. Radiation surveys conducted in Montana and Idaho where slag has been used in construction materials and to pave streets have yielded measurements of ~0.565 uSv/h (65 uR/h) in homes and ~0.435 uSv/h (50 uR/h) on streets that utilized slag.

Table 9. Radionuclide Concentrations in Phosphate Slag

Source: EPA 1993.

There are over 1,300 coal-fired boilers operated by electric utilities and nearly 60,000 industrial boilers in the United States Electric utilities consume the most coal, currently at about 700 million MT (771 million short tons) per year. Domestic coal production has increased, as well as imports, while exports have remained relatively stable. In 1990, 61.6 million MT (67.9 million short tons) of ash and slag were generated, with another 17.2 million MT (18.9 million short tons) of sludges.

Coal consumption generates large amounts of coal ash that requires proper management and disposal, either at the point of use or elsewhere in ash impoundment facilities. Since coal contains naturally occurring uranium and thorium, large quantities of coal ash may present a potential radiological risk to exposed individuals. The degree of risk will depend on the physical and radiological properties of the ash and on how the ash is disposed of or used.

The radioactivity of coal can vary over two orders of magnitude depending on the type of coal and the region from which it is mined. The concentrations of ²³⁸U and ²³²Th in coal average about 0.022 and 0.018 Bq/g (0.6 and 0.5 pCi/g), respectively. The concentrations of the radionuclides in ash will also vary (Table 11). They tend to be enriched in ash compared to coal.

Electrical utility boilers generate ash at a rate of about 10% of the original volumes of coal. Over 95% of the ash is retained. Bottom ash and slag make up about 20% and fly ash makes up the other 75%. Fly ash is formed when flue gases entrain (to draw after oneself) ash. Fly ash is very fine. The remainder of the ash that is to heavy to go off with the gas settles to the bottom of the boiler to become bottom ash.

Ash also typically contains silicon, aluminum, iron and calcium. Liquid slag is produced when the ash melts under intense heat. Treatment of stack exhausts also results in the generation of flue gas desulfurization sludges. About 17 million MT (18.75 million short tons) were produced in 1990.

Table 11. Typical Average Radionuclide Concentrations for Coal Ash

Source: EPA 1993.

The radon emanation coefficient for ash is low because the ash is vitrified. A factor of 0.02 can be used to compare to other coefficients. Radon flux is also low, estimated at 0.018 Bq/m²-sec (0.5 pCi/m²-sec).

About 70 to 80% of the coal ash generated is disposed of in landfills or ponds. There are about 300 off-site coal-ash landfills and surface impoundments. A typical ash disposal landfill may be anywhere from 30 to 60 hectares. It is estimated there are 305 off-site coal-ash landfills and surface impoundments and that there are about 900 on-site disposal facilities. Fly ash, bottom ash, and boiler slags are used as substitutes in cement and concrete, as structural fills, for snow and ice control, and as blasting grits. The potential impact of long-term accumulation of by-products in the biosphere should be considered (Gabbard 1993).

Coal ash is used as an additive in concrete, cement, and roofing materials, land reclamation, paint and undercoatings, and various products and as a structural fill for road construction. About 30%of ash is reused. There is concern that fly ash may become regulated in the future, which would discourage reuse.

The rate of production of domestic crude oil is closely tied to the international price of crude and to fluctuations that depend on world-wide political and economic conditions. Production for the month of November 1995 was estimated at 6.5 million barrels per day (API 1996). Production in 1970 was approximately 9.6 million barrels per day. It is estimated that about 25 thousand MT (27.5 thousand short tons) of TENORM scale and 230 thousand MT (253.5 thousand short tons) of TENORM sludge are generated from domestic production each year, based on 1989 figures.

Radioactivity in oil and gas production and processing equipment is of natural origin and is now known to be widespread, occurring throughout the world. Estimates suggest that up to 30 % of domestic oil and gas wells may produce some elevated TENORM contamination. The geographic areas with the highest recorded measurements were northern Texas and the gulf coast crescent from southern Louisiana and Mississippi to the Florida panhandle. Very low levels of TENORM radioactivity were noted in California, Utah, Wyoming, Colorado, and northern Kansas fields.

Uranium and thorium compounds are mostly insoluble and as oil and gas are brought to the surface, remain in the underground reservoir. As the natural pressure within the bearing formation falls, formation water present in the reservoir will also be extracted with the oil and gas. Some radium and radium daughter compounds are slightly soluble in water and may become mobilized when this production water is brought to the surface. The precipitate consists principally of barium sulfate (BaSO₄), calcium sulfate (CaSO₄), and calcium carbonate (CaCO₃). Because the chemistry of radium is similar to that of barium and calcium (all are Group IIA elements), radium may also precipitate to form complex sulfates and carbonates.

The amount of TENORM material from a producing field generally increases as the amount of water pumped from the formation increases. Since radium concentrations in the original formation are highly variable, the concentrations that precipitate out in sludges and as scale on internal surfaces of oil and gas production and processing equipment are also variable. This scale in these chemical matrices is relatively insoluble and may vary in thickness from a few millimeters to more than an inch. Scale deposits in production equipment may at times become so thick to completely block the flow in pipes as large as 10.1 cm (4 in.) in diameter.

Radium-226 is generally present in scale in higher concentrations than ²²⁸Ra. Typically, ²²⁶Ra in scale is in equilibrium with its progeny, but ²²⁸Ra is not. The nominal activity appears to be about three times greater for ²²⁶Ra than for ²²⁸Ra (Table 12).

The oil and gas production stream passes through a separator where the oil, gas, and water are divided into separate streams based on their different fluid densities (Fig. 7). Most of the solids removed in the separator accumulate there. The product may also be treated using a heater/treater to separate oil from produced water and sludge. The produced water flows from the separators into storage tanks and is often injected into disposal or recovery wells. Scales are usually found in piping and tubing, including oil flow lines, water lines, injection and production well tubing, manifold piping and small-diameter valves, meters, screens, and filters. Concentrations on TENORM occurred in wellhead piping and production piping near the wellhead. Concentrations of radium in scale deposited in production tubing near wellheads can range up to tens of thousands of pCi/g. The concentration of radium deposited in separators is about a factor of ten less than that found in wellhead systems. There is a further reduction of up to an order of magnitude in the radium concentration in heater/treaters. The concentration in granular deposits found in separators range from one to about one thousand pCi/g. The largest volumes of scale have been found in the water lines associated with separators, heater/treaters, and gas dehydrators.

Click Here for Flow Chart.

TENORM radionuclides may also accumulate in gas plant equipment from ²²²Rn decay products, even though the gas is removed from its ²²⁶Ra parent. Rn-222 concentrates in the liquid petroleum gas (LPG) fraction during processing. Gas plant deposits differ from oil production scales, typically consisting of radon decay products plated out on the interior surfaces of pipes, valves, and other gas plant equipment. The only significant radionuclides remaining in gas plant equipment are ²¹⁰Po and ²¹⁰Pb.

Table 12. Average Radionuclide Concentrations in Oil and Gas Scale

Radon flux rates from scale are hard to determine. Several factors, such as particle size, thickness of the deposit, and the presence of oil and other material may reduce radon flux rates. Since much of the waste is internal to components, it may be challenging to characterize net radon flux. A radon emanation coefficient of 0.05 has been assumed for the referenced report (EPA 1993).

Exposure rates vary widely depending on geographic location and the type of equipment. Median exposure rates were measured for water handling equipment in the ~0.261 to 0.348 uSv/h (30 to 40 uR/h) range. Gas processing equipment with the highest levels include reflux pumps, propane pumps, and tanks and lines. Median exposure rates were reported to be in the ~0.348 to 0.609 uSv/h (30 to 70 uR/h) range. For both oil and gas processing equipment, a few measurements were observed to be in excess of ~8.7 uSv/h (1 mR/h).

The origin of TENORM-contaminated sludge is similar to that of scale. As the produced water is subjected to changes in temperature and pressure, dissolved solids may precipitate out of solution and deposit sludge within the oil production system. These deposits are generally in the form of oily, loose material. Sludge often contains silica compounds, but may also contain significant amounts of barium. Some of the solids in the original product stream are removed in the separator and accumulate there as sludge. As the stream is further treated using heater/treaters to separate oil from water, sludge is also separated and allowed to accumulate. The largest volumes of sludge settle out of the production stream and remain in the oil stock and water storage tanks. Radionuclide concentrations in sludge vary from background levels to several hundred pCi/g, with the highest concentrations in the separator and collection areas near the separator (drains, etc.) (Table 13). The levels deposited in heater/treaters and in sludge holding tanks are about a factor of 10 less than those found in the separator. TENORM concentrations in sludge deposits in heater/treaters and tanks are generally around 2.78 Bq/g (75 pCi/g).

Table 13. Average Radionuclide Concentrations in Sludge

Source: EPA 1993.

Radon flux from sludge is also hard to characterize for several reasons. The presence of oil or other petroleum products associated with the sludge may reduce radon flux rates. The presence and concentration of ²²⁶Ra will govern radon flux and diffusion properties from sludge. A radon emanation coefficient of 0.22 was assumed for the referenced report (EPA 1993).

Oil field tubulars and equipment are now surveyed for the presence of radioactivity, and contaminated equipment is either held in storage or sent to a commercial decontamination facility. Tank sludges are also surveyed for radioactivity, dewatered, and held in storage pending disposal.

In some states, production water from oil and gas industry is disposed down hole. In addition, well injection for slurried material at limited concentrations has been permitted for oil field TENORM. Some oil field scale is stored in drums. The industry disposes of scale and sludge wastes removed from production equipment and also discards contaminated components. There are instances where TENORM waste is disposed of off-shore, under license from the United States Mineral Management Service.

Water Treatment Residuals

New Colorado Policy and Guidance for TENORM.

Since water for domestic use comes from streams, lakes, reservoirs, and aquifers, it contains varying amounts of naturally occurring radioactivity. Radionuclides are leached into ground or surface water when water comes in contact with uranium- and thorium- bearing geologic media. The predominant radionuclides found in water include radium, uranium, and radon, as well as their progeny.

Water treatment includes passing the water through various types of filters and devices that rely on physical and chemical processes to remove impurities and organisms. If water containing radionuclides is treated by such systems, it is possible to generate radioactive wastes even if the treatment system was not originally intended to remove radioactivity. Such wastes include filter sludges, ion-exchange resins, alum sludges, ferric chloride residual, granular activated carbon, and water from filter backwash.

Of the over 60,000 public water supply systems, it was estimated that about 700 of them treat water containing elevated NORM radionuclide concentrations. The areas suspected of having the most systems with elevated radionuclide concentrations are the North Central Region, the Piedmont and Coastal Plain Provinces, and portions of Arizona, New Mexico, Texas, Mississippi, Florida, and Massachusetts.

It is estimated that approximately 260,000 MT (287,000 short tons) of water treatment sludge containing elevated levels of TENORM, including spent resins and charcoal, are generated annually (Table 14).

Three technologies are likely to produce the TENORM waste because they generate sludge and are known to remove radioactivity from water. They are lime softening, greensand filtration, and ion-exchange and activated charcoal.

• Lime softening is used on larger systems to soften water by the addition of calcium hydroxide, which raises the pH causing calcium and magnesium in the water to precipitate. The precipitate, along with the suspended solids, is removed by sedimentation and filtration. Eighty to 90 % of the radium in the water is also trapped in the sludge.


• Greensand is made of grains of glauconite often mingled with clay or sand and may also contain natural algae. These large sand bed filtration systems remove nearly 60% of radium found in the water.


• Ion-exchange resins are used to soften water. Cation exchange removes about 95% of the radium. Anion exchange removes about 95% of the uranium. These resins are usually back- washed for reuse. The backwash water is typically discharged or back-washed to another column for further treatment. Radionuclide content eventually builds up in the resin after prolonged use. Activated charcoal is often used in conjunction with ion-exchange systems to remove organics and gases, including radon. Over 95% of the radon, with smaller amounts of uranium and radium can be removed.

Ion-exchange resins generate waste at higher concentrations of those found in sludges but in much smaller quantities. Field data indicate that radium concentrations between 11.8 to 129.5 Bq/L (320 to 3,500 pCi/L) occur in the column rinse and brine. Radium buildup in cation-exchange resins has been observed to average about 0.33 Bq/g (9 pCi/g), with peak concentrations ranging from 0.92 to 1.48 Bq/g (25 to 40 pCi/g).

Selective sorbants specifically designed to remove radium from water are particularly effective, with wastes retaining concentrations of ²²⁶Ra averaging 1.48 kBq/g (40,000 pCi/g) and up to 4.07 kBq/g (110,000 pCi/g). This material is considered discrete NARM, > 74 Bq/g (> 2 nCi/g), and should be treated as low-level waste.

The concentration of radionuclides in water treatment sludge will depend on:

• the amount of naturally occurring radioactivity and radionuclide concentrations in the water supply,


• radionuclide removal efficiency for the system, and


• the amount of sludge produced per unit volume of water processed.

Table 14. Average Radionuclide Concentrations in Water Treatment Sludge

^a For ²²⁸Ra and ²²⁸Th, the values shown in brackets are concentrations after two years of decay and ingrowth.

Adapted from EPA 1993.

Water treatment sludges are placed in lagoons and may include lime sludge, back flush water, spent ion-exchange media, and sand filter elements. Disposal in lagoons results in the accumulation of radium in bottom sediments that may have to be dredged and disposed of properly. Sludge is also disposed of in sanitary landfills, discharged to sewers, injected in deep wells, or spread on agricultural soils, while the decanted water is recycled.

Radon fluxes from disposed sludges are assumed to be near those of typical soils. Radiation exposure rates from sludges are expected to be near those of ambient background levels. Exposure rates from spent resins and charcoal beds, however, would be much higher. Exposure levels as high as several mR/h have been observed on charcoal and resin beds. An average of ~0.748 uSv/h (86 uR/h) was adopted for the referenced report (EPA 1993).

EPA has revisited their guidance on water treatment residues, and should have it published for comment by the end of 2001.  With the recent promulgation of a uranium MCL, the likelihood that more sludges will be generated has caused concern with regulatory agencies as well as generators.

Metal Mining and Processing Waste

The mining and processing of ores for the production of metals generates large quantities of residual bulk solid and liquid wastes. Because the minerals of value make up only a small fraction of the ore, most of this bulk material has no direct use. It is estimated that the mining and processing of ores and minerals, other than uranium and phosphate, has resulted in the production of more than 40 billion MT (44 billion short tons) of mine waste and tailings from 1910 to 1981.

The metals extraction industry typically generates about 1.5 billion MT (1.65 billion short tons) of waste per year, including about 1.0 billion MT (1.1 billion short tons) of waste rock and overburden, 0.4 billion MT (.44 billion short tons) of ore tailings, and less than 0.1 billion MT (.11 billion short tons) of smelter slag. Depending on the original ores and processing methods, some of these wastes contain elevated concentrations of TENORM (Table 15).

It is generally believed by geologists that the level of NORM found in ores depends more on the geologic formation or region rather than on the particular type of mineral being mined. These ores often contain many different minerals, and the radionuclide content of one type of ore or mining operation or its wastes will not be representative of other mines or waste types. For some ores, the refining process may yield a waste process that may contain higher radionuclide concentrations when compared to the original ore. It has been reported that some of the more uncommon metals have highly radioactive waste products. Also, some processes associated with metal extraction appear to concentrate certain radionuclides and enhance their environmental mobility.

Most of the metal mining waste is stored on-site or near the point of generation, in tailings ponds or used to construct dams, dikes, and embankments. About two thirds is mine waste, and one third is tailings. Metal mining processing wastes have only been reused in a limited number of applications, typically for backfilling mined out areas and for construction and road building near the mines. Some mineral processing wastes have been used to make wallboard and concrete.

Some of the mining wastes are stockpiles that are reprocessed several times to extract additional minerals. NRC staff published guidance on September 22, 1995 (NRC 1995); allowing for certain feedstocks containing uranium and thorium to be processed by licensed uranium mills. This will allow the wastes to be disposed of in the uranium mill tailings pile. There are several restrictions on the feedstock.

Table 15. Metal and mining industries known or believed to involve TENORM

Source: EPA 1993.

Rare Earths

The rare earth elements, sometimes called lanthanides, are a group of 15 chemical elements with atomic numbers 57 through 71. Yttrium, which has an atomic number of 39, is also included because it occurs with other rare earth elements and has similar chemical properties. The special properties of the rare earth elements are why they are used in catalysts, ceramics, refractory and metallurgical processes, magnets, etc. They are also used in low-temperature superconductor technology, which may increase their demand in the future. The United States is the world's leading producer of rare earth elements. Rare earth oxides include bastnasite, monazite, and xenotime. Bastnasite (also spelled bastnaesite) can contain up to 75% rare earth oxides, including up to 0.1% ThO₂. Monazite can contain about 60% rare earth oxides, including 4 to 10% ThO₂. Uranium may also occur in monazite at 0.1 to 0.5 % U₃O₈. Thorium can be removed from monazite ores by several methods, resulting in thorium residue wastes. Xenotime can contain elevated levels of thorium and uranium. The ThO₂ and U₃O₈ components from the rare earth metals appear in the waste products. Although some of these wastes have been treated as low-level waste and disposed of properly, some of the TENORM-contaminated wastes remain at the processing sites.

The annual generation rate of waste is assumed to be 20,800 MT (22,900 short tons) per year containing 6% TENORM with relative activities of 144 Bq/g (3,900 pCi/g) for thorium and 666 Bq/g (18,000 pCi/g) uranium. These values are considerably higher that the NRC's 0.05% for source material.

The radon flux rate from rare earth oxide waste piles depends on many factors, such as the radium concentration in the wastes, moisture content, porosity, and depth of the pile. The radon emanation coefficient for these wastes is estimated at 0.3.

Radiation exposure rates associated with these wastes can range from near background to several uSv/h (several hundred uR/h) for monazite wastes. Depending on the source, radiation levels may differ because many of the decay products may no longer be in secular equilibrium with uranium and thorium. A total external radiation exposure rate from thorium and uranium can be up to ~122 uSv/h (14 mR/h).

Other Metals

Zirconium, hafnium, titanium, and tin generate approximately 470,000 MT (518,000 short tons) of waste a year with an average ²²⁶Ra concentration of 1.59 Bq/g (43 pCi/g). Much of the ore from which titanium is obtained originates in sands that also contain monazite. Ores can contain concentrations of uranium and thorium in the range of 0.18 to 0.74 Bq/g (5 to 20 pCi/g). Total radium in sludge from titanium process streams had concentrations as high as 2.85 Bq/g (77 pCi/g). Some ZrO₂ concentrates from South Africa are used in a process that chlorinates the sands and converts the zirconium to tetrachloride. Measurements indicate that ²²⁶Ra concentrations in this ore are about 7.4 Bq/g (200 pCi/g). Direct chlorination of zircon puts the radium into the highly soluble radium chloride chemical form, which can yield high leachate concentrations in liquid waste streams. Values of 1665 Bq/L (45,000 pCi/L) of ²²⁶Ra were detected in water samples at one plant. The high solubility and mobility of radium chloride could pose a potential threat to the environment.

Amang is a general term for the by-products obtained when tin tailings are processed into concentrated ores. It includes minerals such as monazite, zircon, ilmenite, rutile, and garnet. Radium-226 and ²³²Th activities in amang have been reported to range from 15.91 to 17.76 Bq/g (430 to 480 pCi/g) and 42.9 to 326.7 Bq/g (1,160 to 8,830 pCi/g) respectively. Tailings from these ores may have a significant potential to cause elevated radiation exposures.

Measurements made at a tin smelter showed ²³⁸U concentrations up to 1.59 Bq/g (43 pCi/g) and ²³²Th concentrations up to 0.7 Bq/g (19 pCi/g). Gamma survey measurements at a tin smelter showed radiation levels in slag storage areas ranging from ~0.087 to 4.35 uSv/h (10 uR/h to 500 uR/h), with average levels less than ~0.522 uSv/h (60 uR/h). The large industries, including copper and iron, generate over 1.0 billion MT (1.1 billion short tons) of waste per year, with an average ²²⁶Ra concentration of 0.18 Bq/g (5 pCi/g).

Geothermal Energy Production Waste

Geothermal energy in the United States is utilized only in a few places, mostly in California. Solid wastes originating from the treatment of spent brines contain TENORM. Hot, saline fluids from geothermal reservoirs may have a dissolved solids content approaching 30% by weight. The average ²²⁶Ra concentration in this waste is estimated at 4.88 Bq/g (132 pCi/g), with waste generation estimated at 54,000 MT (59,500 short tons).

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