Radiation exposures from the beneficial use of alumina production residue

ABSTRACT

Estimates of radiation exposure are developed over the life cycle of beneficial use in cement of an alumina production residue (APR) waste pile. The life cycle includes radiation exposures that might be experienced by industrial workers involved in excavation and transport of the residue to cement plants, industrial workers at the cement plants, construction workers making use of the cement, members of the public who might be in the proximity of the cement products, and disposal of the cement at the end of its useful life. The results indicate that it is not reasonably likely for exposures related to beneficial use of APR waste in cement to exceed the acceptance criteria delineated in current radiation protection standards for workers and members of the general public.

Implications: Radiation exposure estimates developed over the life cycle of beneficial use in cement of an alumina production residue (APR) waste pile indicate that it is not reasonably likely for exposures to exceed the acceptance criteria delineated in current radiation protection standards for workers and the public. Assumed APR waste characteristics, storage, transport, cement production, uses in concrete, and ultimate disposal are generalizable to many APR situations. The findings demonstrate that beneficial use of APR waste as a cement ingredient can be accomplished safely, with potentially significant benefits to management of the large volume of APR being stored around the world.

Introduction

Evans (2015) reports that 115 million metric tonnes of smelter and chemical-grade alumina were produced in 2015, using primarily the Bayer process, and that 1 to 1.5 tonnes of alumina production residue (APR) are produced per tonne of alumina. On this basis, Evans (2015) estimates that 150 million tonnes of APR are produced annually. Evans (2015) also estimates that the residue is being produced at some 60 active sites and is in storage at an additional 50 closed legacy sites, totaling a combined stockpile of three thousand million tonnes at over 100 locations throughout the world.

This vast volume of APR is stored in a variety of settings with varying levels of containment, stabilization, and water management, especially neutralization and/or dewatering of the highly alkaline overlying and interstitial water. Although the APR contains many valuable minerals and organic compounds, only some 2% to 3% has been put to beneficial use. One of the concerns associated with the beneficial use of APR is that it contains naturally occurring Ra-226 and Ra-228. Nonetheless, there is extensive ongoing research to find economical uses for the residue, including (Evans 2015):

• Recovery of specific components present in the bauxite residue (e.g., iron, titanium, aluminum, lanthanides, yttrium, and scandium)

• Use as a major component in the manufacture of another product (e.g., cement)

• Use of the bauxite residue as a component in a building or construction material (e.g., concrete, tiles, bricks)

• Use for soil amelioration or capping

• Use for some specific property, which might include conversion of the bauxite residue to a useful material by modifying the compounds present (e.g., Virotec process).

This paper explores the beneficial use of APR in cement and the attendant radiological issues associated with the elevated levels of naturally occurring radionuclides and their decay products often present in the residue. The method used to evaluate the potential radiological issues is to construct a reference alumina residue storage facility and model the life cycle of the residue and associated radionuclides in the residue, including the levels of radiation exposures that might be experienced by industrial workers involved in the excavation and transport of the residue, industrial workers at cement plants, construction workers making use of the cement, members of the public who might be in the proximity of the cement products, and issues associated with disposal of the cement at the end of its useful life.

Potential for beneficial use of APR in cement and other building materials

The United States Environmental Protection Agency’s (EPA’s) Sustainable Healthy Communities Program includes a task for “Beneficial Use of Waste Materials,” which is designed to conduct research and analyses to characterize and quantify the risks and benefits of using or reusing waste materials. The EPA, the International Aluminum Institute (IAI), and others have long recognized the need for the safe management of large volumes of various types of residue associated with processing bauxite for the production of aluminum oxide, aluminum sulfate (alum), and aluminum metal. Power, Markus Grafe, and Klauber (2009) provides an overview of current best management practices for bauxite residue. In a review of the literature, Power, Markus Grafe, and Klauber (2009) estimates that the global inventory of bauxite residue for 2015 is 4 billion tons, with the doubling time decreasing each year. They explain that the management practices for the safe storage of this material are continually improving in order to minimize the potential impact of this material on the environment during long-term storage, with the primary focus on reducing its alkalinity, increasing its stability, reducing its potential for contaminating groundwater while in storage, and with a goal of beneficially reusing this material, when possible, rather than bearing the burden of perpetual institutional control.

Red and brown muds produced as waste and by-products of processing bauxite are caustic. However, if properly managed, muds can be used for land reclamation, feed material for other extraction processes, cement production, brick production, and other beneficial uses (International Aluminium Institute 2015).

Fergusson (2014) provides an overview of the beneficial use of alumina refinery residue, citing numerous reports of modified alumina refinery residue being put to beneficial use in the construction industry. A meeting of the ICSOBA (International Committee for Study of Bauxite, Alumina & Aluminum) Bauxite Residue Seminar was held on October 17–19, 2011. The Proceedings stated that only a few percent of the annual residue production of bauxite residue is currently being used, mainly in cement and ceramics, in agriculture as landfill covering, and in road and embankment construction. However, the summary of that meeting concluded that, with proper pretreatment, such as dewatering and neutralization, the residue has many beneficial uses, including as raw material input for Portland cement clinker production

(3–5wt% bauxite reside) and as pozzolanic material in cement (up to 30wt% bauxite residue) for concrete applications.

It is clear that, with proper pretreatment, APR can be used beneficially as an additive to Portland cement. Because of their relatively high cost, calcium aluminate cements are used in a number of restricted applications where performance achieved justifies costs:

• In construction concretes, where rapid strength development is required, even at low temperatures

• As a protective liner against microbial corrosion, such as in sewer infrastructure

• In refractory concretes, where strength is required at high temperatures

• As a component in blended cement formulations, for which various properties are required, such as ultra-rapid strength development and controlled expansion.

• In sewer networks for their high resistance to biogenic sulfide corrosion

Radiological issues associated with beneficial use of APR

APR contains naturally occurring levels of radioactivity. Table 1 shows typical levels of radioactivity present in APR (Miller and Miller, 2016). Naturally occurring material in which the naturally occurring radionuclide concentrations have been technologically increased is referred to as Technologically Enhanced Naturally Occurring Radioactive Material, or TENORM. By this definition, APR can be viewed as TENORM. TENORMs have received a considerable amount of attention from the radiation protection community because of their presence in virtually every aspect of the ore beneficiation and oil and gas industries. No federal regulations explicitly govern the management and disposal of TENORM. However, potentially applicable radiation protection principles have been promulgated (CRCPD 2015):

• Under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA)

• In the U.S. Occupational Safety and Health Administration (OSHA) regulations set forth in 29 CFR 1910.1096

• In the U.S. Nuclear Regulatory Commission (NRC) regulations set forth in 10 CFR Part 20

• In the Clean Water Act (CWA) of 1972

• In the Safe Drinking Water Act (SDWA) of 1974

• In guidance in support of the USEPA Superfund and Resource Conservation and Recovery Act (RCRA) of 1976)

Table 1. Typical radiological characteristics of alumina production residue.

Nuclide	Activity Conc (pCi/g)	Nuclide	Activity Conc (pCi/g)	Nuclide	Activity Conc (pCi/g)
Th‐232	5.49E + 00	Tl‐208	9.09E + 00	Ra‐226	3.25E + 00
Ra‐228	1.24E + 01	U‐238	3.65E + 00	Pb‐214	3.60E + 00
Th‐228	7.22E + 00	Th‐234	2.03E + 00	Bi‐214	3.74E + 00
Pb‐212	1.06E + 01	U‐233/234	3.18E + 00	U‐235	1.44E ‐ 01
Bi‐212	9.26E + 00	Th‐230	2.57E + 00	K‐40	1.42E + 00

In addition, SSRCR Part N, Regulation and Licensing of Technologically Enhanced Naturally Occurring Radioactive Material (TENORM) “ … establishes radiation protection standards for TENORM (CRCPD 2004). These standards include the possession, use, processing, manufacture, distribution, transfer, and disposal of TENORM and of products with TENORM.” These CRCPD standards are available to states for use in developing state-specific TENORM regulations. Because this paper addresses the beneficial use of the alumina residue, the guidance in SSRCR Part N is of particular interest.

In preparing this paper, based on consideration of the above-cited standards and guidelines, the following acceptance criteria are used as the basis for determining the radiation risk-based acceptability of the beneficial use of the alumina residue in cement:

• Exemption level of 5 picoCurie/gram (pCi/g) above the natural background of residual levels of radium in soil following cleanup (not including exposure to radon)

• Exposure limit of 25 millirem/year (mrem/yr) above natural background for the cleanup and release of sites and structures where the residue was formerly stored (not including exposure to radon)

• Exposure limit of 100 mrem/yr for exposures of workers engaged in the handling of the residue and any material produced as a result of the beneficial use and ultimate disposal of this material (not including exposure to radon)

• No individual will experience exposures to radon indoors above 4 picoCurie/liter (pCi/L)

• No individual will experience exposures to outdoor levels of radon in excess of 0.5 pCi/L

• Sources of drinking water regulated under 40 CFR part 141 will not exceed the maximum contaminant levels (MCLs) set forth in those regulations

• The combined annual discharge concentration of Ra-226 plus Ra-228 to receiving bodies of water will not exceed 3.75 pCi/L.

Naturally occurring radionuclides in cement and other building materials

Even in the absence of APR, the materials used to make cement will contain naturally occurring radionuclides. To gain some perspective as to the concentrations of radionuclides in cement not related to APR, a literature review was performed. That review found many publications that provide information on the concentrations of naturally occurring radionuclides observed in cement and other building materials. Table 2 summarizes representative concentrations from the literature. A recurring observation is that the radionuclide content of cement is highly variable and depends on its chemical composition in relation to its geologic source and geochemical characteristics.

Table 2. Naturally occurring radionuclides in cement and other building materials.

	Concentration (pCi/g dry wt)
Reference	Ra-226	Th-232	K-40	Notes
Chong and Ahmed (1982)	2.2	―	―	Cement
Stoulos, Manolopoulou, and Papastefanou (2003)	2.49	―	―	Cement
Hizem, Fredj, and Ghedira (2005)	0.59	―	―	Cement
Xinwei (2005)	1.25	0.77	3.7	Cement bricks
	1.75	1.31	4.36	Cement plaster
Rahman et al. (2008)	0.81	1.1	14.1	Brick
	0.51	0.81	20.8	Sand
	0.49	0.49	6.18	Marble
	0.65	0.49	6.59	Cement
Estokova and Palascakova (2013)	0.23–0.52	0.26–0.71	4.23–13.2	Cement
Trevisi et al. (2018)	1.38	1.32	15.0	Bricks
	1.6	0.92	9.2	Concrete

Life-cycle risk assessment exposure scenarios

As shown in Figure 1, for this evaluation of the potential radiation exposures from the beneficial use of APR, a number of life-cycle exposure scenarios have been postulated. The scenarios explicitly evaluate the potential radiological impacts associated with the beneficial use of this material in Portland cement from its excavation at the residue pond until it is ultimately disposed of in a landfill. In between, exposure scenarios address workers at a cement kiln and at a concrete plant, as well as the drivers who transport the material from one site to another. Many of the end uses of alumina concrete would essentially be inaccessible to ordinary human exposure (e.g., in sewers, as refractory brick); nevertheless, use of the APR-containing concrete as a home’s slab foundation is used here as a reasonable maximum exposure scenario. The final scenario evaluated is based on the assumption that the APR-containing concrete end-use item (e.g., slab foundation, sewer, refractory brick) has reached the end of its useful life and has been crushed and either used in a road bed or sent to a municipal or construction material landfill.

Figure 1. Potential life cycle radiation exposures from beneficial use of alumina production residue (APR).

The results of all these evaluations, as well as the assumptions that were made, are documented in the sections that follow.

Life-cycle stage 1 – excavation of material from the APR pond

The first step was to estimate the amount of time it would take to excavate the 300,000 cy of material from an APR pond. This was necessary to determine the exposure that would be incurred by the workers performing the excavation. The RS Means Company annually publishes data regarding the time and equipment necessary to perform various construction activities, including the bulk excavation of areas. The Means data is usually used by companies to develop cost estimates for proposals, so the guide is published annually. However, the Means data are used here simply to estimate the time required to excavate the APR pond, and those data are not time-sensitive; therefore, the Means publication for the year 2000 (Means 1999) is used in this analysis.

Means presents data for 26 different types and sizes of excavation equipment, including shovels, backhoes, and front-end loaders ranging in size from 0.5 to 5 cubic yards (cy). Using the Means data, it was estimated that between 150 and 1,875 days would be required to excavate the APR pond material. For example, using a backhoe, the reference APR pond would be excavated in 289 days. While determining the exact duration necessary to excavate the pond is beyond the scope of this paper, it can safely be assumed that it would take more than a calendar year to remove all of the material. It is also likely that the same worker would not be involved every day for the total duration of the excavation, and that operators of excavation equipment are in an enclosed driver seat. Thus, it is conservative to assume a one-year dose (i.e., 2,000 hours) to the workers performing the excavation in calculating exposure for comparison to the appropriate regulatory limits.

During excavation, the equipment operator and other onsite construction workers would be exposed via the following pathways: (1) radiation emanating from the material itself, (2) submersion within a plume of material that becomes airborne during the excavation process, (3) inhalation of material that becomes airborne, and (4) ingestion of material adhering to the worker’s fingers. Each of these potential exposure pathways is evaluated in the following subsections.

Direct/shine dose

Federal Guidance Report No. 12 (FGR 12) (EPA 1993) presents radionuclide-specific dose factors that can be used to calculate the dose rate to an individual standing on ground that is contaminated with radioactivity. Rather than standing on the ground, the operator of excavation equipment would be sitting inside a cab located some distance above the ground. Thus, using the FGR 12 dose factors would result in a conservative estimate of the actual dose received by the excavation equipment operator. Also, in addition to the operator, a number of individuals would be present to support the excavation (e.g., foreman, oiler, supervisor, health and safety technician) – it is likely that these individuals would be standing on the contaminated ground, so using the FGR 12 dose factors would be appropriate for them.

The FGR 12 dose factors are provided in per-volume units (i.e., Sv/s per Bq/m³), and the radioactive material in the reference APR pond is provided in per-weight units (i.e., pCi/gm); therefore, it is necessary to multiply the results by the density of the APR pond material, assumed to be 1.1 kg/L.

Using the assumed radionuclide concentrations in the reference APR pond (from Table 1) and the FGR 12 ground shine dose factors, the calculated dose rate from standing on the APR Pond surface was calculated to be 2.0E-03 mrem/hour, dominated by the Th-232 decay product Tl¬208.

Conservatively assuming that the construction workers are exposed for their entire work year (i.e., 2,000 hours) results in an effective whole-body dose of 40 mrem/year.

Inhalation dose

An individual who is submerged within a dust cloud of APR pond material would also inhale some of the material and be exposed via the inhalation pathway. Assuming a dust loading for construction activities of 6.0 × 10–4 g/m3 (ANL 1993), one can calculate the potential radiation exposure due to the inhalation pathway.

In addition to the airborne dust loading, the inhalation dose to a worker depends strongly on the particle size distribution and solubility. ICRP (2002) provides a detailed description of the human respiratory tract model, which establishes the basis for the inhalation dose conversion factors (DCFs) that are used by all regulatory authorities throughout the U.S. and abroad. The inhalation DCFs presented below, which are published by the EPA in Federal Guidance Report No. 13, are based on the ICRP recommendations. The biokinetic behavior and clearance rate of any inhaled aerosol depend on a parameter referred to as the activity median aerodynamic diameter (AMAD). The size, shape, and density of aerosols are highly variable and affect the deposition pattern and clearance rate of the aerosol in the lung. For this reason, particle sizes are expressed in terms of a standardized metric, the AMAD, which is defined as the diameter of a unit density sphere with the same terminal settling velocity in air as that of an aerosol particle whose activity is the median for the entire aerosol (Shleien, Slaback, and Birky 1998); in other words, a sphere with a diameter of 1 micron and density of 1 gram per cubic centimeter. A vast amount of research has resulted in ICRP recommending that the default particle size for deriving inhalation doses from aerosols is a 5-micron AMAD, which is the value used in the inhalation dose calculations described herein.

The inhalation DCFs also depend on the different chemical forms that radionuclides may assume in the environment. This metric, referred to as solubility or transportability, determines the movement of the radionuclide as it is transported through the body following inhalation until its eventual excretion. Federal Guidance Report No. 13 provides inhalation DCFs for a range of plausible solubilities for many radionuclides, referred to as fast, medium, and slow (F, M, or S). In this paper, three categories of DCFs for the various radionuclides of interest were evaluated, and the limiting DCFs are employed.

Using the FGR 13 (EPA 1999) inhalation DCFs for each radionuclide characterizing the reference pond, and an inhalation rate of 2 m³/hour, the hourly dose rate to the construction worker due to the inhalation exposure pathway was calculated to be 6.4E-04 mrem/hour, dominated by Th-232, Th-228, and Th-230.

Conservatively assuming that the construction workers are exposed for their entire work year (i.e., 2,000 hours) results in an effective dose commitment of 13 mrem/year. The term effective dose commitment refers to the effective whole-body dose that the worker is committed to over the remainder of his life due to the radionuclides inhaled in a given year.

Ingestion dose

It was assumed that construction workers would ingest dust for the entire 27-week annual seasonal work period (i.e., 1080 hours). Even with that conservative assumption, the resulting dose was negligible when compared to the direct/shine or inhalation doses (i.e., three orders of magnitude less). Based on this result, the inadvertent ingestion exposure pathway was not evaluated for any of the other exposure scenarios analyzed.

Life cycle stage 2 – transportation of APR to cement production facility

As shown by Means, the excavation rate could vary greatly. For example, assuming a 2-cy backhoe, the excavation rate would be about 130 cy/hour, with a typical dump truck capable of carrying about 20 cy, resulting in 6½ truckloads per hour being generated. If the transport distance is short, then the trucks could be re-used; however, if the transport distance is long, then a fleet of dump trucks would be required.

Material excavated from an APR pond would need to be transported to the cement kiln. It is assumed that transport by truck would require an inordinate amount of time, and transport by rail would be the preferred alternative. However, even if railroad transport is assumed, dump trucks and drivers would still be needed to transport the excavated material to rail spur in the vicinity of an ARP pond. This driver would drive onto the APR pond, wait while the truck is loaded with excavated material, drive off of the APR pond to the rail spur, dump the material into a waiting railcar, and then repeat the process for the entire workday. When on the APR pond, the driver’s exposure pathways would be similar to those of the excavator operators, plus when the truck is loaded, the driver would be exposed to the APR pond material in the bed of the truck, mitigated to some extent by being inside the cab. Alternatively, if trucks are used to transport the material to the cement kiln, then the driver would spend most of his time being exposed to only the material contained within the truck’s bed. Thus, the rail transport scenario would result in the largest exposure to an individual truck driver, and that scenario is assumed in this analysis.

The shine dose to the driver from the material in the truck’s bed has been calculated using the MicroShield computer code, with the dump truck modeled as a 4.5-ft by 7-ft by 17-ft rectangular source.

Based on Table 1 radioactivity concentrations, the calculated dose rate to the loaded dump truck driver is 0.017 mrem/hour. In order to derive the driver’s total annual dose, it is assumed that, on average, the dump truck would be fully loaded half of the time and empty for the other half of the time; thus, the driver’s dose due to material in the truck’s bed is 17 mrem/year. This dose needs to be added to the exposure the driver receives from standing on the APR pond surface and inhaling suspended APR pond material dust. The driver’s dose rates from those two pathways are the same as for the excavator operator; however, because the driver spends some of his time off of the APR pond surface, his annual exposure duration to these two pathways would be less than that of the excavator operator. Thus, assuming that he spends two-thirds of his time on the APR pond, the truck driver’s total dose would be 52 mrem/year from all three exposure pathways.

Life-cycle stage 3 – exposure of workers at the cement kiln facility

A schematic of a typical cement kiln facility is shown in Figure 2. For this analysis, it is assumed that the excavated APR pond material is stored in silos. Thus, any cement kiln personnel employed in operations that occur prior to filling these silos would not be exposed to the APR pond material.

Figure 2. Process flow for a typical cement kiln facility (U.S. DOE 2001).

Materials storage silo

Once the APR material is received at the cement facility, it would be stored in the storage silo. These storage silos can have a range of sizes starting from a few feet in diameter up to 50 feet in diameter and 100 feet or more of height (Ashoka Machines 2019). For this study, it is assumed that the cylindrical storage silo would have dimensions resulting in the same volume as a dump truck. The MicroShield model does not account for shielding provided by the metal walls of the storage silo. The dose point is conservatively selected to be midway up the side of the silo; it would be difficult for a worker to be positioned at that location.

Based on this silo model and Table 1 radioactivity concentrations, the dose rate calculated using MicroShield due to external exposure to the APR material in the storage silo is 0.019 mrem/hour. Notice that this MicroShield-calculated dose rate is nearly identical to the dose rate from standing on an infinity-thick slab of contaminated APR material, which is considered to be the limiting direct/shine exposure scenario. Thus, it is concluded that the size of the storage silo makes little difference to the dose rates received by the worker. A worker who spends 1 hour each day in proximity to the proportioning storage silo would receive a direct/shine dose of 4.8 mrem/year.

Grinding mill

Assuming that the grinding mill is of a similar size as (or smaller than) the storage silo, the direct/shine dose rate due to the APR material in the grinding mill is 0.003 mrem/hour (i.e., 15% of the silo’s dose rate). A worker who spends 1 hour each day in proximity to the grinding mill would receive a direct/shine dose of 0.7 mrem/year.

Assuming that the dust level in the vicinity of the grinding mill is the same as at a construction site (i.e., 6.0 × 10⁻⁴ g/m³), the inhalation dose rate to a worker standing near the grinding mill would be 15% of the dose rate to a construction worker at the APR pond, or 9.6 × 10⁻⁴ mrem/hour. Because the grinding mill is a piece of operating equipment, it is assumed that a worker would need to be in attendance at all times. A worker who spends 2000 hours per year in proximity to the grinding mill would receive an additional inhalation effective dose commitment of 1.9 mrem/year.

Cement Kiln

After leaving the grinding mill, the cement feed material enters a preheater tower. The preheater tower is assumed to be constructed of material similar to the grinding mill; thus, the dose rates in the vicinity of the preheater tower are expected to be similar to the grinding-mill dose rates.

After leaving the preheater tower, the cement feed material enters the kiln. A rotary kiln is made from 15- to 30-mm-thick steel plate, welded to form a cylinder that may be up to 230 m in length and up to 6 m in diameter (Grzella et al. 2005). In order to insulate the steel shell from the high temperatures inside the kiln and protect it from the corrosive properties of the process material, the kiln is lined with firebrick. For this study, the kiln was modeled as a cylinder 230 m long and 3 m in radius. The kiln is assumed to be lined with an 800-cm-thick layer of firebrick, with a density of 2.35 g/cm³. The steel plate that actually forms the kiln is conservatively not included in the MicroShield model.

Although it is realized that, during operation, only a portion of the kiln’s volume is filled with material, for simplicity, it is conservatively assumed that the entire interior kiln volume is filled with material similar to the material in the grinding mill (i.e., 15% APR material and 85% lime and silica). The dose point was conservatively selected to be midway on the side of the kiln.

Based on the MicroShield model and assuming that the kiln contains material at a concentration that is 15% of Table 1 radioactivity concentrations, the MicroShield calculated dose rate due to the APR material in the kiln is 0.0014 mrem/hour. Because the kiln is a piece of operating equipment, it is assumed that a worker would need to be in attendance at all times. A worker who spends 2000 hours per year in proximity to the kiln would receive a direct/shine dose of 2.8 mrem/year.

Cement storage silo

Because APR material is assumed to compose only 15% of the final cement mixture, the dose rate near the cement storage silo would be 15% of the dose rate near the material storage silo, which is assumed to contain 100% APR material. Thus, the dose rate near the cement storage silo would be about 0.003 mrem/hour. A worker who spends 1 hour each day in proximity to the cement storage silo would receive a dose of 0.7 mrem/year.

Life-cycle stage 4 – transportation of cement to ready-mix concrete plant

According to the Portland Cement Association (PCA), ready-mixed concrete accounts for 75 percent of all concrete shipped (Portland Cement Association 2019). Because APR material is assumed to compose only 15% of the final cement mixture, the shine dose rate to the driver from cement in the bed of the truck would be 15% of the shine dose rate to the driver transporting APR material, which is assumed to contain 100% APR material. Thus, the dose rate to the cement truck driver would be about 0.003 mrem/hour. A worker who spends 1,000 hours per year (half his time with a full load and the other half driving back) transporting cement would experience a dose of 3 mrem/year.

Life-cycle stage 5 – exposure of workers at the concrete plant

A concrete plant can have a variety of parts and accessories, but consists primarily of cement storage silos, aggregate storage compartments, weight stations, conveyors, and mixers.

Exposure of workers at the concrete plant would come primarily from the APR-contaminated cement in the storage silos. Thus, the dose rate to a concrete plant worker would be essentially the same as the dose rate in the proximity of the cement storage silo at the cement kiln facility, or about 0.0071 mrem/hr. A worker who spends 1 hour each day in proximity to the cement storage silo would receive a dose of 1.8 mrem/yr.

Life-cycle stage 6 – transportation of concrete

Concrete is a mixture of cement, fine aggregate (i.e., sand), coarse aggregate (i.e., gravel), and water. The proportions of each of the four components vary widely, but a typical concrete mix is shown in Table 3.

Table 3. Typical concrete recipe.

Concrete Component	Amount per cubic foot	Proportion
Cement	22 pounds	15.4%
Sand	41 pounds	28.7%
Gravel	70 pounds	49.0%
Water	10 pounds	7.0%
	143 pounds	100.0%

Based on this composition, concrete is composed of 15.4% cement. Thus, the radionuclide concentration in concrete would be (0.15 × 0.154) 0.023 times the APR material radionuclide concentrations given in Table 1. Likewise, the dose rate to the driver transporting the concrete would be about 2.3% of the dose rate to the driver transporting APR material, or about 0.0004 mrem/hour. Assuming that half the driver’s time is spent with a full load, his annual dose from transporting concrete made from cement that contains APR material would be about 0.4 mrem.

Life-cycle stage 7 – concrete use and exposure incurred by members of the public

As discussed above, calcium aluminate cements are used in a number of restricted applications where performance achieved justifies costs. A quick review of these applications shows that, for many/most of them, daily direct human contact is unlikely. Nevertheless, in order to bound potential public exposures, a simple exposure scenario is postulated. It is assumed that the concrete made from cement containing APR material would be used to form the slab of a one-story house. As for the concrete-truck driver, the radionuclide concentrations in the home’s slab would be (0.15 × 0.154) 0.023 times the APR material radionuclide concentrations given in Table 1, and the direct/shine dose rate to individuals living within the home would also be (0.15 × 0.154) 0.023 times the direct/shine dose rate to a construction worker on the APR pond surface (0.02 mrem/hour, or 0.0005 mrem/hour). EPA (2011) Exposure Factors Handbook, Table 16–1, indicates that the very young (<2 years) and the old (>65 years) could spend up to 24 hours per day in their home. Thus, the calculated direct/shine dose rate to an individual in the home would be about 4 mrem/year.

Life-cycle stage 8 – recycling and disposal of concrete

In 2003, the NRC published a major guidance document for use by licensees to help in assessing the health and safety of the recycling, use, and disposal of a wide variety of material (NRC 2003 also referred to as NUREG-1640). Chapter 6 of that report presents a detailed description of every aspect of the recycling, use, and disposal of a broad range of material, including Portland cement, which might contain trace levels of radionuclides. The outcome of that report is a set of dose factors that present the relationship between the radiation doses to members of the public, including occupationally exposed members of the public, from a myriad of exposure pathways and scenarios.

NUREG-1640 presents a schematic diagram of the flow of recycled and disposed concrete. As is the case in the analysis of other cleared materials, this is a simplified idealization of the actual process. The diagram depicts the sequence of steps that is represented by the exposure scenarios in NUREG-1640. A total of eight concrete recycling and disposal exposure scenarios were evaluated in NUREG-1640 and are listed in Table 4. Other steps and processes are discussed in detail in NUREG-1640, Volume 1, Chapter 6.

Table 4. NUREG-1640 concrete recycling and disposal exposure scenarios.

Scenario abbreviation	Scenario title
Processing concrete	Processing concrete rubble at satellite facility
Truck driver	Truck driver hauling concrete rubble
Road building	Building road using recycled concrete
Driving on road	Driving on road built with recycled concrete
Disposal-industrial	Handling concrete rubble at industrial landfill
Disposal – MSW	Handling concrete rubble at municipal landfill
Leachate – industrial	Ingesting leachate from industrial landfill
Leachate – MSW	Ingesting leachate from municipal landfill

The dose factors are expressed in units of mrem/year per pCi/g of 39 radionuclides, including the radionuclides of interest to this study. The product of these dose factors with the observed concentration of individual radionuclides in concrete yields the annual dose to the particular member of the critical population group for the exposure scenario and pathway of interest.

Dose factors are provided for eight scenarios that depict exposures resulting from the handling and processing of cleared concrete rubble, transportation of the rubble, the use of recycled concrete in road construction, the landfill disposal of concrete rubble, and the infiltration of well water by leachate from landfills containing concrete rubble. The analysis uses data on concrete recycling and disposal, as currently practiced in the United States, and on contemporary U.S. work practices and living habits. Table 5 presents the limiting dose factor for each of the eight scenarios.

Table 5. Normalized mass-based effective dose equivalents to critical groups for concrete.

Radionuclide*	Average Dose Factor (mrem/year per pCi/g)
Th-232	0.33	Truck driver
Ra-228	0.43	Road Building
Th-228	0.68	Road Building
U-238	0.079	Leachate-MSW
U-234	0.085	Leachate-MSW
Th-230	0.066	Truck Driver
Ra-226	0.75	Road Building
K-40	0.069	Road Building

Notes.* While Pb-212, Pb-214, Bi-212, Bi-214, and Tl-208 are not explicitly listed, their contribution to the dose conversion factors (DCFs) are already incorporated into the DCFs of their parents; i.e., Ra-228 and Ra-226.

Because Table 5 dose factors were calculated based on a large volume of concrete being disposed (e.g., the rubblization of a nuclear power plant), they would likely overestimate the exposures associated with the disposal of a single, or a few, home slab foundations containing radiation from APR added to the concrete. Nonetheless, using Table 5 dose factors, Table 1 APR concentrations, and 15% of APR in cement and 15.4% cement in concrete, the exposure to APR-contaminated concrete rubble was calculated to be about 0.27 mrem/year, dominated by Ra-228, Th-228, and Th-232.

Radon buildup and exposures

Radon-226 decays into radon-222 (Rn-222), which in turn decays into the short-lived isotopes: polonium-218 (Po-218), lead-214 (Pb-214), bismuth-214 (Bi-214), and polonium-214 (Po-214). Because it is a noble gas, radon has several exposure pathways that are not included the previous dose calculations. This section investigates dose contributions from the potential radon exposure pathways.

On January 5, 1983, the EPA issued 40 CFR Part 192, Standards for Remedial Actions at Inactive Uranium Processing Sites, which includes the standard (i.e., 40CFR § 192.02(b)(1)) that control measures shall be designed to provide reasonable assurance that the release of Rn-222 to the atmosphere from residual radioactive materials will not exceed 20 pCi/m²-s, when averaged over the entire site and over at least a one-year period (FR 1983).

Based on this EPA (EPA 1983; EPA 520/1-83-008-1, page 3–5) and NRC (NRC 1980; NUREG-0706, Volume 3, page S-2) precedent, a Rn-222 emanation rate of 1 pCi/m²-s per pCi/g Ra-226 has been used in this study. Using this emanation rate and Table 1 Ra-226 concentration, the Rn-222 flux above a reference APR pond would be about 3.25 pCi/m²-s, which is well below the EPA’s limit of 20 pCi/m²-s.

The following sections present the method for converting the radon emission flux into a radon airborne concentration and the calculated in-growth of the radon progeny. The method was taken from the “User’s Manual for RESRAD Version 6” (Yu et al. 2001), Appendix C: Radon Pathway Model.

Outdoor radon and progeny concentrations

The buildup of radon and its progeny outdoors is calculated using RESRAD, Equation C.7. The calculated airborne radon concentration above the reference APR pond is 0.083 pCi/L. This value is appropriately compared to the outdoor radon concentration acceptance criterion of 0.5 pCi/L described above.

Indoor radon and progeny concentrations

Radon emitted from a home built on a slab of concrete incorporating APR as an ingredient would enter the home, and radon progeny would build up inside the home. To derive the concentration of radon and its progeny inside a home or other structure, RESRAD provides Equation C.12 for radon and Equation C.21 for radon progeny.

Assuming that the outside radon/progeny concentrations are negligible, the buildup of radon inside the home is 0.096 pCi/L. For perspective, the EPA’s action level for radon is 4.0 pCi/L (4,000 pCi/m³). The EPA estimates that about 6% (or 5.8 million) homes have radon levels greater than the action level, with 0.7% of U.S. homes having annual average radon levels greater than 10 pCi/L (10,000 pCi/L) (EPA 1992).

Results and conclusion

Table 6 presents the results of the assessment of life-cycle radiation doses to workers and members of the public associated with the beneficial use of APR material in cement. Conservative assumptions about potential exposure scenarios and exposure factors were used in calculating these doses. The resulting compounded conservatism translates into upper-bound dose estimates that are unlikely to actually occur.

Table 6. Summary of calculated radiation exposures from the beneficial use of alumina production residue.

APR Exposure Scenario	Annual Dose (mrem/year)
Excavation of Material from the APR pond	53.
Direct/Shine Dose	40.
Submersion Dose*	Negligible
Inhalation Dose	13.
Ingestion Dose*	Negligible
Transportation of APR to Cement Production Facility	52.
Direct/Shine Dose from Loaded Truck	17.
Direct/Shine Dose from APR pond Surface	27.
Inhalation Dose above APR pond Surface	8.7
Exposure of Workers at the Cement Kiln Facility	–
Materials Storage Silo	4.8
Grinding Mill	2.6
Preheater Tower	2.6
Cement Kiln	2.8
Cement Storage Silo	0.7
Transportation of Cement to Ready-Mix Concrete Plant	3.0
Exposure of Workers at the Concrete Plant	0.7
Transportation of Concrete	0.4
Exposure of Members of the Public	4.0
Recycling and Disposal of Concrete	0.23
Radon on Top of the APR pond	≪0.5 pCi/L
Radon Dose Inside a Home	≪4 pCi/L

Notes.* Calculated, but not included in this paper due to their negligible contribution to the dose.

The results indicate that it is not reasonably likely for exposures related to the beneficial use of APR waste in cement to exceed the acceptance criteria delineated in current radiation protection standards for workers and members of the general public. Workers, as used here, refer to the individuals who excavate the APR pond, the employees at the cement kiln or the concrete plant, and the truck drivers who transport the material. From a radiation protection perspective and as evaluated in this paper, these individuals are not “radiation workers,” and are subject to the same dose limits as members of the public.

The authors postulate that the assumed reference APR waste characteristics, storage, transport, cement processing, uses in concrete, and ultimate disposal are generalizable to many APR situations. As such, the method presented can be used to demonstrate that beneficial use of APR waste as a cement ingredient can be accomplished safely across its entire life cycle, with potentially significant benefits with regard to the management of the large volume and number of APR waste sites around the world.

Additional information

Notes on contributors

Stephen Marschke

Stephen Marschke is Vice President/Nuclear Engineer with SC&A. He has over 45 years of experience performing radiological analyses for both government agencies and private clients.

William Rish

William Rish is a Principal Engineer with ToxStrategies, He has more than 35 years’ experience in engineering, site assessment and remediation, risk assessment, and probabilistic uncertainty analysis. He has prepared hundreds of risk assessments and managed numerous large, complex site investigations and remediation projects, and has been active for many years in the development of federal and state rules, guidance, and policy.

John Mauro

John Mauro holds a Ph.D. in Health Physics from New York University Medical Center (1973), is certified by the American Board of Health Physics since 1976, in 2004 was appointed by the Governor of New Jersey as a Commissioner of Radiation Protection, is a member of the Conference of Radiation Control Program Directors, and was appointed by the National Institute and Science and Technology to the Community Resilience Panel.  Dr. Mauro’s career, which spans over 40 years, has been dedicated to the protection of workers, members of the general public, and environment from the potential harmful effects of ionizing radiation and radioactive materials in the workplace and the environment. Dr. Mauro’s primary clients have been the Environmental Protection Agency, the Nuclear Regulatory Commission, the National Institute of Occupational Safety and Health, the Centers for Disease Control and Prevention, the Department of Energy, and the Republic of the Marshall Islands.