Lauer, and 4 other co-authors reported in 2015 thatThe distribution and enrichment of naturally occurring radioactive materials (NORM) in coal combustion residuals (CCRs) from different coal source basins have not been fully characterized in the United States. The authors provide a systematic analysis of the occurrence of NORM (232Th, 228Ra, 238U, 226Ra, and 210Pb) in coals and associated CCRs from the Illinois, Appalachian, and Powder River Basins. Illinois CCRs had the highest total Ra (228Ra + 226Ra = 297 ± 46 Bq/kg) and the lowest 228Ra/226Ra activity ratio (0.31 ± 0.09), followed by Appalachian CCRs (283 ± 34 Bq/kg; 0.67 ± 0.09), and Powder
River CCRs (213 ± 21 Bq/kg; 0.79 ± 0.10).
Total Ra and 228Ra/226Ra variations in CCRs correspond to the U and Th concentrations and ash contents of their feed coals, and the authors show that these relationships can be used to predict total NORM concentrations in CCRs. They observed differential NORM volatility during combustion that results in 210Pb enrichment and 210Pb/226Ra ratios greater than 1 in most fly-ash samples. Overall, total NORM activities in CCRs are 7−10- and 3−5-fold higher than NORM activities in parent coals and average U.S. soil, respectively. This study lays the groundwork for future research related to the environmental and human health implications of CCR disposal and accidental release to the environment in the context of this elevated radioactivity.
In spite of the rise of natural gas production, coal is still a major source for electricity production in the U.S. Long-term projection by the U.S. Energy Information Administration predicts that coal will continue as a major player in the U.S. electricity sector, up to 32% by 2040. In 2013, 39% of the nation’s electricity was generated from coal sources, resulting in the production of 114 million tons of coal combustion residuals (CCRs) that include fly ash, bottom ash, boiler slag, and flue-gas
desulfurization (FGD) solids.
Naturally occurring radioactive materials (NORM) are among the inorganic constituents that are present in coals and enriched in CCRs following the combustion of coal. The NORM in coal consist of primordial 238U (t1/2 = 4.5 × 109 years) and 232Th (t1/2 = 1.4 × 1010 years) and their decay products, as well as 40K (t1/2 = 1.3 × 109 years). Previous studies have shown that NORM concentrations in CCRs can be as much as an order of magnitude greater than those of their feed coal sources because of the elimination of carbon during combustion and coals commonly having ∼10% ash content. Depending on the ash content of the feed coal, typical NORM concentrations in CCRs range from 3 to 10 times the concentrations in coal. Consequently, the enrichment of NORM in CCRs raises potential human and environmental health concerns associated with the release of CCRs to the environment as either emission from smoke stacks, disposal to landfills, coal ash ponds, and abandoned mines, or spills.
The radioactivity of coals and CCRs has been studied since the 1960s. Because of their relatively long half-lives, the most commonly measured NORM in coals and CCRs are 238U, 226Ra, 210Pb, 232Th, 228Ra, and 40K. It has been suggested that coal rank controls the NORM concentrations such that low-rank subbituminous coals, brown coals, and lignites are more effective at adsorbing metals during coalification and will therefore tend to have higher NORM concentrations compared to higher-rank bituminous coals. Fly ash has been differentiated from bottom ash based on the enrichment of the volatile 210Pb in fly ash and consequent depletion in bottom ash. Similarly, studies have also shown 210Pb, 238U, and small 226Ra variations in varying particle size fractions of fly ash. Despite the long history of previous research on
NORM in CCRs, very few studies have addressed the relationships of NORM concentrations and distribution in CCRs to the specific parent coal basins in the U.S.
The authors hypothesized that the relationship between source coals and derived CCRs has important implications for understanding the major factors and processes that control the radioactivity of CCRs and for evaluating the potential human and environmental health risks associated with coal combustion and disposal of CCRs to the
environment. The objectives of this paper are to provide systematic data of NORM concentrations and ratios in CCRs originating from the three major coal-producing basins in the U.S., the Appalachian, Illinois, and Powder River Basins, which produced 25%, 13%, and 41%, respectively, of the total U.S. coal (in 2013), and to address the potential implications associated with the disposal and accidental release of CCRs to the environment. The authors measured 238U, 226Ra, 210Pb, 232Th, and 228Ra in coals and CCRs from the Appalachian, Illinois, and Powder River Basins, and characterized the 228Ra/226Ra, Th/U, and 210Pb/226Ra ratios as well as the total Ra activity (228Ra + 226Ra).
Assuming radioactive secular equilibrium (i.e., the activity of the parent is equal to the activity of the progeny radionuclides) within the U and Th decay chains, at least through 226Ra and 228Ra in both coals and CCRs, the 228Ra and 226Ra activities and activity ratios in CCRs should reflect the original U and Th concentrations and ratios in the feed coals. Differential volatilization of certain elements during coal combustion could result in different activity ratios in CCRs compared to source coals. Accordingly, The authors hypothesized that CCRs would have NORM concentrations and ratios based on the original U and Th concentrations and the chemical characteristics of the feed coals.
Overall then, this paper shows that the combustion of coal causes an enrichment of NORM in CCRs that correlates with the U and Th concentrations and ash content of the parent coals. Coals and corresponding CCRs have distinct 228Ra/226Ra activity ratios and total Ra activities that are characteristic of the source basin of the feed coals and also distinguishable from the background soil. Volatilization of Pb results in further enrichment of 210Pb and its progeny nuclides in fine fly-ash particles, resulting in a breakage in secular equilibrium and overall higher radioactivity in finer-grained fly ash. The authors concluded that CCRs in the U.S. have total Ra activities typically 7−10 times the activities of coal and 3−5 times the activities of average U.S. soil.
The results of this study serve to better quantify radionuclide concentrations and ratios in CCRs from the major U.S. coal basins and lay the groundwork for future research related to the human and environmental health impacts of coal combustion in the context of CCR radioactivity. Specifically, the results of this study are important for the future estimation of radionuclide concentrations in inhaled air containing suspended CCR particulates, calculation of inhalation doses to high-risk populations such as power-plant workers, and evaluation of radionuclide dissolution/adsorption and mobility in the environment near coal ash disposal ponds.
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