Analysis of naturally occurring radionuclides in fly ash and gypsum samples

Abstract

Coal combustion residues from coal fired power plants are advantageous for use in building and construction materials. These by-products contain trace quantities of natural radionuclides from the uranium and thorium series, as well as naturally occurring radionuclides such as 40K. Analysis was performed for samples of coal fly ash, flue gas desulfurization (FGD) gypsum, and scrubber sludge from the United States. The radioactive content of 234U and 232Th decay series nuclides was determined using gamma photopeaks from progeny 213Pb at 352 keV and 208Tl at 583 keV, respectively; 40K concentrations were determined using the 1461 keV photopeak. The samples were hermetically sealed for several weeks to allow for secular equilibrium between the radium parents and the radon and subsequent progeny. Samples were analyzed in a common geometry using two high purity Germanium photon detectors with low energy detection capabilities. The specific activities (Bq kg-1) were compared to results from literature studies including different building materials and fly ash concentrations. Fly ash bituminous 238U concentrations varied from 30 to 217 Bq kg-1 (mean + 1 sd 119 ± 45 Bq kg-1), and sub-bituminous concentrations varied from 72 to 209 Bq kg-1 (115 ± 40 Bq kg-1); bituminous 232Th concentrations varied from 10 to 120 Bq kg-1 (73 ± 26 Bq kg-1), and sub–bituminous concentrations varied from 53 to 110 Bq kg-1 (81 ± 18 Bq kg-1); bituminous 40K concentrations varied from 177 to 928 Bq kg-1 (569 ± 184 Bq kg-1), and sub-bituminous concentrations varied from 87 to 303 Bq kg-1 (171 ± 69 Bq kg-1). A dosimetric analysis was performed for both external and internal dose components for persons inhabiting buildings with fly ash incorporated in the building materials. The internal dose was calculated for 222Rn using results from pervious literature and the results of the specific activity from study. The external dose was calculated using GEometry ANd Tracking 4 GEant4 Monte Carlo radiation transport simulation environment.