The enrichment behavior of natural radionuclides in pulverized oil shale-fired power plants

The oil shale industry is the largest producer of NORM (Naturally Occurring Radioactive Material) waste in Estonia. Approximately 11–12 million tons of oil shale containing various amounts of natural radionuclides is burned annually in the Narva oil shale-fired power plants, which accounts for approximately 90% of Estonian electricity production. The radionuclide behavior characteristics change during the fuel combustion process, which redistributes the radionuclides between different ash fractions. Out of 24 operational boilers in the power plants, four use circulating fluidized bed (CFB) technology and twenty use pulverized fuel (PF) technology. Over the past decade, the PF boilers have been renovated, with the main objective to increase the efficiency of the filter systems. Between 2009 and 2012, electrostatic precipitators (ESP) in four PF energy blocks were replaced with novel integrated desulphurization technology (NID) for the efficient removal of fly ash and SO₂ from flue gases. Using gamma spectrometry, activity concentrations and enrichment factors for the ²³⁸U (²³⁸U, ²²⁶Ra, ²¹⁰Pb) and ²³²Th (²³²Th, ²²⁸Ra) family radionuclides as well as ⁴⁰K were measured and analyzed in different PF boiler ash fractions. The radionuclide activity concentrations in the ash samples increased from the furnace toward the back end of the flue gas duct. The highest values in different PF boiler ash fractions were in the last field of the ESP and in the NID ash, where radionuclide enrichment factors were up to 4.2 and 3.3, respectively. The acquired and analyzed data on radionuclide activity concentrations in different PF boiler ashes (operating with an ESP and a NID system) compared to CFB boiler ashes provides an indication that changes in the fuel (oil shale) composition and boiler working parameters, as well as technological enhancements in Estonian oil shale fired power plants, have had a combined effect on the distribution patterns of natural radionuclides in the oil shale combustion products. - Highlights: • Oil shale natural radionuclide activity concentrations in PF boilers were measured. • Radionuclide activity concentrations and enrichment factors increase in PF boilers. • Activity concentrations in ash fractions were compared between different boilers. • Radionuclide distributions vary between boilers operating with different technology. • Radionuclide enrichment factors have distinct differences compared to previous data