The high volume of coal used for combustion usually leads to a large amount of coal combustion residues (CCRs), which contain the naturally occurring radioactive materials (NORMs) decayed from U and Th in coals. The high radioactivity of NORMs can cause potential harm to humans if the CCRs are used as building materials. The activities of CCRs not only depend on the concentrations of radionuclides but also largely depend on the variations of ash yields of coal. On the other hand, ash yields significantly vary in coal from less than 1-50%. This indicates that similar concentrations of radionuclides in coal with different ash yields generally do not result in similar activities in CCRs. Therefore, it is significant to build a threshold of U in coals with different ash yield levels. In this research, based on the data of 945 coal samples from China and the selected optimal model using the classification and regression tree algorithm, the threshold of U for the radiation hazard is determined to be 7.98 mg/kg for coals with ash yields higher than 20%, while the threshold of U for the radiation hazard is 5.28 mg/kg for coals with ash yields lower than 20%.
Keywords: building materials; classification and regression tree; coal combustion residues; naturally occurring radioactive materials; radiation hazard.