The level and the behavior of the naturally occurring primordial radionuclides 238U, 226Ra, 210Pb, 232Th, 228Ra and 40K in coals and fly ashes are described. The activity concentrations of the examined coals and originated from coal mines in
Greece ranged from 117 to 435 Bq·kg−1 for 238U, from 44 to 255 Bq·kg−1 for 226Ra, from 59 to 205 Bq·kg−1 for 210Pb, from 9 to 41 Bq·kg−1 for 228Ra and from 59 to 227 Bq·kg−1 for 40K. These levels are comparable to those appeared in coals of different countries worldwide. The activity concentrations of
the examined fly ashes and produced in coal-fired power plants in Greece ranged from 263 to 950 Bq·kg−1 for 238U, from 142 to 605 Bq·kg−1 for 226Ra, from 133 to 428 Bq·kg−1 for 210Pb, from 27 to 68 Bq·kg−1 for 228Ra and from 204 to 382 Bq·kg−1 for 40K. The results showed that there is an enrichment of the radionuclides in fly ash relative to the input coal during the combustion
process. The enrichment factors (EF) ranged from 0.60 to 0.76 for 238U, from 0.69 to 1.07 for 226Ra, from 0.57 to 0.75 for 210Pb, from 0.86 to 1.11 for 228Ra and from 0.95 to 1.10 for 40K.