NATURALLY OCCURRING RADIONUCLIDES OF SOIL AROUND THE COAL MINES OF WEST MACEDONIA, GREECE

摘要

West Macedonia Lignite Center (WMLC), located in Northwest Greece, has four lignite power plants, which release into the atmosphere about 21400 tons/year of fly ash. Lignite contains natural radionuclides, which are emitted from the stacks through the coal combustion and deposited on the ground. In this paper three natural radionuclides were investigated in the topsoil of WMLC basin. More analytically ~226Ra, ~228Ra, ~40K were determined a) in soil samples from both undisturbed and cultivated soil and b) in ash samples produced at the laboratory by lignite combustion. Radiometric measurements were conducted by means of a 73 cm3 SILENA HP-Ge detector with a FWHM resolution of 1.9 keV at 1332 keV for ~60Co and a relative efficiency of 10.6 %. Compared with the global range for coal ashes the ash of the WMLC lignite was found to have ~226Ra concentrations ranging from 138 to 896 Bq/kg, which ranks it among the ashes having a relatively high activity of ~226Ra. From the natural radionuclides of soil, the ~40K has the highest specific activity followed by ~226Ra and ~228Ra. These data are, within the limit of experimental errors, comparable with those previously reported for the same area.There is no correlation of the radionuclide concentrations with the prevailing winds, which come from Northwest to Southeast. Even in the points of the study area, where fly ash particles are mostly deposited the soil radioactivity was found the same as in the places with no ash deposition. The ~226Ra concentration in ash is approximately eight times as high as ~228Ra, but in the soil samples the two radionuclides are almost equal. (The ratio 226Ra / ~228Ra was found to be 1.1). If there was a radioactive pollution of the soil due to fly ash deposition, this ratio would be considerably higher than 1, because the ~226Ra concentration of ash is much bigger than that of the ~228Ra. The above mentioned peculiarities concerning the distribution of the investigated natural radionuclides in the study indicate their natural origin. For the time being both cultivated and uncultivated topsoil of the WMLC basin can therefore be considered, within the experimental uncertainties, to be unaffected by fly ash radionuclides.