Natural and artificial radionuclides in wood pellets and derived ashes

This study presents a radiological assessment of wood pellets and their combustion ashes, with a focus on the activity concentrations of caesium-137 (Cs-137) and potassium-40 (K-40). A total of 23 commercial pellet samples and 16 corresponding ash samples from domestic appliances in Greece were analyzed using high-resolution gamma-ray spectrometry. The results reveal that K-40 activity concentrations in pellets (10-80 Bq/kg) are consistent with values typically reported for wood biomass. In contrast, Cs-137 levels show strong geographical dependence, ranging up to 60 Bq/kg in pellets from Eastern Europe, particularly Ukraine and Romania, reflecting the heterogeneous Chernobyl fallout distribution. Combustion resulted in an increased radionuclide concentration in the ash, with arithmetic mean activities of 2751 ± 40 Bq/kg for K-40 and 281 ± 5 Bq/kg for Cs-137, corresponding to average enrichment factors of 76 and 33, respectively. These values indicate radiological concerns for densely populated areas that burn pellets for heating and align with enrichment ranges reported in comparable European studies. From a regulatory perspective, this concentration effect is of particular significance. Although none of the analyzed pellet samples exceeded the exemption limit of 100 Bq/kg for Cs-137, several ash samples surpassed this threshold due to combustion-induced enrichment. This finding indicates that fuels considered radiologically safe at the point of use may produce ashes that warrant further regulatory consideration and scenario-specific assessment. Spatial mapping of Cs-137 activity, measured in pellets and the corresponding ash from different countries, is consistent with Chernobyl deposition models, underscoring the long-term persistence of radiocaesium in forest ecosystems.