NATURAL AND ARTIFICIAL RADIONUCLIDES IN WOOD BIOMASS USED FOR HEATING – COMPARISON OF NORTH-EAST ITALY AND IMPORTED WOOD PELLETS

The problem of radionuclides contamination in imported wood pellet, used for industrial and domestic heating, emerged for the first time in Italy in June 2009 in a batch from Lithuania, in which a concentration of 137Cs of about 300 Bq/kg was measured, increased to about 40000 Bq/kg after burning. The radioactive fall-out due to the Chernobyl accident (26 April 1986), that deposited over extensive areas of central and northern Europe, affected also areas exploited for agroforestry and forestry resources. The soil contamination occurred almost like “leopard spot” and so in a heterogeneous way also at thousand kilometres away from the Chernobyl site. The eventual radioactive contamination currently present in woody biomasses is almost due to 137Cs radionuclide, which has a half-life of about 30 years and therefore (depending on the contamination of the investigated area) still potentially present in variable quantities in soils and vegetation. The biomass represented by pellet and wood chips are nowadays of a great importance due to its extreme thermal efficiency, its cheapness and its relative low environmental impact in terms of carbon dioxide emissions; the major problem involved in this biomass, in the Italian context, is represented by the fact that the demand for pellets is much greater than the territorial possibility of self-production, therefore there is the need to import this resource from external countries and thus with potentially non-negligible levels of radioactivity. This work, conducted at ICMATE-CNR in Padua, aimed to investigate the activity concentration of 137Cs, 40K and other radionuclides possibly present in 27 samples of forest chips (woody flour mainly produced by Picea abies species) from the autonomous province of Trento and the ashes produced by the same samples burned in similar to domestic combustion conditions (pellet stove at about 550° C). The taken samples were subsequently analysed by high-resolution gamma spectrometry in order to evaluate the activity concentration of 137Cs and of the natural 40K, first in the “fresh” samples of forest chips and afterwards in the same samples incinerated after their pelletization, with the aim of comparing the amount of radionuclides of the former with respect to the latter and to determine the concentration factor. The extensive sampling to the whole province allowed to have a reliable and composite map of distribution of 137Cs radioactivity in the wooded areas on the surface of the Trento province. 134Cs had been also searched for, but it was below the instrumental limits of detection in all the samples. Results thus obtained were then compared and evaluated with respect both to dataset previously obtained from the same ICMATE-CNR laboratory consisting of 65 pellet samples from different areas of Eastern Europe (Bosnia, Croatia, Ukraine, Serbia and Russia) analysed in 2010-2011; and with respect to literature data relating to woody biomass for combustion of European and non-European origin.