Radiological impact of using forest tree biomass for energy and recycling the ash

In Northern Europe the use of tree biomass for production of energy is mostly based on thinning of forests, harvest residues, stumps from final cutting, bark of commercial roundwood, and organic waste of pulp industry. Results based on field experiments allow estimations of 137Cs activity removed from forests in wood fuel, and also the activity returned in ash used as a fertiliser. After one fertilisation treatment in three types of coniferous forests the activity concentration of 137Cs in wood fuel and ash was reduced to a fraction of 0.3 to 08 of activities in products from unfertilised forests. The improved tree growth and lowering activity concentrations of 137Cs in trees and understorey after fertilisation refer to reduction of human radiation exposure in Chernobyl-affected areas in the long term. Thereby radiation doses to workers handling the ash and members of the public using forests will gradually decrease faster than through radioactive decay only. A documented and traceable ash recycling history of forests is important for all users of forests.