Wood residues can accumulate harmful substances such as heavy metals and radioactive materials, although the quantities can vary considerably. The highest concentrations of radioactive materials are found in wood pellet ash and wood briquette ash. For this reason, amongst others, wood pellet ash and wood briquette ash should not be used as garden fertiliser without prior testing.

Wood is a relatively fast-growing renewable resource and is considered an environmentally friendly, largely carbon-neutral fuel. The combustion process produces only ash as a waste product. The gasification process produces both ash and biochar. For each type of wood fuel (wood pellets, wood chips, logs and wood briquettes), there are specially designed biomass systems that are optimally adapted to the product used.

Heating with wood pellets is subsidised by the federal government, the federal states and local authorities. In recent years, an increasing number of private households have been heating their homes with pellet boilers. Nationwide, there are more pellet boilers in households than other types of biomass boilers. Commercial biomass heating plants and biomass CHP plants (CHP stands for combined heat and power) in Austria are currently operated primarily using wood chips, sawmill by-products and bark. During the heating process, the organic component of the wood is burned, whilst the mineral, non-combustible portion remains as ash.

Radioactivity is present everywhere in nature. Natural radionuclides have either existed since the formation of our solar system (primordial radionuclides) or are continuously produced by the interaction of high-energy cosmic radiation with the Earth’s atmosphere (cosmogenic radionuclides). Radioactive substances that do not occur naturally but are produced by humans are called artificial radionuclides.

The soil contains, amongst others, the natural radionuclides potassium-40, radium-226 and thorium-232. Potassium-40 and thorium-232 are primordial radionuclides which, due to their long half-lives, are still present today. Radium-226 is produced in the decay chain of the primordial radionuclide uranium-238.

Partly as a result of the Chernobyl reactor accident (1986), artificial radionuclides such as caesium-137 and strontium-90 were dispersed across several European countries, including Austria. Due to their half-life of approximately 30 years, they can still be detected in the environment in the affected areas.

Plants such as trees absorb these radioactive substances through the soil, which can lead to an accumulation of these natural and artificial radionuclides in the wood residues (e.g. ash, biochar) during the combustion or gasification process.

The radioactivity in the wood itself is harmless from a radiation protection perspective and therefore poses no health risk to the population. However, the accumulation of radioactive substances in the resulting wood residues (e.g. ash, biochar) varies greatly and may be relevant from a radiation protection perspective.

The dose assessments carried out for workers and affected members of the public have shown that no measures are required from a radiation protection perspective.

Nevertheless, we do not recommend spreading wood pellet ash or wood briquette ash in the garden or even on a vegetable patch without first testing it. This is because, in addition to radioactive substances, the ash may also contain concentrations of non-combustible and potentially harmful minerals, salts, heavy metals and organic pollutants.
The disposal of ash in private households is regulated differently in each federal state and often also in each municipality. In most parts of Austria, it is mandatory to dispose of cooled ash in general waste.

In principle, a circular economy should be aimed for in relation to commercially produced wood residues (e.g. ash, biochar). Wood ash can, for example, be used as an aggregate in the production of building materials or for composting (see ‘Guidelines for the proper use of plant ash’ at the bottom of this page under Downloads). If wood ash is reused as an aggregate, the impact of both natural and artificial radionuclides must be assessed. In accordance with Section 123(1) of the AllgStrSchV 2020, the reference value for external exposure in occupied rooms due to gamma radiation from construction products is 1 mSv effective dose per year.

Whilst no measures are required for the general public from a radiation protection perspective, relevant exposures may occur for workers during certain occupational activities. We would like to draw attention to the fact that, in accordance with Section 23 of the StrSchG 2020, a dose assessment must be arranged for workers in the following activities, amongst others: Activities involving exposure from residues, such as maintenance work, in particular the maintenance and removal of heat-resistant cladding made of zirconium-containing material, cleaning or modification and dismantling of pipework and technical installations such as pumps and valves, maintenance as well as modification and dismantling of filter systems and flue gas scrubbers.
 

Detailed technical information on the project can be found in the two final reports, which are available under Downloads.