Forest fires in the Chornobyl area

The Chornobyl reactor accident in Ukraine in 1986 released large quantities of radioactive substances into the environment. While highly volatile substances (e.g. radioactive caesium or radioactive iodine) reached high altitudes because of the high temperatures of the burning reactor and dispersed widely with the wind and weather, non-volatile radioactive substances (e.g. strontium and plutonium) were deposited mainly in the immediate vicinity of the Chornobyl (Russian: Chernobyl) nuclear power plant in Ukraine as well as in the neighbouring areas of Belarus. The immediate vicinity of the nuclear power plant in particular was severely contaminated.

To this day, radioactive caesium and strontium and transuranic elements such as plutonium and americium can be found in the immediate vicinity of the Chornobyl nuclear power plant because of their long half-lives. In 1986, an exclusion zone was set up in order to protect the population from the high levels of radiation in the vicinity of the damaged reactor. Around 10% of the total radioactive substances released in the Chernobyl accident have been deposited within the exclusion zone.

Forest fires in the exclusion zone can have radiological consequences

If trees, bushes, grass and the top organic soil layers dry out due to the weather, the risk of forest fires increases.

In the forests of the exclusion zone around Chornobyl, most of the radioactive substances released during the reactor accident in 1986 and subsequently deposited are found in the uppermost organic soil layers and in the biomass (e.g. in trees, shrubs, and grass).

In the event of a forest fire, the radioactive substances deposited there would be released into the atmosphere, rise to different heights with the fire smoke, and be dispersed with the air currents. The amount and activity of radioactive substances that could be released in a forest fire are considerably lower than in the 1986 reactor accident.

If trees, shrubs, grass and, the top organic soil layers were to dry out because of weather conditions, the risk of forest fires would increase. A forest fire could then be triggered by a lightning strike, for example. The risk of forest fires in the exclusion zone is further increased because there is no forest management, and there are thus large quantities of easily combustible deadwood.

Forest fires can release radioactivity into the atmosphere

The amount of radioactivity released into the atmosphere during a forest fire depends on many factors such as

• the size of the burning area
• the type and activity of radionuclides in the above-ground biomass (e.g. in trees, shrubs and grass) and in the upper organic forest soil layers
• the moisture content of the above-ground biomass and the upper organic soil layers
• the fire conditions, especially the fire temperature
• the weather conditions, especially wind and precipitation

Depending on the intensity of the fire and the contamination of the burning surfaces, people in the immediate vicinity of the fires (in or near the smoke plume) may be exposed to increased radiation through the inhalation of radionuclides released from the biomass and soil. Even outside the exclusion zone around Chornobyl, radioactive substances can be detected in the air during large forest fires.

Radiological consequences of forest fires in the Chornobyl exclusion zone

If contaminated forests were to burn, the radionuclides contained in the biomass and in the uppermost organic soil layers would be partly released into the atmosphere. Because of the thermal buoyancy force, the radioactive particles would be spatially distributed and deposited with wind and weather.

• Larger particles would then be quickly deposited in the immediate vicinity.
• Smaller particles can be transported and deposited over long distances by the wind. The proportion in the air would decrease as the transport path increases. In dry weather, relatively few particles would be deposited during transport. However, if it rained, the radioactive particles would be washed out of the atmosphere and deposited with the rain. This would then lead to the (additional) radioactive contamination of the area concerned.

The BfS assesses the radiological consequences for Germany

If unusual releases of radionuclides are to be expected or have already occurred (e.g. in the case of forest fires in highly contaminated areas), the BfS first examines the possible radiological effects on Germany. Model calculations by the German Weather Service allow us to predict whether the radioactive particles released from the site of the forest fire would reach Germany at all.

Air dust collector at the BfS monitoring station Schauinsland

If air masses with radioactive particles were to reach Germany, the low activity concentrations in the air could be measured only with the help of extremely sensitive measuring systems (e.g. those used by the BfS trace measuring station on the Schauinsland near Freiburg). The activity concentrations in the air would be so low that other early warning systems (e.g. the ODL measuring network) would be unable to detect them. Trace measuring stations in other European countries also continuously measure the activity concentration of radionuclides in the air and exchange this information with each other. As a member of this scientific network, the BfS also receives all relevant data from the other countries.

The BfS assesses the radiological situation in Germany based on its own measurements, the measurements of other European trace measuring stations, and atmospheric dispersion calculations and informs the Federal Environment Ministry, the media, and the general public.