Radioactivity in forests

Radioactive substances received in fallout are very slow to leave the nutrient cycle of northern forests. However, harmful effects of radioactive materials can be reduced with silvicultural measures.

The migration of long-lived radionuclides in the forest nature, cesium-137 in particular, is being studied on a long-term basis. The Chernobyl nuclear accident did not cause any limitations to the use of forests. However, the cesium concentration in game, wild berries and wild mushrooms has decreased more slowly compared with farm produce.

In northern coniferous forests, long-lived radioactive substances remain in the vegetation or the surface layer of soil for a long time. They do not bind to mineral soil as quickly as they do in nutrient-rich farm environments. Therefore, they circulate in the forest ecosystem for a long time. Harmful effects of radioactive substances can be reduced through suitable silvicultural measures, such as forest fertilization.

Radioactive fallout in forest exposes people in the forest to external radiation. The sources of the exposure are gamma radiation-emitting substances in trees, vegetation and soil, such as cesium-137. The distribution of cesium in forest changes quickly after the spreading of the fallout, as fallout dust migrates from treetops and trunks into the undervegetation and soil. Sample measurements give an overview of how the long-lived cesium-137 circulates in the forest ecosystem.

Finnish construction timber

Activity concentration of  Cs-137 in Finnish debarked timber is roughly a few tens of becquerels per kg stock weight (80% dry matter), when the timber is harvested from mature Scots pine forests in the main fallout area contaminated by the Chernobyl accident in 1986. A considerable part of Finnish timber is produced in less contaminated areas where activity concentrations are very low. Only exceptionally the 137Cs concentration in stemwood used for construction of log houses or timber framed detached houses exceeds 100 Bq/kg stock weight.

People living all year round in log houses with wooden ceiling and floor receive a radiation dose due to gamma radiation from the decay of Cs-137. The dose is generally not higher than a few percents of the action level, 1 mSv/year, set for exposure to gamma radiation from natural radioactive substances in building materials originating from rock and soil (ST Guide 12.2 / 17 December 2010, STUK).

Radiation exposure from Cs-137 in furniture made of Finnish timber is in practice negligible because of shorter exposure times and smaller size of the sources compared to houses.

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