After the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March of 2011, a wide area of forests in eastern Japan was contaminated with radionuclides. In particular, radiocesium (137Cs) has the potential to threaten the forestry and wood production in the contaminated area for many decades because it was released in large amounts (10 PBq)1 and has a relatively long half-life (30 years). Radiocesium levels for some wood uses are strictly regulated in Japan (e.g., 40 Bq kg−1 for firewood2 and 50 Bq kg−1 for mushroom bed logs3), meaning that multipurpose uses of wood from even moderately contaminated areas are restricted. Although a guidance level of radiocesium in construction wood has not been declared in Japan, the permissible levels in some European countries (370–740 Bq kg−1)4,5,6 suggest that logging should be precautionary within several tens of kilometers from the FDNPP, where the 137Cs activity concentration in wood potentially exceeds 1,000 Bq kg−1 [refs. 7,8]. To determine whether logging should proceed, the long-term variation in wood 137Cs concentration must be predicted as accurately as possible. Many simulation models successfully reproduce the temporal variations in the early phase after the FDNPP accident, but produce large uncertainties in long-term predictions9. To understand the 137Cs dynamics in forests and trees and hence refine the prediction models, it is essential to provide and analyze the observational data of 137Cs activity concentrations in tree stem parts.
The 137Cs concentration in the whole wood showed an increasing trend in six out of nine plots. In four of these plots, the increasing trend shifted to a flat or decreasing trend, indicating that the 137Cs dynamics in many forests reached apparent steady state at 10 years after the accident. However, the lack of the clear shift to a decreasing trend indicates that the 137Cs root uptake is probably still increasing in some plots. Continuous monitoring surveys and further studies clarifying the complex mechanisms of 137Cs root uptake in forests are needed in order to refine the simulation models and improve their prediction accuracy.Decadal trends in 137Cs concentrations in the bark and wood of trees contaminated by the Fukushima nuclear accident. | Nuclear Information