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What is going to happen to the radiocesium contamination of forest trees in Fukushima in the future? – Newly developed computational model elucidated the contamination mechanism and enabled the future prediction – 

2022.01.06

Nuclear Science and Engineering Center developed a computational model (SOLVEG-R) that predicts land-surface dynamics of radioactive materials in detail. By using the model, we clarified contamination mechanism of wood of forest trees affected by the radiocesium (137Cs) fallout of the TEPCO’s Fukushima Daiichi Nuclear Power Plant (hereinafter referred to as ‘FDNPP’) accident and enabled predictions of the concentration of radiocesium in the wood in the future.

In the forests affected by the FDNPP accident, trees became contaminated with radiocesium and commercial use of wood has been regulated. To enable eventual reactivation of the commercial use, understanding the future evolution of concentration of radiocesium in wood is necessary. In the present study, we propose a model, SOLVEG-R, that calculates dynamic behavior of radiocesium in forests on the basis of water cycle and tree growth in forests influenced by actual meteorology and soil condition. The model was applied to two contaminated forests of cedar plantations and natural oak stands in Fukushima, and the results elucidated the pollution mechanism of wood at these forests. It was shown that the transfer of radiocesium to the trees occurred mostly through surface uptake of radiocesium trapped by needles (cedar) and bark (oak) during the accident and the subsequent contamination of the wood proceeded through enduring internal recycling of the radiocesium absorbed on these tree surfaces. It was suggested that the concentration of radiocesium in the wood begun to decrease from the year 2020 (cedar) or 2017 (oak), as a result of removal of radiocesium from the tree surfaces by precipitation wash-off and leaf-fall. As affected by dilution effect from the tree growth, the decrease of the concentration proceeded at a rate of 3% per year, which was rapider than the decrease through radioactive decay of radiocesium (2% per year).

SOLVEG-R is able to calculate radiocesium dynamics in forests without any calibration of the modeled processes by observational data and therefore it is applicable to various forests in Fukushima. Besides, the model accounts for dynamics of radiocesium in forest-floor organic layer and tree’s growth. The model can be therefore used to predict radiocesium concentration in trees after forest decontamination activities or forest managements such as removal of forest-floor organic layer and new planting of tree seeds after deforestation.

JAEA HP Press release (Japanese only)