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Accidental release of radionuclides caused by nuclear accidents like those in Fukushima and Chernobyl can result in pulses of radioactivity entering the forest environment. Due to intense recycling in the forest, equilibrium between radioactivity concentrations in trees and in soil may not be reached during the period of short-term radionuclide transport following the accident. Another question arises as to whether the equilibrium hypothesis using empirical concentration ratios (CRs) can be applied to the long-term period. Using two atmospheric 137 Cs fallout scenarios in the Fukushima and Chernobyl sites, this study investigated whether the CR approach could provide conservative predictions of 137 Cs levels in trees following 137 Cs fallout events by comparing predictions from the CR approach using data gathered for trees by the IAEA to those from dynamic transfer models and actual measured data. The inter-comparisons also aimed to investigate whether the CR approach could account for the variability of 137 Cs levels across different tree organs. The results showed that caution may be necessary when using the CR approach, which relies on the IAEA dataset, to estimate 137 Cs accumulation in forest trees in the short - and long term following atmospheric 137 Cs fallout events. A calculation by TRIPS 2.0 demonstrated the importance of considering the distribution within tree organs for in-depth analysis of radiological impact of forest trees. Our findings suggest that it may be preferable to use CR values based on site-specific data rather than generic data collected from various sites. This is particularly relevant when studying the sites where the bioavailability of 137 Cs for trees and thus possible exposures are higher. This study also showed that dynamic modeling approaches could offer an alternative means of estimating CR values of the entire tree or specific tree organs in situations where empirically derived values are not available., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Forests cover approximately 70% of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident in 2011. Following this severe contamination event, radiocaesium ( 137 Cs) is anticipated to circulate within these forest ecosystems for several decades. Since the accident, a number of models have been constructed to evaluate the past and future dynamics of 137 Cs in these forests. To explore the performance and uncertainties of these models we conducted a model inter-comparison exercise using Fukushima data. The main scenario addressed an evergreen needleleaf forest (cedar/cypress), which is the most common and commercially important forest type in Japan. We also tested the models with two forest management scenarios (decontamination by removal of soil surface litter and forest regeneration) and, furthermore, a deciduous broadleaf forest (konara oak) scenario as a preliminary modelling study of this type of forest. After appropriate calibration, the models reproduced the observed data reliably and the ranges of calculated trajectories were narrow in the early phase after the fallout. Successful model performances in the early phase were probably attributable to the availability of comprehensive data characterizing radiocaesium partitioning in the early phase. However, the envelope of the calculated model end points enlarged in long-term simulations over 50 years after the fallout. It is essential to continue repetitive verification/validation processes using decadal data for various forest types to improve the models and to update the forecasting capacity of the models., (Copyright © 2021 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The majority of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered with forests. We developed a dataset for radiocaesium ( 137 Cs) in trees, soil, and mushrooms measured at numerous forest sites. The 137 Cs activity concentration and inventory data reported in scientific journal papers written in English and Japanese, governmental reports, and governmental monitoring data on the web were collated. The ancillary information describing the forest stands were also collated, and further environmental information (e.g. climate) was derived from the other databases using longitude and latitude coordinates of the sampling locations. The database contains 8593, 4105, and 3189 entries of activity concentration data for trees, soil, and mushrooms, and 471 and 3521 entries of inventory data for trees and soil, respectively, which were collected from 2011 to 2017, and covers the entire Fukushima prefecture. The data can be used to document and understand the spatio-temporal dynamics of radiocaesium in the affected region and to aid the development and validation of models of radiocaesium dynamics in contaminated forests.

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (F l/t ), the migration center (X c ) and the relaxation depth (α) using 99 soil inventory data sets. F l/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, F l/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only X c increased significantly with time according to generalized mixed model analysis (p < 0.01). However, X c and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high 137 Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of 137 Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of 137 Cs from organic to surface mineral soil layer after the second year. These results indicate that 137 Cs retention capacity of the organic layer can affect the apparent vertical migration of 137 Cs in the underlying mineral soil layer., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)