Calculations for ambient dose equivalent rates in nine forests in eastern Japan from 134Cs and 137Cs radioactivity measurements

Understanding the relationship between the distribution of radioactive Cs-134 and Cs-137 in forests and ambient dose equivalent rates (H)over dot*(10)) in the air is important for researching forests in eastern Japan affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This study used a large number of measurements from forest samples, including Cs-134 and Cs-137 radioactivity concentrations, densities and moisture contents, to perform Monte Carlo radiation transport simulations for (H)over dot*(10) between 2011 and 2017. Calculated (H)over dot*(10) at 0.1 and 1 m above the ground had mean residual errors of 19% and 16%, respectively, from measurements taken with handheld NaI(Tl) scintillator survey meters. Setting aside the contributions from natural background radiation, Cs-134 and Cs-137 in the organic layer and the top 5 cm of forest soil generally made the largest contributions to calculated (H)over dot*(10). The contributions from Cs-134 and Cs-137 in the forest canopy were calculated to be largest in the first two years following the accident. Uncertainties were evaluated in the simulation results due to the measurement uncertainties in the model inputs by assuming Gaussian measurement errors. The mean uncertainty (relative standard deviation) of the simulated (H)over dot*(10) at 1 m height was 11%. The main contributors to the total uncertainty in the simulation results were the accuracies to which the Cs-134 and Cs-137 radioactivities of the organic layer and top 5 cm of soil, and the vertical distribution of Cs-134 and Cs-137 within the 5 cm soil layers, were known. Radioactive cesium located in the top 5 cm of soil was the main contributor to (H)over dot*(10) at 1 m by 2016 or 2017 in the calculation results for all sites. Studies on the Cs-137 distribution within forest soil will therefore help explain radiation levels henceforth in forests affected by the FDNPP accident. The merits of this study are that it modelled multiple forests for a long time period, with the important model inputs being informed by field measurements, and it quantified how the measurement uncertainties in these inputs affected the calculation results.

Automated summary

Understanding the distribution of Cs-134 and Cs-137 in forests is crucial for researching radiation levels in forests affected by the Fukushima Dai-ichi Nuclear Power Plant accident. The main contributors to calculated ambient dose equivalent rates were found to be Cs-134 and Cs-137 in the organic layer and the top 5 cm of forest soil. Uncertainties in the simulation results were mainly due to the accuracy of Cs-134 and Cs-137 activities in the soil layers and their vertical distribution.