Carbon, cesium and iodine isotopes in Japanese cedar leaves from Iwaki, Fukushima

Evaluating remediation of radionuclide contaminated forest near Iwaki, Japan, using radiometric methods

Radiometric surveys have been conducted in support of a project investigating the potential of biofuel power generation coupled with remediation of forests contaminated with radionuclides following the Fukushima Daiichi accident. Surveys conducted in 2013 and 2014 were used to determine the distribution and time dependence of radionuclides in a cedar plantation and adjacent deciduous forestry subject to downslope radionuclide migration, and a test area where litter removal was conducted. The radiocaesium results confirmed enhanced deposition levels in the evergreen areas compared with adjacent areas of deciduous forestry, implying significant differences in depositional processes during the initial interception period in 2011. Surveys were conducted both with and without a collimator on both occasions, which modified the angular response of the detector to separate radiation signals from above and below the detector. The combined data have been used to define the influence of radionuclides in the forest canopy on dose rate at 1 m, indicating that, in evergreen areas, the activity retained within the canopy even by 2013 contributed less than 5% of ground level dose rate. The time dependent changes observed allow the effect of remediation by litter removal in reducing radionuclide inventories and dose rates to be appraised relative natural redistribution processes on adjacent control areas. A 15 × 45 m area of cedar forest was remediated in September 2013. The work involved five people in a total of 160 person hours. It incurred a total dose of 40-50 μSv, and generated 2.1 t of waste comprising forest litter and understory. Average dose rates were reduced from 0.31 μSv h^-1 to 0.22 μSv h^-1, with nuclide specific analyses indicating removal of 30 ± 3% of the local radiocaesium inventory. This compares with annual removal rates of 10-15% where radionuclide migration down-slope over ranges of 10-50 m could be observed within adjacent areas. Local increases were also observed in areas identified as sinks. The results confirm the utility of time-series, collimated, radiometric survey methods to account for the distribution and changes in radionuclide inventory within contaminated forests. The data on litter removal imply that significant activity transfer from canopy to soil had taken place, and provide benchmark results against which such remediation actions can be appraised.

Iodine isotopes in precipitation: Four-year time series variations before and after 2011 Fukushima nuclear accident

Rainwater samples were collected monthly from Fukushima, Japan, in 2012-2014 and analysed for (127)I and (129)I. These are combined with previously reported data to investigate atmospheric levels and behaviour of Fukushima-derived (129)I before and after the 2011 nuclear accident. In the new datasets, (127)I and (129)I concentrations between October 2012 and October 2014 varied from 0.5 to 10 μg/L and from 1.2 × 10(8) to 6.9 × 10(9) atoms/L respectively, resulting in (129)I/(127)I atomic ratio ranges from 3 × 10(-8) to 2 × 10(-7). The (127)I concentrations were in good agreement with those in the previous period from March 2011 to September 2012, whereas the (129)I concentrations and (129)I/(127)I ratios followed declining trends since the accident. Although (129)I concentrations in five samples during the period of 2013-2014 have approached the pre-accident levels, (129)I concentrations in most samples remained higher values in winter and spring-summer. The high (129)I levels in winter and spring-summer are most likely attributed to local resuspension of the Fukushima-derived radionuclide-bearing fine soil particles deposited on land surfaces, and re-emission through vegetation taking up (129)I from contaminated soil and water, respectively. Long-term declining rate suggests that contribution of the Fukushima-derived (129)I to the atmosphere would become less since 2014.