Fukushima: lessons learnt from an extraordinary case of soil decontamination. Effectiveness of landscape decontamination following the Fukushima nuclear accident: a review

Following the accident at the Fukushima nuclear power plant, in March 2011, the Japanese authorities decided to carry out major decontamination works in the affected area, which covers more than 9,000 km². On December 12, 2019, with most of this work having been completed, the scientific journal 'Soil' of the European Geosciences Union (EGU) is publishing a synthesis of approximately sixty scientific publications that together provide an overview of the decontamination strategies used and their effectiveness, with a focus on radiocesium. Soil decontamination, which began in 2013 following the accident at the Fukushima Dai-ichi nuclear power plant, has now been nearly completed in the priority areas identified. Indeed, areas that are difficult to access have not yet been decontaminated, such as the municipalities located in the immediate vicinity of the nuclear power plant. The authors compiled the results of approximately sixty scientific studies published on the topic. This analysis provides new scientific lessons on decontamination strategies and techniques implemented in the municipalities affected by the radioactive fallout from the Fukushima accident. This synthesis indicates that removing the surface layer of the soil to a thickness of 5 cm, the main method used by the Japanese authorities to clean up cultivated land, has reduced cesium concentrations by about 80% in treated areas. Nevertheless, the removal of the uppermost part of the topsoil, which has proved effective in treating cultivated land, has cost the Japanese state about Euros 24 billion. This technique generates a significant amount of waste, which is difficult to treat, to transport and to store for several decades in the vicinity of the power plant a step that is necessary before it is shipped to final disposal sites located outside Fukushima prefecture by 2050. By early 2019, Fukushima's decontamination efforts had generated about 20 million cubic metres of waste. Decontamination activities have mainly targeted agricultural landscapes and residential areas. The review points out that the forests have not been cleaned up - because of the difficulty and very high costs that these operations would represent - as they cover 75% of the surface area located within the radioactive fallout zone. These forests constitute a potential long-term reservoir of radiocesium, which can be redistributed across landscapes as a result of soil erosion, landslides and floods, particularly during typhoons that can affect the region between July and October. Atsushi Nakao, co-author of the publication, stresses the importance of continuing to monitor the transfer of radioactive contamination at the scale of coastal watersheds that drain the most contaminated part of the radioactive fallout zone. This monitoring will help scientists understand the fate of residual radiocesium in the environment in order to detect possible recontamination of the remediated areas due to flooding or intense erosion events in the forests. This research is complementary to the project to develop bio- and eco-technological methods for the rational remediation of effluents and soils, in support of a post-accident agricultural rehabilitation strategy (DEMETERRES), led by the CEA, and conducted in partnership with INRA and CIRAD Montpellier. In cultivated areas within the special decontamination zone, the surface layer of the soil was removed to a depth of 5 cm and replaced with a new 'soil' made of crushed granite available locally. In areas further from the plant, substances known to fix or substitute for radiocesium (potassium fertilizers, zeolite powders) have been applied to the soil. As far as woodland areas are concerned, only those that were within 20 metres of the houses were treated (cutting branches and collecting litter). Residential areas were also cleaned (ditch cleaning, roof and gutter cleaning, etc.), and (vegetable) gardens were treated as cultivated areas.