Following the accident at the Fukushima Daiichi nuclear plant in March 2011, the Japanese authorities carried out major decontamination works in the affected area, covering more than 9000 square kilometers. 

With most of this work completed, the scientific journal SOIL of the European Geosciences Union (EGU) on 12 December published a synthesis of approximately 60 scientific papers, that together provide an overview of the decontamination strategies used and their effectiveness dealing with radiocaesium. This work is the result of an international collaboration led by Olivier Evrard, a researcher at the Laboratory of Climate and Environmental Science at Université Paris Saclay.

Soil decontamination began in 2013 and has now been nearly completed in the identified priority areas in Fukushima Prefecture. A decision was taken in November 2011 to decontaminate eleven municipalities evacuated after the accident, and a futher 40 non-evacuated municipalities affected by lower levels of radioactivity. However, areas that are difficult to access have not yet been decontaminated.

The research, based on 60 studies, focuses mainly on the fate of radioactive caesium in the environmen. Because caesium-137 (C-137) has a half-life of 30 years, it represents the greatest risk to the local population in the medium and long term. Without decontamination caesium-137 will remain in the environment for around three centuries. “The feedback on decontamination processes following the Fukushima nuclear accident is unprecedented because it is the first time that such a major clean-up effort has been made following a nuclear accident,"Evrard said. "The Fukushima accident gives us valuable insights into the effectiveness of decontamination techniques, particularly for removing caesium from the environment,” he aded. 

Notably the research indicates that removing the surface layer of the soil to a thickness of 5cm – the main method used by the Japanese authorities to clean up cultivated land – reduced caesium concentrations by about 80% in treated areas. However, removing the uppermost part of the topsoil, which has proved effective in treating cultivated land, cost the Japanese state about €24 billion ($26.6bn). It also generated a significant amount of waste, which is difficult to treat, transport and store pending shipment to final disposal sites outside Fukushima prefecture by 2050. By early 2019, Fukushima’s decontamination efforts had generated about 20 million cubic metres of waste.

In cultivated areas within the special decontamination zone, the surface layer of the soil that was removed was replaced with a new “soil” made of crushed granite available locally. In areas further from the plant, substances known to substitute for radiocaesium (potassium fertilizers, zeolite powders) were applied to the soil. In woodland, areas with 20m of housing were treated by cutting branches and collecting litter. Residential areas were also cleaned and (vegetable) gardens were treated as cultivated areas.

Decontamination activities have mainly targeted agricultural land and residential areas. Forests have not been cleaned up because of the difficulty and very high costs involved as forests cover 75% of the surface area located within the radioactive fallout zone. These constitute a potential long-term reservoir of radiocaesium, which can be redistributed 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, stessed 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 radiocaesium in the environment in order to detect possible recontamination of remediated areas due to flooding or intense erosion events in the forests.

The analysis recommends further research on:

  • Issues associated with the recultivation of decontaminated agricultural land;
  • Monitoring of the contribution of radioactive contamination from forests to the rivers that flow across the region; and
  • The return of inhabitants and their reappropriation of the territory after evacuation and decontamination.

This research will be the subject of a Franco-Japanese and multidisciplinary international research project, MITATE (Irradiation Measurement Human Tolerance viA Environmental Tolerance), led by the Frane's National Scientific Research Centre in collaboration with other French bodies, including Atomic & Alternative Energies Commission (CEA), as well as Japanese organisations. 

The project will start on 1 January and run for for an initial period of five years. This research is complementary to the project to develop bio- and eco-technological methods for the remediation of effluents and soils, in support of a post-accident agricultural rehabilitation strategy led by the CEA.

Photo: Area of land in Fukushima used for storage of contaminated soil (Credit: Credit: Evrard et al., SOIL 2019)