Research and management challenges following soil and landscape decontamination at the onset of the reopening of the Difficult-to-Return Zone, Fukushima (Japan)
Authors: Evrard Olivier , Chalaux-Clergue Thomas , Chaboche Pierre-Alexis , Wakiyama Yoshifumi , Thiry Yves 
Published: September 2023

Abstract. Twelve years after the nuclear accident that occurred at the
Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in March 2011, radiocesium
contamination (with a large dominance of 137Cs, with a 30-year
half-life) remains a major concern in various municipalities of north-eastern
Japan. The Japanese authorities completed an unprecedented soil
decontamination programme in residential and cultivated areas affected by
the main radioactive plume (8953 km2). They implemented a
complex remediation programme scheme to remediate soils that are fundamental
to life on Earth, relying on different decision rules depending on the waste
type, its contamination level and its region of origin, after delineating
different zones exposed to contrasted radiation rates. The central objective
was not to expose local inhabitants to radioactive doses exceeding 1 mSv yr−1 in addition to the natural levels. At the onset of the full
reopening of the Difficult-to-Return Zone (DTRZ) in spring 2023, the current review
provides an update of a previous synthesis published in 2019
(Evrard et al., 2019). Although this ambitious soil remediation
and reconstruction programme has almost been completed in the 12 municipalities
of Fukushima Prefecture in which an evacuation order was imposed in at least
one neighbourhood in 2011, from the 147 443 inhabitants who lived there
before the accident, only 29.9 % of them had returned by 2020. Waste
generated by decontamination and tsunami cleaning/demolition work is planned
to have been fully transported to (interim) storage facilities by the end of
2023. The cost of the operations conducted between 2011 and 2020 for the
so-called “nuclear recovery” operations (including decontamination) was
estimated by the Board of Audit of Japan in 2023 as JPY 6122.3 billion
(∼ EUR 44 billion). Decontamination of cropland was shown to
have impacted soil fertility, and potassium fertilisation is recommended to
limit the transfer of residual radiocesium to new crops. In forests that
cover 71 % of the surface area of Fukushima Prefecture and that were
not targeted by remediation, radiocesium is now found in the upper mineral
layer of the soil in a quasi-equilibrium state. Nevertheless, 137Cs
concentrations in forest products (including wood for heating and
construction, wild plants, wildlife game, mushrooms) often keep exceeding
the threshold values authorised in Japan, which prohibits their exploitation
in the area affected by the main plume. Radionuclides from forests were shown
to be exported in dissolved and particle-bound forms to downstream river
systems and floodplains, although multiple monitoring records showed the
continuous decrease in radiocesium concentrations in both river water and
sediment across the main plume between 2011 and 2021. Fish contamination is now
generally found to be below the threshold limits although reputational damage
remains a major concern for local fishing communities. The remobilisation of
radiocesium from sediment accumulated in reservoirs of the region is also of
potential concern as it may lead to secondary contamination of fish or
irrigation waters supplied to decontaminated fields. Overall, this synthesis
demonstrates the need to continue monitoring post-accidental radiocesium
transfer in these environments and to keep sharing data in order to refine
our predictive understanding of radiocesium mobility and consolidate the
tools available to model contaminant transfer in ecosystems. In forests in
particular, novel countermeasures and wood uses remain to be developed and
tested. Furthermore, the hydrologic connectivity between soils under
different ecosystems greatly influences long-term radiocesium
transport. The consequences of extreme phenomena (e.g. typhoons, forest
fires) that may become more frequent in the future as a result of global
change in these contaminated environments should be further anticipated.