From zonal containment to spot recovery: Multi-platform geostatistical reassessment of radiation risk boundaries in Fukushima (2019–2024)

Abstract

This study provides a spatiotemporal reassessment of the radiological landscape within the Difficult-to-Return Zone (DTRZ) surrounding the Fukushima Daiichi Nuclear Power Plant (FDNPP), integrating airborne, car-borne, and walk-borne survey data from 2019 to 2024 through Empirical Bayesian Kriging (EBK) to generate high-resolution (50-m) dose-rate surfaces. Applying the official Japanese government occupancy threshold of 3.8 μSv·h⁻¹ (equivalent to 20 mSv·y⁻¹), the 2019 DTRZ boundary demonstrated complete radiological containment, with all threshold-exceeding grid cells located within the zone boundaries in both monitoring years, but exhibited pronounced internal over-restriction: 96.2% of the 337.34 km² DTRZ area comprised grid cells that did not exceed the threshold in 2019, rising to 98.4% by 2024 as the threshold-exceeding area contracted by 56.8% from 12.74 km² to 5.50 km². Radiological assessment of the seven Specified Reconstruction and Revitalization Base Areas (SRRBA) (digitized from published cartography) revealed that mean dose rates across all patches declined from 6.45 mSv·y⁻¹ to 3.08 mSv·y⁻¹ (−52.2%), with all patches recording doses below the 20 mSv·y⁻¹ policy threshold and threshold-exceeding area contracting by 86.1%. Forest classes (Deciduous Broadleaf Forest [DBF], Deciduous Needleleaf Forest [DNF], Evergreen Broadleaf Forest [EBF], Evergreen Needleleaf Forest [ENF], and Bamboo forest) accounted for 60.2% of threshold-exceeding cells in 2019, declining to 56.5% in 2024, with a notable compositional shift from DBF (39.6% to 25.8%) toward ENF (15.1% to 22.8%) reflecting differential radioecological recovery rates. These findings empirically validate the radiological justification for the SRRBA designation and the broader transition from broad zonal restriction toward spot-based recovery management.