This study evaluated environmental radiation within the Specified Living Areas for Returnees (SLARs) in Okuma Town, Fukushima Prefecture, prior to large-scale decontamination and demolition. The objective was to provide field-based baseline data on the ambient dose equivalent rate across public roads and individual residential properties to inform assessments of future remediation effects and to support decisions about evacuation order policies and long-term radiation management.
Following the 2011 Fukushima Daiichi Nuclear Power Plant accident, portions of Okuma Town have remained designated as difficult-to-return zones (DRZs). The Japanese government has created SLARs within DRZs where residents may return as conditions permit; these areas have been prioritized for infrastructure work and decontamination because they are considered more likely to be reinhabited.
Surveys were conducted in SLARs adjacent to the Specified Reconstruction and Revitalization Base Areas of Okuma Town. For analysis, SLARs were divided into three areas according to decontamination status:
Geographic and dose data were collected and mapped to characterize spatial distributions across these areas before full-scale remediation of residential properties.
Monthly car-borne surveys were carried out from November 2024 through September 2025 using a Radi-probe system. The system combined a CsI spectral survey meter with GPS, a compact camera, and control PC to record ambient dose equivalent rates and gamma-ray spectra in real time. The survey meter was installed inside a standard passenger vehicle at approximately 1 m above ground level (front passenger seat).
Measured dose rates were stored with synchronized GPS coordinates to enable spatial plotting. To adjust for signal attenuation by the vehicle, shielding coefficients (ranging from 1.20 to 1.68) were applied to recorded values. Surveys attempted consistent routes and speeds, but minor variability occurred because of traffic and road conditions. Data points were logged at fixed 5-second intervals during drives.
Ambient dose equivalent rate measurements at individual residential properties were performed from November 2024 through February 2025 prior to decontamination and demolition. Measurements included positions such as house entrances to reflect locations where returning residents would spend time. These site-level data provided inputs for estimating potential individual external dose exposures under return scenarios.
Across the SLARs, car-borne survey results showed a significant reduction in road ambient dose rates over the study period. The median ambient dose rate measured by car-borne surveys across all SLARs decreased from 0.49 µSv/h in November 2024 to 0.34 µSv/h in September 2025 (p < 0.05). Spatial mapping of the monthly survey data illustrated heterogeneity among districts consistent with differing decontamination progress and local conditions.
The three-area classification reflected observable differences: Area 1 (little or no decontamination) tended to show higher ambient rates relative to areas with ongoing or completed road decontamination.
Measurements made at residential properties before decontamination found a median ambient dose equivalent rate of 0.96 µSv/h at house entrances. Using these field measurements to estimate annual external doses produced two contextual comparisons against national guidance: the estimated annual external dose satisfied the Ministry of the Environment reference level of 20 mSv (a guideline used when lifting evacuation orders) in most areas, but did not meet the long-term post-accident target level of 1 mSv.
These findings indicate that, prior to property-level remediation, ambient rates at locations where residents would live could lead to estimated annual doses above the long-term target but within the short-term reference level applied for evacuation-lifting decisions.
The study provides pre-decontamination baseline data for SLARs that are prioritized for potential return of residents. The observed decrease in road ambient dose rates over the survey period suggests continuing decay and/or the effects of partial decontamination on public thoroughfares. However, residential property entrance rates measured prior to remediation indicate that additional measures would be required to reach the long-term target of 1 mSv for annual external exposure in many locations.
These baseline measurements can inform expectations about the magnitude of dose reduction achievable through planned decontamination and demolition, and they contribute objective, spatially resolved information for residents and policymakers engaged in decisions about return and long-term management.
The authors report sources of variability including vehicle shielding corrections, route and speed differences during car-borne surveys, and heterogeneity in decontamination progress among districts. Measurements were not always obtained at identical locations each month for practical reasons. All relevant data are stated to be available within the manuscript and its supporting information files.
Field surveys conducted prior to residential decontamination showed a significant decrease in road ambient dose equivalent rates across Okuma Town SLARs between November 2024 and September 2025. Residential entrance measurements before remediation had a median ambient dose equivalent rate of 0.96 µSv/h; estimated annual external doses based on these measurements generally met the Ministry of the Environment reference level of 20 mSv but did not reach the long-term target of 1 mSv. These baseline data support assessment of future dose reductions from remediation and inform ongoing discussions on evacuation order policy and long-term radiation management.