In March 2011, an earthquake off the Pacific coast of Tohoku caused an accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP). This discharged a large amount of radioactive material into the environment. After the accident, a Planned Evacuation Zone based on a macro-scale dose rate distribution map, which was generated from airborne (aircraft) survey observation, was delineated and implemented on April 22, 2011. Airborne survey played a major role in providing information for developing an appropriate response to this emergency.

It has been five years since the accident occurred and the focus must now shift toward environmental recovery and resumption of normal life. Thus, new monitoring methods are required to reveal the micro-scale dose rate distribution, including that in the forest area. We used an Unmanned Aerial Vehicle (UAV), which can fly at altitudes lower than an aircraft, to establish monitoring techniques and methods that could be applied to micro-scale dose rate distribution monitoring and mapping. We found that the dose rate distribution map from the UAV, flown at altitudes of 30 m and 60 m, compared favorably with maps generated from ground-based survey observations. However, in the forest area, the dose rate values observed in the UAV survey were lower than those observed in the ground-based survey due to shielding by trees.

Surveys of dose rate using UAVs that can fly at low altitudes enable the monitoring of micro-scale dose rate distribution in places that are inaccessible to ground-based surveys. Further, the resulting micro-scale dose rate distribution maps provide important base information that assists in decision making of residents who return to the area.