Distribution of strontium-90 in soils affected by Fukushima dai-ichi nuclear power station accident in the context of cesium-137 contamination

90Sr and 137Cs activity concentrations were determined by radiometric methods in 76 soil samples (soil, litter, rain gutter deposit, and roadside sediment samples) affected by the Fukushima Dai-ichi Nuclear Power Station (FDNPS) accident and collected from the Fukushima exclusion zone. The 90Sr and 137Cs activity concentrations were in the range of 3 to 1050 Bq kg-1 (median 82 Bq·kg-1) and 0.7 to 6770 kBq·kg-1 (median 890 kBq·kg-1), respectively (decay correction date: March 15, 2011). A strong positive correlation was found between 90Sr and 137Cs activity concentration and higher mobility of 90Sr was confirmed in Japanese soil samples. The activity ratio of 90Sr/137Cs in 85% of all samples was in the range of 5.0 × 10-5 to 5.0 × 10-4 with a median of 1.2 × 10-4. From the activity ratio values it was concluded that the 90Sr released to the atmosphere was only around 0.0003-0.02 PBq which is negligible compared to the Chernobyl accident (∼10 PBq) or other nuclear accident contaminations. From the standpoints of radioecology and radiation safety, 137Cs remains the primary pollutant of the FDNPS accident.