An early survey of the radioactive contamination of soil due to the Fukushima Dai-ichi Nuclear Power Plant accident, with emphasis on plutonium analysis

摘要

A field survey was carried out soon after the Fukushima Dai-ichi Nuclear Power Plant accident. Gamma-ray emitting radionuclides and plutonium isotopes in a series of soil samples collected from the heavily contaminated areas outside the 20-km exclusion zone, as well as from Okuma Town adjacent to the plant, were measured. Volatile radionuclides such as ~(131)I, ~(134)Cs, and ~(137)Cs contributed largely to the released radioactivity. Higher depositions of these nuclides were observed in areas to the northwestern—including Okuma Town and Iitate Village, which is at a distance of 25–45 km from the plant. The results obtained were consistent with the levels and distributions estimated later by the Japan-USA joint-survey (Asahi Shimbun Company, 2011). Trace amounts of plutonium isotopes originating from the accident were detected mainly in soil samples from Iitate Village and in limited soil samples from Okuma Town. The detected levels of ~(239,240)Pu contamination due to the accident were considered to be less than a millionth those of the ~(137)Cs contamination.