Isotopic Compositions of 236U and Pu Isotopes in “Black Substances” Collected from Roadsides in Fukushima Prefecture: Fallout from the Fukushima Dai-ichi Nuclear Power Plant Accident

Black-colored road dusts were collected in high-radiation areas in Fukushima Prefecture. Measurement of 236U and Pu isotopes and 134,137Cs in samples was performed to confirm whether refractory elements, such as U and Pu, from the fuel core were discharged and to ascertain the extent of fractionation between volatile and refractory elements. The concentrations of 134,137Cs in all samples were exceptionally high, ranging from 0.43 to 17.7 MBq/kg, respectively. 239+240Pu was detected at low levels, ranging from 0.15 to 1.14 Bq/kg, and with high 238Pu/239+240Pu activity ratios of 1.64–2.64. 236U was successfully determined in the range of (0.28 to 6.74) × 10–4 Bq/kg. The observed activity ratios for 236U/239+240Pu were in reasonable agreement with those calculated for the fuel core inventories, indicating that trace amounts of U from the fuel cores were released together with Pu isotopes but without large fractionation. The quantities of U and 239+240Pu emitted to the atmosphere were estimated as 3.9 × 106 Bq (150 g) and 2.3 × 109 Bq (580 mg), respectively. With regard to U, this is the first report to give a quantitative estimation of the amount discharged. Appreciable fractionation between volatile and refractory radionuclides associated with the dispersal/deposition processes with distance from the Fukushima Dai-ichi Nuclear Power Plant was found.