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 U-236 and Pu isotopes and Cs-134,Cs-137 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 Cs-134,Cs-137 in all samples were exceptionally high, ranging from 0.43 to 17.7 MBq/kg,, respectively. Pu239+240 was detected at low levels, ranging from 0.15 to 1.14 Bq/kg, and with high Pu-238/Pu239+240 activity ratios of 1.64-2.64. U-236 was successfully determined in the range of (0.28 to 6.74) X 10(-4) Bq/kg. The observed activity ratios for U-236/Pu239+240 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 Pu239+240 emitted to the atmosphere were estimated as 3.9 X 10(6) Bq (150 g) and 2.3 x 10(9) 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.