Distribution and mineralogy of radioactive Cs in reservoir sediment contaminated by the Fukushima nuclear accident

Abstract

This study investigated the distribution and mineralogy of radioactive Cs in a core sample from reservoir sediment affected by the Fukushima Daiichi Nuclear Power Plant accident. The sample was obtained 19 months after the accident from a reservoir in Iitate-mura, Fukushima Prefecture. The vertical concentrations of radioactive Cs in the sediment core varied little from the surface to 9 cm depth: 20-30 Bq/g for 137Cs. The concentration decreased abruptly at depths greater than 9 cm. The radioactive Cs concentrations were particle-size dependent: finer particles exhibited higher concentrations of radioactive Cs. X-ray diffraction analyses of the size-fractionated sample revealed that the contribution of clay minerals (14 Å minerals, kaolin mineral and mica) increased concomitantly with the decrease of particle size, implying a close relation between radioactive Cs concentration and clay mineralogical compositions. Periodical airborne monitoring showed that the 137Cs inventory in the reservoir catchment area decreased significantly with time after the accident. Results of a 137Cs inventory in the reservoir sediment were almost identical to those for the catchment area immediately after the accident, but were significantly higher than those in the core sampling period. Results might indicate that the reservoir sediments serve as a sink for radioactive Cs.