Radiocesium balance and its behavior in dam on Uda river and Ota river in Fukushima Prefecture

Abstract:

In this study, we researched the annual radiocesium balance in dam reservoir after the Fukushima Dai-ichi nuclear power plant accident, to evaluate the retention function of radiocesium. We investigated at two reservoirs of different contamination level of radiocesium: Matsugabo dam on Uda river, and Yokokawa dam on Ota river, and ¹³⁷Cs was continuously monitored in inflow river water and discharged water from January to December in 2014. Particulate and dissolved ¹³⁷Cs concentration was measured by the cartridge filter method (Tsuji et al., 2014; Yasutaka et al., 2014) once or twice a month, and concentration of suspended solid was calculated by converting the turbidity measured at intervals of 5-10 minutes. Inflow and outflow rate data were contributed by each dam controlling office.

As a result of ¹³⁷Cs concentration, ¹³⁷Cs in suspended solid in discharged water remarkably decreased at Matsugabo dam, probably due to the effect of decontamination in upper catchment. On the other hand at Yokokawa dam, decontamination in upper catchment had not been proceeding, ¹³⁷Cs in suspended solid in discharged water did not decrease. For both reservoir, fluctuating tendency of dissolved ¹³⁷Cs was observed in inflow and discharged water throughout one year.

As to ¹³⁷Cs balance, at Matsugabo dam, 6.6×10⁸ Bq of dissolved ¹³⁷Cs and 4.1×10⁹ Bq of particulate ¹³⁷Cs flowed into the reservoir, 5.7×10⁸ Bq of dissolved ¹³⁷Cs and 3.0×10⁸ Bq of ¹³⁷Cs was discharged in 2014. At Yokokawa dam, 3.7×10¹⁰ Bq of dissolved ¹³⁷Cs and 1.3×10¹¹ Bq of particulate ¹³⁷Cs flowed into, 3.3×10¹⁰ Bq of dissolved ¹³⁷Cs and 1.2×10¹⁰ Bq of particulate ¹³⁷Cs was discharged. In both cases, in spite of the different ¹³⁷Cs contamination level, although particulate radiocesium storage in the both dam was accumulative, the discharge load of radiocesium as dissolved form did not increase.