10:45 AM - 11:00 AM
[3A04] Re-evaluation of the amount of direct discharge from the FNPP1 to the open water: Relationship between 137Cs radioactive concentration in seawater and accumulated precipitation amount /137Cs activity concentration in sediments.
Keywords:Nuclear power plant accident at Fukushima Dai-ichi, dirct discharge, accumurated precipitation amount, sediment, river runoff
We have estimated the amount of direct discharge of 137Cs to the ocean by using 137Cs activity in the seawater close to the FNPP1. But Machida et al. (2019) pointed out that this method was overestimated except immediately after the accident. We again analyzed the long-term trend of 137Cs radioactive concentration in seawater along the coast and the relationship between accumulated precipitation amount and 137Cs concentration in sediments. It was found that 137Cs transported from land, when the 7 days accumulated precipitation amount exceeded several ten mm, contributed to an increase in the radioactivity concentration of 137Cs along the coast, and the apparent half-life residence time of 137Cs activity concentration after each event was about 30 days. It is also suggested that precipitation might enhance discharge of 137Cs to open water in case of not heavy rain event.