[M-AG39] Radioisotope migration: New development for dynamics study of radionuclides from 1F NPP accident

[MAG39-06] Radiocaesiun and other radionuclides in the coastal region of Fukushima since 2011

*Michio Aoyama^1,2, Hideki Kaeriyama³, Daisuke Tsumune⁴ (1.Center for Research in Isotopes and Environmental Dynamic, Faculty of Life and Environmental Sciences, University of Tsukuba, 2.Institute of Environmental Radioactivity, Fukushima University, 3.Japan Fisheries Research and Education Agency, 4.Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry)

Keywords:the Fukushima Daiichi Nuclear Power Plant , radiocaesium, tritium, strontium-90

In this presentation, we describe the behavior of radionuclides in the coastal regions very close to and near the Fukushima Daiichi Nuclear Power Plant (FNPP1). Fission products in the three cores of the FNPP1 directly released radionuclides from the FNPP1 site during the FNPP1 accident. The major radionuclides directly released to the open water were ¹³¹I, ¹³⁴Cs, and ¹³⁷Cs, as well as the atmospheric deposition of these three radionuclides. While tritium and ⁹⁰Sr were also directly released to the open water, the release of plutonium isotopes was negligible.
The estimated total amount of directly released radiocaesium was 3.6±0.7 PBq at the end of 2021, that of tritium was 0.05 PBq, and that of plutonium isotopes was negligible.
The ¹³⁷Cs activity concentration at the site and at the discharge canal of Units 5 and 6 of the cooling water from FNPP1 to open water (hereafter 56N of FNPP1) was very high in April 2011 at up to 68 MBq m^-3 and then decreased considerably. However, its annual average remained within 100–1,000 Bq m^-3 throughout 2013–2021; the minimum annual average was 104 Bq m^-3 in 2018. In 2021, the annual average concentration of ¹³⁷Cs activity in the surface water at 56N of FNPP1 was approximately 180 Bq m^-3. This level is still two orders of magnitude higher rather than the ¹³⁷Cs activity concentration in the surface water in the North Pacific Ocean before the FNPP1 accident.
Stations at Tomioka and Onahama are situated between the FNPP1 site and Hasaki, locates 176 km south of FNPP1, and the coastal current generally flows southward in this region; therefore, the ¹³⁷Cs activity concentration in the surface waters at Tomioka and Onahama were slightly diluted and generally showed activity concentrations between those at FNNP1 and Hasaki.
The ¹³⁷Cs activity concentration generally increased during times of heavy rainfall at Ukedo port, Tomioka, Iwasawa, and Onahama. The correlation coefficient between the ¹³⁷Cs activity concentrations at Ukedo, 56N of FNPP1, Tomioka, and Iwasawa, and the 1–9-day-modified antecedent precipitation index (API) showed a maximum for the 5–7-day-modified APIs. The relationship between the 5–7-days API and the ¹³⁷Cs activity concentration may also reflect the time scale of desorption as stated from two days to one week in previous studies.
It is also confirmed that an aerial radiological survey is a useful way to rapidly monitor the distribution of radionuclides, such as ¹³¹I, ¹³⁴Cs, and ¹³⁷Cs in surface seawater, particularly on an urgent basis.
The ⁹⁰Sr activity concentration gradually decreased after 2013, while the ¹³⁷Cs activity concentration did not decrease after 2016. The mechanism of fluctuation factors of the ⁹⁰Sr/¹³⁷Cs activity ratio is unclear, and a few possible causes of the decreasing trend of the ⁹⁰Sr/¹³⁷Cs activity ratio were discussed.
It was concluded that no plutonium contamination occurred in the seawater from the FNPP1 accident.
Issues are unclear and remain as future tasks; moreover, studies are discussed.