Modelling study of the potential release of contaminated water from storage tanks at the Fukushima Dai-ichi NPP into marine environment

The accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011 led to the uncontrolled release of a significant amount of radioactive materials into the marine environment. To prevent the further release of highly contaminated water, which was used for cooling the overheated nuclear reactor cores, and groundwater, which was continuously pumped out the reactor buildings, a large number of tanks was installed in the area around NPP to collect all this water. However, at the moment the capacity of tanks is almost exhausted. The contaminated water was processed to decrease the activity stored in the tanks, but any decontamination system cannot remove all radionuclides from the water. According to TEPCO (2020) data, about 1.2 million m³ of contaminated water were stored in tanks in March 2020 containing radionuclides with long and moderate half-life, among which 10 radioisotopes (H-3, C-14, Co-60, Sr-90, Tc-99, Ru-106, Sb-125, I-129, Cs-134, Cs-137) are dominant (Buesseler, 2020). Therefore, it is important to estimate the impact on human health of potential release of contaminated water from tanks to the ocean. This impact significantly depends on the ability of radionuclides to concentrate in the marine organisms, which are in the human diet, and the values of dose coefficient. The compartment model POSEIDON-R was applied for calculation the concentration of activity in the water, bottom sediments and biota at different distances from the FDNPP. The area of interest was covered by the system of compartments with specification around FDNPP. The exchanges of activity between compartments were governed by average currents in the region. The maximal concentrations and doses were conservatively estimated for coastal box 4x4 km around the FDNPP. Accumulation of activity in the organisms was calculated by dynamical model taking into account chemical properties of the element, its role in metabolic processes and the positions of organisms in the pelagic and benthic food webs. The potential individual doses of radiation were estimated using average consumption rates of marine products in Japan based only on domestic production. The conservative scenario, when a whole volume of contaminated water will be released into the marine environment at a constant rate during 10 years, was chosen. According to results of modelling for 50 years, the obtained dose even in the coastal box turned out to be significantly lower than the maximum annual effective dose commitment for the public equal to 1 mSv (IAEA, 2011). The main contribution into the dose is expected from I-129 and C-14. Although the activity of tritium (H-3) far exceeds activities of other radionuclides in tanks, its contribution to the total dose is only third due to low ability to concentrate in organisms and low dose coefficient. The dose factors and activity factors for 10 radionuclides at different distances from the FDNPP were obtained to be used for estimation of doses to human and concentration of activities in marine organisms for any long-lasting release scenario.