Transport of H-3 and I-129 in water and their uptake by marine organisms due to the planned release of Fukushima storage water

The 3D model THREETOX was applied for the long-term simulation of the planned release of radioactively contaminated water from Fukushima storage tanks to marine environment. Two radionuclides were considered: ³H that has the largest activity in tanks and ¹²⁹I that can caused the largest dose of radiation to human. The constant release rate of ³H equal to 22 TBq/y according to TEPCO estimations and the constant release rate of ¹²⁹I equal to 361 MBq/y according to estimations from the current study were used in the simulations.

The THREETOX model used monthly averaged currents from the KIOST-MOM model. A dynamic food web model was included in the THREETOX model. In the model, organisms uptake the activity directly from water and through the food chain. The food chain consists of phytoplankton, zooplankton, non-piscivorous (prey) fish, and piscivorous (predatory) fish. In case of ¹²⁹I, macro-algae was also considered. The modelling area covers Fukushima coastal waters and extends for 1600 km from the coast to the East. From North to South this area extends for 1300 km.

From model results, we can see how contamination will spread along the coast in different seasons. For example, in summer time the currents near the coast are directed to the North that leads to contamination of the Sendai Bay. This means that at different points along the coast, the concentration of radionuclides can periodically change according to currents that change during the year. Calculated concentrations of activity at several points along the coast of Japan, which correspond to largest cities in the area of interest, were extracted from model results. For example, calculated concentration of ³H in water in Tomioka point, which is quite close to FDNPP, sometimes can exceed 200 Bq/m³. In Soma point, the concentration will exceed 50 Bq/m³, while in point Iwaki-Onahama – 20 Bq/m³ at some moments of time. In other points, the calculated concentration of ³H in water will not exceed 10 Bq/m³ that is less than background concentration 50 Bq/m³. Concerning ¹²⁹I, its maximum concentration in water will be around 10^-3 – 10^-2 Bq/m³ in points close to FDNPP and around 10^-4 Bq/m³ in points further from the NPP that is around 100 000 times less than the calculated concentrations of ³H.

Calculated concentrations of OBT (organically bounded tritium) in predatory and prey fish are less than 0.01 Bq/kg in all points except FDNPP point where it is around 0.02 Bq/kg. This value is 10 times less than measured concentration of OBT in fish (0.2 Bq/kg) that was made in 2014 in the coastal area near the damaged NPP. Calculated concentrations of ¹²⁹I in predatory and prey fish are in the range 10^-6 – 10^-4 Bq/kg in all considered points. Concentrations of ¹²⁹I in macro-algae are about 100 times higher due to ability of iodine to accumulate in macro-algae.