Verification of direct release rate of oceanic 137Cs from Fukushima Daiichi Nuclear Power Plant Accident by higher resolution ocean dispersion model

The March 11, 2011, Great East Japan Earthquake triggered accidents at the Fukushima Daiichi Nuclear Power Plant (1F NPP), releasing radioactive substances into the ocean. Sparse observational data on ¹³⁷Cs in the ocean led to interpolation and simulation for a comprehensive understanding. The primary focus was on direct release, emphasizing the need for a suitable source.

The direct release rate (Bq/day) was calculated by multiplying the seawater exchange flow rate (m³/day) and observed ¹³⁷Cs concentration (Bq/m³). Using a mesh size of 735 m x 929 m x 8 m on the model, the seawater exchange flow rate at the release point was simulated. The ¹³⁷Cs concentration relied on average observed radioactivity at 5, 6, and the south discharge canals near the 1F NPP. Direct release was estimated at 2.2x10¹⁴ Bq/day from March 26 to April 6, 2011, aligning with rates derived from other methods.

The seawater exchange flow rate's dependency on the model's mesh size was acknowledged. For this estimation, a 735 m x 929 m mesh size encompassing key points was considered reasonable for the seawater exchange flow rate, given the complex transport process from the release source (Unit 2 intake) to observation points (5, 6, and the south discharge point) due to port structures.

A higher resolution model with a 147 m x 186 m mesh (1/5) was used for a detailed analysis of direct release rates. The size of the sea area for determining the volume of seawater exchange flow rate can now be changed. Despite challenges in setting due to damaged ports, using the seawater exchange flow rate in a similar area as the previous resolution was deemed appropriate. The results of the validation of the release rate and the observed results by the relationship equation confirmed the consistency with the amount of seawater exchange obtained by the results of the dye tracer release experiments in the 1970s.

The release of ¹³⁷Cs from the 1F NPP site persists. Estimating direct release rates up to 2016, a long-term simulation with a higher resolution model was conducted for validation. Results showed the oceanic ¹³⁷Cs concentration distribution influenced by coastal currents, eddies, and the Kuroshio Current, leading to spatio-temporal variability. Validation with observed annual mean concentrations revealed good agreement. The higher resolution improved coastal transport reproducibility, addressing ¹³⁷Cs radioactivity underestimation at the Fukushima 2 NPP, 10 km south of the 1F NPP.