Lagrangian radionuclide transport modeling with fast and slow adsorption-desorption processes: application to the Yellow Sea with a hypothetical atmospheric deposition

To cope with the increasing threat of radioactivity release accidents in the Yellow Sea a Lagrangian radionuclide transport model in the region was recently developed coupled in off-line manner with current-wave-suspended sediment modeling system (Brovchenko et al, 2022).  The radionuclide model included as an essential feature the fast adsortion-desorption processes of dissolved and particulate radionuclides in the presence of multi-ftactional sediments. Upgrade is made in this work by including fast and slow adsorption-desorption processes of radionuclides and a novel approach for lagrangian simulation of the radionuclide exchange between near-bottom water-layer and bed sediments. Lagrangian particles in the model can possess several states: dissolved in the water column, adsorbed on suspended sediment of particular size, dissolved in the pore water, adsorbed on the bed sediments of particular size. Note that, If particles are adsorbed on the sediments then it can be in two different states, namely fast and slow reversible forms; if there are Nsed sediment size classes then we have N_tot =2+4N_sed  total states of the radionuclide. Throughout the numerical integration the model calculates the probabilities to transfer into each possible state (that depends on the current state and time step) during the next time step and then chooses the new particular state by comparing with the generated uniformly distributed random number. Hypothetical accident at the Haiyang NPP in China, which is located at the coast of Yellow Sea close to Korea is considered as a scenario of accident. The atmospheric transport and deposition of radionuclides on the sea surface was simulated by the FLEXPART model. The obtained deposition fluxes were used as a source term in the Lagrangian radionuclide transport model. 3D fields of currents, suspended sediment concentration and turbulent diffusion coefficient as well as bed sediment fractional composition are identical to the previous results of the Yellow Sea (Brovchenko et. al. 2022).  Computational domain of the FLEXPART model includes bigger outer area, which covers Yellow and East China Sea, with spatial resolution of 0.15 deg, and inner area, which covers only Yellow Sea with better spatial resolution of 0.05 deg. The source term of ¹³⁷Cs released due to hypothetical accident at the Haiyang NPP was obtained from the 6-day simulations of the FLEXPART model. The total amount of radioactivity that deposited on the calculation area is approximately 55 PBq. The radioactivity budget analysis reveals that almost near 50% of the ¹³⁷Cs was deposited to the bottom sediments and approximately half remained in the dissolved form. About 4% of the total amount remains on the suspended sediments in one-step modelling and about 9% with the use of two-step model. The total bed contamination changed only 1% because for this period bottom contamination fluxes dominated over the bed cleaning process. More differences are expected for simulation with duration of several years when dissolved ¹³⁷Cs concentration in water will decrease and bed cleaning process become more significant.