After the earthquake and tsunami on 11 March 2011 damaged the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), an accidental release of a large amount of radioactive isotopes into both the air and the ocean occurred. Measurements provided by the Japanese agencies over the past 5 years show that elevated concentrations of ¹³⁷Cs still remain in sediments, benthic organisms, and demersal fishes in the coastal zone around the FDNPP. These observations indicate that there are ¹³⁷Cs transfer pathways from bottom sediments to the marine organisms. To describe the transfer quantitatively, the dynamic food chain biological uptake model of radionuclides (BURN) has been extended to include benthic marine organisms. The extended model takes into account both pelagic and benthic marine organisms grouped into several classes based on their trophic level and type of species: phytoplankton, zooplankton, and fishes (two types: piscivorous and non-piscivorous) for the pelagic food chain; deposit-feeding invertebrates, demersal fishes fed by benthic invertebrates, and bottom omnivorous predators for the benthic food chain; crustaceans, mollusks, and coastal predators feeding on both pelagic and benthic organisms. Bottom invertebrates ingest organic parts of bottom sediments with adsorbed radionuclides which then migrate up through the food chain. All organisms take radionuclides directly from water as well as food. The model was implemented into the compartment model POSEIDON-R and applied to the north-western Pacific for the period of 1945–2010, and then for the period of 2011–2020 to assess the radiological consequences of ¹³⁷Cs released due to the FDNPP accident. The model simulations for activity concentrations of ¹³⁷Cs in both pelagic and benthic organisms in the coastal area around the FDNPP agree well with measurements for the period of 2011–2015. The decrease constant in the fitted exponential function of simulated concentration for the deposit-feeding invertebrates (0.45 yr⁻¹) is close to the observed decrease constant in sediments (0.44 yr⁻¹). These results strongly indicate that the gradual decrease of activity in demersal fish (decrease constant is 0.46 yr⁻¹) is caused by the transfer of activity from organic matter deposited in bottom sediment through the deposit-feeding invertebrates. The estimated model transfer coefficient from bulk sediment to demersal fish in the model for 2012–2020 (0.13) is larger than that to the deposit-feeding invertebrates (0.07). In addition, the transfer of ¹³⁷Cs through food webs for the period of 1945–2020 has been modelled for the Baltic Sea contaminated due to global fallout and from the Chernobyl accident. The model simulation results obtained with generic parameters are also in good agreement with available measurements in the Baltic Sea. Unlike the open coastal system where the FDNPP is located, the dynamics of radionuclide transfer in the Baltic Sea reach a quasi-steady state due to the slow rate in water mass exchange in this semi-enclosed basin. Obtained results indicate a substantial contribution of the benthic food chain in the long-term transfer of ¹³⁷Cs from contaminated bottom sediments to marine organisms and the potential application of a generic model in different regions of the world's oceans.

A detailed description of the advanced version of compartment model POSEIDON-R for the prediction of transport and fate of radionuclides in the marine environment is given. The equations of transfer of radionuclides in the water and bottom sediment compartments along with the dynamical food chain model are presented together with dose module to assess individual and collective doses to the population due to the regular and accidental releases of radionuclides. The method for the numerical solution of model equations is also presented. The modelling results for the northeast Atlantic shelf seas were compared with measurements of 137Cs. •The three-dimensional compartment model POSEIDON-R describes the transfer of radionuclides and their daughter products in marine environment as a results of regular or accidental releases. This includes any transfer through the water column and sediments.•The model is complemented by a dynamic food chain model for transfer of radioactivity in pelagic and benthic food webs.•The dose module in the model calculates internal and external doses for humans and non-human biota.

Long-term consequences of radionuclide contamination of the Arabian Gulf as a result of hypothetical accidents at the Bushehr and Barakah nuclear power plants (NPPs) were studied using a chain of models including the atmospheric dispersion model RIMPUFF, the marine compartment model POSEIDON-R, and the dose model. The compartment model POSEIDON-R is complemented by a dynamic model of the biota food chain that includes both pelagic and benthic organisms. The source terms for the hypothetical releases of the selected radionuclides (¹³⁴Cs, ¹³⁷Cs, ¹⁰⁶Ru, and ⁹⁰Sr) in the atmosphere were defined as a fraction of respective reactor inventories available in the literature. Conservative meteorological scenarios for the calculation of the initial depositions of radionuclides were selected. Because the Gulf is shallow, a significant portion of the reactive radionuclides (¹³⁴Cs, ¹³⁷Cs, ¹⁰⁶Ru) remain in the bottom sediments and continue to contaminate water and benthic organisms for a long period of time. The annual dose due to the consumption of marine products can exceed 1 mSv, whereas the annual dose due to drinking the water from desalination plants is expected to be an order less. The contribution of elements to the dose depends on the type of reactor. This is manifested in differences between the contributions of different marine organisms to the dose.