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<p><strong>Abstract.</strong> 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 four years show that elevated concentrations of <sup>137</sup>Cs still remain in sediments, benthic organisms and demersal fishes in the coastal zone around the FDNPP. These observations indicate that there are <sup>137</sup>Cs transfer pathways from bottom sediments to the marine organisms. To describe the transfer quantitatively, the dynamic food chain model 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 feeding by benthic invertebrates and bottom omnivorous predators for the benthic food chain; crustaceans, molluscs 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 Northwestern Pacific for the period of 1945–2010 and then for the period of 2011–2020 to assess the radiological consequences of releases of <sup>137</sup>Cs due to FDNPP accident. The model simulations for activity concentrations of <sup>137</sup>Cs 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 ingesting invertebrates (0.45 y<sup>–1</sup>) is close to the decrease constant for the sediment observations (0.44 y<sup>–1</sup>), indicating that the gradual decrease of activity in the demersal fish (decrease constant is 0.46 y<sup>–1</sup>) was caused by the transfer of activity from organic matter deposited in bottom sediment through the deposit feeding invertebrates. The estimated from 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) due to the biomagnification effect. In addition, the transfer of <sup>137</sup>Cs through food webs for the period of 1945–2020 has been modelled for the Baltic Sea that was essentially contaminated due to global fallout and the Chernobyl accident. The model simulation results obtained with generic parameters are also in good agreement with available measurements in the Baltic Sea. Due to weak water exchange with the North Sea of the semi-enclosed Baltic Sea the chain of water-sediments- biota slowly evolves into a quasi-equilibrium state unlike the processes off the open Pacific Ocean coast where the FDNPP is located. Obtained results demonstrate the importance of the benthic food chain in the long-term transfer of <sup>137</sup>Cs from contaminated bottom sediments to marine organisms and the potential of a generic model for the use in different regions of the World Ocean.</p>

Abstract. 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.

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Thoron (220Rn) exhalation from building materials has become increasingly recognised as a potential source for radiation exposure in residences. However, contrary to radon (222Rn), limited information on thoron exposure is available. The purpose of this study is to estimate the concentration of thoron and its progeny products in a typical Dutch living room using computational fluid dynamics. The predicted thoron concentration is ∼9 Bq m(-3) using a source term of 14 Bq s(-1) for the thoron exhalation from building materials. The concentration varies from 15 Bq m(-3) near the building materials to 2.7 Bq m(-3) in the centre of the living room. The mean effective dose from thoron progeny is calculated as 0.09 mSv y(-1), with a total effective dose from radon and thoron progeny of 0.38 mSv y(-1).