Development of the POSEIDON-GM–box-based radioactivity transport model for the global ocean: preliminary results of Cs-137 and Sr-90 distributions due to global fallout

According to the World Nuclear Association, the capacity of nuclear power plants around the world is growing steadily. In detail, about 60 reactors are under construction among which 29 reactors are to be launched by 2025 with 19 reactors located on the coast. This increases the risk of radioactive contamination of ocean waters in the case of potential nuclear accidents. Rapid growth of NPPs amount around the world requires the development of the global model that can simulate the release of radioactive materials from any NPP and assess its impact on the adjacent local/regional seas and global ocean. The compartment model POSEIDON with the new global system of boxes could be such a model which simulates transport of a wide range of radionuclides in the marine environment including marine food chains and calculates doses to humans from seafood consumption. Also, the model operates with both atmospheric and direct releases of radionuclides to the marine environment.

A new global system of boxes for the POSEIDON-GM (Global Model) has been developed which consists of 437 boxes of variable sizes. Most ocean boxes have 4 vertical layers in the water column: first surface layer with a depth of 30 m, second layer between 30 and 200 m depth, third layer between 200 and 1000 m depth, and fourth layer below 1000 m. Boxes with an average depth of less than 1000 m have fewer vertical layers depending on their depth. Water fluxes between boxes were calculated from the Global Ocean Physical Multi-Year product by Copernicus Marine Environment Monitoring Service, which contains monthly mean circulation data averaged for 1993-2016.

Annual depositions of ¹³⁷Cs and ⁹⁰Sr in each box of the POSEIDON-GM due to the global fallout have been calculated using data of deposition density from UNSCEAR for the 1945-1987 period with extrapolation for the 1988-2020 period. The transport of ¹³⁷Cs and ⁹⁰Sr entered the World Ocean due to global fallout has been simulated by POSEIDON-GM. Calculated concentrations of both radionuclides in water have been compared with measurement data from the MARIS database for the Southern Hemisphere, where the global fallout was the dominant source of radioactive contamination.

The next stage of the POSEIDON-GM development will include consideration of other sources of radioactive contamination, such as local sources from nuclear weapon tests, releases from nuclear reprocessing plants, sources due to Fukushima and Chornobyl nuclear accidents, etc. The model results will be compared with measurement data for concentrations of radionuclides in water and from different trophic levels of marine organisms from around the World Ocean.