Modelling of the Fate of ¹³⁷Cs and ⁹⁰Sr in the Chornobyl Nuclear Power Plant Cooling Pond before and after the Water Level Drawdown

During the accident in April 1986, the Cooling Pond (CP) of the Chornobyl Nuclear Power Plant (ChNPP) was heavily contaminated by fuel particles and radionuclides of cesium-137 (Cs-137) and strontium-90 (Sr-90). Starting from the end of 2014, a gradual decrease of the CP water level began leading to the transformation of the whole reservoir into eight separate sectors and raising the concern of the fate of Cs-137 and Sr-90 in the future. In this study, two mathematical models were applied to reproduce radioactive contamination of the CP from 1986 to 2021 and to provide a forecast of Cs-137 and Sr-90 concentrations in the CP water from 2022 to 2030. The hydrodynamic model THREETOX provided three-dimensional (3D) currents in the CP corresponding to hydrological conditions before and after water level drawdown, and these currents were used in the box model POSEIDON-F for the long-term simulations of the changes in Cs-137 and Sr-90 concentrations in water, bottom sediments, and biota. Seasonal changes in the distribution coefficient (K-d) describing the partition of Cs-137 between water and sediments were considered in the box model, which allowed us to reproduce the observed variations of concentration. Calculated concentrations of Cs-137 and Sr-90 in water and freshwater fish occupying different trophic levels agreed well with measurements for the entire post-accident period. After the water level drawdown, concentrations of Cs-137 in the CP water slightly increased in all eight sectors, while Sr-90 concentrations significantly increased in sectors close to ChNPP, which was explained by an additional Sr-90 source when comparing the simulation results and measurement data. Using the model forecast from 2022 to 2030, we predict that the concentration of both radionuclides will gradually decrease in new water bodies of the Cooling Pond except in the northern sectors, where the suggested additional source of Sr-90 will lead to a stabilization of Sr-90 concentrations.