Model & data based assessment of the impacts of drawdown of the Chornobyl NPP Cooling Pond on the Cs-137 concentrations in water, sediments and biota

Cooling Pond (CP) of the Chornobyl Nuclear Power Plant (ChNPP) is one of the most radioactively contaminated large water bodies over the globe. During the active phase of the ChNPP accident, radionuclides got into the CP in result of atmospheric deposition, release of highly contaminated water from system of accidental cooling, and water used to extinguish the fire. In the years after the accident, the contamination was distributed in the CP due to currents. For this period, three types of hydrological conditions dominated in the CP. Initially, the currents were forced by the cooling system of the ChNPP, which caused a circular movement of water. After the decommissioning of the ChNPP, the natural circulation took place in the CP. Starting from the end of 2014, when pumps that continuously fed the CP with water from the Prypiat River were shutdown, a gradual decrease of water level began. Now the water level has dropped by about 6 m leading to the transformation of the whole reservoir into several small lakes and redistribution of radionuclides in them. The objectives of the study were to calibrate models, which were customized for the CP, using data for the whole post-accident period including data collected during the drawdown period by the joint efforts of Ukrainian and Japanese researchers, and then to provide model based predictions of the future radionuclide concentrations in new water bodies.

During field studies that were carried out in November 2020, the current state of radioactive contamination of the CP was investigated. Samples of water, suspended and bottom sediments and biota were taken in 9 closed or semi-closed water bodies formed after partial drying of the CP. Concentrations of Cs-137 and its distribution in dissolved and particulated forms were measured in the laboratory. For simulations, the modeling system that consists of the 3D model of thermohydrodynamics and radionuclide transfer THREETOX and the box model POSEIDON-R was created. The THREETOX model was used for the obtaining currents in the CP for each type of hydrological conditions. The POSEIDON-R model was applied for the long-term simulations of the changes of activity concentration in the water, bottom sediments and biota starting from the 1986. The system of boxes in the POSEIDON-R model includes shallow and deep-water boxes. It was built in such a way that after the water level in the CP fell, the calculations were performed only in deep-water boxes. Fluxes of water between boxes were calculated based on currents from the THREETOX model. Seasonal changes in distribution coefficient K_d describing the partition of Cs-137 concentration between water and sediments were also taken into account. Calculated concentrations of Cs-137 in water and bottom sediments agree well with measurements for all boxes and for entire modeling period. It has been shown that POSEIDON-R model is able to reproduce changes in the concentrations of Cs-137 in freshwater fish occupying different levels the food chain. Scenarios for the potential changes of Cs-137 concentrations were considered by the variation of basic parameters.