Radionuclide Speciation in the Chornobyl NPP Cooling Pond Bed Following Drawdown

Abstract

The cooling pond (CP) at the Chornobyl nuclear power plant (ChNPP) is one of the most heavily contaminated radioactive water bodies in the Chornobyl exclusion zone (ChEZ). This study investigates the geochemical behavior of irradiated nuclear fuel particles (FPs) and associated radionuclides in the CP sediments in response to the pond drawdown. Unlike in terrestrial environments, the conditions in water bodies slow the destruction of FPs. Sequential extraction of sediment samples confirms that a significant proportion of the radionuclide activity remains confined within the matrixes of the FPs. Contrary to previous predictions, the drained zones of the pond have maintained an alkaline pH of 8 instead of the anticipated acidification to pH 5, chiefly because of the buffering effect of the degradation of Dreissena mussel shells. Exposure of FPs in the drained areas has led to increased leaching of radionuclides, which has caused the residual water bodies to become contaminated via overbank runoff. The quasi-uniform distribution of FPs observed in the bottom sediment profile suggests ongoing redistribution driven by wind-induced resuspension (trans-sedimentation) and methane off-gassing (ebullition). Understanding the long-term behavior of FPs in aquatic environments following nuclear accidents is essential for improving future risk assessments and remediation strategies.