Retrospective Estimation of Potential Mobility of Artificial Radionuclides (60Co, 137Сs, 152Eu, 154Eu) in Bottom Sediments of the Yenisei River

The present study addresses transformation of physicochemical forms (species) of artificial radionuclides in bottom sediments (BS) of the Yenisei River after the shutdown of the reactors at the Mining-and-Chemical Combine (MCC) at Krasnoyarsk. The BS samples collected from the Yenisei at the MCC in 2018-2021 contained Co-60, Cs-137, Eu-152, Eu-154, Eu-155, and Am-241. The species of radionuclides were studied using sequential extraction procedure. Considerable proportions of Co-60 (48-100%) and Cs-137 (72-83%), which are still released to the Yenisei with controlled discharges, were irreversibly bound to BS mineral solids. The percentages of europium isotopes, which have not been released to the Yenisei as part of controlled discharges since 2013, were lower in the irreversibly bound fraction (4.6-9.3% of Eu-152), but their representation as relatively mobile species was more diverse compared to Co-60 and Cs-137. Interestingly, some percentage of Eu-152 (up to 15.5%) was retained in the most mobile, exchangeable, form. The species of Eu-154 and Eu-152 differed, which can be explained by dissimilar origins of these isotopes. Further research is needed to investigate the mechanisms of physicochemical transformation of radionuclides in BS that enable them to be present in mobile species over long time periods and mechanisms of immobilization of artificial radionuclides in the environment.