The chemical behavior of actinides, especially of those with multiple oxidation rates (U, Pu, Np), is highly variable in different environmental conditions and depends both on the initial released form, as in the case of “hot” particles, and on the whole set of environmental conditions (pH-Eh of the natural waters, temperature, the cationic and anionic composition of solutions; composition of host rocks, etc.). An additional problem is to define the role of actinides’ mobile colloidal form in specific geochemical conditions of radioactively contaminated objects. Such objects include historically accumulated radioactive waste (RW) repositories, which, along with traces of accidental radioactive release, are potentially hazardous sources of contamination of the environment and, above all, water reservoirs.
Analysis of speciation and partitioning of actinides in specific nuclear legacy sites is necessary to scientifically substantiate methods of the remediation of contaminatedareas and to predict radionuclide migration behavior in certain geochemical conditions. Assessment and long-term analysis of radiocnulide behavior under the RW disposal facilities conditions include the chemical interaction of radionuclides with corrosion products of engineering barriers, clay minerals and host rocks, determination of reaction mechanismsat nano- and microlevels, as well as numerical methods of calculation of migration behavior of various radionuclides (actinides, main fission products).
These studies include analysis of speciation of radionuclides under geochemical conditions, research of radionuclide behavior at mineral/ water interface with the use of modern physical and chemical methods, and applying the obtained data in numerical models (codes) of migration behavior. The studies include following tasks:
- model experiments to study radionuclide sorption and formation of colloids under strictly controlled conditions;
- thermodynamic simulation of radionuclide sorption under ambient conditions;
- analysis of speciation of radionuclides in real environmental objects, both radioactively contaminated and containing low concentrations of artificial radionuclides.
Together with colleagues from the Geological Department of Moscow State University, experimental data on radionuclide distribution at molecular level are being collected for further use in mathematical models of migration at geological scale. The study of actual samples taken in contaminated areas will allow to verify the modeling, and determine the possible remediation strategies, which are so important today.
The main scientific results:
- We have established speciation of actinides in groundwater contaminated by solutions coming from liquid RW surface repository pool at Mayak Production Association site. Secondary ion mass spectrometry with nanometer resolution (nano-SIMS) and scanning transmission electron microscopy (HAADF STEM) allowed us to confirm the colloidal transport of Pu and U in underground waters in pseudocolloidal form in ferrihydrite aggregates.
- We have defined the mineral phases responsible for the predominant Pu sorption in the conditions of deep liquid RW collector layer.
- A detailed characterization of various “hot” particles ejected due to the explosion at Chernobyl Nuclear Power Plant has been carried out. We have investigated local conditions in the reactor core at the earliest stages of the accident based on the properties and structural parameters of thecertain particles.
- Spectral, diffraction and microscopic characteristics of lava-like samples containing nuclear fuel materials of Chernobyl’s 4th power-generating unit have been obtained, which allowed us to clarify the accident scenario and predict the possible ways of lava destruction.
- Spectral and microstructural characteristics of individual mineral inclusions separated from the vitreous matrix of Chernobyl “lava” by hydrofluoric acid leaching were studied. Individual minerals of high uranium zircon, uranium dioxide, U-Zr-O phase and rutile were analyzed using gamma spectrometry, three-dimensional (3D) optical microscopy employing a Keyence VHX-1000 device at magnifications up to 5000×, vibration spectroscopy, photoluminescence and transmission electron microscopy of thin films cut by focused ion beam (FIB).
- Data from nuclear spectrometry and mass spectrometry of bottom sediment samples from lakes in the North Caucasus allowed us to determine the rate of sedimentation in these water bodies and Lake Huko; the source of technogenic radionuclide contamination – global fallout of 1954-1963 – has been established by Pu isotope ratio.
- Pathways of radionuclide migration in biotic and abiotic environmental components of radioactively contaminated water bodies at Mayak Production Association have also been studied.
- Regularities of 232Th distribution in modern loose mountain valley sediments (Kola Peninsula, Khibiny) were determined.
- Radioactivity of aerosols associated with peat materials was determined (Bryansk region compulsory evacuation zone).
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