Introduction: Tritium and radiocarbon, naturally occurring and anthropogenically released radionuclides, accumulate in plants and food chains due to their high mobility. In living organisms, tritium occurs primarily as tritium in tissue water and as organically bound tritium, whereas radiocarbon is incorporated into organic compounds through photosynthesis. Aquatic systems with altered hydrodynamics play a particularly important role in the retention and distribution of these radionuclides. One such system is the flow accumulation area of the Brežice Hydropower Plant, located downstream of the Krško Nuclear Power Plant, where the reduced flow velocity of the Sava River enables increased retention of substances and their accumulation in aquatic plants. These plants therefore represent important bioindicators of spatial and temporal changes in environmental radionuclide contamination. Purpose: The purpose of this master’s thesis is to analyse the presence and distribution of tritium and radiocarbon in aquatic plants within the flow accumulation area of the Brežice Hydropower Plant and to evaluate the influence of seasonal variations, sampling location, plant type, precipitation regime, and operational characteristics of the Krško Nuclear Power Plant on their accumulation. Methods: The presence of the radionuclides tritium and radiocarbon in plant samples was determined using liquid scintillation spectrometry in an accredited laboratory of the Jožef Stefan Institute. The samples included annual aquatic plants and several terrestrial plant species for comparison. Sample preparation comprised the preparation of samples and measurement media, the measurements themselves, and subsequent data processing and interpretation of results using the processing of secondary data. Results: The analysis of tritium in tissue water and organically bound tritium showed that both fractions were present in all analysed plant samples, with higher and more variable activities observed in tissue water tritium compared to organically bound tritium. No significant differences were detected among the measured radiocarbon values. Discussion and conclusion: The study demonstrated that aquatic plants in the flow accumulation area of the Brežice Hydropower Plant do not exhibit elevated activities of tritium and radiocarbon. Tritium in tissue water proved to be a sensitive indicator of short-term environmental changes, whereas organically bound tritium and radiocarbon reflect a more stable, long-term presence of radionuclides in the ecosystem. Variability in activity levels depends on a combination of environmental factors, including discharges from the Krško Nuclear Power Plant, hydrological conditions, and plant biological activity, highlighting the importance of systematic, long-term monitoring.
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