Radiocarbon is one of the most significant radionuclides affecting the safety margins of near-surface repositories for the disposal of low- and intermediate-level, short-lived radioactive waste, arising from the operation and decommissioning of nuclear power plants (NPPs). One of the goals of the present study was to characterize radioactive waste from Ignalina NPP (Lithuania) (storage tanks TW18B01 and TW11B03) from the spent ion-exchange resins/perlite stream to determine the ¹⁴C-specific activity of inorganic and organic carbon compounds. The approach applied is based on classical radiochemical separation methods, including acid-stripping techniques and wet oxidation with subsequent catalytic combustion. The suitability of the method for ¹⁴C-specific activity determination in ion-exchange resin samples with a minimum detectable activity of 0.5 Bq/g by liquid scintillation counting (LSC) was demonstrated. The extraction efficiency of inorganic and organic carbon compounds based on model samples with known ¹⁴C activity was estimated. The fraction of ¹⁴C associated with organic compounds ranged from 42% to 63% for storage tank TW18B01 and from 30% to 63% for storage tank TW11B03. The specific activity of inorganic ¹⁴C was estimated as 12.6 Bq/g with a relative standard deviation (RSD) of 29% for storage tank TW18B01, and 177.5 Bq/g with a RSD of 35% for storage tank TW11B03. Based on volume and density data, the total ¹⁴C activity for radioactive waste stored in tanks TW18B01 and TW11B03 was estimated as 3.59E + 10 Bq (±32%) and 4.15E + 11 Bq (±28%), respectively.

Water samples from the different basins of the Baltic Sea and North Sea were collected during four cruises taken place in 2016–2017. Altogether 60 water samples for ³H activity concentration and 120 water samples for stable isotopes ratio (δ¹⁸O and δ²H) were analysed. Exceptionally strong inflow activity from North Sea to the Baltic Sea occurred prior our surveys. The data obtained enabled comprehensive evaluation of tritium and stable isotopes distribution in waters of Baltic Sea and contributing inputs, including riverine and atmospheric freshwater flux and saline water inflow through the Danish Straits. By comparing with the previous studies, we noticed a tendency toward an increase in the δ¹⁸O and δ²H values of zero-salinity component over time, which may be due to climate change. In the case of tritium, the influence of artificial sources has also been noticed.

This study presents long-term observations of tritium (³H) concentrations in the Neris River at monitoring sites located near the Belarus–Lithuania border and in the city of Vilnius. Since the commissioning of the Belarusian Nuclear Power Plant (BelNPP), ³H levels in the river have consistently exceeded natural background values, with pronounced temporal variations. These fluctuations are attributed to routine ³H releases from the BelNPP, with increased concentrations observed during scheduled maintenance periods. A ³H transport model was developed to estimate the downstream propagation of releases and to assess the time lag between upstream discharge events and their detection at downstream locations. The model reliably simulates ³H behavior in flowing water and can be adapted to future scenarios and other water-soluble radionuclides, provided that isotope-specific and hydrological data are available. These findings highlight the importance of continued monitoring and further research on the fate and transport of radioactive substances in transboundary river systems.