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Abstract:The present investigation conducted in Čepkeliai peatland (south-eastern Lithuania) deals with two radioisotope methods -carbon-14 ( 14 C) and tritium ( 3 H).14 C was applied to peatland chronology and sedimentation rate estimation and 3 H to peat water dynamics characterization. According to 14 C data, peatland development began since 12650-11350 years cal BP and peat accumulation since 10550-9700 years cal BP with peat accumulation rate of 0.27-0.79 mm/year in the central part of the peatland. The peat water mean residence time and basic groundwater flow was estimated to be respectively about 27±6 years and 62±15 mm based on 3 H data. The obtained data showed relatively undisturbed natural condition of Čepkeliai peatland in the studied sites.

On a first attempt, the determination of 14 C and 36 Cl activity concentrations in basic operational waste (spent ion-exchange resins and perlite mixture), in decommissioning waste (construction concrete, sand, stainless steel and serpentinite) and irradiated graphite from the Ignalina NPP has been performed. The samples for measurement of the specific activity of 14 C and 36 Cl were obtained from the selected places, where the highest values of the dose rate and the activity concentrations of gamma emitters were found. The performed study of the total 14 C and 36 Cl activity concentrations was based on estimated chemical forms of 14 C (inorganic and organic compounds) and 36 Cl as Cl-ion. The tested methods used in this study were found to be suitable for estimation of activity concentrations of measured radionuclides.

The strategy of radioactive waste management of Lithuania provides for evaluating the possibilities of disposal of spent nuclear fuel and long‐lived radioactive waste originated from Ignalina NPP in deep geological formations. The initial studies performed in Lithuania during 2001–2004 focused on screening of all potentially prospective geological formations as host formations. Since most information is available on crystalline basement, this formation was selected for the model case studies. Taking into account the assumptions (canister defect scenario proposed by Swedish experts and evaluated by LEI experts), groundwater flow and radionuclide (iodine‐129 as mobile and long‐lived one) transport modelling using computer code FEFLOW was performed according to geosphere conditions and parameters characteristic of the southern part of Lithuania (0,8×0,6×0,52 km far‐field block). The upward groundwater flow through defected canister located in tectonically damaged zone was simulated. The main results of calculations are the following: in the case of upward groundwater flow, the maximum I‐129 volumetric activity in single tectonic fracture above defected canister will not exceed 1 Bq/1, and in the active water exchange zone, it is close to 10−2 Bq/l. These figures show that doses obtained by human recipient via aquatic pathway should be below the dose constraint (0,2 mSv/y). More complicated scenarios and assumptions should be investigated in future studies.