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.

The assessment of nuclear objects sites in Lithuania, including groundwater characterization, took place in the last few years. Tritium activity in groundwater is a very useful tool for determining how groundwater systems function. Natural and artificial tritium was measured in 8 wells in different layers (from 1.5 to 15 meters depth). The results were compared with other regions of Lithuania also. The evaluated tritium activities varied from 1.8 to 6.4 Bq/L at nuclear objects sites in Lithuania and they are coming to background level (1.83 Bq/L) and other places in Lithuania. The data show, that groundwater at the nuclear power objects sites is not contaminated with artificial tritium. In this work, the vertical tritium transfer from soil water to the groundwater well at nuclear objects site was estimated. The data show that the main factor for vertical tritium transfer to the well depends on the depth of wells.

The Strategy on Radioactive Waste Management of Lithuania (Radioaktyviųjų… 2008) envisages evaluating the possibilities of disposal of spent nuclear fuel and long-lived radioactive waste from operation and decommissioning of Ignalina NPP in a deep geological repository. The crystalline basement and sedimentary cover of south-eastern Lithuania was selected for the current model case studies due to availability of geological and hydrogeological data from previous explorations. Groundwater flow, radionuclide (iodine-129 as mobile and long-lived one) transport and heat transfer, modelling using computer code FEFLOW was performed. The model domain of south-eastern Lithuania comprises Protero-zoic-Archaean aquifer with overlaying aquifers system of sedimentary cover. The upward groundwater flow through defected canister located in tectonically damaged zone was conservatively generated. The main results of calculations are following: in case of upward groundwater flow, the maximum activity concentration of 129I in groundwater of the tectonic fracture zone above defected canister will not exceed 10−4 Bq/l; the maximum temperature in the tectonic fracture will obtain about 30-35°C and will not impact on the radionuclide transport. Location of model domain in south-eastern Lithuania does not mean any reference to the site for deep geological repository. The results show that doses obtained by human via drinking water should be below the dose constraint (0.2 mSv /year). Santrauka Radioaktyviųjų atliekų tvarkymo strategija numato galimybę panaudotą branduolinį kurą ir kitas ilgaamžes radioak- tyviąsias atliekas, susidariusias eksploatuojant Ignalinos AE ir susidarysiančias ją demontuojant, galutinai patalpinti giliai geologinėse formacijose. Dėl didelio geologinės bei hidrogeologinės informacijos kiekio šiame darbe nagrinėti tik Pietryčių Lietuvos kristalinis pamatas bei jį dengiantys nuosėdinių uolienų sluoksniai. Radionuklidų bei šilumos sklaidai kristalinio pamato uolienose vertinti naudota kompiuterinė programa FEFLOW 5.0. Pasirinkta konteinerio defekto scenarijus, taikant skaičiavimus, atliktus LEI ekspertų. Modelis apima archėjaus ir proterozojaus vandeningąjį sluoksnį bei nuosėdinėje dangoje slūgsančius vandeninguosius kompleksus. Modeliuotas požeminio vandens srautas, kylantis per tektoniškai pažeistą zoną, kurioje palaidotas PBK konteineris. Iš pagrindinių modeliavimo rezultatų nustatyta, kad ilgaamžio ir mažai sorbuojamo radionuklido 129I sklaidos intensyvumas mažai priklauso nuo geologinės aplinkos savybių, o šilumos sklaida neturi įtakos radionuklidų sklaidai. Žmogaus gaunama dozė, skaičiavimų duomenimis, nesieks ribinės vertės (0,2 mSv/metai).

A computer code FEFLOW 5.0 was applied for simulating transport of the most mobile radionuclides ( 3 H, 14 C, and 129 I) with groundwater from the designed low and intermediate level radioactive waste near-surface repository at Stabatiškė site. An analysis of the variability of the model parameters of hydrogeological system such as dispersivity, hydraulic conductivity, porosity, and radionuclide solid / liquid distribution coefficient (K d ) has been made. The obtained results show that the activity concentration of the most mobile radionuclides in aquifer will remain low for a long time after the repository closure and the conservatively predicted maximal annual effective dose (8.9·10−3 mSv/y) to the inhabitant will be much lower than the dose constraint (0.2 mSv/y).