Radionuclide transport from near-surface repository for radioactive waste - The unsaturated zone approach

About 100 000 m3 of solid conditioned Low and Intermediate Level Waste (LILW), generated during operation and decommissioning of the Ignalina nuclear power plant (INPP), are to be disposed of in a near-surface repository (NSR) - a 'hill'-type repository with reinforced concrete vaults and with engineered and natural barriers. The northeastern Lithuania and the environment of the INPP in particular were recognized as the areas most suitable for a near-surface repository (Stabatiske Site). The engineered barriers of the repository consist of concrete cells surrounded by clay-based material of low permeability with about the same isolating capacity in all directions. The clay materials must be effectively compactable so that required hydraulic conductivity is reached. The Lithuanian Triassic clay turned out to be sufficiently rich in smectites and was proposed as main candidate for sealing of the repository. When the concrete vaults are filled, the repository cover will be constructed. The surface of the mound will be planted with grass. In this study a computer code FEFLOW 5.0 was applied for simulating the transport of the most mobile radionuclides (3H, 14C, 59Ni and 94Nb) with moisture through an unsaturated vault of the near-surface repository in Stabatiske Site. The HYDRUS-1D analysis was used to assess the radionuclide transport in the repository and to estimate initial activity concentrations of radionuclides transported from the cemented waste matrix. Radionuclide release from the vault in the unsaturated conditions after closure of the repository and consequent contaminant plume transport has been assessed taking into account site-specific natural and engineering conditions and based on a normal evolution scenario. The highest peak radionuclide activity concentrations were estimated applying the FEFLOW code. The highest value of 14C activity concentration(about 1.3x108 Bq/m3) at the groundwater table is predicted after about 10 000 years after repository closure, while its activity concentration in the disposed waste is 4.7x109 Bq/m3. The predicted maximum activity concentrations of the others modeled mobile radionuclides 3H (after 120 years after repository closure), 94Nb (after 350 000 years) and 59Ni (after 233 000 years) at the groundwater table are respectively 4.9x104, 1.3x104 and 8.4x105 Bq/m3. The radionuclide transfer simulation results demonstrate that the investigated long-lived radionuclides will not completely decay in the repository vaults, however the increased activities could be observed in the environment after long time period only. Document available in abstract form only. (authors)