Safety assessment of a near-surface repository of radioactive waste usually assumes a homogeneous distribution of activity within the vaults of the repository. However, in some cases there is radioactive waste, e.g., disused sealed sources, which should result in so-called "hot spots" when disposed of with other radioactive waste. An uneven distribution of waste activity is obtained in that case.

The impact of heterogeneities in source distribution on radionuclide releases from a near-surface repository to the groundwater is analyzed in the paper. The conditions that stand for the application of homogeneous distribution of radioactive waste are revealed.

The assessment has been performed using the methodology of the Improving Long Term Safety Assessment Methodologies for Near Surface Radioactive Waste Disposal Facilities (ISAM) study recommended by the International Atomic Energy Agency for the safety analysis of the near-surface repository. A description of the ISAM methodology and its application for the analysis of heterogeneity including a brief description of the disposal system of radioactive waste, the scenarios for the radionuclide migration, and the developed conceptual models are presented in the paper.

The calculations have been carried out using the DUST and GWSCREEN computer codes, designed to simulate radionuclide transport. The modeling results for the case of homogeneous distribution of radioactive waste in the repository are considered. The impact of heterogeneities has been analyzed by comparing the modeling results of radionuclide transport for the heterogeneous case of waste distribution to the homogeneous case.