The paper presents the results of the best material selection for additional safety barrier construction during uranium-graphite reactors decommissioning. The research findings show that the best material for safety barriers is clay-containing natural material of various types with counter-migrational and counter-filtrational qualities. We have demonstrated a technology for void-free filling of cavities in the reactor space of under-decommissioning uranium-graphite reactors on the site of JSC Pilot and Demonstration Center for Decommissioning of Uranium-Graphite Nuclear Reactors (PDC UGR). This will make it possible to construct reliable man-made geo-barriers and prevent the release of radionuclides from the repository into the environment for thousands of years.