2022

Keto, Paula; Rinta-Hiiro, Ville; Naumer, Sami; Schatz, Timothy

Design basis and performance targets for a landfill type near surface repository have been examined as part of the KYT2022 SURFACE project, as well as the differences between a near surface repository and a landfill for hazardous waste landfill. The main difference comes from legislation and from ensuring passive safety for the repository after closure. The effect of site conditions on the engineered barriers was studied from the perspective of low temperatures during winter in Finland. Numerical modelling shows that freezing of some or all of the engineered barriers in the cover layer can take place during a cold winter, especially in a situation when there is lack of sufficient snow coverage that would provide insulation. In normal and hazardous waste landfills, the frost shall not penetrate to the level of the mineral sealing layer. This leads to a recommendation of performing site and design specific numerical modelling on the frost penetration and, based on the results, considering cover top layer thicknesses that are more than the typical minimum 1 m used in normal and hazardous waste landfills. The potential impacts of post closure forestation also supports use of a thicker top layer to avoid puncture of synthetic liners by tree roots.<br/>Limiting water inflow into the repository through the cover layer was identified as one of the key factors in ensuring long-term and passive safety for the near surface repository. This can be done by combining water tight synthetic liners with a mineral sealing layer. Limiting water flow to the repository decreases the quantity of formed leachates and can slow the generation of landfill gas.<br/>The need for gas management systems depends on the rate at which gas is generated in the waste. In order to minimize gas generation from soft waste pallets containing organic waste, placing this waste into metallic packages was reviewed as an option. This would also enhance the mechanical stability of the repository. Performance of the drainage systems at the foundation structure and collection and handling leachate waters was also assessed as part of this work. Some of the drainage is in any case needed for preventing accumulation of leachate water in the bottom of the repository. However, the effect of the drainage and leachate water collection system for post closure safety requires further considerations. If the cover layer works as expected and the waste is not in direct contact with the water, the generation of leachate water should be minimal.<br/>Final recommendations concerning the design of the landfill type near surface repository will be summarised in 2022 including the analysis of results from KYT2022 SURFACE tasks 1 (radionuclide migration) and 3 (steel corrosion and microbial activity) on the repository design recommendations.