Performance of a Landfill-Type Near Surface Disposal Facility, Summary Report

The KYT SURFACE project was conducted between 2019–2022 as a part of the KYT 2022 Programme. This project examined near surface repositories, especially landfill type, for waste deposition of short-lived very low (VLLW), low (LLW), intermediate (ILW) level nuclear waste. The KYT SURFACE project was divided in three tasks and consisted of three phases. This report gives final recommendations concerning the design of the landfill type of near surface repository, including the analysis of results from KYT SURFACE tasks 1 (radionuclide transport), 2 (biodegradation of waste and steel corrosion), and 3 (performance of engineered barriers). The report also reviews the design basis for the near surface repository, covering the safety functions as well as legislation and other regulations.

According to the results gained in the tasks 1 and 2, utilization of bentonite as a part of the mineral sealing layer mixture with crushed rock or flour was recommended as it increased the Sr sorption capacity approximately 20%. In addition, high performance of the barriers in the cover layer of the repository is required to limit infiltration of water into the system and thus prevent events such as Sr remobilization, corrosion of the metallic waste and waste packages, and formation of biogas (methane and CO2). Due to the possibility of gas formation, the cover layer should be permeable for gas, or a gas collection system should be installed.

Studies in the task 3 of the KYT SURFACE project concentrated on the design of the repository system regarding especially the water infiltration and percolation within the repository as well as freezing and thawing of the mineral sealing layers. In addition, a separate risk analysis was conducted to screen the most probable and detrimental events and processes affecting the long-term performance of the near surface repository. Based on the project results, the landfill type of near surface repository functioned effectively and met its desired targets. However, considering the long lifetime of the repository, some uncertainties should be carefully reviewed. Thickness and density of the top layer covering the drainage and sealing layers should be thick enough to protect these underlying layers from high rates of water percolation, freezing, forest and ground fires, tree growth and root penetration, burrowing animals, and human intrusion. In addition, erosion process could deteriorate the top layer itself. A minimum thickness of 2 m was recommended for the top layer, but site-specific conditions and material properties of the top layer should be taken into account in the design process. Percolation of water through the cover layer to the waste layer should be prevented to minimise the formation biogas (biodegradable waste), corrosion, and possible formation of contaminated leachate water. Thus, sealing layers with combinations of a synthetic and mineral sealing layers was recommended for foundation and cover structures to provide long-term low permeability for the system. In addition to the drainage system in the cover layer, a leachate water drainage system should be used in the foundation layers and monitor the radionuclides in the leachate water. Recommendations for the further studies with near surface disposal facilities included subjects such as monitoring and modelling of propagation of water and escaped radionuclides through the barriers, retention and re-mobilisation of different radionuclides, and post-closure monitoring strategy