Characterization and development of cementitious materials used in storage facilities for immobilization of ILW radioactive waste

This PhD Thesis focuses on the development of cementitious mortars for the immobilization of Low-Level and Intermediate-Level radioactive waste in accordance with the specifications of the Italian National Repository. The repository, designed to ensure the safe confinement of radioactive waste, is based on a multi-barrier system that follows the nuclear principle of DID (Defense-In-Depth). The four main barriers are the waste package (the innermost barrier), the module (which contains multiple waste packages), the cell (which encloses multiple modules), and the multilayer hill (the outermost barrier that encapsulates and covers the entire system). The waste package is a metallic container with an internal cementitious matrix that immobilizes radioactive waste, ensuring mechanical strength, durability, and the capacity to retain radionuclides over the long term. The objective of this research is to characterize and develop a potential cementitious mortar suitable for use in the National Repository as an immobilizing matrix for LLW and ILW radioactive waste. The project initially involves a thorough characterization of the components used, which are sand, cement, water, and superplasticizer additive, to achieve optimal performance in terms of workability of the fresh mix and compressive strength and durability of the hardened matrix. In this step, various water-to-cement (w/c) and sand-to-cement (s/c) ratios have been studied, which are essential parameters for ensuring both the workability of the mortar and its long-term mechanical stability. These preliminary tests allow for narrowing the w/c and s/c ranges to create different combinations of mortars, which, once hardened, are subjected to compressive strength tests. Specimens were partially contaminated with “surrogate radionuclides,” i.e., non-radioactive chemical tracers (Li, Co, Cs, Pb), added to the mixing water to simulate the chemical-physical characteristics of most radionuclides that can be immobilized in the mortars and disposed of in the National Repository. Once a series of "blank" and contaminated samples were produced, new compression and leaching tests were carried out to evaluate both mechanical strength and surrogate retention capacity. To also evaluate the long-term durability characteristics of the mortar, a thermal aging program is proposed, consisting of freeze-thaw cycles ranging from -40 °C to +40 °C within 24 hours for at least 30 days, simulating severe environmental conditions than those expected. The aged samples are then subjected to new compression and leaching tests to assess any degradation in mechanical properties and changes in contaminant retention capacity. The results obtained suggest that the objectives set out in this study have been effectively addressed, providing a solid methodological basis in nuclear waste immobilization. This thesis is part of the National Project “BRiC2022 - ID47 - Procedure innovative per la qualifica di attrezzature di lavoro utilizzate per le attività di decommissioning - Studio di modelli matematici e di procedure di validazione" funded by INAIL, which aims to develop innovative procedures for qualifying equipment used in decommissioning activities.