Thesis title: 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 and intermediate-level radioactive waste (LLW and ILW) 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 immobilize radioactive waste, ensuring mechanical strength, durability, and the ability 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, ensuring long-term confinement.
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.
To determine these parameters, the development phase included workability tests using the reduced Abram’s Cone (mini-slump test) and temperature tests to control heat generation during hydration. 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.
This investigative approach has made it possible to better understand and identify a potential optimized mix design for the preparation of samples to be submitted to characterization tests (compression and leaching tests).
The 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.
Tests were performed partly during the maturation phases and partly after full maturation. 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 more 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 foundation for applications in nuclear waste immobilization.
In conclusion, this research focused on developing cementitious mortars to immobilize LLW and ILW in the context of the Italian National Repository. The main findings include:
• Optimal w/c and s/c ratios were identified through comprehensive characterization of mortar components, enhancing workability and mechanical properties
• The use of surrogate radionuclides successfully simulated the behaviour of actual radionuclides, enabling thorough evaluations of both mechanical performance and retention capacity
• A thermal aging program provided valuable insights into the long-term durability and potential degradation of the mortars
Future research could expand upon these findings by incorporating additional materials, such as geopolymers, to further reduce leaching and improve performance. Overall, this study contributes significantly to advancing safety and effectiveness in radioactive waste management, with ongoing efforts focused on refining mix designs for real-world applications in repository conditions. This thesis is part of the national project "INAIL BRiC ID47/2022" funded by INAIL, which aims to develop innovative procedures for qualifying equipment used in decommissioning activities.