Actinide oxides co-precipitation and reduction of Ca-uranyl-carbonato complexes by iron

Licentiate thesis, 2024

Given the escalating global energy demands and consumption trends, nuclear energy has witnessed a resurgence worldwide. Several nations are reaffirming their commitment to expanding nuclear power generation to achieve energy security, fulfill climate goals, and foster sustainable development. However, the safe disposal of high-level radioactive waste, particularly spent nuclear fuel, remains a significant challenge on a global scale. Proposed solutions include deep geological repositories situated approximately 500-1000 meters underground, intended for the management of this hazardous waste. While these repositories are designed to transition rapidly to anoxic, reducing conditions after closure, the potential breach of safety barriers, such as containment canisters, could lead to localized oxidizing conditions. This scenario arises from the ionizing radiation present in spent fuel, which generates radiolytic oxidants, thereby causing the oxidative dissolution of the fuel. Nevertheless, the presence of dissolved Fe(II) and molecular hydrogen, acting as repository reductants, can significantly suppress the oxidative dissolution of the fuel by consuming the radiolytic oxidants. This study delves into the interaction between corroded metallic iron (canister material) and dissolved U(VI) in simulated groundwater compositions under anoxic conditions, with an investigation on whether the formation of Ca-uranyl-carbonato complexes hinders the reduction of U(VI) by metallic iron. Concurrently, the leaching of used nuclear fuel remains a pivotal aspect of safety analysis for underground repositories. Co-precipitation, initially proposed as a radionuclide retention mechanism, offers promise for lowering the solubility of minor components in nuclear waste disposal scenarios. To explore this phenomenon further, a co-precipitation experiment involving Ce(III) oxide with UO2, a major component of spent fuel, was conducted, to investigate the potential possibility for actinides  ions to co-precipitate with each other under disposal conditions, with Ce serving as a surrogate for Pu. Understanding these interactions would provide insights into the release mechanisms of radionuclides into the environment.

A multidisciplinary approach integrating chemical analysis, spectroscopy technique (ICP-MS, XAS),surface analysis (XPS),crystallography (XRD) and micro structural analysis (SEM-EDX) was employed in these studies for both liquid and solid characterizations. The findings suggest that the concentrations of Ce, other lanthanides or actinides and fission products released by the fuel matrix during oxidative dissolution will  be orders of magnitude lower than their individual solubilities when they co-precipitate with UO2(s) at the iron surface of the canister material. In the Ca-uranyl carbonato complexes studies in the presence of iron ,metallic iron foils efficiently reduced U(VI) to U(IV), leading to its significant sorption and precipitation on the iron foil surfaces as U(IV).