Co-precipitation of actinide hydrous oxides and leaching of unirradiated MOX fuel in the presence of iron and its corrosion products

Doctoral thesis, 2026

With growing global energy demand and the need to ensure energy security, meet climate goals, and support sustainable development, nuclear energy has experienced a renewed global interest. However, the safe disposal and management of high-level radioactive waste, particularly spent nuclear fuel (SNF), remains a significant scientific, societal and political challenge on a global scale.  One Proposed solution is the long-term isolation of the SNF in deep geological repositories (DGRs), where SNF is enclosed in copper canisters with an iron insert, surrounded by bentonite clay, and placed ~500 meters underground in granitic bedrock.
 
While deep geological repositories (DGR) are designed to rapidly evolve towards anoxic, reducing conditions after closure, a potential breach of the canister containment followed by groundwater intrusion to the SNF can generate localized oxidizing conditions through the formation of radiolytic oxidants produced through water radiolysis. Such conditions can induce oxidative dissolution of UO2 matrix, potentially resulting in the mobilization and release of highly radiotoxic radionuclides into the biosphere. Metallic iron, a key component of the engineered barrier system, can simultaneously undergo anoxic corrosion upon contact with groundwater producing Fe(II) and H2.These species could inhibit the oxidative dissolution of the spent fuel matrix. In addition, metallic iron and Fe(II) may also play a crucial role in reducing U(VI) to U(IV) in groundwater systems, thereby limiting its solubility and mobility. Furthermore, the  co-precipitation UO2(s) with minor components of  the SNF may serve as a retention mechanism for radionuclides, further enhancing repository safety. Therefore, a detailed understanding of fuel matrix dissolution, radionuclide migration, and interactions with engineered barrier materials is essential for assessing repository performance over extended timescales.
 
This thesis investigates key chemical processes influencing the behaviour of SNF under DGR conditions.  The findings indicate that anoxic corrosion of metallic iron significantly suppresses radiolytically induced oxidative dissolution of the fuel, leading to lower actinide releases. The coprecipitation studies infer that the concentrations of other actinides, lanthanides and fission products released by the fuel matrix during oxidative dissolution will not be determined by their individual solubilities when they coprecipitate with UO2(s) at the iron surface of the canister insert but will be orders of magnitude lower. Additionally, metallic iron efficiently reduces U(VI) to U(IV), promoting its sorption and precipitation on iron corrosion products. Overall, this thesis provides new insights and a better understanding of uranium redox behaviour in groundwater systems, spent fuel redox stability and actinide oxides co-precipitation processes under repository relevant conditions.