PhD topic at the Belgian Nuclear Research Centre (SCK-CEN) – Promoter Muchez Philippe

The principal type of nuclear fuel material, which remains widely used today, is uranium dioxide (UO₂). It’s main constituent, the element uranium, has the highest atomic weight occurring naturally on earth, and consists of different radioactive isotopes. The environmental impact of uranium-containing materials, with respect to the safe storage and retention of radionuclides, depends strongly on the chemical speciation of uranium. Conditions relevant to different stages of the nuclear fuel cycle are to be considered, from the conversion of uranium-containing materials into fresh fuel, to the intermediate storage of fuel materials and the geological disposal of spent fuel.

Molecular configurations containing U(IV), U(VI), and to a lesser extent U(V) environments occur readily in nature, depending on the type of ligand. This results, for example, in a wide variety of uranium-containing minerals, such as uraninite, schoepite and coffinite. In the nuclear fuel cycle in particular, the uranium oxides present the most relevant species. The chemical speciation of uranium tends to adapt to its environment (such as oxidizing or anoxic conditions, acidity levels, presence of moisture, …). In acidic conditions water-soluble uranyl (UO₂)²⁺ ions are easily formed, which is relevant in the context of radionuclide migration and retention from spent fuel. On the other hand, in normal atmospheric conditions the thermodynamically more stable oxide is U₃O₈. Oxidation of UO₂ into U₃O₈ is associated with a considerable reorganization of the crystal structure, and results in a volume expansion of about 36% which can be detrimental for storage containers.

A systematic approach is required to identify different corrosion mechanisms and the associated reaction kinetics. In order to further improve our understanding it will be key to map the transition and structural relations occurring between the relevant uranium-containing materials. The gained knowledge will allow us to understand the uranium speciation in different environments, and to impose restrictions on the safe storage of nuclear fuel materials.

The PhD candidate will study the effect of various external factors on the chemical speciation of uranium. In-situ techniques such as thermogravimetry, calorimetry and X-ray diffraction will be applied to monitor the response to dry and humid gaseous environments (oxidizing, anoxic, reducing, and combinations thereof). Additionally, experiments will be performed to investigate the exposure of fresh and irradiated nuclear fuel materials to relevant aqueous conditions. Microscopic observations and analysis techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and electron probe micro-analysis (EPMA) will be used to characterize exposed surfaces, and to help determining mechanisms of corrosion. To unravel the compositional and structural relations occurring between the uranium-containing materials, spectroscopic techniques such as Raman and X-ray absorption spectroscopy (XAS), as well as more advanced diffraction techniques such as selected-area electron diffraction and neutron diffraction will be applied.