Abstracts Category : Other

Advanced chemical and structural Characterization of Nuclear Waste Materials related to the Nuclear Fuel Cycle

by Sebastian Peter Bahl

This dissertation reports structural and speciation investigations of simulated and genuine nuclear wastes. Chapters 4 and 5 emphasize on the very first application of the bulk U, Np and Pu M4,5 absorption edge high-energy resolution X-ray absorption near edge structure (HR XANES) method for characterization of U, Np and Pu oxidation states in model and genuine nuclear waste glasses and spent nuclear fuel (SNF). Chapters 6 and 7 address volatilization and precipitation challenges occurring during vitrification of fission products (FP) like Cs, Tc, Ru and Pd. Various approaches are discussed and potential solutions are proposed. Chapter 4.1 describes the results from the studies of Pu doped borosilicate glasses. Pu(III), Pu(IV) and Pu(VI) are for the first time characterized simultaneously present in a borosilicate glass using Pu M5 edge HR-XANES. It is illustrated that the method can be very efficiently used to determine Pu oxidation states which control the solubility limit of Pu in a glass matrix. HR XANES results show that the addition of excess Si3N4 is not sufficient for complete reduction of Pu to Pu(III) which has a relatively high solubility limit (10-25 wt% PuO2) due to its network-modifying behavior in glasses. It is provided evidences that the initially added Pu(VI) is partly preserved during vitrification at 1200/1400 C in Ar atmosphere. Pu(VI) could be very advantageous for immobilization of Pu rich wastes since from U(VI) vitrification a possible glass solubility limit of up to 40 wt% can be deduced. Chapter 4.2 reports the characterization and the structural differences between U, Np and Pu doped model and genuine nuclear waste glasses. The U, Np and Pu M4,5 edge HR-XANES reveals predominant U(VI) and Pu(IV) species in all glasses. But ordered structures involving U, O and likely Si are found only in the genuine waste glass by U L3 EXAFS analyses. Strong synchrotron X-ray induced radiation damage leading to reduction of U(VI) to U(IV) is detected also only for the genuine waste glass by U M4 HR-XANES. This effect might be related to differences in the radioactivity and/or the local atomic U environments in the model and the waste glasses. It might be explained with transfer of electronic charge to U from binding ligands and/or free charges as well as possible U reactions with radicals or charged species. Such reduction of U and potentially other An elements might be inducible by , and/or irradiation processes on a long-time scale and are hence of relevance for the An speciation in HLW glasses stored in an underground repository. Chapter 5 discusses the speciation of U and Pu in commercial and special irradiated high burn up SNF samples as well as of unirradiated UO2 reference materials. The bulk sensitivity of the An M4,5 edge HR-XANES technique is unambiguously demonstrated, whereas X-ray photoelectron spectroscopy (XPS) is sensitive only to species formed on the surface. The U M4,5 HR-XANES method can clearly distinguish between U(IV) and U(V) as well as Pu(IV) andAdvisors/Committee Members: Vitova, Prof. H. Geckeis, T. (advisor).