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

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. ... mehrIt 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 α, β and/or γ 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) and Pu(V)/Pu(VI), which is not always possible with the conventional U and Pu L3 XANES. U(IV) and U(V) likely in the form of U4O9 are found in the two commercial SNF samples. It is shown that the U oxidation continues in ambient conditions as a function of time due to the small particle size of the samples. No phase transformation from UO2 to U3O8 or UO3 is observed for any of the studied SNF samples. The special irradiated high burn-up sample stored in atmosphere with ca. 1% O2 for 20 years has only very minor amount of U(V) which illustrates its high stability against oxidation being advantageous in case of a potential cladding failure scenario for example in extended interim storage. For the first time it is demonstrated that along Pu(IV) also Pu(VI) is present in the bulk of presumably all SNF samples. This Pu(VI) is unlikely formed by oxidation in air but rather is a result of the neutron absorption and subsequent nuclear reactions.
Chapter 6 reports studies of HLW residual materials from a reprocessing and a vitrification plant. They are simulated and the formed compounds are characterized as well as possible host matrices for their immobilization are reviewed. The challenge of Cs and Tc volatilization loss is addressed by careful selection of immobilization processes, i.e., addition of reducing agent and application of host matrices with low melting temperature. A selection of suitable immobilization materials is made with regard to minimized FP volatilization behavior. Potential leaching performance is discussed considering composition and macroscopic appearance of the synthesized samples.
Chapter 7 presents insights obtained from the X-ray CT in-situ study of a vitrification process with emphasize on the precipitation of noble metal particles (NMP) which are known to cause severe problems in industrial vitrification processes in form of drain plugging and short circuits. Results indicate the formation of the particles at 600 700 °C in the lower part of the cold-cap. An exceptional large precipitated specie is observed and described. Its formation might have an impact on the NMPs sedimentation behavior. For the first time, in-situ tracking of their movement in a glass melt is reported. The designed setup has the potential to unambiguously reveal the highly debated NMP sedimentation mechanism. Two counter measures on the particles formation are tested. The use of glass powder instead of glass beads and the addition of 1 wt% Si as reducing agent decreased the NMP precipitation by 40% and 30%, respectively, and simultaneously retain Re used as a surrogate of Tc by a factor of 2-3.