Actinide Interaction with Zr-bearing Phases: Spectroscopic Investigations of An3+ Sorption and Incorporation Reactions with Zirconia

Actinides, especially plutonium (Pu) and americium (Am), are of large concern for the disposal of spent nuclear fuel (SNF). The rather long half-lives of the isotopes Pu-239, Am-241 and Am-243, are causing them to govern the radiotoxicity of SNF from about 500 to 1 million years after removal from the reactor core. Therefore, the safety of a final high-level radioactive waste (HLW) repository largely depends on the mobility of these actinide isotopes. In a worst-case scenario, where water enters a HLW repository, the dissolution of the SNF matrix may lead to the mobilization of actinides. In sub-surface environments under reducing conditions, these actinides can be expected to exist in their tetravalent or trivalent oxidation states, of which the latter one is more soluble and, thus, more mobile. Therefore, the trivalent oxidation state can be considered especially important. Following a release of these trivalent actinides, the multi-barrier concept of a final repository is designed to hinder their spreading into the environment through immobilization reactions such as adsorption to a surface or incorporation via secondary phase formation. One of the first possible interaction partners for actinides is the corrosion layer on the cladding material surrounding the fuel rods, consisting of zirconia (ZrO2). ZrO2 is capable to act as adsorber material for actinides as well as of incorporating large quantities of actinides. Furthermore, zirconia is a promising solid phase for the immobilization of certain waste streams from SNF reprocessing. Therefore, the possible interaction mechanisms between trivalent actinides and zirconia were studied in this thesis. In this work, various methods have been combined to gain comprehensive understanding of the macro scale as well as the molecular interactions taking place in the presence of zirconia. Information of macro scale phenomena in sorption and incorporation studies was obtained in batch-sorption experiments and with powder X-ray diffraction (PXRD), respectively. Luminescence spectroscopy (TRLFS, from time-resolved laser-induced fluorescence spectroscopy) was used in sorption and incorporation investigations to study molecular level interactions of trivalent elements on the surface or in the bulk of ZrO2. The incorporation studies were complemented with extended X-ray absorption fine-structure (EXAFS) spectroscopy. Most experiments were performed using Eu3+ (batch-sorption, TRLFS), or Y3+ (EXAFS) as actinide analogues. Spectroscopic sorption studies and complementary incorporation experiments were performed using the actinide Cm3+ (TRLFS). To study zirconia solid solutions, co-precipitation synthesis of M3+ doped hydrous zirconia, followed by calcination of the resulting phase was performed. A low-temperature hydrothermal synthesis procedure, adapted with the intent to simulate conditions potentially present in a HLW repository, was applied to selected Eu3+ doped ZrO2 compositions. The aim of these studies was to investigate how solid solution formation occurs under such hydrothermal conditions and to compare the incorporation behavior with that of the calcination method. Batch-sorption experiments revealed a favorable pH-dependent behavior for the retention of trivalent actinides in a HLW repository, as complete sorption of Eu3+ was achieved at a pH < 6 for low trivalent metal ion concentrations. The formation of three pH-dependent inner-sphere sorption complexes could be derived with TRLFS. Here, the spectroscopic signature of the third sorption complex differs from the other two. A very strong redshift of the Cm3+ emission peak (612.5 nm) and a long luminescence lifetime (190 ± 40 μs) allows for speculation, whether differing complexing anions, such as carbonates, could play a role or whether differing interaction processes, such as a surface layer incorporation could take place. The incorporation of trivalent cations into zirconia leads to a phase transformation from monoclinic (m) ZrO2, stable without any dopant to the stabilized tetragonal (t) and cubic (c) ZrO2 phases. At doping fractions high enough to stabilize the tetragonal or cubic phase, TRLFS revealed the presence of three differing dopant sites. The introduction of the aliovalent Eu3+ cation into the Zr4+ crystal structure results in the formation of oxygen vacancies to preserve charge neutrality in the crystal structure. Two of these dopant environments could be assigned to structurally incorporated Eu3+ with differing coordination numbers of 8 and 7, i.e. sites with zero or one oxygen vacancy in the first coordination sphere, respectively. The third Eu3+ species could be assigned to incorporation into surface or near-surface layers of zirconia. EXAFS revealed a constant environment of the host (Zr4+) and the dopant (Y3+) within the low doping range as well as within the stabilized zirconia phases. Therefore, the differing sites observed via TRLFS could not be observed here. Incorporation into t- or c-ZrO2 has shown a non-distinguishable spectroscopic behavior meaning that the dopant’s environment in t-ZrO2 and c-ZrO2 is very similar. TRLFS shows a low site symmetry of the dopant in both cases, despite of the high bulk symmetry, i.e. tetragonal or cubic. In the non-stabilized monoclinic crystal structure, Eu3+ incorporation was found to be accompanied by the formation of a secondary phase. The secondary phase is assumed to be nano clusters of the dopant’s oxide, forming inside the zirconia matrix. The hydrothermal synthesis of Eu3+ doped ZrO2 revealed a different phase composition as a function of dopant concentration than observed with the calcination method. At low dopant concentrations where the m-ZrO2 prevails after high-temperature treatment, t- and c-ZrO2 are very abundant after hydrothermal treatment. This is a result of the small crystallite size resulting from the low synthesis temperature and short synthesis time, which causes the stabilization of the tetragonal phase even without any dopant present. At higher doping fractions, phase compositions comparable to the calcination synthesis are obtained. Both, the sorption as well as the incorporation behavior of zirconia studied here show properties advantageous for the retention of trivalent actinides within the environment of a HLW repository. TRLFS studies of the sorption speciation showed the formation of inner-sphere complexes and, possibly surface layer incorporated species, which are more stable under environmental conditions than interactions based on Coulomb interactions only. The speciation of the Cm3+ sorption on zirconia was studied and thermodynamic data was derived via surface complexation modeling for the first time. The very systematic approach of studying the doping throughout a large range resulted in basic understanding of the dopant behavior in zirconia. The incorporation capabilities of actinides into the lattice was observed to be high for t- and c-ZrO2 while rather limited for m-ZrO2. Therefore, the monoclinic structure seems to be unsuitable for incorporating trivalent dopants. Under conditions potentially present in a HLW repository, i.e. hydrothermal synthesis conditions, the amount of m-ZrO2 was observed to be strongly reduced for low overall dopant concentrations. This could facilitate the incorporation of actinides into zirconia even at low concentration levels and therefore, increase its capabilities to act as a retention barrier in a HLW repository. The conclusions of this thesis are of importance in the field of nuclear waste management as they help closing gaps in the understanding of retention processes of trivalent actinides. The obtained molecular information can be built on with experiments designed to obtain reliable thermodynamic data, used in the safety analysis of a HLW repository. Furthermore, the interaction of zirconia with other actinides can be studied in a targeted manner based on the knowledge obtained in this thesis. In the field of material sciences, the molecular information obtained here is of interest as well, as zirconia is a very versatile material. This is due to its abundance of applications ranging from electrolyte material in solid oxide fuel cells to building materials