Keramiken des Monazit-Typs zur Immobilisierung von minoren Actinoiden und Plutonium

The safe disposal of radioactive waste in deep geological formations is a challenging task of present and future generations. Innovative strategies as the conditioning of radionuclides in ceramic matrices can make a contribution here. This work points out monazite-ype ceramics as potential waste forms for minor actinides and Pu. Several aspects concerning nuclear disposal as well as fundamental structural information were investigated. Lanthanide phosphate endmembers (LnPO4) within the stability field of monazite (Ln = La-Gd) were synthesised within the scope of this work. To extend the knowledge of monazite phases, monoclinic TbPO4- and DyPO4-phases were prepared and characterised. Tb- and Dy-phosphates are situated in the xenotime stability field close to that of monazite. They can exist as metastable monazite phases. Structural characterisations of long- and short-range order were performed by X-ray diffraction, infrared (IR) and Raman spectroscopy. Structural data could be complemented, enhanced and gaps of knowledge could be filled by the first systematic consideration of the complete Ln-monazite-series (Ln = La-Dy). Furthermore, this work focuses on Sm-monazite phases. Samarium with an atomic number of 62 is located in the middle part of the lanthanides showing the monazite structure. Accordingly, it has a mean cationic radius within the Ln-monazite-series and hence shows a relative high flexibility regarding the incorporation of radionuclides with different radii. Sintering densities of SmPO4 ceramics were optimised by varying process parameters like pressure and number of pressing steps. An irregular texture as well as densities of 94 % of the theoretical value could be achieved. The resistance of Sm-monazite against ionising radiation were examined. Radiation damages caused by the α-decay of radionuclides incorporated in a ceramic matrix were simulated by computer calculations and experimentally by heavy ion bombardment of SmPO4. Thin layers of the samples bombarded with Au ions show initiating effects of radiation damages enduring doses of Ddpa = 0.02 and 0.06, respectively, whereas a dose of Ddpa = 1.65 produced an amorphisation to a great extent in a lamella sample. However, crystalline areas remained due to recrystallisation processes in the material. The flexibility of the Sm-monazite's crystal structure, necessary for the incorporation of radionuclides, is required for the use of SmPO4 ceramics as a suitable waste form. Therefore, Sm was substituted by other lanthanide cations, that served as surrogates for minor actinides such as Np, Am, and Cm or for Pu. Actinides reveal a similar behaviour as the lanthanides and in case of trivalent actinides (e. g. Am, Cm) also favour the monazite structure as endmembers. For the coupled substitutions, needed for the incorporation of tetravalent actinides (e. g. Np, Pu), also the resulting mean values of cationic radii have to be considered. These radii can reach into a limit range, where the stability field of the monazite structure usually changes into that of the xenotime structure. Hence, knowledge of the structural limitations of monazite is essential. Therefore, on the one hand Sm1-xCexPO4 solid solutions were synthesised in the range of x = 0 to 1 and characterised. The formation of a complete solid solution between the endmembers SmPO4 and CePO4 were examined by structural investigations of the long- and short-range order. On the other hand, Terbium was incorporated gradually in the Sm-monazite up to the complete substitution of Sm. The TbPO4-endmember generally forms the tetragonal xenotime structure. Within the Sm1-xTbxPO4 system, solid solutions show the monazite crystal structure over a wide range up to x = 0.75, before the xenotime structure is formed. Furthermore, in situ high-pressure Raman experiments were carried out on (Sm,Tb)PO4 solid solutions to investigate the resistance of external stress of the different crystal structures. While xenotime phases transform to monazite at high pressures, the crystal structure of monazite remains stable up to the studied pressures of ~ 20 GPa. Systematic investigations of monazite phases as well as the characterisation of Sm-based monazites of this work contribute to considerations of the suitability of monazite-type ceramics as a potential waste form for the immobilisation of minor actinides and Pu.