Synthesis and characterization of solid phases for actinide immobilization
Crystalline ceramics have been suggested as potential immobilization matrices for high-level radioactive wastes (HLW) generated in the late stages of the nuclear fuel cycle. Especially for the immobilization of specific waste streams containing minor actinides (Np4+, Am3+, Cm3+) or plutonium (Pu3+,4+), long-term disposal in glass matrices may be insufficient, owing to the amorphous nature and higher solubility in comparison to crystalline ceramic materials. The selection of crystalline host phases for the incorporation of the above-mentioned transuranium elements should focus on matrices capable of incorporating the trivalent and tetravalent oxidation states, as these will be prevailing in deep underground repositories of high-level waste streams.
Lanthanide orthophosphate (LnPO4) ceramics crystallizing in the monazite structure have been shown to be excellent candidates for the incorporation of trivalent dopants. It is further known, that monazites can accommodate tetravalent actinides. Their incorporation requires coupled substitution reactions which preserve charge neutrality within the phosphate ceramic structure. Other candidate phases for tetravalent radionuclides are those with a tetravalent host cation, such as zirconia (ZrO2). Here, a direct substitution of the tetravalent actinide for the Zr4+ host cation can occur without involvement of additional charge-compensating reactions. The large mismatch between the small Zr4+ host cation and the substantially larger An4+ cation, however, may influence the actinide incorporation reaction and the solid structure.
As part of a large joint project (AcE) with five national partners, this Ph.D. project will study the immobilization of especially tetravalent metal ions in ZrO2 by direct substitution of M4+ for Zr4+ in the crystal lattice or by coupled substitution of M4+ and M3+ and/or M2+. The latter substitution reactions can increase the solubility of the oversized tetravalent dopants in the ZrO2 matrix as a result of oxygen vacancy formation which increases the flexibility of the host structure. In joint efforts with the partners at RWTH-IFK synthesis strategies for M2+M4+ co-doped monazites will also be explored.
The syntheses will be complemented with detailed structural characterizations of the solid phases, comprising atomic scale investigations of the incorporated actinides within the solid structures. The aim is to develop a detailed understanding of tetravalent actinide immobilization in crystalline host phases on an atomic basis as well as to make reliable statements with regard to the long-term stability and the retention capacity of the host matrices under repository conditions.