Characterisation and fabrication of zirconia and thoria based ceramics for nuclear applications

The reduction of the long term radiotoxicity of nuclear waste during disposal is the aim of the research called “Partitioning & Transmutation of minor actinides (MAs)”, which also requires the development of inert ceramic support materials. Moreover, after separation, if the transmutation is not available, the actinides can be conditioned into stable dedicated solid matrices (Partitioning & Conditioning strategy). Yttrium-stabilized zirconia and thoria are discussed in the international nuclear community as candidates for the fixation of long-lived actinides as target material for transmutation and as stable materials for long-term final disposal. The aims of the following work are twofold: determine the impact of the addition of actinides, simulated by cerium on the properties of the matrices and study the possibility of synthesising homogeneous ceramics using simple fabrication routes. Within this framework, (Zr,Y)O$_{2-x}$ - CeO$_{2}$ and ThO$_{2}$ - CeO$_{2}$ powders with variable ceria contents (from 0 to 100 %) were synthesised by a co-precipitation method of nitrate solution. The influence of ceria concentration on the powder' properties, such as thermal behaviour and the evolution of material crystallisation during annealing, was investigated in detail by thermogravimetry (TG) coupled with differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Both systems crystallise at high temperature in a stable solid solution, fcc, fluorite type structure and follow the Vegard’s law for the complete range of ceria. For both systems a critical concentration of 20 mol % has been established. For ceria concentrations lower than 20 %, the properties of the system are mainly controlled by the matrix. Pellets with different ceria concentrations were compacted from these powders by using different technological cycles. In order to obtain materials with reliable properties, the technological parameters of each chosen fabrication route, have been optimised. By employing mild wet methods (calcination at 600°C, wet-grinding in acetone and fractionation in acetone), (Zr,Y,Ce)O$_{2-x}$ pellets with densities of up to 0.97 TD can be obtained. In the case of the (Th,Ce)O$_{2}$ system, pressing by repressing from non-milled powder was selected as the fabrication route, allowing the fabrication of pellets with densities of up to 0.98 TD. In both cases, materials with homogeneous repartition of pores, well formed grains and boundaries and good mechanical properties were obtained

Experimental evaluation of the dissolution rates of Ti and FeTi70 in liquid Fe

During secondary steelmaking, improving alloy yield and engineering inclusion content require understanding and quantification of the alloy distribution in the melt. When additions are dropped in the melt, a steel shell solidifies around them. After this shell has melted, the alloy is spread in the melt. The influence of process parameters on the duration of the shell period for Ti and FeTi70 additions has been experimentally evaluated. For Ti, the melt temperature and the initial addition size were varied and for FeTi70, only the melt temperature was varied. By continuously measuring the apparent weight of submerged samples with a load cell, the shell period and the amount of molten alloy within the shell were determined. The shell period increases at lower superheats and for larger sample sizes. For a certain size of Ti additions, the molten content within the shell increases with increasing shell period. The importance of this period, relative to the total dissolution time, increases at lower superheats. All investigated FeTi70 samples may melt completely within the shell. While the shell period lasts longer for FeTi70 than for the corresponding Ti samples, this fast internal melting yields a net reduction in total dissolution time.status: publishe