Influence of yttria doped thoria microstructure on its compatibility with liquid sodium
Résumé
Among the different concepts developed in the framework of the fourth generation of nuclear reactors, Sodium cooled Fast Reactors (SFR) are particularly studied in France. In such concept, the sodium coolant must contain low and specified oxygen content in order to limit the corrosion of the core’s steel structures (claddings…). In this aim, electrochemical sensors provide a fast and specific measure complementary to others devices (such as plugging-meter). Hence, the solid electrolyte employed should be compatible with sodium at high temperature (about 400°C), mechanically resistant and pure ionic conductor. Then, its microstructure must be carefully controlled, with low grain size, high density, and low impurity content.
The preparation of Th1-xYxO2-x/2 ceramics was then undertaken through oxalic co-precipitation and subsequent heat treatment at 600°C. The influence of the yttrium content on the material microstructure was further investigated to yield sintering maps by correlating the average grain size and the relative density values. Yttrium doping was found allow full densification up to 98%TD and to prevent grain growth, with the lowest yttrium content exhibiting the highest grain size.
The study of the compatibility of the ceramic with liquid sodium was undertaken at 500°C during several hundredth hours. In these experimental conditions, operando monitoring of the microstructure evolution was performed through SEM observations of specified locations of the ceramic (i.e. surface, fracture face). If no sign of corrosion was evidenced at the surface of the samples, the fracture showed two different habits (trans- and intergranular). The latter, located at the surface, indicates a penetration of sodium through the grain boundaries. The thickness of the intergranular zone then allowed us to provide a first estimation of the penetration rate of sodium within the electrolyte. Also, no secondary phase such as Na2ThO3 was detected, even if its formation should be thermodynamically favoured in our working conditions.
Electric characterizations were also undertaken before and after sodium corrosion. While the measurements performed on pristine materials were mostly found in good agreement with those reported in the literature, with a maximum of the conductivity observed between 8 and 15 mol.% of yttrium, those conducted post-mortem revealed a significant variation of the conductivity, especially for the grain boundary contributions.
All these tests confirmed the importance of mastering the electrolyte ceramic from its preparation to its operation within liquid sodium at high temperature. Additional experiments now must be undertaken in real conditions to measure accurately the corrosion rate and qualify the material for its use in the reactor.