Behaviour of cesium in zirconia-based nuclear fuel

A zirconia-based nuclear fuel for plutonium transmutation is being developed at PSI. Its composition would be ZrO2-10%YO1.5-7%ErO1.5-15%PuO2. The solubility and retention of cesium as well as formation of its zirconate need to be assessed, because cesium has been identified as a potential safety relevant fission product. Therefore, a critical study of the system Zr-Cs-O and an approach to describe the structural build-in of cesium ions in zirconia were performed. For comparison, the system U-Cs-O was selected in order to characterize the behaviour of cesium in uranium-free and uranium-based fuel. An extensive literature review was used to assess thermodynamic data for binary and ternary phases and yielded to a basis for calculating the phase relationships by using the software THERMO-CALC. The results are presented in form of isothermal sections (Zr-Cs-O and U-Cs-O at 1000 and 2000 K) and pseudobinary temperature/concentration sections (ZrO2-Cs2O, UO2Cs2O). In addition to the characterization of cubic stabilized zirconia, the formation of other ternary phases like orthorhombic Cs2ZrO3 and tetragonal Cs2UO4 was investigated by calculating ΔGreaction-curves for different oxygen partial pressures. It has already been shown that zirconia can be stabilized by rare-earth oxides, and that their solubility as well as lattice parameters and ionic conductivity show a relationship with the dopant size. Further calculations to clarify the behaviour of alkali- and earth-alkali elements in zirconia are ongoing. (author) 4 refs