Influence of the miscibility gap in the evolution of the microstructure in UO2-based fuel doped with Nd
Résumé
During operational conditions of pressurized water reactors (PWR) nuclear fuel is highly irradiated. This process induces many modifications of the initial properties, such as porosity, thermal conductivity or the apparition of a particular structure named High Burn-up Structure (HBS), characterized by sub micrometric grain size and high population of closed pores, among others (1). This work focuses on the evolution of the microstructure of simulant materials to model nuclear fuel under conditions similar to those found in the HBS. The chosen system to study was U-Nd-O, being Nd used as dopant since it is the most abundant lanthanide fission products. At low temperature and low concentrations of Nd, the system is partially miscible and presents a region where it is biphasic. This region is formally called as miscibility gap. Previous studies carried out on the U-Nd-O ternary system highlighted that the miscibility gap at room temperature exists when the concentration of Nd is higher than 9 at% (2,3). With rising temperature, the solubility increases and the biphasic system becomes monophasic. The objective of this paper is to analyze the system when it is submitted to a strain energy that appears when the initially homogeneous phase at elevated temperature crosses a biphasic domain at a lower temperature (enters the miscibility gap). Thus, the most salient feature of this study will be to observe if this internal strain would produce dislocations which would contribute to the formation of the HBS. In the current work, three different samples with 4, 17 and 25 at% of Nd were studied. The evolution of the crystalline structure in temperature and the presence of dislocations at room temperature as a consequence of phase separation are analyzed through means of High Temperature-X-Ray Diffraction (HT-XRD), High-Temperature Scanning Electron Microscopy (HT-SEM), and Transmission Electron Microscopy (TEM). Experimental results are confronted to thermodynamic modelling calculated through the CALPHAD method using the Thermo-Calc code. In this paper a first approach on the evolution of the microstructure as a function of the concentration of Nd is also proposed.