Analysis of the behavior of ATF micro-cell UO2-Mo and UO2-Cr fuels under irradiation: Thermomechanical effects of an enhanced conductivity

• Various 3D micro-structures of micro-cells CERMET are modeled • The effect of micro-cells shape and defects on the CERMET effective conductivity is investigated • Effect of the enhanced thermal conductivity on the thermomechanical behavior of the fuel rod is assessed • Simulations show that CERMET fuel pellets are expected to have a reduced tendency to crack and to expand • Thermodynamic based calculation of the evolution of the oxygen potential and phase composition under irradiation This work studies the behavior of micro-cell UO 2 fuels by numerical simulations performed at different scales, the metallic additive being molybdenum or chromium. At the scale of a volume element, the effective conductivities of UO 2 -Mo and UO 2 -Cr mixtures are modeled to understand the experimental results reported in [1]. This analysis employs homogenization techniques combining theoretical approaches [2] with microstructures simulations. These simulations allow the quantitatively study of the effects of a discontinuity in the metal network. Still at the scale of a representative volume element, thermodynamic calculations are used to assess the stability of the constituents at various temperatures. Then, at the scale of a fuel rod, the ALCYONE fuel performance code is used to investigate the impact of increased thermal conductivity, representative of CERMET conductivity, on the fuel behavior under both nominal and transient irradiation conditions. As expected considering the modeled conditions, these CERMET fuels exhibit a behavior similar to standard UO 2 but reach a lower temperature under comparable irradiation conditions. This results into a reduced tendency to expand and to crack as well as a delayed gap closure kinetics in nominal conditions. In transient conditions, these lower temperature variations increase significantly the melting margins and lower the pellet-cladding mechanical interactions.