New recommendation for the thermal conductivity of irradiated (U, Pu)O2 fuels under Fast Reactor conditions. Comparison with recent experimental data.

New experimental results obtained at the Joint Research Centre in Karlsruhe (JRC-Karlsruhe) within the European Sustainable Nuclear Industrial Initiative Plus project (ESNII + ) were used to develop an updated recommendation for the thermal conductivity of (U, Pu)O 2 fuels irradiated in fast reactor (FR) conditions. Fresh and irradiated FR fuels were characterised, and their thermal diffusivity and heat capacity were measured by the laser flash technique. The thermal conductivity of those fuels was calculated, using hydrostatic density measurements when available or using calculated density. The thermal conductivity model, developed in this work, describes different effects induced by irra- diation on the degradation of thermal conductivity: soluble fission products (FPs), precipitated FPs, ra- diation damage, and porosity. Those effects are considered in separate corrective factors, which depend on the chemical composition of the irradiated fuel, its microstructure, and the temperature. To estimate these parameters, thermomechanical calculations (PLEIADES-GERMINAL V2) and thermochemical calcula- tions (Thermo-Calc V4.1 with the TAF-ID V11 database) were carried out in conjunction. This paper discusses the uncertainties related to the irradiation conditions and the data provided by the GERMINAL-Thermo- Calc calculation.JRC.G.I.2 - Nuclear Material Researc