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

Abstract

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)O2 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 irradiation on the degradation of thermal conductivity: soluble fission products (FPs), precipitated FPs, radiation 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 calculations (Thermo-Calc V4.1 with the TAF-ID V11 database) were carried out in conjunction. The data provided for each fuel sample consisted of fuel temperature, burn-up, end-of-life (EOL) isotopic composition, molar fraction of the dissolved FPs in the matrix, molar fraction of the oxide, and the metallic precipitated phases, density, produced and released fission gases.In this work, we showed a clear discrepancy between the experimental data and the most widely used thermal conductivity model for irradiated fuels (Lucuta et al., 1996). Lucuta's model was then modified and a new recommendation for FR (U, Pu)O2 fuels was provided. This work demonstrates that by introducing the neodymium content as the parameter driving the thermal conductivity of the matrix, a better agreement between the model and the experimental data can be achieved. This paper discusses the uncertainties related to the irradiation conditions and the data provided by the GERMINAL-Thermo-Calc calculation.