Solid-solid conductance in nuclear fuel: An assessment of it in-code models performance

In the nuclear industry the preservation of cladding integrity should be always ensured under any expected or unexpected event. Given the current trend towards higher fuel burn-up, conditions of fuel-rod performance are becoming more demanding and, at the same time, safety assessments are moving from a conservative to a best-estimate approach, where more accurate models are used to reduce too large conservative margins. Heat transfer from the fuel rod plays an essential role for an accurate description of fuel performance.The present paper is focused on heat transfer from the fuel to the inner side of the cladding under high burn-up conditions. Fundamentals of fuel-to-clad heat transfer and their implementation in “steady-state” performance codes have been reviewed. Major variables in the process have been identified (burn-up, pressure contact, interfacial temperature, morphology), and the main hypotheses and approximations used in each code to estimate them, have been examined and compared. Finally, model predictions have been compared to experimental data taken from the open literature. Even though the reduced number of data does not allow to settle general conclusions about model performance, substantial discrepancies have been found in their predictions. They have highlighted two important facts: the large scattering of already available data base and the scarcity of data under anticipated high burn-up conditions.