Evaluation of the mobility behaviour of radionuclides under highly saline and alkaline conditions is a major concern for the performance assessment of radioactive waste disposal. The aim of this study was to determine the effect of up to 2.8 mol/kg$_{solution}$ content of NaNO$_3$, on the solubility and the retention of U(VI) at 22 °C onto a hardened cement paste (HCP) prepared from ordinary Portland cement (CEM I). To avoid the interference of the high salt concentration and ionic strength,and because of the expected low solubility of uranium under such alkaline conditions, time-resolved laser fluorescence spectroscopy (TRLFS) was selected to accurately measure U(VI) concentration in solution using the standard addition method in 85% H$_3$PO$_4$. This allows both limiting the dilution and matrix effects and determining the resulting [U(VI)] in solution with acceptable precision for the distribution factor (Rd) in both sorption and desorption experiments. The operational solubilitylimit measured at high ionic strength lowered by a factor of three compared to the reference cementitious condition, and its Rd values decreased by a factor ca. four. The sorption of U(VI) appears to be reversible under these conditions.