[en] The method of determination of U in ²³⁵U slag by X-ray-fluorescence (XRF) analysis is developed. U in acidolysis solution and dregs of ²³⁵U slag are determined by thin sample technique and standard added method. The total U in slag is given. The results of XRF are agree well with those obtained from FIA and high performance liquid chromatography (HPLC). The results are compared with two non-destructive assay (NDA) methods, the calibration data are provided to DNA