Estimation of the isotopic composition of the fuel is an important task for the nuclear industry. Several basic techniques are known for determining the isotopic composition of heavy elements, such as uranium, both more accurate and longlasting, and express. For production with a large number of samples, the method of total complete evaporation is optimal, providing a high degree of automation and acceptable accuracy of analysis. Total Full evaporation also allows to reduce the qualification level in sample preparation, thereby reducing errors in the results. In this work, standard samples of the composition NBS U010, NBL U030a, and NBL U050 were tested by the method of complete evaporation using a Phoenix thermal ionization mass spectrometer (Isotopx, UK). Statistical processing of the results of a series of experiments showed that, in comparison with others, the total complete evaporation method better meets the accuracy requirements for determining the 235 U/ 238U isotopic ratio and is consistent with published data.