[en] The determination of small amounts of ²³⁵U in samples having a volume up to 200 l within the scope of the state safeguards system is described. Two methods of measurement have been realized: irradiation of the sample by neutrons from an Sb-Be source which induce nuclear fissions in the ²³⁵U, and passive measurement of the gamma-rays accompanying the alpha decay of ²³⁵U by means of an absorption-corrected gamma-scanner. These methods allow to detect minimally 0.1 g of ²³⁵U during a measuring time of 30 min with an error of 10% depending on the material in which the uranium is embedded. The neutron method is simulated with a Monte-Carlo model for optimizing the design parameters and calculations of correction factors for interpretation of the measurements. The results of computation and measurement are compared with each other. (author)