A set of six samples, collected worldwide from various uranium ore mining facilities, was analysed for uranium isotopic composition by high accuracy isotope mass spectrometry. The goal of this article was twofold: to measure isotopic variations between samples of different geographical origin and to produce calibrated isotope ratios with the smallest achievable uncertainty las defined according to the ISO Guide to the Expression of Uncertainty in Measurement). In the first step, the molar ratio of the isotopes U-235 and U-238, n(U-235)/n(U-238), was measured using a UF6-gas-inlet isotope mass spectrometer (VARIAN MAT 511). This instrument was calibrated against gravimetrically prepared synthetic isotope mixtures thus allowing SI-traceable measurements to be made. The ratios of the "minor isotopes" to U-238 [n(U-234)/n(U-238) and (U-236)/n(U-238)] were determined in a second step using a thermal ionisation mass spectrometer with high abundance sensitivity (Finnigan MAT262-RPQ-PLUS). The mass-fractionation correction was done internally using the result of the (U-235)/n(U-238) measurement. As a result, the complete measured uranium isotopic composition is traceable to the SI system. For all ratios n(U-234)/n(U-238), n(U-235)/n(U-238), and n(U-236)/n(U-238) significant differences for samples of different origin were found. Regarding the n(U-236)/n(U-238) results, only two samples, one of them from the Oklo reactor in Gabon, showed significant presence of U-236. For all other samples an upper limit for n(U-236)/n(U-238) of about 6 x 10(-10), mainly dependent on the instrumentation, was found. As a result of this study we propose values for the isotope abundances of natural uranium for the "Best Measurement from a Single Terrestrial Source" and the "Range of Natural Variations" in the IUPAC-table of the "Isotopic Composition of the Elements." (Int J Mass Spectrom 193 (1999) 9-14) (C) 1999 Elsevier Science B.V.
