A METHODOLOGY FOR THE ISOTOPIC CHARACTERIZATION OF NATURAL URANIUM TO BE USED AS REFERENCE MATERIAL FOR THERMAL IONIZATION MASS-SPECTROMETRY

Owing to natural variations in the abundance of 235U and 234U, nuclear grade U3O8 with a natural isotopic composition was analyzed by thermal ionization mass spectrometry (TIMS) and characterized (i.e. mean isotopic ratio values, 95% confidence limit) to be used as isotopic reference material. The mass discrimination bias factor was calculated for the aforementioned situation using isotopic reference materials within the range U-002 to U-015 for the 235U:238U isotopic ratio (R 5 8) and with the isotopic reference materials U-005, U-010, U-015 and U-900 for the 234U:235U isotopic ratio (R 4 5). As the variances obtained for the isotopic ratio measurements were not constant in the investigated range, weighted least-squares regression analysis was applied. The atomic weight of U calculated for the sample was consistent with the accepted value published by the Commission on Atomic Weights and Isotopic Abundances from IUPAC. © 1990.