Year
2004
Abstract
IRMM-072 was a highly successful set of (15) mixtures of the uranium isotopes 233U, 235U and 238U in which the isotopic ratio n(235U)/n(238U) was held constant and the ratio n(233U)/n(235U) was varied for each member of the set from 1.0 down to 2.10-6. The reference material was popular for calibration of mass-spectrometer responses and has been exhausted. At IRMM, the replacement series is being prepared and certified. This will be IRMM- 074. The enriched isotopic uranium materials have been purified using identical methods and chemicals, but in separate clean glove-boxes to eliminate crosscontamination. The oxides undergo a sintering process to convert them to U3O8 simultaneously in a controlled environment and in the first stage, 235U and 238U oxides are weighed together and dissolved in nitric acid. The 233U oxide is dissolved separately. The series is produced by careful weighing of amounts of the two solutions. The uncertainties contributing to the final uncertainties of the isotopic ratios are the weighing errors, the measured impurities in each isotopic material, the stoichiometry of the oxides and the isotopic abundances of each. The main uncertainty is expected to be the measurement of the chemical impurities and for this reason a number of measurement techniques are being employed. The methods for the preparation and mixing will be described and the progress towards certification reported.