Improvements in the determination of uncertainties of isotopic ratios for uranium samples thanks to IAEA Round Robin
Résumé
In the field of nuclear energy, the role of analytical chemistry is growing thanks to an increasingly widespread usage of powerful instruments. Among analytical instruments, ICP/MS equipped with multiple collectors (MC ICP/MS) allows measurements of isotopic composition of various elements, with a very low uncertainty. During MC ICP/MS measurements, all ion currents are biased by mass discrimination, and other effects (detector gain, abundance sensitivity background, dead time). So, various corrections must be applied to all measurements to determine accurate and precise isotope ratios. The use of certified isotopic reference materials (IRM) associated to a sample standard bracketing (SSB) method allows us to determine accurate isotope ratios. The ratios are determined taking into account in particular the mass discrimination, the uranium hydride ions, blanks and isotopic reference materials measurements. The uncertainties associated to those ratios are then calculated. For example, in the case of uranium, atom ratio of $^{235}$U to $^{238}$U is measured with a relative uncertainty better than 0.1%. For the $^{234}$U and $^{236}$U isotopes, the uncertainties associated to the atom ratios of $^{234}$U to $^{238}$U and $^{236}$U to $^{238}$U are greater due to the lowest abundances of those isotopes. The performance of the laboratory to determine uranium isotope ratios and the uncertainties associated is evaluated by participating to inter-laboratory tests.The nuclear material Round Robin proficiency testing scheme (NM RORO), organized by the International Atomic Energy Agency (IAEA) is a reliable and objective tool for assessing and monitoring analytical performance of laboratories in the area of nuclear material analysis. We have participated to three consecutive NM RORO (2015, 2017 and 2019). These RORO allow us to testify the accuracy of the SSB method to determine uranium isotope ratios. Concerning the calculation of the uncertainties, the 2015 NM RORO helped to improve the determination of the uncertainty associated to the 236U/238U ratio (which was beyond-acceptable-range zeta-score). The 2017 NM RORO lead to good results for the determination of uranium ratios and their associated uncertainties and allows us to validate our method.
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