Simultaneous uranium/plutonium separation and direct isotope ratio measurements by using CO_2 as the gas in a collision/reaction cell based MC-ICPMS

摘要

The measurement of U and Pu isotope ratios is of prime interest in nuclear research fields particularly for the management of radioactive waste. One of the major drawbacks in analysing Pu isotopes is the occurrence of ~(238)U-~(238)Pu isobaric interference. In this work we propose to suppress this interference by adding CO_2 as a reactive gas in the collision-reaction cell of the MC-ICPMS. We have demonstrated that the different reactivities of Pu and U towards CO_2 gas allow accurate and precise Pu and U isotopic measurements on Pu~+ and UO_2~+ forms, by using two different CO_2 gas flow rates: 0.4 mL min~(-1)for Pu~+ and 1.5 mL min~(-1) for UO_2~+. Parameters like optimum CO_2 gas flow in the cell, oxides isotope abundance, instrumental mass bias and memory blank, were carefully evaluated and corrected. The developed analytical method was validated using a transmutation ~(235)U target sample irradiated in the Phenix French fast neutron reactor. A good agreement between isotope ratios of U and Pu measured with CO_2 in the cell and U and Pu measured after chemical separation was obtained. The relative difference of the average isotope ratio values between the chemically purified fraction and the direct measurements using CO_2 was found to be 0.38% and 0.11% for U and Pu, respectively.