Simultaneous uranium/plutonium separation and direct isotope ratio measurements by using CO2 as the gas in a collision/reaction cell based MC-ICPMS

Abstract

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 ²³⁸U–²³⁸Pu isobaric interference. In this work we propose to suppress this interference by adding CO₂ 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 CO₂ gas allow accurate and precise Pu and U isotopic measurements on Pu⁺ and UO₂⁺ forms, by using two different CO₂ gas flow rates: 0.4 mL min⁻¹ for Pu⁺ and 1.5 mL min⁻¹ for UO₂⁺. Parameters like optimum CO₂ 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 ²³⁵U target sample irradiated in the Phenix French fast neutron reactor. A good agreement between isotope ratios of U and Pu measured with CO₂ 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 CO₂ was found to be 0.38% and 0.11% for U and Pu, respectively.