Certification of the New Uranium Nitrate Solution Isotopic Reference Materials: the IRMM-2019-2029 and IRMM-3000 Series

This paper firstly describes the certification of the uranium isotopic composition in the IRMM-2019-2029 series of uranium nitrate solution isotopic reference materials. Originally the IRMM-019-029 series of eleven UF6 reference materials was certified in the 1980's-1990's. For the re-certification performed in 2014, the UF6 materials were converted to uranium nitrate solutions for homogeneity and characterisation studies. The uranium isotopic composition of these solutions were characterised by Thermal Ionisation Mass Spectrometry (TIMS), by taking advantage of newly established measurement procedures such as the Modified Total Evaporation (MTE) and Double Spike (DS) methods (ASTM C1832 and C1871a, respectively), and using a new set of certified uranium isotope reference materials, which were prepared by gravimetrical mixing of highly enriched 233U, 235U, 236U and 238U oxides or solutions. The results of the characterisation measurements for the IRMM-019-029 series were also confirmed by Gas Source Mass Spectrometry (GSMS) measurements using the original UF6 gases.For the certification of the IRMM-2019-2029 series, each of the uranium nitrate solutions remaining after the certification of IRMM-019-029 was diluted to a concentration of 2 mg uranium per g solution and dispensed into cleaned screw-cap quartz ampoules. Each ampoule contains 5 mL of solution. Subsequently, process control measurements of the uranium isotopic composition by TIMS using the same methods were performed on three ampoules of each of the reference materials. The results agreed well with the certified values for the IRMM-019-029 series of UF6 materials. Therefore, the certified values for IRMM-019-029 were also assigned to the uranium solution reference material series, IRMM-2019-2029. The materials are intended for the calibration of instruments and methods and for quality control purposes. The concentration of 2 mg uranium per g solution, although not certified, is ideal for TIMS/MTE measurements and the solutions may be diluted for TIMS/TE and MC-ICPMS analyses.Secondly, the certification of the uranium isotopic composition in the IRMM-3000 series of isotopic reference materials is ongoing at JRC-G.2, in collaboration with IAEA and ORNL. This series consists of five uranium nitrate solutions with 235U enrichments ranging from 20 % to 90 %. Preliminary results of the certification process will be presented.