Preparation and certification of the uranium nitrate solution series IRMM-2019-2029: revision of the IRMM-019-029 series of UF6 materials : Certified reference materials for the U isotopic composition

This report describes the certification of the IRMM-2019-2029 series of uranium nitrate solution reference materials and the revision of the uranium hexafluoride (UF6) reference materials series IRMM-019-029, both certified for the uranium isotopic composition. The certified values and their uncertainties were assigned following ISO 17034 [1], ISO Guide 35 [2] and the Guide to the Expression of Uncertainty in Measurement [3]. The IRMM-019-029 series of eleven uranium hexafluoride (UF6) reference materials was originally produced and certified in the 1980's-1990's. For the first re-certification performed in 2014, the UF6 materials were converted into eleven uranium nitrate solutions to perform the homogeneity and characterisation studies. These materials were characterised in compliance with ISO 17034 [1] 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, 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 were also confirmed by Gas Source Mass Spectrometry (GSMS) measurements using the original UF6 gases [4, 5]. For the certification of the isotopic composition of the IRMM-2019-2029 series each of the remaining uranium nitrate solutions used for the certification of IRMM-019-029 was diluted using one molar nitric acid to a concentration of about 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" (PCM) by TIMS using the MTE and DS methods were performed for three ampoules for each of the reference materials. The results for the isotopic composition from the process control measurements on these uranium nitrate solutions agreed well with the certified values for IRMM-019-029 series of UF6 materials. Therefore no separate new characterisation and homogeneity studies were performed, and the certified values for IRMM-019-029 were assigned as well to the uranium solution reference material series IRMM-2019-2029. Moreover, certified values for both the IRMM-019-029 series of UF6 reference materials and the IRMM-2019-2029 series of uranium nitrate solution reference materials were established using new published values for the atomic masses a