50 YEARS OF URANIUM ISOTOPIC REFERENCE MATERIALS AT JRC-GEEL

The history of uranium isotopic reference materials certified at JRC-Geel and provided to customers during the last 50 years is reviewed, by presenting the methods for preparation and certification as well as the most relevant applications for several examples of certified reference materials (CRMs). These include the IRMM 019 029 series of UF6 (uranium hexafluoride) materials and several series of uranium nitrate solutions, namely the IRMM 072, IRMM 073, IRMM 074, IRMM 075, IRMM 183 187, IRMM 2019 2030, IRMM 3636a,b and the IRMM 3000 series.  The IRMM-3000 series of HEU materials is ideal for investigating or verifying the linearity of the 235U enrichment scale between 20 % and 90 % for various types of isotope mass spectrometers, such as TIMS/MTE, TIMS/DS, ICP, MC-ICPMS, with various detection systems and measurement methods. This is illustrated in the Explanatory Figure (see Figure 1 below), by using all available verification measurements performed for the IRMM 3000 series solutions (blue filled squares), performed by JRC-Geel (TIMS/MTE and TIMS/DS), the IAEA (TIMS/MTE and MC-ICPMS) and ORNL (TIMS/MTE and MC-ICPMS). As a conclusion regarding the application of the IRMM-3000 series for a variety of modern mass spectrometers and methods, the verification of 235U/238U ratios for the enrichment scale from 20 % to 90 % can be confirmed at a level of better than 0.025% for both uncertainties and accuracy. The verification of the enrichment scale has been recently extended to uranium oxide micro-particles analyzed by LG-SIMS. For this purpose, uranium oxide (U3O8) particles were generated from IRMM 3050 and IRMM-3090 solutions using a VOAG (vibrating orifice aerosol generator) at the Forschungszentrum Juelich (Juelich/Germany). Verification measurements were performed by the HIP LGSIMS at the University of Heidelberg (Germany), the IAEA and by MC-ICPMS at ORNL, and confirmed the isotopic composition of the FZJ-3050P and FZJ-3090P particles to be in agreement with the certified isotopic compositions of the IRMM-3050 and IRMM-3090 solutions (blue circles). The certified isotope ratios for all of these above mentioned isotope reference materials from JRC-Geel are traceable to the SI via the gravimetrical preparation, either directly through gravimetrical mixing of highly enriched base materials, or indirectly by using the existing gravimetrically prepared reference materials for calibration of the mass spectrometers used for the certification measurements. Due to developments of isotope mass spectrometers and standardized analytical methods during the past decades, the performance of certification measurements in terms of precision, accuracy and uncertainties has improved significantly. This has led to a comprehensive set of state of the art uranium isotopic reference materials offered by JRC-Geel, which is available for nuclear safety and security purposes but also much appreciated in the scientific community. JRC-Geel nuclear reference materials are tailored to support Euratom and international safeguards 1  but are equally suited for measurements in nuclear forensics and environmental sampling. They support EU policies beyond the nuclear applications by reaching out to earth sciences, geo- and cosmochemistry being an intrinsic component for research in these fields. The needs for the nuclear mass spectrometry and nuclear safeguards communities are regularly reviewed among users and producers at special technical meetings at the IAEA, INMM, ESARDA and at the JRC.