An improved method was recently developed for the isotopic analysis of single-reference uranium oxide particles for nuclear safeguards. This method is a combination of analytical tools including in situ SEM micromanipulation, filament carburization and multiple ion counting (MIC) detection, which is found to improve sensitivity for thermal ionization mass spectrometry (TIMS) isotope ratio analysis. The question was raised whether this method could be applied for the detection of nuclear signatures in real-life particles with unknown isotopic composition. Therefore, environmental dust was collected in different locations within a nuclear facility. The screening of the samples to find the uranium particles of interest was performed using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) detector. The comparison of the measurement results to reference data evaluated by international safeguards authorities was of key importance for data interpretation. For the majority of investigated particles, detection of uranium isotopic signatures provided information on current and past nuclear feed operations that compared well with facility declarations.

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Uranium hexafluoride gas source mass spectrometry at IRMM is based on two foundations, firstly the operation of a UF 6 gas source mass spectrometer (GSMS) and secondly the preparation of primary UF 6 reference materials, which were converted from gravimetrically prepared mixtures of highly enriched oxides of 235 U and 238 U. Recently a new GSMS for uranium isotopic measurements using UF 6 gas, the "URANUS" from Thermo Fisher, was installed at IRMM, which also allows measurements of the so-called "minor" isotope ratios n( 234 U)/n( 238 U) and n( 236 U)/n( 238 U). In this paper the design and the implementation of measurement techniques for the new URANUS GSMS are described. This includes the "single standard" and the "double standard" (DS) method as well as the newly developed "memory corrected double standard" method (MCDS). This required a detailed investigation of memory effects within the GSMS instrument, in particular regarding the dependence of memory effects on the isotope ratios of samples and standards. The results of this study led to new recommendations for the selection of the standards for a given sample and for suitable measurement procedures. The measurement performance for the "major" isotope ratio n( 235 U)/n( 238 U) as well as the "minor" isotope ratios n( 234 U)/ n( 238 U) and n( 236 U)/n( 238 U) is presented and compared with other mass spectrometric techniques. With the installation and validation of the new URANUS GSMS instrument IRMM has established two new complementary techniques for measuring the full isotopic composition of uranium samples. UF 6 GSMS in combination with the MCDS method is considered the preferred technique for samples in the UF 6 form and for smaller uncertainties for measurements of the major ratio n( 235 U)/n( 238 U), while thermal ionization mass spectrometry (TIMS), in combination with the "modified total evaporation" (MTE) method as well as ion counting and high abundance sensitivity for the detection of 236 U, provides a superior measurement performance for the minor isotope ratios n( 234 U)/n( 238 U) and n( 236 U)/n( 238 U).

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