DETERMINATION OF URANIUM ISOTOPE RATIOS IN UF_6 BY USING SWAB SUBSAMPLES: A NEW RAPID TURNAROUND APPROACH FOR URANIUM ENRICHMENT VERIFICATION

摘要

The International Atomic Energy Agency (IAEA) verifies the declared U-235 content in uranium materials under safeguards. High-precision determination of uranium isotope ratios of gram-sized UF_6 samples is routinely made at the IAEA's Nuclear Material Laboratory (NML). The entire process of UF_6 sample collection, analysis, and data evaluation usually takes several weeks. Due to strict regulations for the safe transport of radioactive material, the most time consuming component of the process is usually the sample shipment. This study assessed the feasibility of using small cotton swabs to subsample the UF_6 samples immediately after their collection in the facility. Swab subsamples with uranium quantities below excepted package limits, when properly packed, may be hand carried by inspectors, thus significantly reducing the shipment time component. Still, such subsample contains an amount of uranium sufficient for mass spectrometric analysis. For analysis of swab subsamples, multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) was used. In addition, a large-geometry secondary ion mass spectrometry (LG-SIMS) was utilized to assess the heterogeneity of uranium isotope composition on the particle level. For a proof of principle, a set of gram-sized UF_6 samples was analyzed by the standard technique in NML, which is multi-collector thermal ionization mass spectrometry (MC-TIMS). Determination of U-235 enrichment was carried out using the total evaporation method, while the abundances of uranium minor isotopes (U-234, U-236) were measured using the modified total evaporation method. The U-235/U-238 ratios measured from the swab subsamples by MC-ICPMS and from the bulk samples by MC-TIMS were in agreement well within International Target Value limits for all analyzed samples except one, in which the MC-ICPMS results were systematically higher than the TIMS values by approximately 0.16% relative. The typical relative uncertainties of U-235/U-238 ratios measured by MC-ICPMS were in the range of 0.10% to 0.25% (k=2). The U-234/U-238 ratios measured in swab subsamples by all methods were in good agreement, taking into account the respective measurement uncertainties. The typical relative uncertainties of U-234/U-238 ratios measured by MC-ICPMS were in the range of 1.0-1.5% (k=2). The measured U-236/U-238 ratios were below 10~(-7).