Combined HRGS-TIMS Method for Uranium Concentration Measurements in Solutions

The International Atomic Energy Agency (IAEA) Safeguards Analytical Laboratory includes the Nuclear Material Laboratory (NML), which is located in Seibersdorf, Austria. The core mission of this laboratory is the treatment and measurement of nuclear material samples collected during IAEA inspections. The majority of nuclear material samples submitted to the NML are relatively pure product samples with high uranium concentrations that are characterized with high accuracy using destructive analysis (DA) methods e.g., potentiometric titration and isotope dilution mass spectrometry (IDMS). In contrast to the product samples, the NML also receives samples taken from waste streams. These samples are collected for material characterization purposes and therefore do not require the low uncertainties typical of DA methods. Such samples typically represent solutions with uranium contents below 1 wt. % and often contain precipitates or some other solid fraction that need to be filtered and treated separately. This paper presents a new method that was developed at NML for treating this type of samples. The method combines the Non-Destructive Assay (NDA) result acquired using a high-resolution gamma spectrometry (HRGS) system for the U-235 content with the enrichment measurement result obtained using thermal ionization mass spectrometry (TIMS). The uncertainty of the new HRGS-TIMS method is approximately 5%, and is primarily limited by the uncertainty of the HRGS measurement. Here we present a summary of the method and the comparative analysis results from multiple inspection samples using the routine IDMS method and the new HRGS-TIMS approach.