Uranium Hexafluoride Assay Measurement Using Femtosecond Laser-Induced Breakdown Spectroscopy

Most modern uranium enrichers use centrifuges to separate ²³⁵U [Uranium 235] and ²³⁸U [Uranium 238] in the gaseous form, UF₆ [uranium hexafluoride]. Monitoring the enrichment level of the material being produced in a Gaseous Centrifuge Enrichment Plant (GCEP) or in a single centrifuge is important to the enricher as well as those tasked with monitoring the enrichment activities of declared facilities, such as the International Atomic Energy Agency (IAEA). Many enrichers currently select mass spectrometry as the only alternative that can reliably provide the accuracy necessary for optimizing the performance of their centrifuges during research and development or certifying the resulting products of their commercial enrichment plants to their customers. Unfortunately, this method is costly in terms of personnel requirements, equipment investment, and difficulty obtaining a continuous measurement of the enrichment. Continuously conducting measurements of UF₆ enrichment in a foreign facility essentially precludes mass spectrometry due to the costs noted above and the strong desire of the foreign enricher not to be subjected to destructive sampling of their enriched material. The commercial enrichment industry has been able to work around the shortcomings of the measurement techniques currently available. However, nuclear security interests have been sufficiently underserved that significant research and development is being invested to provide cheaper, more accurate, robust systems which can be installed in declared enrichment facilities to provide a continuous monitor of the enrichment activities conducted within those facilities.

The research developed methods for determining UF₆ assay using a femtosecond laser-induced plasma. The resulting methods yielded assay measurements within 0.28% of the actual ²³⁵U content with a standard deviation of 0.90%. The research also includes results of investigation of High Order Generalized Singular Value Decomposition (HOGSVD) as a means of analyzing spectral data for assay measurement. HOGSVD achieved 95.15% accuracy in spectral classification testing. Additionally, ARESLab models were built using the spectral data collected during this research and ARESLab models built with HOGSVD-based numerical features. The HOGSVD-based model’s ability to predict assay is compared with the spectra-based model. HOGSVD was also demonstrated to be of value identifying additional isotopic shifts for inclusion in multivariate fitting algorithms for determining UF₆ assay. The contribution of this research is an accounting of the effectiveness and applicability of the femtosecond laser and HOGSVD to the measurement of UF₆ assay through Laser-Induced Plasma Emission Spectroscopy.