Criticality Analyses of Enriched Uranium-Hexafluoride Containing Impurities

Due to its chemical and physical properties, enriched UF6 presents several challenges to criticality safety. Recently, criticality safety evaluations of packages for the transport of enriched UF6 have been increasingly subject to comprehensive examinations and verifications by the competent authorities worldwide. In this paper, criticality safety of packages for the transport of enriched UF6 (up to 5 wt.%) containing impurities is investigated. The investigation is based on variation calculations of criticality relevant parameters such as geometry and material composition for individual packages in isolation and arrays of packages. Single packages and infinite 3D arrays of packages were simulated to figure out most reactive arrangements and prove criticality safety for the DN30 overpack designed by DAHER-NCS. Since UF6 in 30B cylinders is under-moderated, the consideration of conservative amounts of moderation and the study of the impact of their geometrical distribution on reactivity are thereby of paramount importance. This involves the determination of all possible moderation sources resulting not only from inherent impurities in UF6 (up to 0.5 wt.% HF) but also from the chemical interaction of uranium hexafluoride with possibly in-leaking water vapor. Once these quantities of moderation have been determined, criticality analysis is performed for a variety of geometrical fuel-moderator distributions by means of conservative calculation models taking into account rigorous accident conditions of transport and effects related to the physical properties of UF6. The criticality safety was proven for the DN30 overpack, even though a very conservative, theoretical approach was taken in all assumptions and such hypothetical configurations may not be likely to be encountered in actual packaging, transportation, and storage configurations. The high degree of similarity between UF6 overpacks with regard to their geometry and manufacturing materials as well as the standardized use of 30B cylinders allows the applicability of the knowledge acquired in this analysis as well as the used methods to criticality safety assessments associated with other overpacks dedicated to the transport of enriched UF6