Initial Assessment of TeF₆ Adsorption

Research and development that supports the management of off-gases from nuclear fuel reprocessing has historically been focused on the off-gas streams that arise from aqueous reprocessing technology. However, as Gen-IV reactor development pathways move toward deployment, alternative spent nuclear fuel (SNF) processing and disposition pathways have been considered more actively. This work is focused upon aspects of fluoride volatility (FV) processing. The versatility of this method for deployment against multiple types of spent fuel encourages the continued advancement of both the primary separations processes and the secondary processes, including waste treatment, material control and accountability, and engineering designs. Recent work noted that TeF₆, the most highly volatile fluorinated compound produced during FV processing, did not have a clear abatement technology recommended in the literature. Thus, this work performed scoping tests on activated alumina and copper shot to assess whether they could be used to remove TeF₆ from gas streams that bear F₂. Previous work on this topic was not well described in the literature and was not performed with excess F₂ in the stream as would be typical of spent fuel processing via FV. To support an understanding of the concentration of TeF₆ contacting the adsorbent beds, a series of preliminary testing identified the TeF₆ production rate and the equilibrium concentration of TeF₆ in the gas stream contacting the adsorbent. Excess F₂ was determined to not affect the ability of activated alumina to remove TeF₆ from the gas stream quickly and completely. A determination of whether excess F₂ affected the distribution depth of Te in the sorbent bed is still pending analysis of the used sorbent. Literature suggests that when activated alumina is near saturation, TeF₆ could migrate from the sorbent bed. Future testing should investigate this possibility. Copper metal did not adsorb TeF₆ in the presence of F₂ across the sorbent temperature range of 50 to 335°C. F₂ was fully removed by the copper bed. Although preliminary thermodynamics would indicate adsorption to be energetically favorable, other factors that impact adsorption (e.g., slow kinetics, excess fluorine on the copper surface, an unfavorable transition state) are likely preventing the adsorption of TeF₆ at an easily measurable rate. The work also investigated TeF₆ production from multiple forms of Te in the temperature range of 100 to 250°C. No previous study had assessed the initial reaction rates for this process. A carefully designed study allowed determination of the activation energy for the production of TeF₆ from Te metal. The substantial amount of data collected during this study merits analysis beyond what is described here. A more in-depth kinetic analysis will be pursued. Additional analytical results will provide the ability to benchmark adsorption coefficients for TeF₆, understand the distribution of TeF₆ within the alumina bed, and better understand the effect of F₂ partial pressure on Te fluorination. The data from this report, as supplemented by these additional analyses, will be submitted to a peer-reviewed journal.