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Advances in Americium-241 processing
Arko, Brian T.
Arko, Brian T.
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2023
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2024-10-18
Abstract
Since its initial discovery less than 80 years ago by Glenn Seaborg \emph{et. al.}, Americium-241 (\ce{^{241}Am}) has found its way into household and industrial applications ranging from smoke detectors to well logging. Although \ce{^{241}Am} continues to grow in demand, US production stopped \emph{circa} 1984 causing stockpiles to dwindle and increased reliance on capricious foreign supplies. Motivated by this vulnerability and the potential to reduce volumes of radioactive waste streams, Los Alamos National Laboratories (LANL) launched a campaign to isolate \ce{^{241}Am} from plutonium in a diverse set of feed stocks using the Chloride Extraction And Recovery Line (CLEAR). Since its inception, CLEAR had produced high-purity \ce{^{241}Am} using Extraction Chromatography (EC) which has demonstrated effectiveness for recovering and purifying \ce{^{241}Am}, but there are serious limitations with EC and how it interfaces with CLEAR.
Among the limitations, the most hindering are (\underline{\emph{1}}) \ce{^{241}Am} breakthrough and low \ce{^{241}Am} recovery, (\underline{\emph{2}}) ill-defined lifespan of EC material from harsh conditions from \ce{^{241}Am} processing, and (\underline{\emph{3}}) appreciable quantities of neptunium-237 (\ce{^{237}Np}, the \ce{^{241}Am} daughter) in product streams. This research is focused on providing the necessary tools needed for CLEAR to be utilized more effectively. First, \ce{^{241}Am} adsorbtion was characterized for the EC material currently used known as the Mckee Resin (\emph{m}-CMPO\textsuperscript{TBP}) in terms of acid and matrix concentrations. Results suggested the \emph{m}-CMPO\textsuperscript{TBP} was outperformed by the industrially-available Rare Earth (RE) and Diglycolamide (DGA) EC materials. To determine longevity, all of these materials were also compared in terms of acid (HCl$_{(aq)}$) as well as $\gamma$-radiation exposure (using a \ce{^{137}Cs} radiation source). The \emph{m}-CMPO\textsuperscript{TBP} material which has been used to process over 150 g of \ce{^{241}Am} in harsh processing environments were compared to the artificial $\gamma$-radiation exposure experiments. Results showed similarities in morphology features (swelling) as well as \ce{^{241}Am} extraction performance but variances in degradation chemistry. The DGA resins were shown to outperform the (\emph{m}-CMPO\textsuperscript{TBP}) on many fronts. Lastly, the oxidation properties of \ce{^{237}Np} (the \ce{^{241}Am} daughter) were characterized in matrices relevant to \ce{^{241}Am} processing. Results suggested Np currently partitions to the 4+, 5+, and 6+ oxidation states in current operations which causes it to leach into Pu and \ce{^{241}Am} product streams. Depending on the matrix identity, the Np oxidation with \ce{NaClO_2} pathway and kinetics changed suggesting that limiting the acid in feed materials while using the \ce{NaClO_2} oxidizing agent may effectively control the Np oxidation. Better defining the operational constraints for CLEAR would equip engineers and technicians to have efficient \ce{^{241}Am} production in order to have better \ce{^{241}Am} yield, defined lifespan for EC materials used, and higher product purity.
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