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López Poyato, José ManuelSupervisor

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The catalytic generation of tritium-substituted methanes and their analysis using Raman spectroscopy and mass spectrometry

Date read: 2024-01-30 {_iPublic_txt_centro_}: | ID: 10486/713420

Author: Díaz Barrero, Deseada

Director: Telle, Helmut H. ; López Poyato, José Manuel ; Lopez Poyato, J Manuel

Abstract

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química Física Aplicada. Fecha de Lectura: 30-01-2024 (General); Over many years tritium, the radioactive isotope of hydrogen (-decay), has evolved into a key tool for identifying and understanding a plethora of processes in, e.g., organic chemistry and metabolic studies in biology, namely in the form of tritium-labeling (or “tagging”). While these applications utilize only trace amounts of tritium, there are now also scientific and industrial uses in which substantial amounts of tritium are encountered in circulating gas streams, such as in the Karlsruhe Tritium Neutrino (KATRIN) experiment and in nuclear fusion developments for the International Thermonuclear Experimental Reactor (ITER) and the proposed class of DEMO reactors. The interaction of these high fluxes of tritium leads to chemical / radiochemical interactions with the flowconfining materials of tubes and vessels: amongst other molecular species, tritium-substituted methane and higher-order hydrocarbon compounds are generated. While their existence is well known, understanding of their (radio-) chemistry is still incomplete. In this research isotope-substituted methane, CQ4 (Q=isotopes of hydrogen), has been investigated, specifically the isotopic family of CTxH(4-x) molecules. These compounds have been synthesized catalytically in the reactors of the CAPER facility, the tritiated waste purification and disposal system at the Tritium Laboratory Karlsruhe (TLK). During several synthesis campaigns, using flows of methane (CH4) and tritium (T2), samples of significant quantity (up to about 5 liter at ~850mbar) were produced, under controlled reaction condition, and the synthesis could be directed towards the analytes of interest. Small aliquots (~8cm3) of these samples were analyzed, primarily using laser Raman spectroscopy as the analytical tool, complemented by mass spectrometry. Exploiting the technique of depolarization Raman spectroscopy, spectra covering the full complement of vibrational bands expected for CQ4 were recorded. From these a number of bands, hitherto unobserved, could be extracted for CT4 and CT3H. The technique also allowed for semi-quantitative determination of the relative concentrations in a CQ4/Q2 mixture; up to the order of 10-15% for individual methane-T species were found. Finally, both analytical techniques show that not only CQ4 species were present in the mixture, but higher-order tritiated hydrocarbon molecules were identified (primarily in mass spectrometric measurements); indications for the presence of gas-phase ethylene-T were found in the Raman spectra. Even deposits visible to the naked eye, appear on the windows of the Raman cells that are evidence of chemical reactions, possibly of polymerization between the molecules present in the gas phase. All these findings provide advanced information on tritium chemistry and radiochemical evolution of tritiated molecular species, which could be of great importance both for scientific experiments (like KATRIN), and for the operation of fusion reactors (like ITER) (Summary)

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