Gas chromatography at the tritium laboratory Karlsruhe

Abstract

The knowledge of the gas composition in any gas processing plant is a fundamental pre-condition for the high quality of the products to be achieved and for their characterisation. In tritium processing plants the continuous control of the various product streams is even more important because the composition of gases containing tritium may change not only because of the tritium decay to helium-3, but also because of the radiochemical processes which are induced by the energy released during the tritium decay. Thermodynamically stable gas species may be converted into ions, radicals, fragments and excited species. Finally even new gas species may be generated which were not present before in the gas mixture. Among the analytical techniques (mass spectrometry, laser Raman spectroscopy, gas chromatography, use of ionisation chambers) employed at the Tritium Laboratory Karlsruhe (TLK), gas chromatography plays a prominent role. The main reasons for that are the simplicity of the gas chromatographic separation process, the small space required for the equipment, the low investment costs in comparison to other methods, the robustness of the equipment, the simple and straightforward analysis and the fact that all gas species of interest (with the exception of water) can easily be detected by gas chromatographic means. This is, for example, not the case in laser Raman spectroscopy where noble gases can not be characterised by means of vibrational excitations or where the quantitative analysis of even simple gas species such as methane becomes already almost too difficult to perform. Higher hydrocarbons with their even larger possibility of vibrational excitations, especially when all three hydrogen isotopes are present, are even more difficult. Also mass spectrometry can become too complex for a quantitative analysis when too many hydrocarbons are present. Peaks of gas species may start to overlap with the cracking products other gases. The situation becomes even worse when trimers have to be considered and three hydrogen isotopes are present in the gas mixture and in the hydrogen containing molecules. The conventional gas chromatographs GC1 and GC2 used in the Tritium Measurement Techniques (TMT) System of the TLK and the gas chromatograph GC3 of the experiment CAPER are presented in detail, by discussing their flow diagrams, their major components, the chromatograms measured by means of various detectors, shortcomings and possible improvements. One of the main disadvantages of the conventional gas chromatography is the long retention times required for the analysis of hydrogen gas mixtures. To overcome this disadvantage, micro gas chromatography for hydrogen analysis was developed. Reduction of the retention times by one order of magnitude was achieved. This development requires the modification of conventional micro gas chromatographs. The installation of a special external analytical capillary column is necessary to cool the column to the low temperatures required for the separation of the hydrogen molecules. Furthermore, the usefulness of conventional micro gas chromatography for the detection of impurities in gas mixtures similar to the ones to be processed in future power producing fusion devices is demonstrated by the analysis of different impurity gas mixtures.