First Calibration Measurements of an FTIR Absorption Spectroscopy System for Liquid Hydrogen Isotopologues for the Isotope Separation System of Fusion Power Plants

Fusion facilities like ITER and DEMO will circulate huge amounts of deuterium and tritium in their fuel cycle with an estimated throughput of kg per hour. One important capability of these fuel cycles is to separate the hydrogen isotopologues.  For this purpose the Isotope Separation System (ISS), using cryogenic distillation, as part of the TRitium Enrichment Test Assembly (TRENTA) is under development at Tritium Laboratory Karlsruhe. Fourier transform infrared absorption spectroscopy (FTIR) has been selected to prove its capability for inline monitoring of the tritium concentration in the liquid phase at the bottom of the distillation column of the ISS. The actual R&D work is focusing on the calibration of such a system. Two major issues are the identification of appropriate absorption lines and their dependence on the isotopic concentrations and composition. For this purpose the Tritium Absorption IR spectroscopy experiment has been set up as an extension of TRENTA. For calibration a Raman spectroscopy system is used. First measurements, with equilibrated mixtures of H₂, D₂ and HD demonstrate that FTIR can be used for quantitative analysis of liquid hydrogen isotopologues and reveal a nonlinear dependence of the integrated absorbance from the D,₂ concentration in the 2nd vibrational branch of D₂ FTIR spectra.