Design of a dedicated experimental facility and development of a QMS-based methodology for hydrogen isotope permeation studies at near-natural D/H ratios

Hydrogen isotope permeation through metals is a key issue for tritium management in fusion reactors, affecting both the radiological safety and fuel-cycle performance. Although reliable permeation data are essential for the design of tritium processing and isotope separation systems, many evaluations of tritium permeation have relied on experiments using protium and deuterium, due to the practical and regulatory constraints associated with tritium. However, most available H/D permeation data were obtained using percent-level isotope mixtures, which are not representative of the dilute minority-isotope regimes often relevant in many fusion-related permeation and separation scenarios. In this work, a dedicated experimental facility for studying hydrogen isotope separation by permeation through metallic membranes was designed and its functionality tested, with particular emphasis on near-natural deuterium abundance regimes. Thus, a central aspect of this study is the accurate measurement of very low HD / H 2 ratios using quadrupole mass spectrometry (QMS). A comprehensive experimental investigation of H 3 + interference suppression was performed using controlled nitrogen addition and increased ion-extraction voltage, and an extended formation–destruction model for H 3 + ions was developed and shown to reproduce the experimentally observed suppression behaviour. Furthermore, by exploiting the N 2 H + /N 2 + signal evolution as an internal process indicator, a novel approach that allows self-normalization of variations affecting the ion source was introduced, providing improved long-term stability of both the H₃-factor correction and isotope-ratio determination at ppm-level HD/H 2 variations. Measurements performed over periods of up to almost two months yielded relative standard deviations below 2% for both fitted slopes and intercepts. The methodology was validated using natural-abundance hydrogen and a certified hydrogen-deuterium reference mixture, and demonstrated an estimated incremental resolution of a few tens of ppm in deuterium content. Additional calibration experiments showed that nitrogen can also be used as a reference gas for accurate determination of total hydrogen permeation rates. The results demonstrate that the proposed facility and analytical methodology provide a robust basis for future permeation experiments at isotope concentrations representative of fusion-relevant tritium-processing applications.