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This post was last edited by Shaobin Fluid on 2026-4-30 at 10:14. The unique gas retention and transport properties of Group V elements (V, Nb, Ta) have long attracted great interest, especially in the field of nuclear fusion. The nominal high hydrogen isotope permeability and diffusivity at the intended operating temperature, along with a negative solubility activation energy, make these materials promising candidates for manufacturing tritium recycling structures. In the field of fusion, the transport of tritium is a matter of particular concern. Due to its radioactivity, strict control over its inventory is necessary not only for the availability of fuel but also for safety reasons. The design of tritium-related systems and the potential for leaks to other components may be underestimated, thereby affecting the safety of the reactor. To this end, the deuteron permeability data for the three materials were obtained within the relevant temperature range, meeting the requirements generally established for cladding design. The experimental difficulties as well as the effect of surface oxidation were also evaluated; surface oxidation could become a major issue in practical applications. For the experiments, high-purity (99.9%) niobium and tantalum samples, as well as vanadium samples with thicknesses of 0.5, 1.0, and 0.9 mm respectively, were cut to meet the dimensions required by the system, and ground/polished as necessary to minimize the impact of surface treatment. A new gas permeation system (Thermoperm II) has been commissioned for these experiments; it is a replica of the Thermoperm I device developed within the framework of the F4E project and is used for studying hydrogen/metal interactions, allowing for the acquisition of data on hydrogen/deuterium permeation through metal membranes at temperatures ranging from ~300 to 550°C and pressures from 0.1 to 1000 mbar. This experimental procedure involves maintaining a driving gas pressure level on the surface of a sample, while keeping a vacuum condition (~ 10-3 mbar) at the opposite position. Under steady-state conditions, the permeation flux in the concentration range from high to low was measured at each temperature using a Pfeiffer intelligent leak detector (with a sensitivity of 10-12 mbar l/s). The system was carefully inspected and verified using high-purity α-iron and 316-L stainless steel (as part of the aforementioned F4E contract, these two materials were thoroughly characterized by different laboratories). Figures 1 and 2 show the reproducibility of this method, comparing the Thermoperm I and II data for stainless steel and iron, respectively. To ensure adequate surface conditions, a systematic procedure was followed, including pre-washing the samples with a cleaner and in an ultrasonic bath, as well as annealing them in high vacuum at around 400°C for approximately 70 hours before the experiment began, once the samples had been fixed. Figure 1. Penetration of 316L stainless steel obtained in Thermoperm I and II. Figure 2. α-Fe permeability obtained in Thermoperm I and II. This set of materials exhibits very high hydrogen permeability (for example, about 3 orders of magnitude higher than that of α-Fe at 550°C). Furthermore, a negative activation energy is given, which originates from the solubility behavior (solubility decreases as temperature drops). These properties make these materials particularly suitable as membranes for gas separators in fusion, providing effective tritium permeation while also minimizing gas absorption at operating temperatures. However, we must be aware that the permeability data are derived from experimental solubility and diffusion rates, as shown in Figure 3. Figure 3. Hydrogen permeability in vanadium, niobium, and tantalum. The shaded area represents the calculated value. *Uncertain temperature range validity/inference. Results: Osmosis under ideal conditions Figure 4 shows the results of 100 mbar deuterium permeation through V, Nb, and Ta. Positive Arrhenius activation energies were observed for all three materials, as well as permeabilities at 500°C that were 2–3 orders of magnitude lower than the semi-empirical predictions. The equation for the variation of the permeability of each material with temperature is as follows: Figure 4. Experimental deuteron permeabilities of V, Ta, and Nb (drive pressure ~ 100 mbar). Illustration: Functional relationship between permeation flux and pressure at 400°C. Penetration under real conditions: Additional tests were conducted on vanadium samples under deliberately relaxed cleaning conditions, with the usual vacuum pre-annealing step being omitted. There are two objectives: on the one hand, to evaluate the actual tritium permeation capacity that must be expected under fusion operating conditions; on the other hand, to address the experimental difficulties related to surface conditions, which may be the cause of the large variation in the data. Figure 5 shows the permeability results as a function of inversion before and after different treatments. Figure 5. Effect of surface condition on DC magnetic permeability. “The deuteron permeability of the “installed” samples shows two different trends (green symbols in the graph). The slope above approximately 400°C matches very well with the permeability data reported by Deventer et al. The results also show that annealing in vacuum at 300°C can prevent this effect, which is consistent with the decomposition of vanadium oxide at 290°C as reported by Kiss et al. Discussion: For the measurements described here, certain measures were taken to avoid surface effects. The ~P1/2 correlation of the permeation fluxes obtained at Ciemat at pressures ranging from 0.7 to 1000 mbar indicates a low surface effect; however, the presence of a surface effect cannot be completely ruled out. Although the data are scattered and all three materials are considered without distinction, a general trend can be observed from the data collected in Figure 3. The slope of the Arrhenius plot decreases with temperature; therefore, different mechanisms seem to govern H isotope permeation depending on the temperature range. Sherman & Birnbaum provided the only experimental evidence for negative activation energy under high-temperature (> 636°C) and carefully controlled oxidation conditions. However, the data obtained at lower temperatures gradually show an opposite temperature dependence. Conclusion The deuteron permeability through niobium, vanadium, and tantalum was measured in the temperature range of approximately 300–500°C. Comparison with existing data reveals a complex correlation with temperature. Although it is clear that further analysis and experiments should be conducted over a wider range of pressures and temperatures to fully characterize the permeability of H isotopes in these materials, it is also clear that the traditionally accepted values should be avoided. Furthermore, in practical applications, special attention should be paid to the surface oxidation of these materials, as the natural oxides formed at room temperature and their high stability result in a permeability that decreases by more than three orders of magnitude at around 450°C. Among them, tantalum performed the best.