Modelling tritium adsorption and desorption from tungsten dust with a surface kinetic model
Résumé
During tokamak operation, nano-to-micrometer-sized dust will be created due to plasma-wall interactions. The loading of such dust with tritium can be problematic as it may significantly rise the in-vessel tritium inventory causing a safety issue. This work contributes to the assessment of the monitoring of tritium inventory in dust through modelling adsorption and desorption of tritium gas on the tungsten (W) dust surfaces.
We based our approach on a classical kinetic model of sticking and desorption of tritium molecules. The desorption energies are parametrized with density functional theory (DFT) calculations which have shown that it depends on the hydrogen coverage of the surface (1.4 eV for bare surface and <0.8 eV for saturated surface).
Using this model, we simulate and successfully reproduce tritium loading and desorption from 10μm-sized dust typical of what has been collected in WEST. For such dust, the calculated and measured tritium activity after loading under 1 bar of T2 at 743 K is about 500 MBq/g. The tritium desorption from these types of dust is also well reproduced with the DFT desorption energies: in 200 h, the tritium dust lose 200 MBq/g. The model proposed, focused on surface only, will be compared to a more classic rate- diffusion approach in order to evaluate the maximum tritium retained in the dust volume over time.
Then, we show that untritiated dust placed in conditions relevant with the floor of the ITER machine are loaded with a high amount of tritium even at very low in-vessel tritium pressure (10-5 Pa). We investigate possible scenarios for tritium recovery from the W dust using isotopic exchange. The simulations show that exposures of tritiated dust at 1×104 Pa of H2 at 300 K (room temperature for an outside-the-vessel scenario) or 1 Pa of D2 at 373 K (for an in-vessel scenario) are efficient ways to recover the total quantity of tritium as soon as H2 or D2 remain the main isotopes in the gas phase. It is a possible way to validate this surface model (by designing isotopic exchange experiments) and offers opportunity to handle the tritium inventory in W dust, and by extension to any W surfaces, during ITER operations