Speaker
Description
Liquid metals, and lithium (Li) in particular, are increasingly considered strong candidates for next-generation divertor concepts in fusion reactors due to their excellent power-handling capabilities and self-healing behavior. However, lithium’s high affinity for hydrogen isotopes and its tendency to form stable hydrides introduce important challenges for tritium (T) inventory control and fuel-cycle management in future power plants. Since some degree of tritium retention in Li-based divertors appears unavoidable, the development of efficient T-extraction methods is essential. In this work, we investigate hydrogen isotope exchange (IE) in lithium as a potential tritium mitigation technique and directly compare its efficiency with conventional thermal outgassing. Because of tritium’s radiological constraints, deuterium (D) and hydrogen (H) are used as experimental proxies.
We present the first dynamic isotope-exchange experiments performed in lithium using the newly commissioned linear plasma device Upgraded Pilot-PSI (UPP). UPP uniquely enables ITER-relevant plasma exposure combined with operando ion beam, allowing us to directly monitor Li and D contents during plasma interaction. Four identical lithium-filled capillary porous structures (CPSs) were exposed to deuterium plasma leading to Li evaporation. Co-deposited LiD layers formed on a nearby heated stainless-steel foil. Following complete lithium depletion, the CPS was exposed to H plasmas, converting the co-deposited LiD layers into LiH.
Isotope-exchange behavior was investigated from 220 °C to 400 °C. Across all conditions, deuterium concentration during hydrogen plasma exposure followed an exponentially decaying temporal behaviour, with decay constants exhibiting Arrhenius scaling. This temperature dependence demonstrates that isotope exchange in lithium is a thermally activated process. To evaluate reversibility, two samples were subsequently re-exposed to deuterium plasma, restoring LiD with an approximately 1:1 Li:D atomic ratio. This confirms that the IE mechanism is fully reversible.
For the remaining two samples, IE was directly compared to thermal outgassing. After D plasma exposure, the LiD layers were first held in vacuum at the same temperatures used during IE. During this period, Li₂O rapidly formed on the LiD surface, significantly reducing long-term D release through thermal desorption alone. Nevertheless, subsequent hydrogen plasma exposure demonstrated that isotope exchange remained highly effective, even in the presence of oxidized surfaces, and substantially faster than thermal outgassing.
Our results demonstrate that isotope exchange is a highly efficient deuterium removal method, capable of extracting essentially all retained D. IE can potentially be employed in tokamaks before in-vessel maintenance or integrated into a lithium loop system for tritium recovery, enhancing the tritium extraction rate.