Speaker
Mr.
Alfonso de Castro Calles
(Fusion National Laboratory-CIEMAT)
Description
The development of magnetic fusion reactors needs to solve the challenging power/particle exhaust issues to assure a long pulsed/steady state operation avoiding unacceptable damage to the Plasma Facing Materials (PFM’s) that would limit their useful life and the feasibility of such power plants. Although nowadays the use solid tungsten (W) components constitute the main investigated option for these requirements, there are serious concerns over their limitations, especially for the case of unmitigated heat flux and transients handling on the divertor. Liquid metal (LM) divertor concepts explore an alternative solution as their surfaces are, in principle, renewable and unscathed to permanent degradation and disintegration. Moreover, evaporation and high edge non coronal radiation (vapor shielding) can help in this task, reducing the power loads to the surrounding walls. Among LM’s, lithium (Li) is the most promising and studied material. Its employment has shown important advantages in terms of improved H-mode plasma confinement and heat handling capabilities. In such scenario, a possible combination of tungsten at the first wall and liquid Li at the divertor could be an acceptable solution, but several issues related with this material compatibility must be investigated. The co-deposition of Li and hydrogen isotopes on W components could increase the associated tritium retention and might represent an important hazard in terms of radioactive safety. In this work, the co-deposition of Li and deuterium (D) on tungsten at different surface temperature (200ºC-400ºC) has been studied by exposing W samples to Li evaporation under several D2 gaseous environments. Deuterium retention in the W-Li films has been quantified by using Laser Induced Desorption Spectroscopy (LIDS). Additional techniques as Thermal Desorption Spectroscopy (TDS), Secondary Ion Mass Spectrometry (SIMS), profilemetry and Flame Atomic Emission Spectroscopy (FAES) were implemented to corroborate the retention results and for the qualitative and quantitative characterization of the W-Li films. The results show a negligible (below the limit of detection) D uptake by the W-Li layer at Tsurface=225ºC, when it is exposed to simultaneous Li evaporation and low pressure (0.67 Pa) D2 gas exposition. Pre-lithiated samples were also exposed to higher D2 pressures (133.3 Pa) at different superficial temperatures (200ºC-400ºC). A non-linear drastic reduction in the D retention was found for increasing temperatures on the W-Li films that determined D/Li atomic ratios lower that 10-4 at 400ºC. The implication of these results in the potential implementation of this PFM’s solution is considered. Based on the experimental results, an extrapolation of the D co-deposition on W-Li first wall areas in DEMO reactor designs is performed, showing that the associated fuel retention in such hot first wall may be compatible with the tritium inventory limitations.
Primary author
Mr.
Alfonso de Castro Calles
(Fusion National Laboratory-CIEMAT)
