Deuterium retention in Sn-filled samples exposed to fusion-relevant flux plasmas

Abstract

Tin (Sn) is an attractive option for a liquid metal wall material for future fusion reactors. Control of tritium inventory is key for the successful operation of these reactors, but little data exists up until now on hydrogen isotope retention in Sn. Free surface Sn targets and Sn-based capillary porous structure targets were exposed to deuterium (D) plasma in nano-PSI and magnum-PSI respectively. The retained D inventory was determined using the methods of thermal desorption spectroscopy and nuclear reaction analysis. The retention dependence is somewhat complex due to the mixed composition of the exposed samples as well as their liquid nature. The D retained in both types of Sn targets was found to increase with increasing D plasma fluence. For free surface liquid Sn targets, both thermal desorption spectroscopy and nuclear reaction analysis measurements showed a negative relationship between D retention and sample temperature. For capillary porous structure Sn targets, D retained in the top layer measured by nuclear reaction analysis decreased with temperature while the total D retained measured by thermal desorption spectroscopy remained approximately constant. By extracting pure Sn pieces from the targets it was found that the amount of D retained in pure Sn was much lower than that in the whole Sn-based targets and was estimated to be about 10 −7 -10 −4 D/Sn. D retained at the Sn-wall interface was found to dominate the total amount of D retained in the whole sample and observed cavities between deposited Sn droplets and the wall are the leading candidates responsible for this. Cavity formation is proposed to be the main retention mechanism for D in liquid Sn targets, although enhanced solubility leading to supersaturation under a D plasma environment is mainly responsible for the observed higher D retention in pure Sn compared with normal solubility under D gas. When compared with tungsten, D in Sn samples is of the same order of magnitude at temperatures below 300 °C, but at higher temperatures at least one to two orders of magnitude higher, most likely due to D trapped in cavities.

Bubble formation in liquid Sn under different plasma loading conditions leading to droplet ejection

“…One extreme example is bubble formation due to liquid boiling at boiling point. In our previous work [25], we have observed Sn droplet ejection due to boiling in vacuum of 10 -5 Pa at T ~ 800 °C .…”

“…In [40], Duchemin also simulated the bubble bursting process and concluded that the radius of the first droplet due to jet formation is about 10% of the initial bubbles radius. In [25], the pits in the range of 0.1 -1 µm are observed on Sn surface after exposure. 1 µm-scaled bubbles in liquid Sn exposed to hydrogen plasma are also reported by Ruzic [28].…”

“…Some experimental data of hydrogen solubility in liquid Sn can be found in the literatures [47][48][49]. Among these data, Bever's are more reliable as already discussed in [25]. For dilute solvent, approaches unity and this is the case of H dissolution in liquid Sn.…”

“…It is also not explosively reactive with water and air, unlike Li, thus is much safer. A lower hydrogen isotope inventory in Sn is expected [23][24][25] as there is no stable hydride for Sn above room temperature [26], unlike for Li [27]. On the other hand, due to its high atomic number, the influx of Sn entering into the core plasma must be strictly limited to around 0.5% that of lithium [2].…”

CPS Based Liquid Metal Divertor Target for EU-DEMO

“…Regarding the H retention of the liquid Sn, the results that can be found in literature are rather discordant: very low retention was found in [4], while higher values are reported in [5] and [6]. In any case, even the most pessimistic results are not such as to discourage the use of Sn.…”