Smart Tungsten-based Alloys for a First Wall of DEMO

During an accident with loss-of-coolant and air ingress in DEMO, the temperature of tungsten first wall cladding may exceed 1000 °C and remain for months leading to tungsten oxidation. The radioactive tungsten oxide can be mobilized to the environment at rates of 10–150 kg per hour. Smart tungsten-based alloys are under development to address this issue. Alloys are aimed to function as pure tungsten during regular plasma operation of DEMO. During an accident, alloying elements will create a protective layer, suppressing release of W oxide. Bulk smart alloys were developed by using mechanical alloying and field-assisted sintering technology. The mechanical alloying process was optimized leading to an increased powder production by at least 40 %. Smart alloys and tungsten were tested under a variety of DEMO-relevant plasma conditions. Both materials demonstrated similar sputtering resistance to deuterium plasma. Under accident conditions, alloys feature a 40-fold reduction of W release compared to that of pure tungsten.</p

On the plasma suitability of WCrY smart alloys—the effect of mixed D+Ar/He plasmas

Tungsten-chromium-yttrium (WCrY) smart alloys are foreseen as first wall materials for future fusion devices such as DEMO. While suppressing W oxidation during accidental conditions, they should behave like pure W during plasma operation due to preferential sputtering of the lighter alloying elements Cr and Y causing W enrichment at the surface. This paper reports on the results of the simultaneous exposure of WCrY and pure W reference samples to mixed D + 1%Ar+5 %He plasma in the linear plasma device PSI-2. Further, a comparison with exposures to pure D and D + 1 %Ar plasma is made. At incident ion energies of 120 eV, exposure to pure D plasma results in a W-enriched alloy surface due to the preferential sputtering of Cr and Y, while the addition of Ar leads to enhanced erosion for W and WCrY and reduces the W enrichment in smart alloys. With the addition of He to the plasma, erosion of WCrY is enhanced compared to that of pure W. To investigate the plasma impact on the oxidation behaviour, plasma-exposed and reference samples were oxidised in controlled dry oxygen-containing atmosphere at 1000\,^\circ {\rm{C}}. The sample geometry has a great impact on the oxidation behaviour. Yet, it can be shown that the good oxidation-suppressing properties of WCrY smart alloys are preserved during plasma exposure