Development and analyses of self-passivating tungsten alloys for DEMO accidental conditions

Tungsten is considered the main candidate material for the first-wall in DEMO due to its high melting point, low erosion yield and low tritium retention. Nevertheless, it can cause a substantial safety issue in a loss-of-coolant accident (LOCA) in combination with air ingress into the plasma vessel, due to the formation and sublimation of volatile neutron activated tungsten oxide. Self-passivating tungsten alloys introduce a passive safety mechanism by forming a stable chromic oxide scale on the surface acting as a diffusion barrier for oxygen and preventing the formation of tungsten oxide. Self-passivating tungsten alloys optimized for oxidation resistance containing ∼12 wt.% Cr and ∼0.6 wt.% Y are investigated under conditions of argon-oxygen, humid argon and humid air atmospheres at different partial pressures and temperatures ranging from 1073 to 1273 K. Thin films with ∼3.5 μm thickness produced by magnetron sputter deposition are used as a model system. The oxidation resistance of these films in an argon-20 vol.% oxygen atmosphere is sufficient to prevent formation and release of tungsten oxide at temperatures of from 1073 to 1273 K. The sublimation of Cr in nitrogen-oxygen-water atmosphere at T ≥ 1273 K is discussed. A deeper understanding of the governing processes for oxygen/Cr diffusion under different atmospheres is gained, supported by SEM/EDX in combination with FIB cross-section and TGA measurements.