AbstractTungsten brush accommodates thermal stresses and high heat flux in fusion reactor components such as plasma facing surfaces or armor. However, inherently higher surface areas are introduced with the brush design. We have tested a specific design of tungsten brush in steam between 500 and 1100°C. Hydrogen generation and tungsten volatilization rates were determined to address fusion safety issues. The brush design prepared from 3.2-mm diameter welding rods had a packing density of 85%. We found that both hydrogen generation and tungsten volatilization from brush, fixtured to represent a unit within a larger component, were less than projections based upon the integrated total surface area (TSA). Steam access and the escape of hydrogen and volatile oxide from void spaces within the brush are restricted compared with specimens with more direct diffusion pathways to the test environment. Hydrogen generation rates from restrained specimens based on normal surface area (NSA) remain about five times higher than historic rates based on total surface area. Volatilization rates from restrained specimens based upon normal surface area (NSA) were only 50% higher than our historic cumulative maximum flux plot (CMFP) for tungsten. This study has shown that hydrogen generation and tungsten volatilization from brush do not scale according to predictions with earlier determined rates. Volatilization rates from brush with higher packing density could, in fact, approach those from flat surfaces.