The void fraction ε within the reactor cartridge when using a packed bed of coarse or fine magnesium turnings was 0.58 or 0.51, respectively. Thus, a denser magnesium packing can be achieved when using smaller magnesium turnings. From the particle size distributions of the representative Mg turning samples and the void fraction, the total magnesium surface within a filled reactor cartridge can be obtained: 0.33 m 2 for the fine and 0.14 m 2 for the coarse turnings, with specific surface areas of 21,350 m 2 /m 3 and 10,934 m 2 /m 3 , respectively.