(a) The measured yield of hydrogen gas released from the cylindrical geopolymer sample irradiated with water at 100% relative humidity was lower than that of the powder. The evaluated value immediately after irradiation is the one of the solid line (1). Since only hydrogen diffusion proceeds after irradiation, the evaluation value approaches the analytical value 2 of the dashed line when all hydrogen diffused to the gas phase. (b) Model representation where a portion of the hydrogen generated under irradiation is consumed by recombination during diffusion to the sample surface.