The hydrogen gas (H-2) production of wasteforms is a major safety concern for encapsulating nuclear wastes. For geopolymers, the H-2 produced by radiolytic processes is a key factor because of the large amount of water present in their porous structure. Herein, the hydrogen production was measured under Co-60 gamma irradiation. The effect of water saturation and sample size were studied for pure geopolymers, or using zeolites as an example waste. To interpret results, a simple model was used, considering only hydrogen production, a potential recombination and its diffusion in the geopolymer matrix. When geopolymer monolithic samples were large and saturated by water, the hydrogen released was measured up to two orders of magnitude lower with a 40-cm long cylinder samples (1.9 x 10(-10) mol/J) than a sample in powder form (2.2 x 10(-8) mol/J). Knowing the diffusion constant of the matrix, the model was able to reproduce the evolution of the hydrogen release as a function of the water saturation level and predict accurately the evolution when sample size is increased up to 40 cm.