Abstract : We have studied the H 2 production under ionizing radiation of water confined in synthetic saponite and montmorillonite as a function of the relative humidity. The H 2 radiolytic yields in the dry systems are very similar to that measured in a non-swelling clay mineral. They are 2–3 times higher with one water layer in the interlayer space, evidencing very efficient energy transfers and efficient recombination reactions due to a high confinement. With two water layers, the H 2 yields decrease as compared to the previous case, but remain higher than in bulk water, proving that recombination reactions of hydrogen atoms are less efficient. Electron paramagnetic resonance measurements evidence that reactivity changes significantly with the number of water layers. Saponite and montmorillonite give similar results, showing that reactivity is driven by the amount of water and that the details of the clay structure play a less important role. Lastly, the behavior of natural vs. synthetic swelling clays is discussed. The presence of impurities, even in small quantities, significantly alters energy transfers and has a positive implication for the geological nuclear waste management.
