Behavior of Portlandite upon Exposure to Ionizing Radiation: Evidence of Delayed H₂ Production

Abstract

When used under radiation (reactor vessel well concretes, cemented radioactive waste), cementitious materials undergo radiolysis with O−H bonds leading to dihydrogen production. In order to best assess the dihydrogen (H 2 ) risk in nuclear facilities, it is then mandatory to check whether the solid phases of the cement matrix constitute a significant source of radiolytic H 2 in addition to that that arises from pore water. Herein, we focus on portlandite (Ca(OH) 2 ) as the main hydration product of Portland cement together with hydrated calcium silicate. In the absence of water molecules, the Ca(OH) 2 powder leads to an apparent H 2 radiolytic yield under accelerated electrons and γ irradiation of (3.0 ± 0.7) × 10 −9 mol•J −1 and (8.1 ± 0.5) × 10 −9 mol• J −1 , respectively. Interestingly, the H 2 production measured at the end of irradiation does not represent the whole H 2 produced; a portion of it remains trapped in the crystal lattice as evidenced by 1 H NMR measurements. These trapped H 2 molecules can be released after heating the sample at 453 K, leading to a total H 2 radiolytic yield for accelerated electrons and γ irradiation of (1.1 ± 0.2) × 10 −8 mol•J −1 and (1.4 ± 0.2) × 10 −8 mol•J −1 , respectively. The immediate H 2 production corresponds to the fast diffusion, if possible, of H 2 precursors to the surface with H 2 formation near or at the surface; the delayed H 2 production is due to the slow release of trapped H 2 within the material. Our work shows that the fundamental knowledge of these samples under irradiation is crucial to a better description of materials of interest in the nuclear industry. Furthermore, the results highlight that measuring H 2 production in a solid such as portlandite is a delicate task.