Design and characterization of cement-based materials for dihy-drogen trapping

Overview & Synopsis

Mitigating the release of dihydrogen (H2) is an important issue for the disposal of certain types of cemented waste packages, containing for instance metallic waste (such as aluminium, magnesium, uranium…) or highly irradiating waste. The dihydrogen production results from corrosion of the metals encapsulated in the cement matrix, and/or radiolysis of water. CEA has been working for several years on the sequestration of dihydrogen using an oxide getter of the -MnO2/Ag2O type [1]. This getter shows several advantages: reasonable cost, good resistance to irradiation and irreversible trapping of H2 by reduction of MnO2 into MnOOH. The objective of this work is thus to design and characterize self-levelling mortars, incorporating the getter, which may be used as conditioning matri-ces for wastes leading to problematic dihydrogen release. The binders should advantageously have a high chemical water demand, making it possible to obtain significant desaturation of the pore network by the sole hydration reactions, since the efficiency of the getter decreases under wet environment. Two mortar formulations were first optimized using a magnesium phosphate cement (MPC) and a calcium sulfoaluminate cement (CSA). In both cases, the getter slightly influenced the rate of cement hydration at early age (acceleration for MPC, retardation for CSA), but had no effect at later age. Sorption of ions released by the dissolution of cement phases onto MnO2 was evidenced. The pro-cess depended both on pH (which controlled the surface charge of MnO2) and on the type of ions. Conversion of Ag2O into Ag3PO4 was also noticed in the MPC matrix. In a second stage, the influence of the chemical evolution of the getter in the cement matrix on its trapping properties was investigated: metallic aluminum was incorporated in the mortars as a powder (MPC mortar) or as a rod (CSA mortar). In both cases, the hydrogen release was strongly reduced as compared to reference materials prepared without any getter, showing that the getter kept its efficien-cy in the cement matrices.