Nanoscale simulation of reactive transport: application to nuclear glass dissolution

When nuclear glass is altered in a confined environment, a silicated and nanoporous alteration layer called 'gel' forms at the glass/solution interface, limiting the transport of reactive species and leading to a decrease in the glass alteration rate. These passivating properties may partly find an origin in a decrease of water mobility/reactivity in a nanoconfined environment. In order to better understand the origin of this passivating effect, a study coupling classical Molecular Dynamics (MD) and experiments is performed. Six different glasses with five-oxides each are simulated, displaying various B, Al and Si content, deviating from nuclear glass compositions to try establishing a global alteration model. A gel simulation method is developed, based on glass simulation followed by the removal of soluble elements (B, Na). Both the porosities and their size distribution depend on the quantity of soluble elements in the 'dry' gels. The greater the initial system destabilization, the longer the reorganization time. Moreover, tricoordinated Al are formed at the pores' surface, which is an artefact of the interatomic potentials. Two types of water are identified during the gel hydratation thanks to the use of dissociative potentials: 'pore' water inside the pores and 'reticulated' water inside the gel network. A continuous reequilibration between these two types of water is observed. The study of the diffusion inside the pores highlights a greater water mobility at the pores' center, and the presence of calcium in pore water does not impact water dynamics in a nanoconfined environment. Model glasses were synthesized and numerous experiments were conducted, in diluted (initial rate) and silica-saturated (residual rate) conditions. Results indicate the existence of a continuum between two gel formation mechanisms: either by dissolution/reprecipitation or by hydrolysis/condensation. The location between these two modes depends on the Al content in the glass, reinforcing the vitreous network: it is beneficial in the initial alteration rate regime but noxious in the residual rate regime, slowing down the reorganization of the silicate network into a passivating gel. This is not reproduced by the simulations because tricoordinated Al induce a greater reorganization of its environment. At low B content, B and Na behave differently: B is not that mobile compared to Na which is exchanged through interdiffusion with H⁺/H₃O⁺. At high content, B percolation opens paths allowing for Na release, and passivation may be due to closing of these canals during gel reorganization. Isotopic tracing indicated a low pore accessibility, consistent with numerical simulations except for one glass. Water diffusion in the gel would thus not depend on the porous network topology (open vs closed porosity) but instead may be controlled by more or less favorable exchange reactions depending on the nature and position of the sites, which become slower as the gels mature. (author)