Structure of International Simple Glass and properties of passivating layer formed in circumneutral pH conditions

Nuclear glass: developing a new layer of understanding A combination of experiments and simulations has provided insight into the structural changes of a glass of nuclear interest in solution. Borosilicate glasses are used as containment matrices for the highly radioactive waste that results from spent-nuclear-fuel reprocessing. They are stored in deep geological repositories and while this is seen as a relatively long-term solution, how they respond to their environment is of obvious importance. A team, led by Stephane Gin at the CEA, Marcoule, France, have now studied the changes that a reference glass known as International Simple Glass (ISG) undergoes when exposed to water. Their simulations and analytical experiments provide insight into the structure of pristine ISG while also observing the generation of a passivating layer, which partly inherits the structure of the pristine glass but is less reactive with water. Knowing the structure of a material is necessary to understand its evolution under various influences; here, the alteration by water of a reference glass of nuclear interest, called International Simple Glass (ISG), is studied. The ISG atomic structure has not yet been thoroughly characterized. Short- and medium-range order in this six-oxide glass was investigated by molecular dynamics (MD) methods. Combining the simulated data with experimental observations acquired from both pristine and altered ISG provided new insight into the formation of surface layers and passivation of the underlying glass. In the tested conditions of 90 degrees C, silica-saturated solution, and pH(90 degrees C)7, the passivating layer partly inherits the structure of the pristine glass network despite the release of mobile elements (Na, B, and some Ca), with a reorganization of the silicate network following B release. The layer appears to minimize its internal energy by relaxing strain accumulated during glass quenching. The resulting passivated glass shows a strong resistance to hydrolysis. The nanopores of this hydrated material, displaying a mean pore size of similar to 1 nm, are filled with various water species. Water speciation determination inside the nanopores is therefore an achievement for future water dynamic study in the passivated glass.