Bulk, surface structures and properties of sodium borosilicate and boroaluminosilicate nuclear waste glasses from molecular dynamics simulations

Abstract

The bulk and surface structures, as well as physical properties such as ionic diffusion and mechanical moduli, of sodium borosilicate and boroaluminosilicate model nuclear waste glasses with composition similar to the international simplified glass (ISG) have been studied using molecular dynamics simulations with the recently developed partial charge composition dependent potentials. Short and medium range structures of these glasses were analyzed and it was found that, for glass former cations, silicon ion is all four-fold coordinated by oxygen, aluminum is also mainly four-fold coordinated, while boron exists in a mixture of 3- and 4-fold coordination, with the fraction of 4-fold coordinated B (N4 value) close to but slightly higher than the experimental and theoretical values. The medium range structure features such as network former cation oxygen polyhedral connectivity, sodium distribution around the network formers, and network ring size distribution were characterized. The surface structure was generated from the simulated bulk glass structures and showed sodium enrichment on the glass surface. Addition of alumina to borosilicate glasses increased B N4 value and overall glass network connectivity that led to improved mechanical properties. Ion diffusion properties, mechanical properties as well as surface characteristics for the simplified ISG have also been investigated with simulations and compared with available experimental data. This detailed atomic structure information provides insight of the physical properties of these glasses and a step further in understanding the chemical durability and dissolution behaviors.