A Study of Vitrified Nuclear Wasteforms by Molecular Dynamics, Electron Microscopy and Raman Spectroscopy

In this study an attempt is made to create molecular dynamics (MD) models of borate glass, alkali borosilicate glasses, and UK vitreous High Level Radioactive Wasteforms. The study also includes experimental studies of vitrified wasteforms by helium pycnometry, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray florescence spectroscopy (XRF) and Raman spectroscopy.

Molecular dynamics models of alkali borosilicate glasses were created using Buckingham and BHM potentials in the constant pressure and temperature ensemble. The models using BHM potentials showed more realistic boron coordination numbers than those using Buckingham potentials. However structural features such as Si-O, Li-O and Na-O nearest neighbour distances and O Si O and O B O bond angles were considered satisfactory using Buckingham potentials.

SEM images showing phase separation in four different vitrified wasteforms are presented. The chemical composition of the phases were determined using SEM EDX. XRF spectroscopy was obtained from the wasteforms in powder form and show qualitative agreement with nominal compositions.

Raman spectroscopy also revealed the presence of MoO4 tetrahedra in a glass environment and in phases such as CaMoO4 and Na(Gd,Nd)(MoO4)2. The presence of ruthenium, cerium and zirconium phases were also found in the Raman spectra of wasteforms.

MD models of three simplified vitrified wasteforms were created using Buckingham potentials. Two models of each wasteform were created. The first models used only two-body potentials and showed MoO6 octahedra connected to borosilicate network formers. In the second model of each wasteform, an additional O Mo O three-body potential was applied. The results of the second models showed MoO4 tetrahedra detached from the borosilicate network which is a realistic feature in comparison to the experimental observations.