Two simulated (non-radioactive) borosilicate high-level waste (HLW) glasses were
supplied by Nexia Solutions; Blend glass and Oxide glass. This work is primarily
concerned with identifying the crystal phases present in both as-cast and heat treated
(simulating the retarded cooling experienced by 'real ' (radioactive) borosilicate HLW
glasses after pouring) samples of these two glasses; as well as determining the
aqueous durability of these samples. An as-cast Oxide glass sample contained a
'yellow phase' inclusion which was also investigated.
Combined direct current plasma atomic emission spectroscopy (DCP-AES)
and X-ray fluorescence spectroscopy (XRF) determined the chemical composition of
both glasses. Differential thermal analysis (DTA) identified their glass transition
temperatures, whilst Archimedes' principle yielded their densities.
X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) identified crystal phases and, in the case of
microscopy, determined microstructural homogeneity. As-cast Blend glass samples
were compositionally homogeneous and contained RU02 crystals. Heat treated Blend
glass samples contained Cel- (x+y)ZrxGdy0 2- (y/2), (Sr,Nd,La) molybdate and lanthanide
(Nd,Gd,La,Ce) silicate crystals. As-cast Oxide glass samples were either
compositionally homogeneous (Type I microstructure) or compositionally
inhomogeneous (Type II microstructure). RU02, metallic Pd-Te and (Cr,Fe,Ni)304
crystals were observed in both Type I and Type II microstructures, with
Na3Li(Mo04)2 . 6H20 crystals occasionally detected. Zrl- (x+y)CexGdy0 2- (y/2) and
lanthanide (Nd,Gd,La,Ce) silicate crystals were only detected in Type II
microstructures. Heat treated Oxide glass samples displayed either: extensive
crystallisation and matrix cracking (Type A microstructure) or 'banded' crystallisation
(Type B microstructure), depending on their parent (as-cast) microstructure (Type I or
Type II respectively). (Na,Sr,Nd,La)Mo04, Cel- (x+y)ZrxGdy0 2- (y!2) and aNi-rich
crystal phase were detected in both Type A and Type B microstructures. a-cristobalite
crystals were found exclusively in Type A microstructures, whilst lanthanide
(Nd,Gd,La,Ce) silicate and zektzerite crystals were only detected in Type B
microstructures.
A modified product consistency test (modified PCT-B) determined the
aqueous durability of as-cast and heat treated samples of both glasses. Heat treatment
of Blend glass improved its aqueous durability marginally. Heat treatment of Oxide
glass decreased its aqueous durability significantly (as-cast and heat treated Oxide
glass samples had Type I and Type A microstructures respectively).
Combined DCP-AES and XRF revealed the chemical composition of the
'yellow phase' inclusion, showing it to be enriched in Mo, Cs, Na, Li, Cr, Ba, Sr and
Te compared to Oxide glass. XRD identified the 'yellow phase' inclusion as
crystalline, containing CsLiMo04, Na3Li(Mo04)2 . 6H20, (Na,Sr,Nd)Mo04 and BalxSrxMo04
(where 0 <x < 0.5) crystals.