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The Crystallisation and Aqueous Durability of Borosilicate Nuclear Waste Glasses

Rose, Peter Bramwell (2008) The Crystallisation and Aqueous Durability of Borosilicate Nuclear Waste Glasses. PhD thesis, University of Sheffield.

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Abstract

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.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Identification Number/EthosID: uk.bl.ethos.489679
Depositing User: EThOS Import Sheffield
Date Deposited: 18 Jul 2013 13:27
Last Modified: 08 Aug 2013 08:52
URI: http://etheses.whiterose.ac.uk/id/eprint/3654

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