Glass - Back to the Future!

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Michal Miekina is presently a PhD student in the Dept. of Materials Science and Engineering in the University of Sheffield, UK. He is working on nuclear waste immobilization in collaboration with National Nuclear Laboratory (NNL). His project focuses on understanding the conditions under which crystal phases form in Ca/Zn product glasses containing simulated High Level Waste (HLW, POCO waste), and the effects of these crystal phases on the properties of the resulting glassy waste products.

Immobilisation of Mo and Zr rich nuclear wastes in borosilicate glasses

M. Miekina, J.R. Stevens*, N.C. Hyatt and R.J. Hand
Immobilisation Science Laboratory, Dept. of Materials Science & Engineering, University of Sheffield, Mappin Street Sheffield, S1 3JD, UK
* National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park, Warrington, WA3 6AE, UK

A consequence of fuel reprocessing in the UK is liquid high level waste (HLW) which is stored in the Highly Active Storage Tanks (HAST) prior to vitrification in borosilicate glass. A “heel” has developed at the bottom of these tanks, which is believed to mainly contain zirconium molybdate (ZrMo2O8) and caesium phosphomolybdate phases, both of which pose challenges for vitrification as both MoO3 and ZrO2 have a low solubility (~ 1 wt.% each) in borosilicate glasses [1, 2]. Post Operational Clean Out (POCO) of these tanks is planned during which the phases in the heel will need to be incorporated into a vitrified product hence compositional modifications enabling increased loading of these phases are required [3]. Three glass families have been investigated CZ and MZ and CMZ. All 3 families are soda-lithia borosilicates with additions of ZnO, and either CaO (CZ), MgO (MZ) or MgO and CaO (CMZ). Further additions of Fe and Mn oxides (7-9 wt.%) in combination have been found to have a positive effect on suppressing crystallisation at simulant POCO waste loadings of up to 15 wt.% in CMZ glasses. However, greater waste loadings (18, 20 and 25 wt.%) resulted in the formation of CaMoO4, BaMoO4 and SiO2 crystals in the glass. Similar results were obtained with MZ glasses, although this time higher loadings of simulant POCO waste (18-20 wt.%) resulted in formation of BaMoO4, ZrSiO4 and ZrO2 (Figure 1). Details about the crystallisation processes in the glass compositions under investigation will be presented.

Figure 1. Backscattered electron (BSE) image (a) with X-ray elemental mapping in an MZ type glass with 20 wt.% loading of simulant POCO waste (b), (1) BSE image, (2) O-K, (3) Mg-K, (4) Si-KA, (5) Mn-KA, (6) Fe-KA, (7) Zr-KA and (8) Mo-LA