The dissolution of simulant UK Ca/Zn-modified nuclear waste glass: Insight into Stage III behavior
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.50
The dissolution of simulant UK Ca/Zn-modified nuclear waste glass: Insight into Stage III behavior Adam J. Fisher1, Mike T. Harrison2, Neil C. Hyatt1, Russell J. Hand1 and Claire L. Corkhill1
1 NucleUS Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, S1 3JD, UK
2
National Nuclear Laboratory, Central Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, UK
ABSTRACT
The dissolution of the United Kingdom’s vitrified high-level-waste simulant, CaZn MW28, was investigated following the Product Consistency Test-B protocol for 112 d at 90 oC and in ultra-high-quality water. Residual rate dissolution (stage II) and rate resumption (stage III), after 28 d, was observed. Thermodynamic modelling suggested that solutions were saturated with respect to Mg- and Zn-bearing phases, and the presence of Mg- and Zn-smectite clays was tentatively observed. The formation of these phases was concurrent with a significant increase in the dissolution rate, similar to Stage III behavior seen in other nuclear waste simulant glass materials, indicating that the addition of Mg and Zn to high-level-waste glass (7.3 wt. % combined) significantly influences the dissolution rate.
INTRODUCTION The current treatment strategy for high-level-waste (HLW) vitrification in the UK utilises a modified borosilicate base glass, which incorporates Al2O3, ZnO and CaO into the traditional Mixed Windscale (MW) glass formulation. With a nominal composition (in wt. %) of 4.2Li2Ox8.6Na2Ox6.1CaOx5.9ZnOx4.2Al2O3x23.4B2O3x47.6SiO2, this glass is referred to as CaZn MW. The transition to CaZn MW is ongoing, with the aims of: (i) incorporating higher quantities of Mo (through the crystallisation of CaMoO 4) arising from post operational clean out (POCO) of the high-activity-liquor storage tanks [1], (ii) reducing the melt viscosity, thereby inducing better melt homogenization and increased levels of waste loading at lower processing temperatures, thus, (iii) enhancing the melter 103
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durability, and, (iv) improving the chemical durability in geological disposal environments [2,3]. Controversy surrounds the chemical durability of Zn and Ca containing glass, whereby Zn addition has been reported to increase the chemical resistance to dissolution at the forward rate (Stage I - hydrolysis of the glass network) [4], but has a negative impact at later reaction progress, causing aggressive residual stage II dissolution due to the precipitation of secondary Zn-containing phases [5] or possibly triggering rate resumption (stage III), related to the formation of zeolites [6]. The effect of Ca is generally positive, increasing the chemical resistance to dissolution at the residual rate (stage II) due
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