2 results
Chemical durability of vitrified wasteforms: effects of pH and solution composition
- C. A. Utton, S. W. Swanton, J. Schofield, R. J. Hand, A. Clacher, N. C. Hyatt
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- Journal:
- Mineralogical Magazine / Volume 76 / Issue 8 / December 2012
- Published online by Cambridge University Press:
- 05 July 2018, pp. 2919-2930
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- Article
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Vitrification is used for the immobilization and conditioning of high-level waste (HLW) arising from the reprocessing of spent nuclear fuel in the UK. Vitrification is also under consideration for the immobilization of certain intermediate-level wastes (ILW), where there may be advantages of volume reduction and removal of uncertainties in long-term waste behaviour, compared to encapsulation in a cement grout. This paper gives an overview of recent work into the chemical durability of UK vitrified wasteforms to inform the technical specification for the disposal facilities for these waste products and the treatment of their long-term behaviour in post-closure performance assessment. This has included: (1) measurements of the initial glass dissolution rates of a simulated HLW Magnox waste glass in a range of groundwater types representative of potential UK host geologies and in simulated high pH near-field porewaters relevant to potential disposal concepts, using Product Consistency Test type-B (PCT-B) at 40°C; and (2) durability testing of three simulant ILW glasses in a saturated calcium hydroxide buffered solution to simulate conditions in cement-based disposal vaults, using PCT-B tests at 50°C.
The experimentally defined initial rate of HLW Magnox waste glass dissolution in a range of simulated groundwater compositions appears to be similar regardless of the ionic strength and major element composition of the solution. The release of caesium from HLW Magnox waste glass appears to be sensitive to solution composition. Caesium is selectively retained in the glass compared to other soluble components in the two low ionic strength solutions, but is released at similar rates to other soluble components in the three groundwaters and Ca(OH)2 solution. Whether this change in caesium retention is an ionic strength effect or is related to changes in the nature of the surface alteration layer formed on the glass, has yet to be established. For HLW Magnox waste glass, dissolution is accelerated at high pH in NaOH solution, however, the presence of calcium acts to mitigate the effects of high pH, at least initially. In Ca(OH)2 solution, calcium is found to react with all the glasses studied leading to the formation of calcium-containing alteration products. The initial dissolution behaviour in Ca(OH)2 solution varies with glass composition and in particular appears to be sensitive to the boron content.
Reactions in Cemented Nuclear Waste forms – the Need for a Toolbox of Different Cement Types
- Neil B Milestone, Yun Bai, Paulo R Borges, Nick C Collier Jean-Phillipe Gorce, Laura E Gordon, Anthony Setiadi, Claire A Utton, Qizhi Zhou
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- Journal:
- MRS Online Proceedings Library Archive / Volume 932 / 2006
- Published online by Cambridge University Press:
- 21 March 2011, 49.1
- Print publication:
- 2006
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Encapsulation of low and intermediate level waste (LLW and ILW) into a monolithic waste form is a key method for preparing such wastes for storage and ultimate disposal. One of the most efficient ways of doing this is by cementation, successfully used for much of today's operational waste in the UK. Formulations are based on composite cements where high levels ofpulverised fuel ash or blast furnace slag replace ordinary Portland cement (OPC). These provide cementing systems with good fluidity, lowered heat of hydration and a matrix with lower porosity and permeability. Further the alkaline hydrated calcium silicate matrix ensures insolubility of many metal hydroxides and hydrated oxides. Dealing with historic or legacy wastes presents a challenge as for many, a suitable cementing formulation has yet to be devised. To achieve ultimate durability, likely interactions between the waste and the cementing matrix need to be well understood when choosing the cement system to be used. The alkaline OPC based matrix causes corrosion with metals such as Al and Mg and porous silicates and layered metal hydroxides used to selectively remove radionucleides react in the alkaline environment, potentially releasing the adsorbed species. Slurries of materials normally considered inert show interactions within the OPC composite systems, although these are not considered detrimental for durability. This paper describes the chemistry of some interactions that occur within OPC composite cements. It also details some results for alternative cementing systems that may provide a toolbox of cement types that may address many of the interaction issues. These include calcium sulphoaluminate cements, inorganic polymers or geopolymers, and activated slag systems.