Abstract
Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and hydrotalcite (Mg6Al2(OH)16CO3·4H2O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing UVI from radioactive waste streams. U LIII-edge X-ray absorption spectroscopy (XAS) was used to probe the UVI coordination environment in contact with these minerals. For the first time, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these UVI-contacted minerals and make inferences on the UVI coordination, in conjunction with the X-ray spectroscopy analyses. The U LIII-edge XAS analysis of the UVI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with 27Al NMR analysis, indicated that a disordered Ca- or Al-bearing UVI secondary phase also formed. For the UVI-contacted hydrotalcite phases, the XAS and 27Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.
Highlights
Cementitious binders are used extensively in radioactive waste management
It was concluded that the discrepancy in the uptake between the two mineral phases was due to the significant amount of dissolved carbonate released from the hydrotalcite phases, leading to UVI-carbonate complex formation in solution, increasing the UVI solubility and decreasing the amount of UVI uptake by the solid phase (Pshinko et al, 2013)
This indicates that the uranyl moiety [O U O]2+ was maintained, and with reference to the EXAFS model fits this is likely to be in a pentagonal bipyramidal coordination given that a total of NOeq = 5 were fitted for samples of ettringite exposed to 0.5 mM and 10 mM UVI
Summary
Cementitious binders are used extensively in radioactive waste management. In particular, intermediate- and low-level radioactive waste (ILW and LLW, respectively) are generally suitable for cementitious encapsulation as they are non-heatgenerating; in the UK these are encapsulated using cement blends with high replacement levels of Portland cement (PC) by supplementary cementitious materials (SCMs), including blast-furnace slag (BFS) and fly ash (FA) (Ojovan & Lee, 2005; Batchelor, 2006). ILW streams in particular may still contain measurable radioactivity, some of which will arise from the presence of actinides (Nuclear Decommissioning Authority, 2019). One of the options for management of this material, comprising 238UO3 and 238U3O8 powders, is encapsulation within a cement, or mixing with concrete to form a depleted uranium aggregate (DUAGG) which could potentially be used to line vaults in a geological disposal facility (Nuclear Decommissioning Authority, 2014; Radioactive Waste Management, 2016). In these scenarios, cement grout will come into direct contact with U-bearing material. Since the primary decay product of Pu is U, cements that encapsulate PCM will, in the future, contain U isotopes
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