The effect of phosphate melt cooling rate on phase composition and leach resistance of final waste form
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The effect of phosphate melt cooling rate on phase composition and leach resistance of final waste form Konstantin V. Martynov, Elena V. Zakharova, Sergey V. Stefanovsky, Boris F. Myasoedov Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Leninskii av. 31/4 Moscow 119071 Russia
ABSTRACT
Slow cooling of phosphate melt at liquid nuclear waste solidification yields glass-crystalline material. Partial crystallization during melt solidification results in elemental partitioning among crystalline phase and glass: Al, Cr, Fe are concentrated in the crystalline phosphate phase while Ca, Ni, La, U enter predominantly in the residual glass. Glass dissolution rate and leach rate of La and U as rare earth and actinide surrogates depends strongly on the glass composition, for example reduction of Al2O3 content in the glass to ~10-12 wt.% increases leachability by three orders of magnitude as compared to the glass with specified composition (~18-22 wt.% Al2O3).
INTRODUCTION According to current accepted concept, liquid high-level waste (HLW) must be solidified and long-term stored in geological repositories to be isolated from biosphere. Currently the only solidification technology implemented at an industrial level is vitrification yielding to borosilicate or aluminophosphate (in Russia only) vitreous waste forms [1]. Their structure and properties depends markedly on chemical composition, and temperature regime of melting and cooling. Even formally homogeneous glasses may contain crystalline inclusions of low soluble (spinels, phosphates, zircon, etc.) or incompletely dissolved (silica, zirconia) phases [2-5]. Moreover, by now a number of glass-crystalline materials (GCM) composed of target crystalline phase(s) distributed in the matrix vitreous phase are designed [5-10]. They are produced by either spontaneous or induced (with nucleating agent) crystallization. Most of them are silicate- or borosilicate-based (see, for example, [6-10]). It should be noted that the heating and cooling conditions are even more important for the phosphate-based glasses due to their higher tendency to devitrification as compared to borosilicate glasses. Devitrification of phosphate glasses may yield either crystalline or residual vitreous phases with lower chemical durability than the target glass and result in an increase of elemental leach rate from the final waste form or elemental partitioning among the phases [11-14]. Melt cooling rate is easily controlled under laboratory conditions and to produce a homogeneous glass not containing crystalline phases it has to be as high as possible (quenching). The glassmelt produced in an EP-500 Joule heated ceramic melter Downloaded from https://www.cambridge.org/core. North Carolina State University, on 30 Dec 2017 at 09:41:23, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/adv.2017.616
at PA “Mayak” at temperature of up to 1000 C is poured into 220 L canisters [15]. Using the Newton-Richman thermal equation it
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