Abstract

The diffusivity of uranium (VI) has been measured in a supercooled borosilicate melt over the temperature range 560-800 °C. The results, determined by a diffusion couple technique, are: D = 8800 −5000 +11400 exP + (299500 ± 6400/ RT ), where the activation energy is 299.5 kj/mol and the diffusion coefficient is in cm 2 /s. The activation energy of U(VI) diffusion is consistent with a UO 2 2+ complex being the diffusing species. Based on the present data set and data on the speciation of U(IV) and U(V) from the literature it is proposed that the diffusivities of the three primary uranium valence states in borosilicate melt and glasses decrease in the order U(V1) > U(IV) > U(V). Therefore, U(V) may be the preferred valence state for uranium in borosilicate nuclear waste glasses.

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