Abstract
ABSTRACTMagnesium oxide (MgO) is the only engineered barrier being emplaced in the Waste Isolation Pilot Plant (WIPP), a U.S. Department of Energy repository for transuranic waste. MgO will lower dissolved concentrations of actinides by consuming CO2 from possible microbial activity, by buffering the pH between 8.5 and 9.5, and by reducing the amount of free water in the repository. This paper discusses results from experiments measuring the hydration of MgO. Results suggest that periclase (MgO) hydrates rapidly to brucite (Mg(OH)2) in de-ionized water and 4 M NaCl solution at 90°C; the hydration rate decreases as temperature decreases. In ERDA-6, a NaCl-rich WIPP brine, MgO hydrates directly to brucite; in GWB, a high-Mg brine, periclase hydrates to magnesium chloride hydroxide hydrate(s) until the dissolved Mg2+ concentration decreases, and brucite becomes the stable phase. Under humid conditions MgO fully hydrates at higher humidities(> 50%). All data are consistent with diffusion-limited hydration reactions.
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