Experimental Work Conducted on MgO Characterization and Hydration
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Experimental Work Conducted on MgO Characterization and Hydration Haoran Deng1, Yongliang Xiong1, Martin Nemer2 and Shelly Johnsen1 1 Repository Performance Dept. 6712 2 Performance Assessment and Decision Analysis Dept. 6711 Sandia National Laboratories Carlsbad Programs Group, Carlsbad, NM 88220 ABSTRACT Magnesium oxide (MgO) is the only engineered barrier certified by the EPA for emplacement in the Waste Isolation Pilot Plant (WIPP), a U.S. Department of Energy repository for transuranic waste. MgO will reduce actinide solubilities by sequestering CO2 generated by the biodegradation of cellulosic, plastic, and rubber materials. Demonstration of the effectiveness of MgO is essential to meet the U.S Environmental Protection Agency’s requirement for multiple natural and engineered barriers. In the past, a series of experiments was conducted at Sandia National Laboratories to verify the efficacy of Premier Chemicals LLC (Premier) MgO as a chemical-control agent in the WIPP. Since December 2004, Premier MgO is no longer available for emplacement in the WIPP. Martin Marietta Magnesia Specialties LLC is the new MgO supplier. MgO characterization, including chemical, mineralogic, and reactivity analysis, has been performed to address uncertainties concerning the amount of reactive constituents in Martin Marietta MgO. Characterization results of Premier MgO will be reported for comparison. Particle size, solid-to-liquid ratio, and stir speed could affect the rate of carbonation of MgO slurries. Thus, it’s reasonable to hypothesize that these factors will also affect the rate of hydration. Accelerated MgO hydration experiments were carried out at two or three levels for each of the above factors in deionized water at 70 ºC. The Minitab statistical software package was used to design a fractional-factorial experimental matrix and analyze the test results. We also fitted the accelerated inundated hydration data to four different kinetic models and calculated the hydration rates. As a result of this study we have determined that different mechanisms may be important for different particle sizes, surface control for large particles and diffusion for small particles. INTRODUCTION The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy repository for defense-related transuranic waste. It is located in southeast New Mexico at a depth of 655 m in the Salado Formation, a Permian bedded-salt formation. Cellulosic, plastic, and rubber materials are included in the waste containers as actinide-contaminated waste. This includes lab coats, plastic bottles, rubber gloves, as well as waste-emplacement materials such as wood palettes, plastic wrap, and slip sheets. Possible microbial degradation of cellulosic, plastic, and rubber materials could generate carbon dioxide (CO2). Carbon dioxide dissolution would acidify the brine, which would directly affect actinide speciation and solubilities. Therefore, MgO is emplaced in the WIPP repository to reduce actinide solubilities by sequestering CO2. In addition, after t
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