Kinetics of MgO Dissolution and Buffering of Fluids in the Waste Isolation Pilot Plant (WIPP) Repository
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ABSTRACT DOE has proposed to add MgO to backfill in the WIPP repository to stabilize pH and reduce gas pressures and spallation. Experiments show that MgO dissolution rates are proportional to surface area, nearly independent of pH, and decrease with increasing dissolved NaCI. The probable repository MgO dissolution rate will be < 10 to 20 l.mol m' min'. Simulations indicate that MgO dissolution will buffer pH and provide stabilizing cement; however, under highly reducing conditions MgO will not control total gas pressures. Adding oxidizing agents or materials containing oxidized species, for example anhydrite, to the backfill, might control gas pressures.
INTRODUCTION The U. S. Department of Energy (USDOE) has received permission to store low-level, transuranic, mixed nuclear waste in a repository, the Waste Isolation Pilot Plant (WIPP), located near Carlsbad, New Mexico. The repository is located at a depth of about 650 meters below the surface in the Salado Formation, a Permian bedded salt comprised largely of halite with minor anhydrite and clay seams [1]. The transuranic waste is composed of a wide variety of materials including: paper, plastic, wood, cellulose, solidified organics and inorganics, iron metal and alloys, soils, cement, and lead packaging [2]. In some performance assessment scenarios, decomposition of cellulose, rubber, plastic, wood, and other organic material in the waste to form CO 2 which may increase gas pressure in the repository to near-lithostatic levels. If drilling inadvertently penetrates the repository under such conditions, the flow of high-pressure gas could carry fragments of radioactive waste to the surface. This process is called spallation, and is the most likely scenanio for repository failure [3]. In order to decrease CO 2 gas pressures and reduce the possibility of spallation, the WIPP has proposed that excess MgO be added in repository backfill [3]. As envisioned, MgO will react with repository solutions to neutralize acidity and reduce gas pressures by reactions such as: MgO MgO MgO MgO MgO
+ + + + +
2 2H+ = Mg + + H20 2H 20+ 2CO2 = Mg2+ + 2HCO3- + H 20 H2 0 = Mg(OH) 2 CO 2 + H20 = MgCO 3*3H 2 0 CO 2 = MgCO 3
(brucite) (nesquehonite) (magnesite)
Thus, dissolution of MgO could increase the pH, remove CO 2 gas, and cause precipitation of cements that could stabilize the repository [3]. Some effects of MgO backfill in the repository have been assessed by Pappenguth et al. [3]. However, they did not measure the
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MgO dissolution rate to determine whether it would dissolve fast enough to neutralize acidity and gases, nor did they fully characterize the nature of equilibrated repository fluids. The present study is in two parts: (1) Measuring the rate of MgO dissolution and some factors influencing that rate. Can MgO dissolve fast enough to neutralize gases and acidity in the repository system? (2) Modeling fluid compositions which might result from degradation of organic waste and metallic iron, dis
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