THE CARBONATION OF MAGNESIUM OXIDE IN

THE WASTE ISOLATION PILOT PLANT

Anna C. Snider

Sandia National Laboratories

Carlsbad Programs Group

Carlsbad, NM 88220

The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy repository designed for the disposal of defense-related, transuranic waste. The repository is located in southeast New Mexico at a depth of 655 m in the Salado Formation, a Permian bedded salt formation.

Magnesium oxide (MgO) is the only engineered barrier certified for WIPP by the U.S. Environmental Protection Agency. MgO will decrease actinide solubilities by consuming CO2 from possible microbial degradation of organic materials and buffering pH at about 9 and at 10-5.5 atm. MgO will also reduce the amount of water in the repository if subsequent carbonation produces hydrous magnesium carbonates. Formation of magnesite (MgCO3) would release water. The MgO currently being emplaced is supplied by Premier Chemicals of Gabbs, Nevada.

Our investigations of the efficacy of MgO are designed to simulate repository conditions (30°C, PCO2 at 10-5.5 atm). Studies focus on hydration and carbonation rates, and characterization of the products formed under humid and inundated conditions. Carbonation of MgO will be the focus topic for this conference presentation.

Carbonation of MgO at room temperatures, exposed to compressed room air, proceeds at a slow rate. Hydromagnesite (Mg5(CO3)4(OH)24H2O) eventually forms in de-ionized water, 4M NaCl, ,and in the two simulated WIPP brines. Hydromagnesite will control the PCO2 of the repository, which will result in low actinide solubilities.

This research is funded by WIPP programs administrated by the Department of Energy.

Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.