UNCERTAINTIES AFFECTING MGO EFFECTIVENESS AND CALCULATION OF THE MGO EFFECTIVE EXCESS FACTOR FOR THE WASTE ISOLATION PILOT PLANT

Magnesium oxide (MgO) is emplaced in the Waste Isolation Pilot Plant to react with carbon dioxide that would be generated if microbes consume carbon in the transuranic waste materials after closure of the repository. On April 10, 2006, the U. S. Department of Energy requested permission from the U. S. Environmental Protection Agency to reduce the quantity of MgO currently emplaced in the repository from 1.67 to 1.2 moles of MgO per mole of organic carbon in the waste materials. Sandia National Laboratories developed an analysis that evaluated uncertainties related to MgO effectiveness to support the DOE’s request. We provide an overview of the uncertainties considered and how they were integrated into the MgO Effective Excess Factor to demonstrate that reducing the quantity of emplaced MgO would not impact the long-term performance of the repository.