USE OF MGO AS THE ENGINEERED BARRIER IN THE WIPP
MgO will decrease actinide solubilities by consuming essentially all aqueous or gaseous CO2 produced by microbial consumption of cellulosic, plastic, and rubber (CPR) materials (even if microbes consume all CPR materials in the repository), thereby buffering fCO2 and pH within ranges favorable from the standpoint of actinide solubilities. The DOE is emplacing significantly more MgO than required to consume all CO2. MgO could also consume significant quantities of H2O from brine or the gaseous phase.
MgO is being emplaced in polypropylene supersacks on top of the stacks of waste containers. Each supersack contains 1905 ± 23 kg of MgO. Emplacement in supersacks: (1) facilitates handling and emplacement of MgO; (2) minimizes potential worker exposure to dust; (3) minimizes the exposure of periclase, the main reactive constituent of MgO, to atmospheric CO2 and H2O during handling and emplacement, and prior to panel closure.
The supersacks contain dry, granular MgO, of which less than 0.5 % can exceed 9.5 mm in diameter. Emplacement of granular MgO instead of powder (1) results in a bulk density high enough that sufficient MgO can be emplaced without causing operational difficulties, (2) reduces the likelihood of release of dust in the event of premature rupture of a supersack, and (3) ensures that the permeability of the material is high enough to promote complete reaction with CO2.
Creep closure of WIPP disposal rooms will rupture the supersacks and disperse the MgO among and within ruptured waste containers. This will expose the MgO to the room atmosphere, to CO2, and to brine and water vapor.
Acknowledgement: this research is funded by WIPP programs administered by the U.S. Department of Energy.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energys National Nuclear Security Administration under contract DE-AC04-94AL85000.