Effects of Magnesium Oxide Backfill on Brine Chemistry and Pu Concentrations

James Conca and Ningping Lu

EES-12, Earth and Environmental Science Division, LANL-Carlsbad Office, MS A141, Los Alamos National Laboratory, Carlsbad, NM 88220, 505-628-3018, jconca@lanl.gov

Magnesium oxide (MgO) is used as a backfill material for geologic repositories because of its capacity to maintain the pH of the repository at basic levels ( > 8), thereby reducing the solubility of radionuclides. MgO also is its able to sequester carbon dioxide that may be produced by microbial degradation of organic materials in the waste, adsorb free water in the repository through hydration reactions, and immobilize Pu by either sorption or precipitation accompanying the overall changes in aqueous chemistry brought about by reactions with the backfill. Two brines, synthetic Castile and synthetic Salado, with Pu at 10-7 M (about 11 nCi), were infused into MgO backfill at three different water contents and equilibrated for various times up to 110 days, then subjected to Pu desorption in the presence and absence of hypochlorite, both under agitated and non-agitated conditions. After Pu(VI)-brines equilibrated with MgO backfill for 68 days, the solution pH and alkalinity changed dramatically, while 99% to 100% of the Pu was removed from the brines. Subsequently, after 36 days of desorption under non-agitated batch conditions, no Pu released from the Pu-loaded MgO-Castile brine even after 110 days and even in the presence of OCl- up to 10-3 M. Only a small amount of Pu (<0.3% after 100 days) was released from the Pu-loaded MgO-Salado brine. XAS results for the Pu-loaded MgO-agglomerates indicate Pu(IV). The MgO reacted with intruding Mg-Cl brine to form a variety of solids including as brucite, bischofite, tachyhydrite, körshunovskite and other Mg-hydroxychlorides. Separate experiments were performed in these Pu-containing brines without MgO in which the pH was adjusted using NaOH instead. Changing the pH alone could not account for the removal of Pu from the MgO-Brine G solutions, but pH change alone did account for almost all of the removal of Pu from MgO-Brine E solutions. Therefore, in NaCl brines the buffering capability of the MgO backfill alone may be adequate to prevent Pu migration, while in MgCl brines, the backfill is performing a sorptive function in addition to simple pH buffering that enhances its ability to retard Pu migration.