Experimental Investigation of Sodium Bentonite Stability in Hanford Basalt

摘要

Sodium bentonite is a candidate material for the waste package backfill component in a repository in basalt at the Hanford Site. Preliminary hydrothermal experiments have been conducted under near-field geochemical conditions expected to occur in the reference repository location in Grande Ronde Basalt. Experiments have been conducted in the basalt/groundwater, bentonite/groundwater, and basalt/bentonite/groundwater systems. The experiments have been conducted at 300 exp 0 C using a simulated Grande Ronde groundwater, reference Umtanum basalt, and sodium bentonite. Key data generated by the experiments include experimental solution analyses as a function of time and preliminary solids analysis by scanning transmission electron microscopy and x-ray diffraction. Solution trends of the major aqueous species were similar in the three systems and are characterized by: (1) the gradual reduction of the pH value from approx.9.75 to a steady-state value of approx.6, (2) an initial rapid increase followed by a gradual decreasein silica concentration, and (3) a slight or negligible increase in sodium, sulfate, and chloride concentrations. In the bentonite/groundwater experiment, small amounts (<1%) of an albite reaction product were observed. Conserversely, the formation of illite, a common bentonite alteration product, was not observed. These results indicate tjhat sodium bentonite will remain sufficiently stablee at 300 exp 0 C under hydrothermal conditions in basalt to permit its use as a backfill material. (ERA citation 08:023174)