Basalt/water system: considerations for a nuclear waste repository

High-level nuclear waste emplaced in a repository in basalt will lead to elevated temperatures and chemical reactions between the basalt and repository groundwater. The resultant changes in groundwater chemistry and the formation of secondary minerals will affect radionuclide release rates from the repository. In this study, Grande Ronde Basalt and synthetic groundwater were reacted at temperatures of 1000, 1500, and 3000C at a pressure of 30 MPa. Dickson-type sampling autoclaves were used to follow solution composition changes with time. Solution pH remained weakly alkaline, while steady state concentrations of F-, Cl-, SO4-2, and total carbon were similar to or lower than initial values. Alteration assemblages at 3000C included silica, zeolites, potassium feldspar, and iron smectite. These assemblages are metastable, and prediction of alteration in the basalt and packing material depends on accelerated tests results and must be supplemented by study of metastable assemblages in natural analog systems and by tests in open systems with varying flow rates. Experimental data are consistent with rapid adjustment of Eh to reducing values; however, more work on reaction rates and buffering capacity is in progress. 22 references, 1 figure, 3 tables