[en] This report presents results of an experiment in which synthetic Grande Ronde groundwater was reacted with basalt from the Umtanum flow of the Columbia Plateau and simulated spent fuel at a temperature of 100 degree C, pressure of 30 MPa, and a basalt: simulated spent fuel:groundwater mass ratio of 1:1:20 for a duration of ca. 7000 hours. Reaction products that formed during the 100 degree C experiment included: illite, barite, coffinite, uranophane, and crytolite. The solution composition was modified during the experiment. Concentrations of uranium, thorium, samarium, cerium, rhenium, yttrium, and palladium were low during the experiment. Molybdenum, cesium, and iodide were present in higher concentrations and thus, were fairly mobile. The calculated steady-state high-temperature pH was 7.6. This value is very near the calculated pH value in the basalt/water system at 100 degree C and suggests that the addition of simulated spent fuel does not greatly alter the calculated steady-state high-temperature pH. The redox state is altered by the presence of simulated spent fuel, however. The oxygen fugacity at 100 degree C, as indicated by equilibrium of UO₂ and USiO₄ with the dissolved uranium species concentration, is 10^-36 and 10^-35, respectively. These values are more oxidizing than those predicted from basalt/water experiments and suggests that at a 1:1 basalt:simulated spent fuel mass ratio the simulated spent fuel controls the redox state of the system. 23 refs., 10 figs., 6 tabs