Groundwater interaction with bentonite

In this chapter we will pay most attention to the isolation function of the innermost of the barriers, the waste matrix, and its potential interactions with the contacting water. In addition, and because of the similarities in the processes involved, we will also discuss the key processes that control the mobility of some of the critical components of waste in ground-waters. These key processes are bentonite/ groundwater interactions, which can exert a large influence on the processes controlling the master pH/pe variables, iron corrosion processes responsible for poising the redox potential of the system and the interactions between the waste matrix itself and the contacting fluids, which produce radiolysis reaction processes. 

Fig. 3. Simulations calculated with the PHREEQC geochemical code (Parkhust Appelo 1999) (a) time-dependent diagram for the pH evolution of the Aspo ground water/bentonite interaction (b) time-dependent diagram for the pe evolution of the Aspo groundwater/bentonite interaction. Curves correspond to different initial partial oxygen pressures. Initial calcite and pyrite contents are 0.3 wt% and 0.01 wt% respectively, except for the curve of log/02 = —0.22 where calcite and pyrite contents are 1.4 wt% and 0.3 wt%, respectively, pe calculated stands for the cases where the oxygen fugacity is obtained from the groundwater redox potential (Bruno et at. 1999).

Initially, Oz diffuses through the bentonite and granitic domains, controlling the redox state of the system. Once 02 is exhausted, granitic groundwater controls the redox state of the system. The results of these calculations performed with the PHREEQC geochemical code (Parkhust Appelo 1999) clearly indicate that there is a substantial variability in pH/pe space along the temporal and spatial evolution of the near field of a repositoiy. This has clear consequences for the subsequent interactions with the Fe canister material and finally with the spent fuel matrix. 

While bentonite is transported by flowing, unaltered groundwater, interaction with unaltered basalt surfaces appears to agglomerate and to bind it to the basalt. 

Thus far we have discussed only the interaction of analog groundwater with unaltered bentonite and basalt. Table V, which contains data from the 1000 y analog experiment (Exp. 4, Table I), shows the quite different, and quite limited, effect... 

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