Abstract
Bentonite, when used as buffer/backfill material in the deep disposal of high‐level radioactive waste (HLW), could undergo desiccation shrinkage or even cracking due to the heat released from HLW, impairing the efficiency of the barrier system. Furthermore, in‐service buffer materials are inevitably in contact with the groundwater, which sometimes contain high salt concentrations. The groundwater salinity may modify the properties of bentonite and hence affect the process of desiccation and its performance. To investigate this effect, in this study, a series of temperature‐controlled desiccation tests was conducted on compacted specimens of Gaomiaozi (GMZ) bentonite preliminarily saturated with two different saline solutions (NaCl and CaCl2) at the concentration varying from 0.5 to 2.0 mol/L. The experimental results indicated that, as the concentration of saline solution increases, the initial saturated water content of bentonite decreases, whereas the residual water content at the completion of the desiccation test increases. The water evaporation rate is reduced for the specimens saturated with a high‐concentration saline solution, and CaCl2 has a more significant influence on water evaporation than NaCl. The evolution of cracks on the sample surface during the desiccation process can be divided into four stages: crack growth, maintenance, closure, and stabilization; an increase in the salt concentration effectively inhibits crack development. It was shown that the infiltration of saline solutions alters the microstructure of bentonite by changing the arrangement of clay particles from a dispersed pattern to more aggregate state, which results in a decrease in shrinkage strain and shrinkage anisotropy.
Highlights
Bentonite has been widely adopted as buffer/backfill material in the deep geological repositories for safe disposal of highlevel radioactive waste (HLW)
Prior to the desiccation test, a series of scanning electron microscopy (SEM) observations was made on the duplicate bentonite samples, and some typical SEM images are presented in Figure 4. e sample in an as-compacted state, as illustrated in Figure 4(a), demonstrates that there is obvious face-to-face contact between clay particles that are oriented approximately perpendicular to the compaction direction
After being saturated with 2.0 mol/L CaCl2 solution, the bentonite specimen, as shown in Figure 4(d), exhibits an apparent aggregate structure with complete disappearance of flat-shaped stacks of clay platelets, and the aggregated particles are arranged in a disordered manner with the presence of some interaggregate pores. e observed flocculation of clay particles induced by the saline solution is in reasonable agreement with existing studies [30, 31]
Summary
Bentonite has been widely adopted as buffer/backfill material in the deep geological repositories for safe disposal of highlevel radioactive waste (HLW). The long-term decay of nuclear waste can produce a large quantity of heat, which may induce shrinkage or even desiccation cracking of bentonite and damage the barrier performance [7]. Since groundwater salinity can alter the properties of bentonite, it follows that the desiccation and shrinkage behavior of bentonite are affected by water salinity. A clear understanding of how the water salinity influences bentonite’s desiccation behavior is of importance for assessing the efficiency of the barrier system. Backfill with a lower density is more favorable for groundwater inflow and prone to shrinking and cracking when subjected to desiccation
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