Abstract
Bentonite has been considered as a candidate buffer material for the disposal of high-level radioactive waste (HLW) because of its low permeability, high sorption capacity, self-sealing characteristics and durability in a natural environment. Bentonite erosion caused by groundwater flow may take place at the interface of the compacted bentonite and fractured granite. Surface erosion of bentonite flocs is represented typically as an erosion threshold. Predicting the erosion threshold of bentonite flocs requires taking into account cohesion, which results from interactions between clay particles. Beyond the usual dependence on grain size, a significant correlation between erosion threshold and porosity measurements is confirmed for bentonite flocs. A fractal model for erosion threshold of bentonite flocs is proposed. Cohesion forces, the long-range van der Waals interaction between two clay particles are taken as the resource of the erosion threshold. The model verification is conducted by the comparison with experiments published in the literature. The results show that the proposed model for erosion threshold is in good agreement with the experimental data.
Highlights
Several countries plan to use bentonite clay as the buffer and backfill material in nuclear waste repositories.[1,2,3] Compacted bentonite clay around the canister protects it from chemical and mechanical disturbances
We proposed a fractal framework for the surface erosion threshold characteristics of clays, and took effort to relate the surface erosion threshold to the floc size, the effective density and the solid volume fraction of the clay flocs, respectively
The erosion threshold is related the floc size as τc = αa(da/dp)−D/3, and the erosion threshold is expressed by the solid volume fraction and the effective density as τc = βa(φs)−D/3(D−3) and τc = βa[ρe/(ρp − ρw)]−D/3(D−3), respectively
Summary
Several countries plan to use bentonite clay as the buffer and backfill material in nuclear waste repositories.[1,2,3] Compacted bentonite clay around the canister protects it from chemical and mechanical disturbances. No reliable method exists to estimate the erosion threshold for clay based on the clay properties.[23] It is well known that clay particles tend to form complex structures, called flocs, due to cohesive or adhesive forces, and the floc networks tend to be self-similar.[24,25] Based on the scale invariance of a self-similar fractal structure, the number of bonds in an erosion plane does not depend on the size of flocs. Where τc[0] is a scaling parameter, da is the floc diameter, dp is the particle diameter, γ is an empirical coefficient, ranges between 0.5 and 1.5.28 Following the definition of the solid’s volume fraction within flocs, it can be concluded that the erosion threshold is not constant but a function of the floc structure and size.
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