Drying Shrinkage And Protective Storage Of Compacted Bentonite-Sand Blocks

In a high-level radioactive waste(HLW)repository,the performance of buffer barriers around the waste canister is a key factor affecting the functionality of the repository.The compacted bentonite-sand blocks(referred to as“blocks”)used for buffer construction control the engineer properties of the buffer.Since bentonite is sensitive to environmental humidity,blocks can adsorb moisture from a humid environment to swell,and can lose moisture to a dry environment,as known as(a.k.a.)drying.Blocks usually have a high initial water content(17%),increasing the risk of drying during storage and transport before the installation in a disposal hole.The drying of blocks results in volumetric shrinkage and drying cracks,and thus compromises the quality of blocks.This research proposed three steps to face this challenge:1)investigate the shrinkage mechanism of bentonite-sand mixtures(BSMs),trying to reduce the shrinkage of BSMs and limit the development of cracks;2)blocks were exposed to the atmosphere for drying.The shrinkage and cracking of the blocks were monitored,trying to provide justifications for developing block storage techniques;3)based on those results,a block storage method is proposed and evaluated.The research methods included:1)paste-like or compacted BSMs were used to study the shrinkage characteristics of BSMs.The mixtures were desiccated in a temperature-controlled room,and the relationship between void ratio and volumetric water ratio was recorded,a.k.a.,soil shrinkage characteristic curve(SWCC).The effects of sand percentage(R_S),chemistry of Beishan groundwater,and particle size of sand on the shrinkage of BSMs were evaluated;2)to bridge the lab-scale samples and industry-scale blocks,lab-scale samples with different dry densities were desiccated to study the shrinkage characteristics.Then,drying shrinkage and crack developments of blocks were monitored by exposing blocks to the atmosphere.To monitor the shrinkage,a specific instrument equipped with dial indicators was developed and digital image correlation(DIC)was also used for comparison;3)a block was sealed with plastic wrap and stored for five months.The moisture and density distributions of the block after the storage were evaluated by computed tomography(CT)scan and direct measurements using subsamples separated from the block.Based on 1)the studies of BSMs,the results demonstrate that sand can reduce the volumetric shrinkage and limit the crack development of BSMs when R_S>30%.R_S,chemistry of porewater,and particle size of sand can affect the volumetric shrinkage of BSMs,and the key factor that controls the ultimate shrinkage(such as shrinkage limit and final void ratio)of BSMs is the particle size/grain size distribution.Clay content or plasticity is not that important in controlling the ultimate shrinkage of BSMs as usually identified.Based on 2)the studies of blocks,The results indicate that permeability of BSMs decreases with the increase of compacted density,resulting in disconnection of water in BSMs.The disconnected water contributes to the separated desiccation of BSMs,i.e.,at the initial of desiccation,the surface of BSMs tends to be dried while the inside of the BSMs remains wet.This separated desiccation results in the secondary shrinkage of compacted BSMs,i.e.,the inside of compacted BSMs can shrinkage after the shrinkage of the surface parts.The secondary shrinkage becomes distinguished during the shrinkage of blocks.The shrinkage of a block is concentrated on the surface while the inside of the block presents relatively small shrinkage.The total shrinkage strain of the block is 2%,which is less than 1 cm.Drying cracks are also generated on the surface of the block at a depth of～2 cm,with a few cracks penetrated into the block.The uniaxial compaction of blocks results in the orientation of clay plates and aggregates in horizontal directions.The oriented clay plates contribute to the higher swelling pressure in the vertical direction(compaction direction),and the oriented clay aggregates contribute to less shrinkage.Also,fewer cracks developed along the vertical direction due to the particle orientation.Sands mixed in blocks can develop preferential flow around them and thus results in isotropic hydraulic conductivity of blocks.Based on 3)the evaluation of a block sealed with plastic wrap after five months,the results indicate that only a 3%decrease in water content happened during the storage.No obvious deterioration was observed on the surface of the block.Moisture was distributed uniformly in the block,compared with the fresh block.Those results provide sufficient justifications that the sealing method is a promising technique to protect blocks during storage.