Research On Swelling Properties Of Bentonite Buffer Block For HLW Disposal

Deep geology disposal is a universally acknowledged effective method for HLW disposal.Bentonite is regarded as an effective buffer/backfill material because of its engineering properties,for example,it can retard migration of underground water and nuclides effectively.Expansibility is one of key parameters for the design of HLW depository.However,the samples used in laboratory were small-sized,and the test conditions could be well controlled,which can be regarded as ideal conditions.There are several advantages for laboratory samples,such as,selection of materials,improvement of optimum mixture ratio and control of test conditions.The existing test results showed when water content is 12%,the performance of compacted blocks is best and the dry density is 1.87 g/cm~3.During the actual construction of HLW depository,batches of large bentonite blocks would be compacted industrially.There will be many problems during preparing of blocks,including:mixing method,sizes of blocks and machinery.It is difficult to stimulate these conditions effectively and the amplification tests should be conducted to solve these problems.As the distribution of stress inside the blocks would be more complex than in the laboratory small samples,the structures inside blocks would be heterogeneous,which would cause the difference of expansibility inside blocks.As a result,the performance of engineering barrier was deteriorated.So,it is essential to study the expansibility inside blocks spatially and directionally.To analyses distribution of dry densities and water contents inside the blocks which prepared by large machinery and the difference of expansibility among different positions and different directions,the GMZ bentonite and 30%(in weight)quartz sand mixtures had been selected as test materials,three blocks had been compacted in the same dry density(1.87 g/cm~3)and water content(12.0%).The cylindrical coordinate system had been constructed,and the blocks had been sliced into small blocks.The cutting ring samples are taken from these small blocks.These cutting ring samples had been used to test their swelling presses by one-dimension oedometers.The normal directions of planes of Cutting ring samples taken from the same blocks directed to the same direction which is Z-direction,R-direction and?-direction,respectively,for each blocks.Remaining parts of small blocks,after cutting ring samples had been taken,had been used to test their dry densities and water contents.The same sampling means had been used to test swelling strains.The results showed that,the dry densities of small blocks in the same stratum are between 1.81 to 1.88 g/cm~3 and the water contents of small blocks in the same stratum are between 10.6 to 12.1%after slicing,which showed that distribution of dry densities and water contents were homogeneous.In the loading side of the block,the dry densities of small blocks in the central of the stratum were smaller than the other parts.As the distance was increasing from loading side,the dry densities in the central part were increasing(between 0~0.03g/cm~3),while those of other parts were decreasing(between 0~0.04 g/cm~3).Swelling pressure of all cutting ring samples are increasing exponentially as time lapsed.At the early-and Medium-period of development of swelling pressure,the horizontal swelling pressure developed faster than the vertical one.However,the time needed to achieve the final swelling pressure was almost the same for samples along three directions.The maximum swelling pressures are between 900?1250 kPa,800?1075kPa and 800?1062.5kPa,respectively,for Z-direction,R-direction and?-direction.The ratios of swelling pressure along R-and Z-direction are among 0.86 to 0.90.The ratios of swelling pressure along?-and Z-direction are among 0.85 to 0.91;In the semi logarithm coordination,swelling strains were S-shape increasing as time lapsed for samples along three directions.At the later period,swelling strains developed faster in Z-direction than in the other two directions.The maximum swelling strains are between 109~127%,99~112%and98~115%,respectively,for Z-direction,R-direction and?-direction.Mean dry densities and water contents were 0.87 g/cm~3 and 76.0%.The distribution of dry densities and water contents of expanded samples showed that dry densities of the upper and lower parts were smaller than those of medium part;water contents of the upper and lower parts were larger than those of medium part.The distribution of final swelling strains had a bad unbiasedness along Z-direction,while the distribution of final swelling strains had a good unbiasedness along R-and?-direction.So,the block showed a transverse isotropy.