Healing Mechanism And Effectiveness Of Buffer Block Joint In HLW Disposal Barrier

As an important artificial barrier for deep geological disposal of the high-level radioactive waste,bentonite buffer barrier forms a tight encapsulation system with the external surrounding rock,which can block the outward migration of nuclides and ensures the reliability of the whole barrier system.Two methods,the in situ backfilling and highly compacted bentonite block assembly,are used to construct the actual buffer systems.However,both ways will produce a certain number of gaps with different shapes,size and distribution directions between the blocks themselves or between the blocks and the canister or host rock.As a"congenital defect"of disposal barrier,the technology gaps will damage the integrity of buffer structure,become a potential concentrated seepage channel of groundwater,block the effective outward transfer of radiant heat,and worsen the engineering performance of barrier.In this paper,three types of bentonite materials,bentonite powder(P),bentonite particle and powder mixture(P-P)and bentonite slurry(S),were chosen as the joint sealing material from the perspective of engineering disposal.A series of constant volume expansion test were conducted and the swelling stress development of block and joint area were monitored to analyze the healing mechanical mechanism of joint,identify the main factors affecting the joint heling,and establish the expansion force prediction model of the sealing joint-block system.After that,the water migration law in the unsaturated stage and the permeability of the block system were studied through a two-stage permeability test.Then,the healing degree of sealed joint was quantitatively analyzed with the help of X-CT images,thermal conductivity and final dry density.Finally,the mercury intrusion porosimetry(MIP)test and scanning electron microscope(SEM)test were conducted to figure out the micro healing mechanism of joint and the optimal sealing material of joint is selected from both the macro and micro aspects.The swelling test results show that:the existence of joints changes the dynamic mechanism of the swelling stress development of the whole block system.Swelling stress develops preferentially to the"low stress area"(joint),and the radial stress increases faster than the axial stress,which is beneficial to the healing of joints.With the hydration of bentonite in the core area of block,bentonite colloid continuously intrudes into the joint area to seal the large pores.The stress compensation in the axial direction will slow down the pressure increase rate and decrease the final value of swelling stress.The degree of stress compensation in the axial direction is inversely proportional to the dry density of the block,and directly proportional to the sand content.With the increase of the proportion of joint volume,the swelling stress in both axial and radial direction decreases,with a higher decreased rate perpendicular to the joint direction.Under the same block dry density and filling density of joint,the type of sealing material have significant influence on the hydration process and seepage path of the whole block system.Blocks sealed with bentonite P-P got the highest value of swelling stress in both directions,however,the hydration time is the longest because of a lower hydration speed of the highly compacted bentonite particles.Blocks sealed with bentonite P got the fastest hydration rate,but the lag time of axial swelling stress is the longest.Blocks sealed with bentonite S got a faster stress development speed in the early stage and it became slower in the later stage,meanwhile,the the final value of the swelling stress is the lowest.Sealing can significantly improve the swelling property of the block system and shorten the time and amplitude of stress adjustment of the joint-block system.Based on the assumation that the expansion force of the joint-block system is all from the water absorption expansion of montmorillonite,the final expansion force of the joint-block system is predicted by using the montmorillonite void ratio e_m,in which the joint/sealing joint is considered as part of the porosity of the block system.The data analysis results show that there is a single linear relationship between the final expansion force?and montmorillonite void ratio e_m in the double logarithm coordinate,which is basically consistent with the law of the intact blocks.Based on this model,the basic design parameters of the compacted block and sealing joint can be calculated from a comprehensive aspect.The permeability test results show that joint sealing can improve the impermeability of the total block system.Under the same condition,the hydraulic conductivity of sealed block system is at the same order as that of intact block,and blocks with a blank joint is one order lower than that of intact block.There is an exponential relationship between the hydraulic conductivity of the sealed block system and the bentonite void ratio e_b.During the hydration process,the density of adjacent block area decreases and the density of the joint area increases,forming a certain width of transition zone.With the increase of the initial filling dry density of joint,the depth of sealing zone and the angel of sealing zone increase.The final healing degree of the joint is positively correlated with the initial dry density of the filling material.After hydration,the thermal conductivity of the block area is slightly higher than that of the joint area,which is consistent with the final dry density and water content distribution.Meanwhile,the thermal conductivity of the joint area fluctuates less,revealing a uniform healing degree.The sealed joint area is still a hydraulic channel that can not be ignored during the hydration process.Affected by the water migration path of joint,the water content of the joint area is higher than that of the block,and the water content in the top and bottom area of block is higher than that in the middle area.The looser the soil in the joint area is,the more significant the hydraulic path effect of the joint is.The microstructural analysis result shows that the soil in the block area and joint area were"welded"together and the boundary disappear after hydration.For the sealed joint system,the pore size range of samples from joint area became wider and the distribution curve moves to the right along the coordinate axis.Meanwhile,the number of intra-aggregate pores decreases and the number of inter-aggregate pores increases compared with the soil from the block area.The CT scanning images and three-dimensional pore model indicate that the bentonite material in the block area continuously"flows"to the joint area,which can promote the compaction and healing of the joint.However,the number and diameter of pores in the joint area are significantly higher than those in the block area are.There are still a certain number of macropores in the healing boundary,and the complete homogenization may not achieve for the whole block system.In general,a joint backfilled with a bentonite P-P mixture has a higher swelling capacity,lower permeability,and denser structure compared with joint backfilled with bentonite P and bentonite slurry.For the block with a blank joint,the CT and SEM image show that the joint area was not closed completely after hydration,and there are some fissure-like 2D pores in the joint area,which indicated that it is still a potential hydraulic access and structural defect of the whole buffer barrier.