Study On The Engineering Properties Of Rubber-bentonite Mixture For External Barrier Of Nuclear Waste Storage Tank In Groundwater Salinization Environmen

As a potential storage tank for highly radioactive nuclear waste,the groundwater in Beishan area of Gansu Province has the hydrochemical characteristics of salinity,which will affect the safety of the storage tank.The saline groundwater will harden the external bentonite cushion of the nuclear waste storage tank,weaken the deformation buffer capacity,and lead to the surface damage of montmorillonite particles on the bentonite cushion layer and decrease the permeability.Then the groundwater erosion to the external structure of the storage tank,resulting in the leakage of nuclide,harm the surrounding ecological environment.In addition,the operation time of the high-level nuclear waste storage tank is hundreds of years,so it is necessary to consider the change of the shear capacity of the external cushion under the stress history.Therefore,in order to improve the engineering characteristics of bentonite cushion in saline groundwater environment,rubber particles are added into bentonite cushion to improve its compression deformation,deformation buffer,seepage resistance and shear resistance.In this study,different concentrations of sodium chloride solution were used to simulate the groundwater environment with different degrees of salinity.The engineering properties of bentonite cushion improved by rubber particles were studied,and the changes of compression properties,expansion permeability and shear strength of rubber-bentonite mixed materials with different rubber particle incorporation ratios were investigated under chemistry-force field coupling environment as well as under loading conditions.The study draws a number of conclusions.（1）One-dimensional consolidation compression test was carried out to explore the influence of sodium chloride solution on consolidation compression changes of MX80bentonite/rubber mixture.When the ion concentration of sodium chloride solution increased from 0mol/L to 0.1mol/L,the microstructure of montmorillonite particle surface inside the sample changed from agglomeration state to flocculation state,from face to face structure to side to side structure.The change of particle surface structure reduces the compression coefficient and increases the compression modulus of the sample.（2）On the consolidation instrument,the expansion rate and the corresponding expansion amount of the sample will decrease due to the action of salt solution after the sample is expanded at 50k Pa for 48 hours.With the increase of salt solution concentration,the expansion amount and expansion rate of the sample will also decrease under the same salt solution environment.And increasing the weight ratio of rubber particles can reduce the expansion amount and expansion rate,and reach the stability time is shorter.When the rubber content exceeds 50%,compression will occur in medium and high concentration salt solution environment.（3）The permeability coefficient of the mixed material was calculated according to the data of one-dimensional consolidation and compression experiment,and the changes of the mixed material under different salt solution concentrations were studied.The permeability of sample B6T4 is relatively stable and lower than 1×10^-9cm/s under the influence of external stress in any salt solution environment.This indicates that the ratio of B6T4（bentonite mass ratio of 60%,rubber particle mass ratio of 40%）has excellent anti-seepage ability against the erosion of saline groundwater.（4）The influence characteristics of salt solution environment on the strength of mixed materials under long-term stress history were studied,and based on this,direct shear tests were conducted.When the ion concentration of salt solution increases from 0mol/L to0.1mol/L,the shear strength of sample will decrease.The thickness of the diffuse double electron layer between sample particles is inhibited due to the erosion of salt solution ions.Under the influence of sodium chloride ion,the molecular force of the sample will have an effect,thus reducing the repulsive force between particles,and the cohesion of the sample will also be correspondingly reduced,thus reducing the effect of blocking the movement of particles.In spite of this,sample B6T4 showed excellent shear strength under long stress history under the influence of salt solution ions.