Study On The Permeability Of HLW Buffer Block Joint With Salt Solutions

During the construction and long-term operation of underground repositories,barrier performance of bentonite materials will evolve to a certain extent over a long period,which involves many factors,such as radiation,temperature rise produced by radiation heat,groundwater immersion.Groundwater is a kind of solution with a certain salt content,which will effect on the impermeability of the buffer barrier and cause the degradation of the barrier.Block joints will lead to the preferential seepage of groundwater.Salt in the groundwater will influence the healing degree of buffer block joints,so it is necessary to pay attention to the influence of groundwater on the performance of buffer block joints.GMZ bentonite and quartz were selected as the main material,and 30%quartz was added.Samples were pressed into a round cake shaped sample by static pressure method.There was a joint with a width of 2mm in the middle of the sample,and 70%granular bentonite was filled in the joint.Rigid wall osmotic instrument was used to carry out the penetration test.In this penetration test,the infiltration fluid is artificial simulated groundwater in Beishan,Gansu Province.Solute in the solution is NaCl and Na₂SO₄ with a mass ratio of 2:1,and the total dissolved solid（TDS）was set as 0,0.5,1,2,5,10g/L.The purpose of this study was to investigate the effect of groundwater at different concentrations on the healing of buffer block joints.In addition,thermal conduction tests,porosity and microscopic observations of each sample after penetration test were performed.The degradation degree of buffer block under simulated groundwater solution was evaluated.Results show that,the infiltration rate of samples increases with the increase of infiltration solution concentration,which indicates that the permeability increases.Infiltration rate fluctuates to a certain extent at the early stage,and becomes stable at the later stage.This is because the infiltration solution flows into the bottom of samples initially,and the bottom hydrates and expands preferentially,and the expansion of samples is not uniform at the early stage.The higher the concentration of infiltration solution,the greater the conductivity of exudate solution,but it is always lower than the conductivity of infiltration solution,indicating that some ions are adsorbed in the sample.Saturated permeability coefficient of the sample increases from 3×10^-10cm/s to8.5×10^-9cm/s with the increase of infiltration solution concentration from TDS=0g/L to 10g/L.When the infiltration solution concentration is 1g/L,the permeability coefficient reaches 1×10^-9cm/s,from 10^-10 to 10^-9 order of magnitude.When the infiltration solution concentration continues to increase from TDS=1g/L to 10g/L,the permeability coefficient is always in the order of 10^-9,which shows that the permeability of the sample increases.After penetration tests,dry density of joint zone in the sample increased and the dry density of non-joint part decreased,which indicated that the sample had a certain degree of healing.With the increase of infiltration solution concentration,the dry density of joint zone decreases and the dry density of non-joint zone increases,the degree of healing decreased gradually.The coefficient of thermal conduction decreases with the increase of infiltration solution concentration,and the distribution range is between 1.50-1.97W/（mK）.With the increase of solution concentration,the gap of thermal conduction coefficient between joint zone and non-joint part of the sample increases,and the variance of thermal conduction coefficient in joint zone increases,thermal stability decreases.The mercury injection test showed that,when the concentration of infiltration solution increases,the number of pores under any pore size all increases.It was found by SEM that,when the concentration of infiltration solution was low,the bentonite in joint zone of the sample showed flocculent shape,compact particle arrangement and the high degree of hydration.With the increase of salt,the microscopic morphology gradually tends to be round granular and loose particle arrangement.The healing degree of joint zone declined gradually,which explained the reason for the increase of permeability from the microscopic point.