Study On Bentonite Extrusion Process Into Fracture And Erosion By Seepage In The Nuclear Waste Repository

Bentonite is often chosen as buffer/backfill material of high-level nuclear waste repository due to its high expansibility,low permeability,filling between the waste canister and surrounding rock.Once connect with water,bentonite swells and extrudes into the fracture,forming gel under the action of groundwater,which may be eroded by seepage.If the quality of bentonite which extrudes into fracture and eroded by seepage of bentonite exceed a certain degree,the density of the bentonite buffer will decrease,may lead to a drop in performance of the repository and even lost.Therefore,the quantitative understanding of the bentonite extrusion process into the fracture and the erosion by seepage,is vital significant toensure the long-term stability of repository.To this end,this paper made the following research:(1)Expounds the montmorillonite microstructure and swell mechanism,the only swell mineral in the bentonite.Introduces the electric double layer theory which describes the swell process of bentonite,and focused on the defects of electric double layer model,bring in the crystalline force to modify the model.In addition,the modified electric double layer model and the benchmark test results are compared.(2)Basic mechanism of bentonite extrusion into the fracture could be thought of free swelling of bentonite,bentonite free swelling model was improved,utilizing the modified double electrical layer model.compared with results of the bentonite free swelling test.the sensitivity analysis of some important parameters in the model have be carried out.(3)Based on bentonite free swelling model,using the numerical method,Na-bentonite in the buffer extrusion into the fracture has been researched under the condition of no seepage,and the influences of electrolyte concentration,initial dry density on bentonite extrusion distance and quality are analyzed.(4)Combining the extrusion process of bentonite with the migration process of cationic and seepage process of groundwater,the erosion model to bentonite by seepage model is established,to simulate the steady-state bentonite extrusion distance and erosion rate with the change of seepage velocity,and the influence ion concentration and bentonite density on the process are analyzed.