HLW Research Area Of ​​regional Groundwater Flow Simulation And Rock Permeability Characteristics Preselected Beishan Disposal Repository

Deep geological disposal of high-level radioactive waste is considered to bemore stable and safer way to isolate high-level radioactive waste from humanenvironment. Normally, the surrounding rock is expected to be low-permeable matrix.The safety of geological disposal depends on the screen effect of surrounding rockand fluid flow with nuclear waste transport through complex fractured rocks.Therefore, it is critical to conduct hydrogeological assessment at potential disposalsites of interest. Based on the project of “Technical study on the selection andevaluation of potential sites and their relevant geological disposal issues in GansuBeishan region”, this study, at the scale of potential selection site, further made effortson characterizing groundwater circulation, developing a regional numerical model,analyzing controlling factors on groundwater circulation, macroscopically capturinggroundwater flow path, and providing hydrogeological evidences for high-levelradioactive disposal at Beishan region. Additionally, at the scale of potential selectionsite, we conducted researches on capturing spatial distribution of hydraulicconductivity for tentative rocks of interest, developing discrete fracture networkmodeling to simulate hydraulic properties of surrounding rocks, building up a methodto evaluate permeability of matrix, all of which were aimed to provide statistical andtheoretical evidences for selecting and evaluating potential sites.The study developed a hydrogeological conceptual model based on the analyzingand mapping corresponding hydrogeological aspects, including groundwater budget,flow net, and relevant spatial distribution of hydraulic properties. Furthermore, wedeveloped stead-state groundwater numerical model through GMS and used observeddata to calibrate the numerical model. According to the numerical results, we analyzedwater budget for the studied area, and used particle tracking method to simulatenuclear waste movement travelling through the fractured reservoir. Results from thisstudy show that:(1) The average annual recharge to studied groundwater system isaround12042.19×104m3/a, while the average annual discharge is12128.50×104m3/a, with deficiency amounting to-86.31×104m3/a. The recharge magnitude to thegroundwater system is mainly composed of infiltration from precipitation and leakagefrom flood in valleys, which is1418.19×104m3/a. On the other hand, groundwater ismainly discharged in terms of evapotranspiration, which is around9589.17×104m3/a,followed by the lateral discharge to adjacent groundwater system, among which with1068.74×104m3/a is discharged to the downstream of Hehei plain area, whereas with1178.21×104m3/a is discharged to Hexi Corridor region;(2) Groundwater flows bylarge from west to east, where the studied discharge areas are the downstream ofHehei plain and the Hexi Corridor region;(3) According to the particle trackingmethod, it takes around7.2~16.3×106years for particle traveling from theMazongshan area to the Hexi Corridor region, while the shortest and longestresidence time for particle traveling from the Mazongshan area to the downstream ofHehei plain are19.6×106and68×106years. As expected, it takes very long path forparticle circulating through the studied groundwater system and thus results in longresidence time. Moreover, little water mixing and exchanging during circulationprocess. Therefore, Beishan area is considered to be a favorable site for the purpose ofhigh-level radioactive disposal.Based on the data from in-site fracture measurements, this study statisticallyanalyzed hydraulic properties of surrounding rocks around drilling well BS16. Theresults show that:(1) Hydraulic conductivity of surrounding rocks about BS16variesfrom10-7m/s~10-14m/s, and shows a fluctuating distribution in the vertical directionwithout a determined spatial pattern;(2) The principle directions of hydraulicconductivity for surrounding matrix are35°,298°and70°, respectively. Theanisotropic ratio C1ranging between1.3~1.8with average value to be1.46; theaverage anisotropic ratio C2is around3, taking up25%of ensemble samples.Upon analyzing the characteristics of fracture development around BS16, thisstudy developed discrete fracture network through Fracman software, and furtheranalyzed the connectivity of fractures. The results show that:(1) The preferentialattitudes of fractures can be categorized into five groups, which are239∠74°, 303∠66°,56∠63°,136∠72°and97∠12°, respectively. The inclinations anddip angles of those groups normally follow normal Gaussian distribution, except forthe inclination of the fifth group. Additionally, the fracture spacing and length follownegative exponential function. Note that there is a clear vertical stratification forfractured rocks, which can be categorized into six layers according to the fracturedensity distribution. Fracture attitude follows normal Gaussian distribution.(2)According to the results from discrete fracture network modeling at BS16, hydraulicconnection for fracture network is concentrated on the depth between260m~480m,with preferential fracture orientations amounting to320°,70°, and100°. Below thedepth about500m, there is poor hydraulic connection for fracture network, and thus itis a favorable depth for geological disposal of high-level radioactive waste.