Lithologic And Physico-mechanical Characteristics Of Mudstone In Tamusu Candidate Sector For High-level Radioactive Waste Geological Disposal Repository

The deep geological disposal of high-level radioactive waste(HLW)is currently recognized as an effective and feasible disposal method.The prerequisite for deep geological disposal is to screen out suitable repository sites.The current clay rock(mudstone)geological repository site selection in China is still in the stage of planning and site selection,and the Tamusu candidate sectors in Inner Mongolia were selected for future favorable sites.The thesis takes the mudstone at the target horizon in the candidate sector as the research object.Systematical studies on lithologic and physico-mechanical characteristics of the mudstone in the candidate sector have been conducted,and the influence of mudstone lithologic characteristics on physico-mechanical characteristics and on the construction conditions of repository was preliminary evaluated for further studies.1.The mudstone in the candidate sector is lacustrine deposits as a whole,which has the advantages of good continuity,wide underground extension,high uniformity,and relatively complete rock as the host rock of the repository.The structural faults near the candidate sector are basically basin-controlling faults,and there has been basically no active structural fault affecting the candidate sector since the Quaternary.The groundwater in the area is mainly recharged by atmospheric precipitation.Groundwater system of mud and sand,which can effectively prevent the migration of radioactive hazardous materials in the repository with the groundwater system or affect human water use.2.The target layer in the candidate sector mainly develops gray and dark gray mudstone.The whole mudstone is composed of analcime,dolomite(iron dolomite),albite,etc.,and contains a small amount of clay minerals(mainly illite);also see gypsum,hematite,pyrite,etc.Mudstone clay minerals are mainly illite and montmorillonite,with a small amount of chlorite.The tectonic environment of the mudstone source area of the target layer is mainly continental island arcs and active continental margins,with crustal felsic rocks above the parent rock dominated.The mudstone at the target horizon in the candidate sector is composed of analcime and dolomite as the main minerals.It has strong adsorption performance and can effectively hinder,delay and retain the migration of radionuclides.It is a good natural barrier for the host rock of the repository.The mudstone permeability coefficient in the candidate sector is 10^-12m/s,the natural porosity is2.58-6.64%,the average water content is 3.88%,the thermal conductivity coefficient is1.12-1.64W*m^-1K^-1,the average block density can be up to 2.45g/cm³,compared with the related parameters of the clay rock in underground laboratory in France and in other countries,the physical properties of the mudstone in Tamusu sector are superior.3.The combination of high dolomite,analcime,and low clay minerals in the candidate sector of mudstone improves the engineering mechanical properties of the rock.The tensile strength can reach up to 16Mpa and the lowest is 4Mpa.The compressive(uniaxial)strength has reached 59.48-117.6Mpa(target horizon mudstone),and the lowest is about 25Mpa.Compared with the clay rock related parameters of the clay rock underground laboratory in France and other countries,the foundation mechanical properties of the mudstone in this section are more superior.The mudstone in the candidate sector has a higher bearing stress and a certain degree of self-healing after being broken,and the basic mechanical properties are better with the increase of temperature and pressure.4.The overall mechanical properties of mudstone in the candidate sector increase with the increase of dolomite content(dolomite changes significantly before it is less than 40%),and the content of clay minerals will have a significant weakening effect on rock mechanics(clay minerals are more than 15%after the impact is significant).When the content of dolomite in the rock is about 30%,the content of clay minerals is about 16%,and the content of analcime is between 16%and 30%,the candidate sector mudstone reaches the optimal mineral component content range for engineering mechanical properties and rock adsorption performance.5.The candidate sector has dry climate,low rainfall,backward socio-economic conditions,low population density(less than one person within a hundred kilometers square area in the candidate sector)and no resources for exploitation nearby.The establishment of the repository will not be subject to severe natural disasters.The impact of this will not affect normal human life and production.The transportation in the area is relatively convenient.The candidate sector is 150km away from the Lince railway station entrance and only 120km away from the Jingxin Expressway(G7),which is conducive to the transportation of nuclear waste and construction materials.The above research results are helpful to promote the site selection of clay rock(or mudstone)for geological disposal of HLW,and also provide the useful parameters for the following assessment of the candidate sectors in Tamusu area.