| Deep geological disposal is internationally recognized as the safest and most effective disposal method for high-level radioactive waste(HLW).Therefore,it is particularly important to select suitable disposal repository sites.At present,the screening process of clay rock sites in China is still in the stage of planning and site selection.In order to promote the process of clay(mudstone)site selection in China,combined with the implementation of the research project of Geological Disposal Planning of high-level radioactive waste of the State Administration of Science,Technology and Industry for National Defense.In this paper,an evaluation method suitable for clay rock(mudstone)as the host rock for disposal repository is proposed,and the suitability of mudstone layer in Tamusu candidate sector is preliminary evaluated.At the same time,according to the current research progress of site selection,through numerical simulation and theoretical analysis,the radiation safety of the disposal repository to be built in the mudstone layer of Tamusu candidate sector after closing is preliminarily studied.The main conclusions are as follows:The newly established host rock suitability evaluation system QHLWC includes RQD(Rock quality designation),Ja(Joint alteration number),Jr(Joint roughness number),Jn(Joint set number),Jw,HLWC(Permeability performance index)?SRFHLWC(Strength/stress ratio index)?Fa(Activity index of fault structure)a total of seven indicators.The Tamusu target mudstone shows the characteristics of large scale,moderate burial depth and strong adsorption performance.Target mudstone stratum have good integrity,very low permeability coefficient,good mechanical properties and relatively stable regional tectonic environment.The comprehensive score of QHLWC evaluation system is 403.5,which belongs to the appropriate level and is beneficial to play the role of natural barrier for disposal repository.The list of FEPs(the characteristics of repository,the events and the processes affecting the characteristics of repository)suitable for China's clay rock(mudstone)geological disposal repository is preliminarily screened out,and the normal evolution scenario is constructed that the disposal repository to be built in mudstone will reach the containment period of 1000 years after it is closed,the waste canister will be damaged,and the radionuclide will migrate and diffuse with the groundwater to the host rock and biosphere after it passes through the buffer layer.At the same time,it is proposed the abnormal evolution scenario that the future geological activities or earthquakes may produce dominant water-conducting fracture in the mudstone that run through the disposal repository,there could be a rapid hydraulic channel between the engineering barrier and the early faults,and the radionuclides will directly migrate to the dominant water-conducting fracture after releasing from the engineering barrier.The three-dimensional stratigraphic model and fracture model of the candidate sector are initially constructed,and the numerical simulation is carried out combined with the digital model.It is assumed that the repository to be built in the Tamusu mudstone stratum will dispose of 75000 waste containers.Under the normal evolution scenario,the individual effective dose of 10 000 years and 1 million years is 7.08×10-11mSv/a and2.90×10-8mSv/a respectively,which is far lower than the minimum dose constraint value of0.1mSv/a in France,Belgium,Switzerland and in other countries,which choose clay rock as the host rock of HLW disposal repository,and it is lower than the dose constraint value of 0.25msv/a by ICRP.Under the abnormal evolution scenario of the dominant water-conducting fracture penetrating the repository,the individual effective dose of 10000years and 1 million years is 3.133×10-8mSv/a and 3.12×10-6mSv/a respectively.It is also lower than 0.25mSv/a of individual dose constrain of ICRP and 1.0mSv/a of individual dose constraint for 1 million years in the United States.It can be concluded that the overall safety performance of the mudstone in the Tamusu candidate sector is favorable,which can meet the requirements of radionuclide's safety isolation to biosphere for more than10000 years. |
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