| Safety assessment is critical to build confidence in safety of geological disposal of radioactive waste.It is also the important support to site selection and design of repositories.This paper focus on developing the long-term evolution scenarios for a potential high-level radioactive waste?HLW?repository in Beishan,China,which is the most potential area for China's HLW repository.Then,theoretical analysis and numerical simulation methods are used to study the post-closure radiological effects of the potential repository.Also,the uncertainty analysis is conducted.In addition,the relevant design of the repository is optimized according to the results of safety assessment.The main conclusions are as follows:?1?Based on analysis and description of expected evolution of FEPs?Features,Events and Processes?,the normal evolution scenario of the radionuclides release and migration after closure of the Beishan repository is constructed.This scenario assumes that the waste canister will be damaged after 1000 years,and then the radionuclides begin to release and migrate into underground water.According to the likelihood of occurrence,four typical abnormal evolution scenarios are constructed,including initial damage of waste canister,fast water-conducting path appeared in host rock,in buffer layer due to earthquake,and in both rock and buffer layer at the same time.?2?According to the characteristics of site conditions and groundwater migration in Beishan area,the post-closure normal evolution scenario of the repository is modeled and analyzed.The results show that the maximum individual biosphere dose is 0.0016mSv/a by the potential repository in Xinchang,3.93×10-88 mSv/a in Shazaoyuan and0.017mSv/a in Suanjingzi,lower than the dose limit 0.3 mSv/a from ICRP?International Radiation Protection Commission?,based on the assumption that 60,000waste canisters are disposed in each of those potential repositories.In general,the potential repositories in these 3 sites can meet the safety requirements for HLW disposal.?3?The abnormal evolution scenarios of the Xinchang potential repository are modeled and analyzed.The results show that the repository can still meet the safety requirement under the above four typical abnormal scenarios.The uncertainty analysis of the parameters of the engineering barriers and the natural barrier indicates that the results of safety assessment are greatly influenced by the uncertainty of parameters.The results of the uncertainty analysis of the scenarios and parameters reflect that those key parameters of the site,such as permeability coefficient,hydraulic gradient and distribution coefficient,should be the focuses during site investigation and laboratory research.Also,the results show that it is important to identify the fracture distribution and water conducting structure,as well as the tectonic activity and blind structures in the site.The above parameters are important to reduce the uncertainty of radionuclide migration and unexpected events disturbance after the closure of the repository,and enhance the confidence in safety assessment.?4?Repository design optimization is conducted for the Xinchang sub-area based on Monte Carlo stochastic simulation methods.On one hand,the safety avoidance distance between disposal vaults in the intact rock and fractured zones is studied through modelling or parameter adjustment.If the hydraulic conductivity of fractured zone is about 4.11×10-7 m/s,the avoidance distance larger than 7 m can guarantee the safety of the whole repository.On the other hand,the optimization of buffer thickness is also studied through modelling and parameter adjustment.The results show that the buffer thickness of 33 cm is enough to meet the safety requirements,even if a fast water-conducting path with a hydraulic conductivity of 4.11×10-7 m/s exists in the intact rock. |
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