Underground radioactive waste repositories: assessment of post-closure safety under extreme events

拟议的研究涉及对低级和中级核废料的深层地质库(DGR)进行长期安全评估。在许多拥有核电生产的国家，DGR正在被考虑。这些国家包括瑞士、法国、瑞典、美国、德国、英国和日本等。第一个进入建设阶段的项目是芬兰的Onkalo储存库，该项目于2015年11月获得监管部门批准。在加拿大，选址工作目前正在进行，最近的工作重点是安大略省南部的泥质地层(Cobourg石灰岩)，作为DGR的可能选址。该项目的主要目标是检查现有方法中的差距，并建立一些新的方法来评估在泥质岩中建造的DGR关闭后的安全性。这项研究将包括四个主要研究主题，即。(I)Cobourg石灰岩的力学和水力特性的描述；(Ii)裂隙介质中流动的水-力耦合分析；(Iii)与开挖损伤区(EDZ)近场评估相关的案例研究；以及(Iv)极端事件下DGR关闭后的安全性评估。在第(I)部分中，所讨论的基本问题将是石灰岩的强烈非均质性组构与其机械/水力特性之间的关联。织物的量化将采用基于体视学的显式张量测量，该测量将提供与方位相关的信息。对于水力特性，电导率张量将被定义为孔隙网络几何形状的函数，并将假定其与应力场相关的演化规律。第二部分，裂隙介质渗流的流体力学耦合分析研究。将开发一种新的公式，将场算符平均纳入与宏观裂纹相邻的参考体积内。最后两部分，即(三)和(四)，将涉及拟议办法的数字实施和验证。重点将放在评估DGR在发生极端事件时的长期安全性，如强烈地震和气候变化，即。冰川作用和/或永久冻土。力学分析将辅之以放射性核素通过地球圈的运输时间和逃逸到环境中的可能性的评估。该项目的主要成果将是加强对与核废料长期地下储存有关的物理-机械过程的了解，并制定可靠的数字程序来评估发生极端事件时的安全裕度。

The proposed research is concerned with a long-term safety assessment of Deep Geological Repository (DGR) for low and intermediate-level nuclear waste. DGRs are being considered in many countries with a nuclear power production. Those include Switzerland, France, Sweden, USA, Germany, UK and Japan among others. The first project to enter the construction stage is Onkalo repository in Finland, which obtained regulatory approval in November 2015. In Canada the site-selection process is currently under way and the efforts have recently focused on argillaceous formations of southern Ontario (Cobourg limestone) as a possible location for DGR. The primary objective of this project is to examine the gaps in the existing approaches and to establish some new methodologies for evaluating post-closure safety of DGR constructed in argillaceous rocks. The study will include four major research themes, viz. (i) description of mechanical and hydraulic properties of Cobourg limestone; (ii) coupled hydro-mechanical analysis of flow in fractured media; (iii) case studies related to near field assessment of excavation damaged zone (EDZ), and (iv) evaluation of post-closure safety of DGR under extreme events. In part (i), the fundamental issue addressed will be a correlation between the strongly heterogeneous fabric of the limestone and its mechanical/hydraulic properties. The quantification of fabric will employ an explicit tensorial measure, based on stereology, that will provide an orientation dependent information. For hydraulic properties, the conductivity tensor will be defined as a function of geometry of the pore network and an evolution law will be postulated to correlate it with the stress field. In part (ii), the research on the coupled hydro-mechanical analysis of flow in fractured media will be conducted. A new formulation will be developed that incorporates an averaging of the field operators within the referential volume adjacent to macrocrack. The last two parts, i.e. (iii) and (iv), will deal with a numerical implementation and verification of the proposed approaches. The emphasis will be placed on assessing the long-term safety of DGR in case of extreme events, such as strong earthquakes as well as climatic changes, viz. glaciation and/or permafrost. The mechanical analysis will be supplemented by the assessment of the radionuclide transport times through the geosphere and the possibility of escape to the environment. The primary outcome of this project will be an enhanced understanding of the physico-mechanical processes associated with long-term underground storage of nuclear waste and development of reliable numerical procedures for assessing the safety margins in case of extreme events.