力-热-中子辐照作用下核石墨力学行为的细观机理与多尺度建模研究

Nuclear graphite is the key material to be independently developed for new generation of reactors in our country. Due to the complex internal structure and the severe service-environment, the mesoscopic mechanism of some special mechanical behaviors of nuclear graphite under neutron irradiation, thermal and mechanical stresses is not clear. As a result, reliable theoretical model is not available. In this proposed project, the electron microscope and microcomputed tomography (Micro-CT) would be utilized to measure and characterize the internal structure and texture of nuclear graphite in mesoscopic scale, thereby a geometric model would be built. Then combining with the energy minimization method and the phase field method, a mesostructural numerical model would be established to predict the strong nonlinear behaviors including mesoscopic self-organized cracking and microbuckling of graphite grains, and to simulate the macroscopic mechanical behaviors of nuclear graphite under ion irradiation, thermal and mechanical stresses. On this basis, a multi-scale mechanical model would be established. Then constitutive equations reflecting macro-meso relations and evolution equations describing mesostructural and property changes would be derived. At the same time, experiments with ion irradiation, thermal and mechanical stresses would be carried out, in order to test and verify the theoretical model. Finally, considering the analogies between the ion irradiation effect and neutron irradiation effect, the models for ion irradiation would be extended and applied to the mechanism analysis of macroscopic mechanical behaviors induced by neutron irradiation. Through the deep understanding of mesoscopic mechanism of macroscopic mechanical behaviors of nuclear graphite, the research would make it possible to predict the irradiation-induced mechanical behaviors from the original structure and properties of un-irradiated graphite, in order to provide theoretical foundation for the development, analysis and application of nuclear graphite.

核石墨是我国新一代核能技术中亟待自主研发的关键材料，因其复杂的内部结构和极端的服役环境，其在力、热、中子辐照作用下的诸多特殊力学行为的细观机理还不清楚，尚缺乏可靠的理论模型。本项目拟采用电镜观测和显微CT三维重建等方法观测核石墨的细观组织结构，建立几何模型。然后结合最小能量方法和相场方法分析细观自组织开裂、晶粒微屈曲等强非线性过程，建立细观数值模型，模拟力、热、离子辐照作用下的力学行为；并在此基础上建立多尺度力学模型，推导反映宏-细观关联关系的本构方程和描述组织结构、性能变化规律的演化方程；同时进行力、热、离子辐照加载实验，对理论模型进行考核验证。最后基于离子辐照与中子辐照效应的相似性，将模拟方法和理论模型拓展应用到中子辐照力学行为的细观机理分析。以期通过深入理解核石墨宏观力学行为的细观机理，实现根据其初始细观组织结构、性能预测其辐照力学行为的目标，为核石墨的研发、分析和应用提供理论依据。

核石墨可以作为中子慢化、中子反射和结构材料，是我国新一代核能技术中亟待自主研发的关键材料。核石墨的辐照力学性能对核反应堆的安全性和寿命有重要影响，因其复杂的多尺度内部结构和堆内极端的服役环境，其在力、热、中子辐照作用下的诸多特殊力学行为的微细观机理还不清楚，尚缺乏可靠的理论模型。本研究以微结构观测为基础，建立了多晶核石墨的多尺度力学模型，根据其初始组织结构、性能预测了其辐照力学行为，深入分析了其特有宏观力学行为的微细观机理，为核石墨的研发、分析和应用提供了理论依据。.首先，对核石墨的微结构和性能进行了多尺度观测和测试。在细观和微观尺度上，采用扫描电镜观测到了清晰的针状骨料颗粒、以弯曲微晶构成的粘结剂炭桥和部分尺寸较大孔隙。在亚微观和纳观尺度上，采用透射电镜观测到了石墨晶粒内平行于片层方向的微裂纹、部分微晶内的片层弯曲、平行排列的石墨片层。由此建立了多晶核石墨的多尺度内部结构模型。.之后，建立了石墨多晶有限元模型。基于特征应变思想，建立了石墨单晶本构模型；基于粘聚力思想建立了微晶内、微晶间开裂模型。模拟了自组织开裂、晶粒弯曲等强非线性过程，分析了微裂纹、片层弯曲、单晶性能变化、蠕变和辐照温度对粘结剂炭桥辐照力学行为的影响，揭示了核石墨特有辐照力学行为的内在机理。.最后，建立了多晶核石墨多尺度理论模型。在微观多晶模型的基础上，引入微孔的影响，建立了粘结剂炭桥的均匀化模型。将核石墨看成由各向同性粘结剂基体及焦炭骨料颗粒、大尺寸孔隙组成的非均匀材料，建立了其细观力学理论模型，预测结果的主要特征得到了实验验证。分析了核石墨不同组分体积分数、骨料颗粒形貌对宏观性能的影响规律。.同时，围绕核石墨数值模拟和理论建模研究的需要，还开展了非均匀材料力学行为的多尺度分析方法研究，对于包含纳米尺度非均匀相、强各向异性微结构、高密度微裂纹的非均匀材料的弹性、粘弹性问题，提出了多种解析和高效数值分析方法。