不同氧化程度锆合金失水事故降温过程应力场演变研究

Study on stress field evolution of zirconium alloys with different oxidation degrees during cooling process of Loss of Coolant Accident

摘要:

为了研究不同氧化程度的锆合金在失水事故过程应力场的演变规律，以失水事故后三层结构的Zr-4包壳为研究对象，将α-Zr(O)体积分数代表不同氧化程度的锆合金包壳，建立三种不同氧化结构的锆合金模型并进行1200 ℃到800 ℃再到室温的两步冷却有限元模拟。计算结果表明：氧化膜内的应力为压应力，且氧化膜应力随温度降低的变化梯度最大，导致α-Zr(O)层与基体的交界处产生较大的残余应力；基体组织体积占比越大的模型降温后内部残余应力越小；低氧化程度模型中α-Zr(O)层在降温末期应力基本不变，但在氧化程度较高时有明显的应力集中现象，高氧化程度模型在降温时会产生较大的压应变，并导致基体产生较大变形。对降温过程不同氧化程度模型的各组织进行了相应的本构分析，发现淬火初期各层组织的应力先降低后增加，基体靠近α-Zr(O)层边缘组织与心部组织力学性质存在差异，氧化程度较高的模型，同样应变下基体心部组织压应力高于边缘组织。

Abstract:

Base on finite element method, the stress field evolution of zirconium alloys with different oxidation degrees during the Loss of Coolant Accident process is analyzed in this paper, in which the Zr-4 cladding with three-layer structure after Loss of Coolant Accident process has been modeled, with representing different oxidation degrees of the zirconium alloy cladding by the volume fraction of α-Zr (O). The simulation of two-step cooling from 1200 ℃ to 800 ℃ then to room temperature is carried out. The results show that the stress in the oxide film is compressive stress, and the stress gradient of oxide film with the decrease of temperature is the largest, which leads to a larger residual stress at the interface between α-Zr (O) layer and matrix, and the larger the proportion of matrix volume, the smaller the internal residual stress after cooling. In the low oxidation model, the stress of α-Zr (O) layer is basically unchanged at the end of cooling,。However, there is obvious stress concentration when the oxidation degree is higher. The larger compressive strain has been produced in the high oxidation degree model, which leads to greater deformation of the matrix when cooling down. Accordings to the analysis on the corresponding constitutive equations of each structure in different oxidation degree models, it is found that the stress of each layer in the early stage of quenching decreases first and then increases. There is difference of mechanical properties between the matrix close to the edge of α-Zr(O) layer and that close to core structure. For the model with higher oxidation degree, the core compressive stress of matrix structure is higher than that of edge structure under the same strain.