核燃料包壳管疲劳破坏边界面法研究

Nuclear fuel cladding tubes are the first barrier to prevent nuclear fuel leakage, whose integrity and reliability are closely related to the safety operation of nuclear power plant. Due to the reactor power fluctuations and the flow of the coolant, the fatigue failure of the cladding tubes occurs easily under cyclic deformation. Because of some inherent defects existing in finite element method, the stress singularity problems in cladding tubes which are thin-walled structures cannot be accurately and efficiently analyzed. In this project, the boundary face method is employed for fatigue and fracture analysis of nuclear fuel cladding tubes. The contents of this project are as follows: (1) high accuracy and fast boundary face method under thermal-mechanical conditions, which can accurately and efficiently obtain the temperature, displacement, stress and strain fields of the cladding tubes; (2) high accuracy method for hypersingular and nearly hypersingular integrals, which can calculate the irregular integrals accurately; (3) new singular elements for different defects, which can accurately capture the displacement and stress fields around the singular points of different defects; (4) geometric characterization techniques of defects, which can accurately reflect the geometric shape and size of defects. The aim of this project is to develop a general boundary face method with high efficiency and accuracy, which can accurately analyze the fatigue failure process of nuclear fuel cladding tubes and predict the fatigue propagation life.

核燃料包壳管是防止核燃料泄漏的第一道屏障，其完整性和可靠性与核电厂的安全运行密切相关。由于核反应堆功率的波动和冷却剂的流动，使包壳管容易产生循环变形，导致包壳管发生疲劳破坏。有限元法存在一些固有缺陷，无法精确高效分析包壳管这类薄壁结构的应力奇异性问题。本项目拟采用边界面法对核燃料包壳管进行疲劳断裂分析，主要研究内容包括：（1）热力耦合条件下高精度快速边界面算法，能够精确高效计算包壳管的温度场、位移场和应力、应变场；（2）高精度超奇异积分和近超奇异积分方案，用于计算超奇异积分方程中的非正则积分；（3）针对不同缺陷的新型奇异单元，能够在统一框架下精确捕捉各类缺陷奇异点附近的位移场和应力场；（4）缺陷几何表征技术，能够真实反映缺陷的几何形状和尺寸。目标是开发一套高效高精度边界面法计算理论和程序，分析核燃料包壳管疲劳破坏过程，精确预测其疲劳扩展寿命。

核燃料包壳管完整包覆燃料芯块，是保证核安全的重要屏障。由于核反应堆功率的波动和芯块-包壳相互作用，使包壳管容易产生循环变形，导致包壳管发生疲劳破坏。为此，本项目开发了一套基于双层插值边界面法的核燃料包壳管疲劳裂纹扩展寿命预测程序。. 本项目主要研究内容、重要结果、关键数据及科学意义如下：（1）高效高精度近奇异积分和奇异积分计算方案。研究了近奇异积分和奇异积分基本形式，根据复变函数基本理论，发现了奇异性同时存在于积分的两个方向。提出了(α,β)变换与非线性变换相结合的方法来计算近奇异积分和奇异积分。当边长比例达到100，最大计算误差保持在1e-13左右，顶端张角达到170度，最大计算误差保持在1e-14左右。（2）高精度新型奇异单元。根据裂纹尖端位移场的性质，研究了基于双层插值法的新型奇异单元构造方法，推导了奇异单元形函数，并验证了新型奇异单元的有效性。（3）核燃料包壳管疲劳裂纹扩展寿命预测程序。重新设计了双层插值边界面法程序框架，研究了如何在程序中实现裂纹自动扩展；在程序中更新了近奇异积分和奇异积分算法，添加了奇异单元类型；此外，对核燃料包壳管材料锆合金进行了疲劳裂纹扩展试验，测得了疲劳参数C和m。最终开发了一套针对核燃料包壳管的疲劳裂纹扩展寿命预测程序，可为核燃料包壳管的设计与安全服役提供参考依据。