Multi-level cross entropy (MCE) method is introduced to realize the efficient and accurate estimation of thermal-hydraulic reliability (TH-R) of passive systems.The method is applicable to reliability problems with small failure probability and implicit model.Under six update strategies, the TH-R of an experimental facility is evaluated.Numerical results show that system's failure probability can be estimated accurately with less computational cost based on MCE method, and the coefficient of variation is small.Relatively small intermediate failure probability and smoothing parameter can make the sampling center of importance sampling closer to the important area near the design point, and the evaluation accuracy is higher.Related methods and results can lead to further understanding of thermal-hydraulic uncertainty of passive system from the view of reliability, and can be used as reference for passive system's performance evaluation and optimization.%为高效、精确地进行非能动系统热工水力可靠性(thermal-hydraulic reliability,TH-R)评估,引入多层交叉熵(multi-level cross entropy,MCE)方法,该方法适用于小失效率、隐式模型问题.以某型核动力装置非能动余热排出试验系统为算例进行验证,给出6种分布参数更新策略下的系统失效率估计过程.仿真结果表明:MCE方法能在较少运算量下精确估计出系统失效率,且估计值变异系数较小;选择相对较小的中间失效率和平滑参数,能使重要抽样密度函数的采样中心更加靠近设计点附近的重要区域,提高评估精度.相关方法和结论有助于从可靠性角度增强对非能动系统热工水力过程不确定性的认识,可作为非能动系统性能评估与优化的参考.
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