Best estimate plus uncertainty (BEPU) analysis method has been widely used to analyze various transient accidents of pressurized water reactor (PWR). However, the traditional BEPU method has some limitations: (1) The input parameters are not clearly defined, resulting in inaccurate conclusions in the sensitivity analysis. (2) The uncertainty quantification and sensitivity analysis usually share the same set of samples, but they have different requirements for the sample size. In this work, an improved BEPU method is proposed, which can alleviate the above defects. The improved BEPU method possesses the following two characteristics: (1) The sensitivity analysis is performed for the steady-state and transient calculation, respectively. It provides more comprehensive results than does the traditional BEPU method. (2) The sensitivity analysis is performed before the uncertainty analysis to reduce the number of uncertainty analysis inputs. A small-break loss-of-coolant accident (SBLOCA) is simulated by Reactor Excursion and Leak Analysis Program (RELAP) 5 to verify the accuracy and applicability of the improved BEPU method. By the sensitivity analysis, the coolant pump inlet roughness, main flow rate, core heat channel temperature, break area, and PRZ pressure have moderate or higher relationships with the peak core outlet temperature. The fission product yield factor has a moderate positive relationship with the peak cladding temperature (PCT). The sensitivity analysis by the improved BEPU method shows that the peak core outlet temperature has strong relationships with main flow rate, core heat channel temperature, and PRZ pressure, which is not captured by the traditional BEPU method. As a result, it is more reasonable to take steady-state parameters as inputs in the sensitivity analysis of transient. Only those parameters with high correlation coefficients are sampled for uncertainty analysis. Meanwhile, the results of the uncertainty analysis obtained by the improved BEPU are consistent with the results of the traditional method. Neither the PCT nor the peak core outlet temperature will exceed their limits. The results illustrate that the improved BEPU method can reduce the size of samples but maintains the desired accuracy.

最佳估计加不确定度（BEPU）分析方法已被广泛用于分析压水堆（PWR）的各种瞬态事故。但是，传统的BEPU方法有一些局限性：（1）输入参数未明确定义，导致灵敏度分析的结论不准确。（2）不确定度定量分析和灵敏度分析通常共享同一组样本，但是对样本大小有不同的要求。在这项工作中，提出了一种改进的BEPU方法，可以减轻上述缺陷。改进的BEPU方法具有以下两个特点：（1）分别对稳态和瞬态计算进行灵敏度分析。它提供了比传统BEPU方法更全面的结果。（2）在不确定性分析之前进行敏感性分析，以减少不确定性分析输入的数量。反应堆偏移和泄漏分析程序（RELAP）5模拟了一个小中断的冷却液损失事故（SBLOCA），以验证改进的BEPU方法的准确性和适用性。通过敏感性分析，冷却剂泵的入口粗糙度，主流量，堆芯热通道温度，断裂面积和PRZ压力与堆芯出口峰值温度具有中等或较高的关系。裂变产物的屈服因数与峰值包层温度（PCT）具有适度的正关系。通过改进的BEPU方法进行的灵敏度分析表明，峰值堆芯出口温度与主流量，堆芯热通道温度和PRZ压力有很强的关系，传统的BEPU方法无法捕获。结果，在瞬态灵敏度分析中将稳态参数作为输入更为合理。仅对那些具有高相关系数的参数进行采样才能进行不确定性分析。同时，改进的BEPU获得的不确定性分析结果与传统方法的结果一致。PCT和核心出口峰值温度都不会超过其限制。结果表明，改进的BEPU方法可以减小样本大小，但保持所需的精度。同时，改进的BEPU获得的不确定性分析结果与传统方法的结果一致。PCT和核心出口峰值温度都不会超过其限制。结果表明，改进的BEPU方法可以减小样本大小，但保持所需的准确性。同时，改进的BEPU获得的不确定性分析结果与传统方法的结果一致。PCT和核心出口峰值温度都不会超过其限制。结果表明，改进的BEPU方法可以减小样本大小，但保持所需的精度。