截面一致性和协方差数据库对不确定性分析的影响研究

Research on impact of cross section consistence and different covariance library in the process of uncertainty analysis

核数据是反应堆物理计算中最基本的输入参数,同时也是反应堆物理输出响应的不确定度重要来源之一。由于存在数值近似以及模型简化等原因,导致积分实验测量的截面值会存在一定的不确定度。为了更好地规范核数据不确定性分析流程并评估分析过程中相关因素对分析结果的影响,基于自主开发统计抽样法的SUACL程序,研究了影响核数据不确定性分析的两大重要因素:截面初值与协方差矩阵。利用三哩岛核电厂1号机组(Three Mile Island unit 1, TMI-1)栅元基准题分析两者对不确定性分析结果的影响。发现设定扰动一定,计算截面初值越大,引入的不确定度更大,且计算截面初值与截面的理论值越匹配,分析结果与参考值更符合。而基于不同核评价数据库产生的协方差数据下,分析的不确定度结果有明显偏差,以238U的裂变截面为例,基于JEFF3.2产生的协方差数据库分析的不确定度比基于ENDF/B-VII.1与JENDL4.0产生的协方差计算的值小4个数量级。说明采用准确的截面初值与基于与分析堆型相关核评价数据库产生的协方差数据对截面不确定性分析非常重要。

[Background] The nuclear data is the fundamental input parameter of reactor physics computation and it is also one of the significant uncertainty sources to the result of nuclear physics calculation. Usually, the cross section is measured by the integral experiment. However, the cross section obtained from the experiment is always imprecise due to numerical approximation and model simply in the experiment. [Purpose] This study aims to evaluate the effect caused by relative factors of cross section consistence and different covariance library in the process of uncertainty analysis. [Methods] The home-developed code SUACL based on statistical sampling method was developed for the uncertainty analysis, and the Three Mile Island unit 1 （TMI-1） benchmark was taken to evaluate the impact of both the initial value of cross section and the various covariance matrices. [Results] It is found that the larger of the cross section value, the higher uncertainty caused by it in the same degree of perturbation. Therefore, it is better to use the practical initial value of cross section. The uncertainty of infinite effective multiplication factor would be different based on different evaluated neutron data library such as ENDF/B-VII. 1, JENDL4.0 and JEFF3.2. For example, the uncertainty of ^238U fission cross section based on JEFF3.2 is smaller four orders of magnitude than that of ENDF/B-VII. 1 and JENDL4.0. [Conclusion] The appropriate covariance data and precise unperturbed initial value of cross section are critical to analysis uncertainty. Both of them have great impact on the uncertainty result.