Research on the Influence on Neutronic Characteristics of Fine Nuclide Density in EBR-II Core based on LoongSARAX

[Background] Solving benchmark problems is a significant step in the validation of numerical simulation programs. The Experimental Breeder Reactor II (EBR-II) is a famous benchmark for sodium-cooled fast reactors (SFR), with a complicated spatial distribution of nuclide density. Therefore, simplification to the spatial distribution of nuclide density was adopted in many studies on EBR-II benchmark calculation. [Purpose] This study aims to contrast the difference between the results of the fine model and the simplified model, evaluating the rationality of the simplification. [Methods] In this study, both the fine model and the simplified one were built using LoongSARAX, a neutronic numerical program for fast reactors developed by Xi’an Jiao Tong University. Some approximations were adopted in the models: one-dimensional homogenization was adopted for the half-worth driver assembly to handle its complex radial geometry and the super-assembly method was used in the cross-section generation of poison elements. [Results] The results show that in the simplified model, 1) the spatial distribution of fuel nuclide density presents strong asymmetry and strong non-uniformity, 2) calculation time spent in the simplified model is one-tenth of that in the fine model, 3) the effective multiplication factor (keff) is 1383 pcm lower than in the fine model, 4) the spatial distribution of neutron flux is lower in the center and higher in the outer core, compared to that in the fine model, 5) the maximum relative deviation between neutron flux in two models is 4.25%. [Conclusion] In summary, the simplified model has a much lower calculation cost but limited numerical accuracy in keff and neutron flux, compared to the fine model.

[背景] 基准问题求解是数值模拟程序验证工作中的重要环节。实验增殖堆二号（Experimental Breeder Reactor II, EBR-II）是钠冷快堆（sodium-cooled fast reactors, SFR）领域极具代表性的基准题，其核素密度的空间分布极为复杂。因此在诸多针对EBR-II基准题的计算研究中，学界普遍采用对核素密度空间分布进行简化的处理方案。
[目的] 本研究旨在对比精细模型与简化模型的计算结果差异，以此评估该简化处理方案的合理性。
[方法] 本研究采用西安交通大学自主研发的快堆中子学数值程序LoongSARAX，分别搭建了EBR-II的精细模型与简化模型。上述模型中采用了两类近似处理：其一，针对半价值驱动组件的复杂径向几何结构，采用一维均匀化方法简化其几何描述；其二，在毒物组件的截面生成环节，采用超组件（super-assembly）方法。
[结果] 计算结果显示，相较于精细模型，该简化模型呈现以下特征：1）燃料核素密度的空间分布具有显著的非对称性与强不均匀性；2）简化模型的计算耗时仅为精细模型的十分之一；3）有效增殖因数（effective multiplication factor, k_eff）较精细模型低1383 pcm；4）中子通量的空间分布呈现“中心低、外围高”的特征，与精细模型的分布趋势存在差异；5）两个模型的中子通量最大相对偏差为4.25%。
[结论] 综上，相较于精细模型，该简化模型的计算成本大幅降低，但在有效增殖因数与中子通量的数值精度上仅表现有限，整体误差水平仍需关注。