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.