Neutronics analysis of a research reactor using a two-step method with the superhomogenization method

Research reactors are characterized by significant neutron leakage, tight neutron coupling, and complex core geometries, which make accurate neutronics calculations using the two-step method challenging. This paper analyzes and quantifies the errors introduced by the two-step method for research reactor neutronics calculations. Based on the LVR-15 research reactor, the effects of several key factors, such as the number of energy groups, the homogenization model, and the S_N order, are studied in detail by comparing the computed k_eff and power with reference values. The numerical results indicate that the factors affecting calculation accuracy, in descending order of impact, are the number of energy groups, the homogenization model, and the S_N order. The number of energy groups has the most significant impact on calculation accuracy. Specifically, using too few energy groups, such as 2-group energy structures, leads to significant overestimations of k_eff. To further improve accuracy, an improved superhomogenization (SPH) method is proposed. It can stably maintain the k_eff error below 500 pcm and reduce power prediction errors from 3.08 ∼ 4.67 % and 1.69 ∼ 2.80 % to 1.40 ∼ 3.49 % and 0.60 ∼ 1.98 % in the control-rod-in and control-rod-out cases, respectively. These findings provide valuable reference guidelines for other researchers aiming to achieve more accurate research reactor neutronics calculations based on the two-step method.