Numerical investigation on the thermohydraulic performance of LBE-cooled helical cruciform fuel assembly

Applying helical cruciform fuel (HCF) assembly to the lead-bismuth eutectic (LBE) fast reactor can achieve reactor miniaturization and modular design. The heat transfer performance of a lead-bismuth fast reactor differs significantly from that of a traditional pressurized water reactor. Therefore, studying the flow and heat transfer characteristics of the LBE coolant within HCF assembly is of significant importance. This paper presents a numerical investigation of the flow and heat transfer process of the LBE coolant through the 7-pin HCF assembly. The thermohydraulic characteristics within rod bundle channels under the standard working condition are calculated and analyzed, with focus on the effects of inlet velocity (V_in = 1–2.25 m/s), inlet temperature (T_in = 443.15–503.15 K), fuel rod wall heat flux (q_w = 325–500 kW/m²), and helical pitch (H = 150–900 mm) on heat transfer and flow characteristics and pressure drop characteristics. The results show that the helical structure of the HCF rod significantly influence the flow field, promoting transverse mixing and convective heat transfer of the coolant. And due to the periodic variation in the rod gaps, the flow and heat transfer characteristics fluctuate periodically. Compared with the standard working condition, a lower inlet velocity (1 m/s) reduces the pressure drop by 50.3 % and increases the JF factor by 21 %. A lower inlet temperature (443.15 K) increases the Nusselt number by 1.3 % and enhances the JF factor by 4.8 %. A shorter helical pitch (150 mm) raises the Nusselt number by 13 % and increases the JF factor by 5 %. Considering the comprehensive thermohydraulic performance, the LBE-cooled HCF assembly performs better under the conditions of a lower inlet velocity, a lower inlet temperature and a shorter helical pitch. Meanwhile, the fuel rod heat flux density has almost no impact on the flow and heat transfer characteristics, and the core power density can be moderately increased to enhance the system efficiency.