Thermal-hydraulic investigation of the lead-bismuth alloy-cooled wire-wrapped fuel assembly with different pitch to diameter ratios and wire shapes

This study develops a numerical model to investigate the effects of wire wrapping on the flow velocity and temperature distribution in a seven-rod fuel bundle cooling by lead–bismuth eutectic alloy. The enhanced cooling performance of the wire-wrapped rod is compared with the bare rod, and the oscillated local Nusselt numbers along the axial direction in the inter, corner and edge subchannels are analyzed. The Nusselt number oscillation is found strongly correlated with the area change in different subchannels induced by the wrapping wires. In order to reduce the temperature gradient along the axial direction and eliminate local hot spots, the impact of different pitch-to-diameter (P/D) ratios and wire geometries on the flow and heat transfer performances are examined while considering the fixed rated power of the rod bundles. For the central subchannel, a higher P/D ratio leads to significant fluctuation in transverse velocity, reduced temperature and increased Nusselt number. With P/D of 1.29, the average temperature of the central rod is reduced by 13.2 °C. The influence of P/D in the corner and edge subchannels are not significant due to wall effect. Among the wires with circular, rectangular and trapezoidal shapes, the trapezoidal wire shows the highest Nusselt number and the highest TSEF (Transverse Secondary Flow) value indicating the best heat transfer performance.