Impact of PbO Fouling Layer on Steady-state Heat Transfer Performance of Wire-wrapped Fuel Assemblies for Lead-Bismuth Reactors

To investigate the effects of surface fouling on thermal-hydraulic performance of wire-wrapped fuel assemblies in lead-bismuth cooled reactors, a steady-state numerical model was developed for a 19-rod bundle with uniform lead oxide (PbO) deposits. Considering the fluid-solid conjugate heat transfer between lead-bismuth and fuel cladding, wire-wrapped and fouling layer, the effects of fouling layer thickness, lead-bismuth mass flow rate and total heating power on temperature and flow fields of the fuel assembly were analyzed. And then the distributions of lead-bismuth temperature and flow velocity in the assembly and the variation of fuel cladding wall temperature were obtained. Results show that PbO fouling layer with a thinner surface coverage(≤40 μm)has little effect on the overall heat transfer coefficient of the fuel assembly, but it will cause the cladding temperature to rise. Increasing fouling thickness produces uniform axial distribution of maximum circumferential temperature on the assembly cladding outer wall while inducing nonlinear circumferential variations across sections. After increasing lead-bismuth flow rate and reducing heating power, the maximum temperature rise of fouling cladding outer wall decreases. Meanwhile, the influence of boundary conditions on cladding temperature intensifies with increasing fouling thickness. Compared with the case where the surface is uniformly covered fouling layer, local blockage caused by fouling deposition and component shedding will lead to a sudden increase in the local temperature of the cladding, which should be avoided.