Multi-physics Coupling Characteristics of Lead-bismuth Reactor Fuel Assembly Based on Oxidative Corrosion Behaviors

Oxygen is the most promising non-metallic inhibitor in lead-bismuth cooled fast reactors (LFRs). The addition of oxygen to the coolant can format a protective oxide layer on the surface of structural materials, which will effectively alleviate the corrosion of structural materials by the liquid lead-bismuth eutectic (LBE). LFR is a complex environment characterized by the interaction of multiple physical fields. For instance, the growth and removal behaviors of the oxide layer are influenced by various factors such as temperature, oxygen concentration, coolant velocity, and time. Moreover, the formation of the oxide layer changes the thermal-hydraulic characteristics and neutronics parameters of the reactor core. Therefore, studying the coupled effects of oxidation corrosion, thermal-hydraulics, and neutronics is of paramount importance for the development, design, and safety assessment of LFRs. A multi-physics framework that couples neutron physics, thermal-hydraulics, and material corrosion was proposed to investigate the multi-physics coupling characteristics of the fuel assembly in LFRs. Within the coupling framework, neutronics calculations were performed using the open-source neutron diffusion equation solver Moltres, thermal-hydraulic calculations were conducted using the Navier-Stokes module included in the multi-physics object-oriented simulation environment (MOOSE) platform, and corrosion calculations were carried out using the Seal module developed based on the MOOSE. The coupling framework involves two types of coupling parameter transfer relationships: 1） The oxidation corrosion field obtains coolant temperature and flow velocity from the thermal-hydraulic field to compute the oxide layer thickness and transfers the oxide layer thickness to the thermal-hydraulic field to calculate the convective heat transfer coefficient; 2） The neutron physics field receives temperature distribution from the thermal-hydraulic field to compute keff, neutron flux distribution and power distribution, and transfers the power distribution to the thermal-hydraulic field for thermal-hydraulic calculations. In terms of numerical system solving, the coupling framework employs the concept of directly coupled equations of the three physical fields and solves them using the Newton-Krylov iteration method. A neutronics-thermal-hydraulics-material coupling problem of a 19-rod bundle fuel assembly in an LFR was computed using the coupling framework, and the effects of oxygen concentration and coolant inlet temperature on the temporal variations of key coupling parameters and the distribution of oxide layers were investigated. The results indicate that under the benchmark condition, after 10 000 hours of oxidation corrosion, the average thickness of the oxide layer on the fuel assembly cladding surface is approximately 9.86 μm. The maximum temperature rises of the fuel and cladding are 13.36 K and 5.63 K, respectively, with a decrease in keff of 7 pcm. Increasing oxygen concentration is beneficial for inhibiting magnetite dissolution and enhancing the self-repair ability of the oxide layer, but the promotion effect of increasing oxygen concentration on the growth of Fe-Cr spinel is limited after reaching a certain concentration. Although raising the coolant inlet temperature leads to an increase removal rate of magnetite on the inner surface of the cladding at the center of the assembly, it significantly promotes the growth of Fe-Cr spinel.