Numerical study on cladding melting and melt migration behavior in lead-bismuth coolant using the improved MPS method

The behavior of cladding melting and melt migration is an important phenomenon during the early stage of core disruptive accidents (CDA) in lead-cooled fast reactors (LFRs), which can affect the accident evolution trend and severity. In this paper, the Moving Particle Semi-implicit (MPS) method with the advantage of phase-interface capture was employed to further reveal this key phenomenon. In the simulation, the cladding was regarded as a high-viscosity fluid, and the viscosity function related to solid fraction was used. Meanwhile, the modified algorithm with the viscosity term being placed after the pressure term was adopted to ensure the cladding was stationary before the temperature reached the melting point. The computational model was verified by the experiments of tin cladding tube melting and melt migration in seven-pin bundle. After that, the effects of surface tension coefficient, fuel initial temperature and coolant flow speed on cladding melting and melt migration in lead-bismuth coolant were investigated with the improved MPS method. The results indicated that surface tension promoted the melt to adhere on fuel rod surface and the melt-coolant contact area decreased when the surface tension was introduced. The fuel initial temperature with "center-peaked distribution" caused a temperature gradient in the flow channel and led to re-solidified metal at the upper part of the fuel rod. Compared with the "uniform distribution" case, the size of molten droplet in "center-peaked distribution" case was smaller and more fragmented droplets could be observed. When the lead-bismuth coolant was flowing, the enhanced heat transfer accelerated the cooling of melt as well as softened cladding, and the solidified melt blocked the flow channel at the midstream and downstream regions.