Investigating eutectic behavior and material relocation in B4C-stainless steel composites using the improved MPS method

In nuclear severe accidents, eutectic reactions induce early melting of stainless steel (SS) cladding and boron carbide (B4C), leading to control rod failure and eutectic melt relocation. To accurately simulate eutectic melting, we modified the standard Moving Particle Semi-implicit (MPS) method. The conventional MPS model is inadequate due to its simplistic treatment of surface tension, and viscosity. By revising these parameters and incorporating mass diffusion and eutectic reaction criteria based on the Fe-B phase diagram, the enhanced MPS method can effectively capture the complex behaviors of eutectic melting in both 2D and 3D simulations. The study aims to measure boron concentration through the unidirectional diffusion of boron within the stainless steel (SS) layers while evaluating the updated model's ability to replicate melt relocation behavior and geometry. In the current MPS simulations, one scenario employed dummy walls as heat sources, while another scenario used SS surface particles as heat sources to avoid interference with the melt flow as it reached the bottom of the specimen. The results indicate that upon eutectic reaction, boron diffuses into the SS wall, initiating melting at the B4C-SS interface and leading to melt flow following SS cladding penetration. Also, we observed that as temperature increases, there is a proportional rise in boron concentration within the melt due to enhanced unidirectional diffusion of boron atoms into SS cladding. Additionally, the effect of gravity on boron transport has been assessed, revealing its impact on the diffusion rate. The primary focus of this study lies in assessing the eutectic reaction model in the updated MPS code, particularly examining the formation of the eutectic melt, the concentration of B4C within it, and the resemblance of the final formed melt to the experimental observations.