Study on Eutectic Melting Behavior of Control Rod Materials in Severe Accidents of Sodium-Cooled Fast Reactors (4) Analyzing Eutectic Melting and Relocation Dynamics in B4C-Stainless Steel Using the Moving Particle Semi-Implicit (MPS) Method

Abstract

In severe nuclear accidents, eutectic reactions cause stainless steel (SS) cladding and boron carbide (B₄C) to melt early, leading to control rod failure and eutectic melt relocation. To simulate this, a modified moving particle semi-implicit (MPS) method is used for 2D and 3D simulations of a real control rod B₄C-SS specimen, considering mass diffusion and eutectic reaction criteria based on Fe-B phase diagram. The eutectic reaction model implemented in the MPS code was validated against our prior experimental data on B₄C-SS. The objective of the study is to numerically measure the boron concentration within the solidified eutectic melt. The results show that once eutectic reaction occurs, the boron diffuses into the SS wall. Melting initiates at the B₄C and SS interface, with melt flow following SS cladding penetration. As temperature increases, boron concentration within the melt proportionally rises due to enhanced unidirectional diffusion into SS cladding. However, within the SS layers at a specific temperature, boron concentration decreases away from the interface due to concentration gradients. The current model accurately replicated the eutectic melt's geometry, aligning closely with experimental observations. It also reflected a nearly 13% rise in boron's atomic composition within the eutectic melt, similar to the approximately 15% increase noted in our experiments. The updated MPS method indicates a computational capability of the eutectic reaction model used to effectively analyze control rod eutectic reactions, simulating severe accidents, and their subsequent relocation to understand the effect of B₄C ingress into the core.