Dynamic visualization of eutectic reaction between boron carbide and stainless steel

To investigate the eutectic reaction process of control-rod materials in a boiling water reactor (BWR), fundamental tests using boron carbide (B₄C) powder inserted between stainless steel (SS) plates were performed and dynamically visualized. The eutectic reaction process near the contact area of the two materials and the behavior of molten material and B₄C powder were visualized in real time. The temperature, reaction area, and maximum reaction-layer thickness were obtained. The average temperature range of the test was 1455–1481 K. Through dynamic visualization, some important and previously undiscovered phenomena were observed. The solid part of the SS plate and the strong surface tension of the melt retained the melt inside the specimen, preventing it from flowing out from the surface; the melt then invaded the B₄C powder region during the reaction. Diffusion of the B₄C powder and migration of the nonreacted B₄C powder from the B₄C powder region through the retained melt to the SS region were observed. This migration accelerated the local reaction growth rate. The time-resolved observation of these dynamic phenomena offers significant insights to the improvement of numerical calculation codes for severe accident analyses of BWRs, including the Fukushima Daiichi nuclear reactors.